中国物理B

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An extension of integrable equations related to AKNS and WKI spectral problems and their reductions

耿献国, 翟云云

2018 (4): 40201-040201. doi: 10.1088/1674-1056/27/4/040201

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A novel hierarchy of integrable nonlinear evolution equations related to the combined Ablowitz-Kaup-Newell-Segur (AKNS) and Wadati-Konno-Ichikawa (WKI) spectral problems is proposed, from which the Lax pair for a corresponding negative flow and its infinite many conservation laws are obtained. Furthermore, a reduction of this hierarchy is discussed, by which a generalized sinh-Gordon equation is derived on the basis of its negative flow.

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Integrability classification and exact solutions to generalized variable-coefficient nonlinear evolution equation

刘汉泽, 张丽香

2018 (4): 40202-040202. doi: 10.1088/1674-1056/27/4/040202

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This paper is concerned with the generalized variable-coefficient nonlinear evolution equation (vc-NLEE). The complete integrability classification is presented, and the integrable conditions for the generalized variable-coefficient equations are obtained by the Painlevé analysis. Then, the exact explicit solutions to these vc-NLEEs are investigated by the truncated expansion method, and the Lax pairs (LP) of the vc-NLEEs are constructed in terms of the integrable conditions.

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Constructing (2+1)-dimensional N=1 supersymmetric integrable systems from the Hirota formalism in the superspace

王建勇, 唐晓艳, 梁祖峰

2018 (4): 40203-040203. doi: 10.1088/1674-1056/27/4/040203

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The N=1 supersymmetric extensions of two integrable systems, a special negative Kadomtsev-Petviashvili (NKP) system and a (2+1)-dimensional modified Korteweg-de Vries (MKdV) system, are constructed from the Hirota formalism in the superspace. The integrability of both systems in the sense of possessing infinitely many generalized symmetries are confirmed by extending the formal series symmetry approach to the supersymmetric framework. It is found that both systems admit a generalization of W_∞ type algebra and a Kac-Moody-Virasoro type subalgebra. Interestingly, the first one of the positive flow of the supersymmetric NKP system is another N=1 supersymmetric extension of the (2+1)-dimensional MKdV system. Based on our work, a hypothesis is put forward on a series of (2+1)-dimensional supersymmetric integrable systems. It is hoped that our work may develop a straightforward way to obtain supersymmetric integrable systems in high dimensions.

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Shannon information entropies for rectangular multiple quantum well systems with constant total lengths

M Solaimani, 孙国华, 董世海

2018 (4): 40301-040301. doi: 10.1088/1674-1056/27/4/040301

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We first study the Shannon information entropies of constant total length multiple quantum well systems and then explore the effects of the number of wells and confining potential depth on position and momentum information entropy density as well as the corresponding Shannon entropy. We find that for small full width at half maximum (FWHM) of the position entropy density, the FWHM of the momentum entropy density is large and vice versa. By increasing the confined potential depth, the FWHM of the position entropy density decreases while the FWHM of the momentum entropy density increases. By increasing the potential depth, the frequency of the position entropy density oscillation within the quantum barrier decreases while that of the position entropy density oscillation within the quantum well increases. By increasing the number of wells, the frequency of the position entropy density oscillation decreases inside the barriers while it increases inside the quantum well. As an example, we might localize the ground state as well as the position entropy densities of the 1st, 2nd, and 6th excited states for a four-well quantum system. Also, we verify the Bialynicki-Birula-Mycieslki (BBM) inequality.

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Comparative investigation of freezing phenomena for quantum coherence and correlations

杨连武, 韩伟, 夏云杰

2018 (4): 40302-040302. doi: 10.1088/1674-1056/27/4/040302

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We show that the freezing phenomenon, exhibited by a specific class of two-qubit state under local nondissipative decoherent evolutions, is a common feature of the relative entropy measure of quantum coherence and correlation. All those measurement outcomes, preserve a constant value in the considered noisy channels, but the condition, property and mechanism of the freezing phenomenon for quantum coherence are different from those of the quantum correlation.

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Dynamics of entanglement protection of two qubits using a driven laser field and detunings: Independent and common, Markovian and/or non-Markovian regimes

S Golkar,M K Tavassoly

2018 (4): 40303-040303. doi: 10.1088/1674-1056/27/4/040303

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Preventing quantum entanglement from decoherence effect is of theoretical and practical importance in the quantum information processing technologies. In this regard, we consider the entanglement dynamics of two identical qubits where the qubits which are coupled to two independent (Markovian and/or non-Markovian) as well as a common reservoir at zero temperature are further interacted with a classical driving laser field. Then, we study the preservation of generated two-qubit entanglement in various situations using the concurrence measure. It is shown that by applying the classical driving field and so the possibility of controlling the Rabi frequency, the amount of entanglement of the two-qubit system is improved in the off-resonance condition between the qubit and the central cavity frequencies (central detuning) in both non-Markovian and Markovian reservoirs. While the central detuning has a constructive role, the detuning between the qubit and the classical field (laser detuning) affects negatively on the entanglement protection. The obtained results show that long-living entanglement in the non-Markovian reservoir is more accessible than in the Markovian reservoir. We demonstrate that, in a common reservoir non-zero stationary entanglement is achievable whenever the two-qubit system is coupled to the reservoir with appropriate values of relative coupling strengths.

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Controlling the entanglement of mechanical oscillators in composite optomechanical system

张俊, 穆青霞, 张闻钊

2018 (4): 40304-040304. doi: 10.1088/1674-1056/27/4/040304

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A controllable entanglement scheme of two mechanical oscillators is proposed in a composite optomechanical system. In the case of strong driving and high dissipation, the dynamics of the movable mirror of the optomechanical cavity is characterized by an effective frequency in the long-time evolution of the system. Considering the classical nonlinear effects in an optomechanical system, we investigate the relationship between the effective frequency of the movable mirror and the adjustable parameters of the cavity. It shows that the effective frequency of the movable mirror can be adjusted ranging from ω_m (the resonance frequency of the coupling oscillator) to -ω_m. Under the condition of experimental realization, we can generate and control steady-state entanglement between two oscillators by adjusting the effective frequency of the movable mirror and reducing the effective dissipation by selecting the parameter of the cavity driving laser appropriately. Our scheme provides a promising platform to control the steady-state behavior of solid-state qubits using classical manipulation, which is significant for quantum information processing and fundamental research.

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Geometrical optics-based ray field tracing method for complex source beam applications

高敏, 杨峰, 崔学武, 王瑞

2018 (4): 40401-040401. doi: 10.1088/1674-1056/27/4/040401

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Due to the fact that traditional ray field tracking approaches require a large number of geometrical optical (GO) ray tubes, they are very inefficient in many practical applications. An improved ray model scheme for a complex source beam (CSB) tracking technique is proposed in this paper. The source field can be expressed by a superposition of CSBs, then every CSB basis function has a Gaussian-type amplitude distribution and is suitable for replacing a GO ray tube in the ray tracing approach. The complex phase matching technique is adopted to find the reflected beam in the reflection point where local approximation is used to represent the curved surface in its neighborhood. A new solution to multiple reflections using the conventional right-handed reflected system is used to track the field easily. Numerical results show the accuracy of the proposed method.

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Nucleus-acoustic solitary waves in self-gravitating degenerate quantum plasmas

D M S Zaman,M Amina,P R Dip,A A Mamun

2018 (4): 40402-040402. doi: 10.1088/1674-1056/27/4/040402

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Nucleus-acoustic (NA) solitary waves (SWs) propagating in a self-gravitating degenerate quantum plasma (SDQP) system (containing non-relativistically degenerate heavy and light nuclei, and non-/ultra-relativistically degenerate electrons) have been theoretically investigated. The modified Korteweg-de Vries (mK-dV) equation has been derived for both planar and non-planar geometry by employing the reductive perturbation technique. It is shown that the NA SWs exist with positive (negative) electrostatic (self-gravitational) potential. It is also observed that the effects of non-/ultra-relativistically degenerate electron pressure, dynamics of non-relativistically light nuclei, spherical geometry, etc. significantly modify the basic features (e.g., amplitude, width, speed, etc.) of the NA SWs. The applications of our results, which are relevant to astrophysical compact objects, like white dwarfs and neutron stars, are briefly discussed.

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Stochastic resonance and synchronization behaviors of excitatory-inhibitory small-world network subjected to electromagnetic induction

张晓函, 刘深泉

2018 (4): 40501-040501. doi: 10.1088/1674-1056/27/4/040501

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The phenomenon of stochastic resonance and synchronization on some complex neuronal networks have been investigated extensively. These studies are of great significance for us to understand the weak signal detection and information transmission in neural systems. Moreover, the complex electrical activities of a cell can induce time-varying electromagnetic fields, of which the internal fluctuation can change collective electrical activities of neuronal networks. However, in the past there have been a few corresponding research papers on the influence of the electromagnetic induction among neurons on the collective dynamics of the complex system. Therefore, modeling each node by imposing electromagnetic radiation on the networks and investigating stochastic resonance in a hybrid network can extend the interest of the work to the understanding of these network dynamics. In this paper, we construct a small-world network consisting of excitatory neurons and inhibitory neurons, in which the effect of electromagnetic induction that is considered by using magnetic flow and the modulation of magnetic flow on membrane potential is described by using memristor coupling. According to our proposed network model, we investigate the effect of induced electric field generated by magnetic stimulation on the transition of bursting phase synchronization of neuronal system under electromagnetic radiation. It is shown that the intensity and frequency of the electric field can induce the transition of the network bursting phase synchronization. Moreover, we also analyze the effect of magnetic flow on the detection of weak signals and stochastic resonance by introducing a subthreshold pacemaker into a single cell of the network and we find that there is an optimal electromagnetic radiation intensity, where the phenomenon of stochastic resonance occurs and the degree of response to the weak signal is maximized. Simulation results show that the extension of the subthreshold pacemaker in the network also depends greatly on coupling strength. The presented results may have important implications for the theoretical study of magnetic stimulation technology, thus promoting further development of transcranial magnetic stimulation (TMS) as an effective means of treating certain neurological diseases.

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A new nonlinear oscillator with infinite number of coexisting hidden and self-excited attractors

唐妍霞, Abdul Jalil M Khalaf, Karthikeyan Rajagopal, Viet-Thanh Pham, Sajad Jafari, 田野

2018 (4): 40502-040502. doi: 10.1088/1674-1056/27/4/040502

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In this paper, we introduce a new two-dimensional nonlinear oscillator with an infinite number of coexisting limit cycles. These limit cycles form a layer-by-layer structure which is very unusual. Forty percent of these limit cycles are self-excited attractors while sixty percent of them are hidden attractors. Changing this new system to its forced version, we introduce a new chaotic system with an infinite number of coexisting strange attractors. We implement this system through field programmable gate arrays.

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A new four-dimensional chaotic system with first Lyapunov exponent of about 22, hyperbolic curve and circular paraboloid types of equilibria and its switching synchronization by an adaptive global integral sliding mode control

Jay Prakash Singh, Binoy Krishna Roy, 魏周超

2018 (4): 40503-040503. doi: 10.1088/1674-1056/27/4/040503

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This paper presents a new four-dimensional (4D) autonomous chaotic system which has first Lyapunov exponent of about 22 and is comparatively larger than many existing three-dimensional (3D) and 4D chaotic systems. The proposed system exhibits hyperbolic curve and circular paraboloid types of equilibria. The system has all zero eigenvalues for a particular case of an equilibrium point. The system has various dynamical behaviors like hyperchaotic, chaotic, periodic, and quasi-periodic. The system also exhibits coexistence of attractors. Dynamical behavior of the new system is validated using circuit implementation. Further an interesting switching synchronization phenomenon is proposed for the new chaotic system. An adaptive global integral sliding mode control is designed for the switching synchronization of the proposed system. In the switching synchronization, the synchronization is shown for the switching chaotic, stable, periodic, and hybrid synchronization behaviors. Performance of the controller designed in the paper is compared with an existing controller.

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Time-varying formation for general linear multi-agent systems via distributed event-triggered control under switching topologies

王金环, 许玉玲, 张建, 杨德东

2018 (4): 40504-040504. doi: 10.1088/1674-1056/27/4/040504

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This paper investigates the time-varying formation problem for general linear multi-agent systems using distributed event-triggered control strategy. Different from the previous works, to achieve the desired time-varying formation, a distributed control scheme is designed in an event-triggered way, in which for each agent the controller is triggered only at its own event times. The interaction topology among agents is assumed to be switching. The common Lyapunov function as well as Riccati inequality is applied to solve the time-varying formation problem. Moreover, the Zeno behavior of triggering time sequences can be excluded for each agent. Finally, a simulation example is presented to illustrate the effectiveness of the theoretical results.

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Symmetry and asymmetry rogue waves in two-component coupled nonlinear Schrödinger equations

李再东, 霍丛哲, 李秋艳, 贺鹏斌, 徐天赋

2018 (4): 40505-040505. doi: 10.1088/1674-1056/27/4/040505

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By means of the modified Darboux transformation we obtain some types of rogue waves in two-coupled nonlinear Schrödinger equations. Our results show that the two components admits the symmetry and asymmetry rogue wave solutions, which arises from the joint action of self-phase, cross-phase modulation, and coherent coupling term. We also obtain the analytical transformation from the initial seed solution to unique rogue waves with the bountiful pair structure. In a special case, the asymmetry rogue wave can own the spatial and temporal symmetry gradually, which is controlled by one parameter. It is worth pointing out that the rogue wave of two components can share the temporal inversion symmetry.

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Quantum parameter estimation in a spin-boson dephasing quantum system by periodical projective measurements

杨乐, 戴宏毅, 张明

2018 (4): 40601-040601. doi: 10.1088/1674-1056/27/4/040601

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In this paper, we explore how to estimate the phase damping parameter γ and the tunneling amplitude parameter △ from a spin-boson dephasing quantum model by periodical projective measurements. The preparation of initial states is accomplished by performing the period measurements in our scheme. The parameter γ can be always estimated when projective measurement bases are chosen as θ=π/2 and φ=0. Based on the estimated value of γ and the interval information of △, we can select another measurement bases (θ=π/4 and φ=π/2) to obtain the estimated value of △. A coherent control is indispensable to estimate △ if γ is in the interval of △; whereas the control is not necessary if γ is out of the known interval of △. We establish the relation between the optimal period time and the parameter γ or △ in terms of Fisher information. Although the optimal measurement period cannot be selected beforehand, the aforementioned relation can be utilized to adjust the measurement period to approach the optimal one.

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Highly-sensitive NO, NO₂, and NH₃ measurements with an open-multipass cell based on mid-infrared wavelength modulation spectroscopy

陈祥, 杨晨光, 胡迈, 沈建康, 牛二超, 许振宇, 范雪丽, 魏敏, 姚路, 何亚柏, 刘建国, 阚瑞峰

2018 (4): 40701-040701. doi: 10.1088/1674-1056/27/4/040701

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A compact prototype based on mid-infrared wavelength modulation spectroscopy (WMS) is developed for the simultaneous monitoring of NO, NO₂, and NH₃ in the urban area. Three quantum cascade lasers (QCLs) with central frequencies around 1900.0 cm^-1, 1600.0 cm^-1, and 1103.4 cm^-1 are used for NO, NO₂, and NH₃ detections, respectively, by time-division multiplex. An open-path multi-pass cell of 60-m optical path length is applied to the instrument for high sensitivity and reducing the response time to less than 1 s. The prototype achieves a sub-ppb detection limit for all the three target gases with an average time of about 100 s. The instrument is installed in the Jiangsu environmental monitoring center to conduct performance tests on ambient air. Continuous 24-hour measurements show good agreement with the results of a reference instrument based on the chemiluminescence technique.

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Selection rules for electric multipole transition of triatomic molecule in scattering experiments

田红春, 徐龙泉, 朱林繁

2018 (4): 43101-043101. doi: 10.1088/1674-1056/27/4/043101

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In the electron or x-ray scattering experiment, the measured spectra at larger momentum transfer are dominated by the electric dipole-forbidden transitions, while the corresponding selection rules for triatomic molecules have not been clearly elucidated. In this work, based on the molecular point group, the selection rules for the electric multipolarities of the electronic transitions of triatomic molecules are derived and summarized into several tables with the variation of molecular geometry in the transition process being considered. Based on the summarized selection rules, the electron energy loss spectra of H₂O, CO₂, and N₂O are identified, and the momentum transfer dependence behaviors of their valence-shell excitations are explained.

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High-level theoretical study of the evolution of abundances and interconversion of glycine conformers

刘凡, 于静, 黄艳茹

2018 (4): 43102-043102. doi: 10.1088/1674-1056/27/4/043102

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The relative conformer energies of glycine are evaluated by using a focal point analysis expressed as (HF →MP2 →MP3 →CCSD →CCSD(T)). The conformer abundances at various temperatures (298-500 K) are calculated based on the relative energies and Boltzmann statistical thermostatistical analysis with and without considering internal hindered rotations. A comparison between the available Raman spectrum and the electron momentum spectrum confirms that the influence of rigid-rotor hindered rotation on the conformational proportions of glycine is considerable, especially for the Ⅲp structure. The conformational interconversions are discussed. It is found that with increasing temperature, the mole fraction of Ⅱn keeps constant and Ip structure can convert into IVn and Ⅲp, leading to the decrease in the weight of Ip and the increase in the weights of IVn and Ⅲp conformers, which is in accordance with experimental observations.

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Density functional theory study of structural stability for gas hydrate

郭平, 邱奕龙, 李龙龙, 罗强, 赵建飞, 潘意坤

2018 (4): 43103-043103. doi: 10.1088/1674-1056/27/4/043103

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Using the first-principles method based on the density functional theory (DFT), the structures and electronic properties of different gas hydrates (CO₂, CO, CH₄, and H₂) are investigated within the generalized gradient approximation. The structural stability of methane hydrate is studied in this paper. The results show that the carbon dioxide hydrate is more stable than the other three gas hydrates and its binding energy is -2.36 eV, and that the hydrogen hydrate is less stable and the binding energy is -0.36 eV. Water cages experience repulsion from inner gas molecules, which makes the hydrate structure more stable. Comparing the electronic properties of two kinds of water cages, the energy region of the hydrate with methane is low and the peak is close to the left, indicating that the existence of methane increases the stability of the hydrate structure. Comparing the methane molecule in water cages and a single methane molecule, the energy of electron distribution area of the former is low, showing that the filling of methane enhances the stability of hydrate structure.

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Novel potential energy surface-based quantum dynamics of ion-molecule reaction O⁺+D₂ →OD⁺+D

王宪龙, 高峰, 高守宝, 张路路, 宋玉志, 孟庆田

2018 (4): 43104-043104. doi: 10.1088/1674-1056/27/4/043104

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According to a novel electronic ground-state potential energy surface of H₂O⁺(X⁴A"), we calculate the reaction probabilities and the integral cross section for the titled reaction O⁺+D₂ →OD⁺+D by the Chebyshev wave packet propagation method. The reaction probabilities in a collision-energy range of 0.0 eV-1.0 eV show an oscillatory structure for the O⁺+D₂ reaction due to the existence of the potential well. Compared with the results of Martínez et al., the present integral cross section is large, which is in line with experimental data.

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Dynamic stabilization of Na atom in an intense pulsed laser field

郭晓丽, 赵松峰, 王国利, 周效信

2018 (4): 43201-043201. doi: 10.1088/1674-1056/27/4/043201

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We investigate theoretically the ionization properties of the valence electron for the alkali metal atom Na in an intense pulsed laser field by solving numerically the time-dependent Schrödinger equation with an accurate l-dependent model potential. By calculating the variations of the ionization probabilities with laser peak intensity for wavelengths ranging from 200 nm to 600 nm, our results present a dynamic stabilization trend for the Na atom initially in its ground state (3s) and the excited states (3p and 4s) exposed to an intense pulsed laser field. Especially a clear “window” of dynamic stabilization at lower laser intensities and longer wavelengths for the initial state 4s (the second excited state) is found. By analyzing the time-dependent population distributions of the valence electron in the bound states with the different values of principal quantum number n and orbital quantum number l, we can attribute the dynamic stabilization to the periodic population in the low-excited states since the valence electron oscillates rapidly between the lowly excited states and the continuum states.

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Corrections to atomic ground state energy due to interaction between atomic electric quadrupole and optical field

胡洁, 陈宇, 白伊秀, 何培松, 孙青, 纪安春

2018 (4): 43202-043202. doi: 10.1088/1674-1056/27/4/043202

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We study the ground state energy of an atom interacting with an oscillating optical field with electric dipole and quadrupole coupling. Under the rotating wave approximation, we derive the effective atomic Hamiltonians of the dipole/quadrupole coupling term within the perturbation theory up to the second order. Based on the effective Hamiltonians, we analyze the atomic ground-state energy corrections of these two processes in detail. As an application, we find that for alkali-like atoms, the energy correction from the quadrupole coupling is negligible small in comparison with that from the dipole coupling, which justifies the so-called dipole approximation used in literatures. Some special cases where the quadrupole interaction may have considerable energy corrections are also discussed. Our results would be beneficial for the study of atom-light interaction beyond dipole approximation.

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Quantitative evaluation of space charge effects of laser-cooled three-dimensional ion system on a secular motion period scale

杜丽军, 宋红芳, 陈邵龙, 黄垚, 童昕, 管桦, 高克林

2018 (4): 43701-043701. doi: 10.1088/1674-1056/27/4/043701

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In this paper, we introduce a method of quantitatively evaluating and controlling the space charge effect of a laser-cooled three-dimensional (3D) ion system in a linear Paul trap. The relationship among cooling efficiency, ion quantity, and trapping strength is analyzed quantitatively, and the dynamic space distribution and temporal evolution of the 3D ion system on a secular motion period time scale in the cooling process are obtained. The ion number influences the eigen-micromotion feature of the ion system. When trapping parameter q is~0.3, relatively ideal cooling efficiency and equilibrium temperature can be obtained. The decrease of axial electrostatic potential is helpful in reducing the micromotion heating effect and the degradation in the total energy. Within a single secular motion period under different cooling conditions, ions transform from the cloud state (each ion disperses throughout the envelope of the ion system) to the liquid state (each ion is concentrated at a specific location in the ion system) and then to the crystal state (each ion is subjected to a fixed motion track). These results are conducive to long-term storage and precise control, motion effect suppression, high-efficiency cooling, and increasing the precision of spectroscopy for a 3D ion system.

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Design of scale model of plate-shaped absorber in a wide frequency range

袁黎明, 许勇刚, 高伟, 戴飞, 吴琪琳

2018 (4): 44101-044101. doi: 10.1088/1674-1056/27/4/044101

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In order to design the scale model in a wide frequency range, a method based on the reflective loss is proposed according to the high-frequency approximation algorithm, and an example of designing the scale model of a plate-shaped absorber is given in this paper. In the example, the frequency of the full-size measurement ranges from 2.0 GHz to 2.4 GHz, the thickness of the full-size absorber is 1 mm and the scale ratio is 1/5. A two-layer scale absorber is obtained by the proposed method. The thickness values of the bottom and top layer are 0.4 mm and 0.5 mm, respectively. Furthermore, the scattering properties of a plate model and an SLICY model are studied by FEKO to verify the effectiveness of the designed scale absorber. Compared with the corresponding values from the theoretical scale model, the average values of the absolute deviations in 10 GHz~12 GHz are 0.53 dBm², 0.65 dBm², 0.76 dBm² for the plate model and 0.20 dBm², 0.95 dBm², 0.77 dBm² for the SLICY model while the incident angles are 0°, 30°, and 60°, respectively. These deviations fall within the Radar cross section (RCS) measurement tolerance. Thus, the work in this paper has important theoretical and practical significance.

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All-fiber linearly polarized laser oscillator by fiber coiling loss control

黄昱升, 肖起榕, 李丹, 王泽晖, 王雪娇, 闫平, 巩马理

2018 (4): 44201-044201. doi: 10.1088/1674-1056/27/4/044201

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In this paper, we demonstrate an all-fiber linearly polarized fiber laser oscillator. The single polarization of the oscillator is achieved through the careful designing of the active fiber coiling. The relationship between fiber coiling diameter and polarization extinction ratio and optical efficiency is studied, whose results lead to an optimized system. The thermal management of the oscillator is also refined, which allows the oscillator to reach a maximum output power of 44.1 W with an optical-to-optical efficiency of 57.9%. A high average polarization extinction ratio of 21.6 dB is achieved during a 2-hour stability test. The oscillator also owns a narrow 3-dB bandwidth of 0.1 nm, as well as near-diffraction-limit beam quality of M²~1.14.

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Observation of self-Q-switching in bulk Yb: GdYSiO laser

公爽, 田金荣, 郭于鹤洋, 董自凯, 许昌兴, 张文平, 宋晏蓉

2018 (4): 44202-044202. doi: 10.1088/1674-1056/27/4/044202

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We report the first self-Q-switched Yb-doped GdYSiO₅ (Yb:GYSO) laser operating at 1080 nm. Stable Q-switched pulses with a repetition rate of 18.1 kHz and pulse duration of 9.45 μs are obtained. A maximum average output power of 452.6 mW is achieved with a 1% transmission output coupler. With a 3% transmission output coupler, the shortest pulse width of 6.6 μs is obtained. It demonstrates that Yb:GYSO crystal can be simultaneously employed as a saturable absorber and a gain medium to generate microsecond Q-switched pulses in the near infrared region.

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Fundamental and dressed annular solitons in saturable nonlinearity with parity-time symmetric Bessel potential

王红成, 魏亚东, 黄晓园, 陈桂华, 叶海

2018 (4): 44203-044203. doi: 10.1088/1674-1056/27/4/044203

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We theoretically study the existence and stability of optical solitons in saturable nonlinearity with a two-dimensional parity-time (PT) symmetric Bessel potential. Besides the fundamental solitons, a novel type of dressed soliton, whose intensity looks like a ring dressed on an intensity hump, are presented. It is found that both the fundamental solitons and dressed solitons can exist when the propagation constant is beyond a certain critical value. The propagation stability is investigated with a linear stability analysis corroborated by a beam propagation method. All the fundamental solitons are stable, while dressed solitons are unstable for low values of saturable parameter. As the value of saturable parameter increases, the dressed solitons tend to be stable at high powers.

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Photonic crystal structures: Beam deflector and beam router

Utku Erdiven,Erkan Tetik,Faruk Karadag

2018 (4): 44204-044204. doi: 10.1088/1674-1056/27/4/044204

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We investigate the optical characteristic, transverse magnetic (TM) and transverse electric (TE) band of two-dimensional (2D) square lattice photonic crystal structure, which is composed of cylindrical air regions positioned at the corners of the square shaped dielectric rods. We obtain the wide photonic bandwidths between TM1-TM2 and TM3-TM4 bands. According to the results, we demonstrate the band gaps close to each other in the TM and TE frequencies for proposed structures. The resulting photonic gaps are formed to be about 8% at the higher frequencies of TE modes (TE4-TE5) and TM modes (TM7-TM8 and TM9-TM10). In addition, we examine isotropically generated structures for light guiding properties and observe that the light is directed in a particular route without using any deflection. We also investigate the self-collimation effect with the designed structure. The obtained results reveal the influences of the radius of cylindrical air holes and the angle between these air holes on absolute and partial photonic band gaps. Moreover, we observe the TM and TE band gaps that overlap. It is thought that the obtained band overlap will provide an easy way to produce the photonic crystals in practical applications like photonic insensitive waveguide. It is also believed that these results can provide the photonic crystal structures to work as a beam deflecting and beam router in integrated optical circuit applications.

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Head-on collision between two solitary waves in a one-dimensional bead chain

王扶刚, 杨阳阳, 韩娟芳, 段文山

2018 (4): 44501-044501. doi: 10.1088/1674-1056/27/4/044501

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The head on collision between two opposite propagating solitary waves is studied in the present paper both numerically and analytically. The interesting result is that no phase shift is observed which is different from that found in other branches of physics. It is found that the maximum amplitude in the process of the head on collision is close to the linear sum of two colliding solitary waves.

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Oxidation during the production of FGH4095 superalloy powders by electrode induction-melt inert gas atomization

峰山, 夏敏, 葛昌纯

2018 (4): 44701-044701. doi: 10.1088/1674-1056/27/4/044701

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Super-clean and super-spherical FGH4095 superalloy powder is produced by the ceramic-free electrode induction-melt inert gas atomization (EIGA) technique. A continuous and steady-state liquid metal flow is achieved at high-frequency (350 kHz) alternating current and high electric power (100 kW). The superalloy is immersed in a high-frequency induction coil, and the liquid metal falling into a supersonic nozzle is atomized by an Ar gas of high kinetic gas energy. Numerical calculations are performed to optimize the structure parameters for the nozzle tip. The undesired oxidation reaction of alloying elements starts at 1000℃ with the reaction originating from the active sites on the powder surfaces, leading to the formation of oxides, Me_xO_y. The role of active sites and kinetic factors associated with the diffusion of oxygen present in the atomization gas streams are also examined. The observed results reveal that the oxidation process occurring at the surface of the produced powders gradually moves toward the core, and that there exists a clear interface between the product layer and the reactant. The present study lays a theoretical foundation for controlling the oxidation of nickel-based superalloy powders from the powder process step.

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Particle-in-cell simulation for the effect of magnetic cusp on discharge characteristics in a cylindrical Hall thruster

梁圣涛, 刘辉, 于达仁

2018 (4): 45201-045201. doi: 10.1088/1674-1056/27/4/045201

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The cylindrical Hall thruster has the good prospect of serving as a miniaturized electric propulsion device. A 2D-3V particle-in-cell plus Monte Carlo (PIC-MCC) method is used to study the effect of the magnetic cusp on discharge characteristics of a cylindrical Hall thruster. The simulation results show that the main ionization region and the main potential drop of the thruster are located at the upstream of the discharge channel. When the magnetic cusp moves toward the anode side, the main ionization region is compressed and weakened, moving upstream correspondingly. The ionization near the cusp is enhanced, and the interaction between the plasma and the wall increases. The simulation results suggest that the magnetic cusp should be located near the channel exit.

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Phase shift effects of radio-frequency bias on ion energy distribution in continuous wave and pulse modulated inductively coupled plasmas

薛婵, 高飞, 刘永新, 刘佳, 王友年

2018 (4): 45202-045202. doi: 10.1088/1674-1056/27/4/045202

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A retarding field energy analyzer (RFEA) is used to measure the time-averaged ion energy distributions (IEDs) on the substrate in both continuous wave (CW) and synchronous pulse modulated radio-frequency (RF) inductively coupled Ar plasmas (ICPs). The effects of the phase shift θ between the RF bias voltage and the RF source on the IED is investigated under various discharge conditions. It is found that as θ increases from 0 to π, the IED moves towards the low-energy side, and its energy width becomes narrower. In order to figure out the physical mechanism, the voltage waveforms on the substrate are also measured. The results show that as θ increases from 0 to π, the amplitude of the voltage waveform decreases and, meanwhile, the average sheath potential decreases as well. Specifically, the potential drop in the sheath on the substrate exhibits a maximum value at the same phase (i.e., θ=0) and a minimum value at the opposite phase (i.e., θ=π). Therefore, when ions traverse across the sheath region above the substrate, they obtain less energies at lower sheath potential drop, leading to lower ion energy. Besides, as θ increases from π to 2π, the IEDs and their energy widths change reversely.

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Wettability of Si and Al-12Si alloy on Pd-implanted 6H-SiC

汪婷婷, 刘桂武, 黄志坤, 张相召, 徐紫巍, 乔冠军

2018 (4): 46101-046101. doi: 10.1088/1674-1056/27/4/046101

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SiC monocrystal substrates are implanted by Pd ions with different ion-beam energies and fluences, and the effects of Pd ion implantation on wettability of Si/SiC and Al-12Si/SiC systems are investigated by the sessile drop technique. The decreases of contact angles of the two systems are disclosed after the ion implantation, which can be attributed to the increase of surface energy (σ_SV) of SiC substrate derived from high concentration of defects induced by the ion-implantation and to the decrease of solid-liquid surface energy (σ_SL) resulting from the increasing interfacial interactions. This study can provide guidance in improving the wettability of metals on SiC and the electronic packaging process of SiC substrate.

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Superconductivity in electron-doped arsenene

孔鑫, 高淼, 闫循旺, 卢仲毅, 向涛

2018 (4): 46301-046301. doi: 10.1088/1674-1056/27/4/046301

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Based on the first-principles density functional theory electronic structure calculation, we investigate the possible phonon-mediated superconductivity in arsenene, a two-dimensional buckled arsenic atomic sheet, under electron doping. We find that the strong superconducting pairing interaction results mainly from the p_z-like electrons of arsenic atoms and the A₁ phonon mode around the K point, and the superconducting transition temperature can be as high as 30.8 K in the arsenene with 0.2 doped electrons per unit cell and 12%-applied biaxial tensile strain. This transition temperature is about ten times higher than that in the bulk arsenic under high pressure. It is also the highest transition temperature that is predicted for electron-doped two-dimensional elemental superconductors, including graphene, silicene, phosphorene, and borophene.

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The influence of surface effects on Frederiks transition in nematic liquid crystal doped with ferroelectric nanoparticles

Erfan Kadivar,Mojtaba Farrokhbin

2018 (4): 46801-046801. doi: 10.1088/1674-1056/27/4/046801

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Motivated by our recent work, in this work, we present the numerical study of the anchoring effect on the Frederiks threshold field in a nematic liquid crystal doped with ferroelectric colloidal nanoparticles. Assuming weak anchoring conditions, we employ the relaxation method and Maxwell construction to numerically solve the Euler-Lagrangian differential equation for the total free energy together the Rapini-Papoular surface energy to take into account anchoring of nematic liquid crystal molecules at the substrates. In this study, we focus our attention on obtaining the phase diagrams of Frederiks transition for different values of anchoring strength which have been not computed in our previous work. In this way, the effect of nanoparticle radius, nanoparticle volume fraction, nanoparticle polarization, and cell thickness on the Frederiks transition for different values of anchoring conditions are summarized in the phase diagrams. The numerical results show that by increasing the nanoparticles size and nanoparticle volume fraction in the ferronematic system, the Frederiks threshold field is strongly reduced.

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Two-dimensional electron gas characteristics of InP-based high electron mobility transistor terahertz detector

李金伦, 崔少辉, 徐建星, 崔晓然, 郭春妍, 马奔, 倪海桥, 牛智川

2018 (4): 47101-047101. doi: 10.1088/1674-1056/27/4/047101

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The samples of In_xGa_1-xAs/In_0.52Al_0.48As two-dimensional electron gas (2DEG) are grown by molecular beam epitaxy (MBE). In the sample preparation process, the In content and spacer layer thickness are changed and two kinds of methods, i.e., contrast body doping and δ-doping are used. The samples are analyzed by the Hall measurements at 300 K and 77 K. The In_xGa_1-xAs/In_0.52Al_0.48As 2DEG channel structures with mobilities as high as 10289 cm²/V·(300 K) and 42040 cm²/V·(77 K) are obtained, and the values of carrier concentration (N_c) are 3.465×10¹²/cm² and 2.502×10¹²/cm², respectively. The THz response rates of InP-based high electron mobility transistor (HEMT) structures with different gate lengths at 300 K and 77 K temperatures are calculated based on the shallow water wave instability theory. The results provide a reference for the research and preparation of InP-based HEMT THz detectors.

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The effect of replacing pnictogen elements on the physical properties of SrMg₂X₂ (X=N, P, As, Sb, Bi) Zintl compounds

G Murtaza,Abdul Ahad Khan,M Yaseen,A Laref,Naeem Ullah,Inayat ur Rahman

2018 (4): 47102-047102. doi: 10.1088/1674-1056/27/4/047102

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The effect of replacing the anion from N to Bi down the group in the periodic table is investigated on SrMg₂X₂ (X=N, P, As, Sb, Bi). A full potential linearized augmented plane wave plus local orbitals method is used along with different exchange-correlation potentials to obtain the lattice constants, phonons, electronic, and optical properties of the SrMg₂X₂ (X=N, P, As, Sb, Bi) Zintl compounds. A good agreement is achieved and our calculations are validated by previous experimental and theoretical data. All compounds have shown stable dynamical behavior with gamma centered longitudinal response having no imaginary frequencies. Electronic band structures reveal the semiconducting nature of the compounds. The Pnictogen (X)-p state contributed mainly in the valence band and the Sr-d state forms the conduction of the compounds. Relative charge transfer and low overlapping of the atomic densities indicates the preferable ionic bonding character of these materials. In the optical properties, real and imaginary parts of dielectric function, complex refractive index, birefringence, reflectivity, and optical conductivity are calculated. These compounds can be utilized in the optical and optoelectronic devices.

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Passivation of carbon dimer defects in amorphous SiO₂/4H-SiC (0001) interface: A first-principles study

张轶杰, 尹志鹏, 苏艳, 王德君

2018 (4): 47103-047103. doi: 10.1088/1674-1056/27/4/047103

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An amorphous SiO₂/4H-SiC (0001) interface model with carbon dimer defects is established based on density functional theory of the first-principle plane wave pseudopotential method. The structures of carbon dimer defects after passivation by H₂ and NO molecules are established, and the interface states before and after passivation are calculated by the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional scheme. Calculation results indicate that H₂ can be adsorbed on the O₂-C=C-O₂ defect and the carbon-carbon double bond is converted into a single bond. However, H₂ cannot be adsorbed on the O₂-(C=C)'-O₂ defect. The NO molecules can be bonded by N and C atoms to transform the carbon-carbon double bonds, thereby passivating the two defects. This study shows that the mechanism for the passivation of SiO₂/4H-SiC (0001) interface carbon dimer defects is to convert the carbon-carbon double bonds into carbon dimers. Moreover, some intermediate structures that can be introduced into the interface state in the band gap should be avoided.

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Influence of spin-orbit coupling on spin-polarized electronic transport in magnetic semiconductor nanowires with nanosized sharp domain walls

刘恋, 陈文祥, 王瑞强, 胡梁宾

2018 (4): 47201-047201. doi: 10.1088/1674-1056/27/4/047201

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Influence of spin-orbit coupling on spin-polarized electronic transport in magnetic semiconductor nanowires with nanosized sharp domain walls is investigated theoretically. It is shown that the Rashba spin-orbit coupling can enhance significantly the spin-flip scattering of charge carriers from a nanosized sharp domain wall whose extension is much smaller than the carrier's Fermi wavelength. When there are more than one domain wall presented in a magnetic semiconductor nanowire, not only the spin-flip scattering of charge carriers from the domain walls but the quantum interference of charge carriers in the intermediate domain regions between neighboring domain walls may play important roles on spin-polarized electronic transport, and in such cases the influences of the Rashba spin-orbit coupling will depend sensitively both on the domain walls' width and the domain walls' separation.

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Band engineering and precipitation enhance thermoelectric performance of SnTe with Zn-doping

陈志禹, 王瑞峰, 王国玉, 周小元, 王正上, 尹聪, 胡庆, 周斌强, 唐军, 昂然

2018 (4): 47202-047202. doi: 10.1088/1674-1056/27/4/047202

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We have systematically studied the thermoelectric properties in Zn-doped SnTe. Strikingly, band convergence and embedded precipitates arising from Zn doping, can trigger a prominent improvement of thermoelectric performance. In particular, the value of dimensionless figure of merit zT has increased by 100% and up to~0.5 at 775 K for the optimal sample with 2% Zn content. Present findings demonstrate that carrier concentration and effective mass play crucial roles on the Seebeck coefficient and power factor. The obvious deviation from the Pisarenko line (Seebeck coefficient versus carrier concentration) due to Zn-doping reveals the convergence of valence bands. When the doping concentration exceeds the solubility, precipitates occur and lead to a reduction of lattice thermal conductivity. In addition, bipolar conduction is suppressed, indicating an enlargement of band gap. The Zn-doped SnTe is shown to be a promising candidate for thermoelectric applications.

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Synthesis and thermoelectric properties of Nd-single filled p-type skutterudites

吴宏, Nusrat Shaheen, 杨恒全, 彭坤岭, 沈星辰, 王国玉, 卢旭, 周小元

2018 (4): 47203-047203. doi: 10.1088/1674-1056/27/4/047203

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We report the synthesis of Nd-filled and Fe substituted p-type Nd_xFe_3.2Co_0.8Sb₁₂ (x=0.5, 0.6, 0.7, 0.8, and 0.9) skutterudites by the solid-state reaction method. The influences of Nd filler on the electrical and thermal transport properties are investigated in a temperature range from room temperature to 850 K. A lowest lattice thermal conductivity of 0.88 W·m^-1·K^-1 is obtained in Nd_0.8Fe_3.2Co_0.8Sb₁₂ at 673 K, which results from the localized vibration modes of fillers and the increase of grains boundaries. Meanwhile, the maximum power factor is 2.77 mW·m^-1·K^-2 for the Nd_0.9Fe_3.2Co_0.8Sb₁₂ sample at 668 K. Overall, the highest dimensionless figure of merit zT=0.87 is achieved at 714 K for Nd_0.9Fe_3.2Co_0.8Sb₁₂.

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Se substitution and micro-nano-scale porosity enhancing thermoelectric Cu₂Te

史晓曼, 王国玉, 王瑞峰, 周小元, 徐静涛, 唐军, 昂然

2018 (4): 47204-047204. doi: 10.1088/1674-1056/27/4/047204

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Binary Cu-based chalcogenide thermoelectric materials have attracted a great deal of attention due to their outstanding physical properties and fascinating phase sequence. However, the relatively low figure of merit zT restricts their practical applications in power generation. A general approach to enhancing zT value is to produce nanostructured grains, while one disadvantage of such a method is the expansion of grain size in heating-up process. Here, we report a prominent improvement of zT in Cu₂Te_0.2Se_0.8, which is several times larger than that of the matrix. This significant enhancement in thermoelectric performance is attributed to the formation of abundant porosity via cold press. These pores with nano-to micrometer size can manipulate phonon transport simultaneously, resulting in an apparent suppression of thermal conductivity. Moreover, the Se substitution triggers a rapid promotion of power factor, which compensates for the reduction of electrical properties due to carriers scattering by pores. Our strategy of porosity engineering by phonon scattering can also be highly applicable in enhancing the performances of other thermoelectric systems.

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Effect of Nb doping on microstructures and thermoelectric properties of SrTiO₃ ceramics

刘达权, 张玉伟, 康慧君, 李金玲, 杨雄, 王同敏

2018 (4): 47205-047205. doi: 10.1088/1674-1056/27/4/047205

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Nb-doped SrTiO₃ thermoelectric ceramics with different niobium concentrations, sintering temperatures and Sr-site vacancies are successfully prepared by high energy ball milling combined with carbon burial sintering. For fully understanding the effect of niobium doping on SrTiO₃, thermoelectric transport properties are systematically investigated in a temperature range from 300 K to 1100 K. The carrier mobility can be significantly enhanced, and the electrical conductivity is quadrupled, when the sintering temperature rises from 1673 K to 1773 K (beyond the eutectic temperature (1713 K) of SrTiO₃-TiO₂). The lattice vibration can be suppressed by the lattice distortion introduced by the doped niobium atoms. However, Sr-site vacancies compensate for the lattice distortion and increase the lattice thermal conductivity more or less. Finally, we achieve a maximum value of figure-of-merit zT of 0.21 at 1100 K for SrTi_0.9Nb_0.1O₃ ceramic sintered at 1773 K.

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Multinary diamond-like chalcogenides for promising thermoelectric application

张旦, 白洪昌, 李志亮, 王江龙, 傅广生, 王淑芳

2018 (4): 47206-047206. doi: 10.1088/1674-1056/27/4/047206

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Thermoelectric (TE) materials have been considered as a strong candidate for recovering the waste heat from industry and vehicles due to the ability to convert heat directly into electricity. Recently, multinary diamond-like chalcogenides (MDLCs), such as CuInTe₂, Cu₂SnSe₃, Cu₃SbSe₄, Cu₂ZnSnSe₄, etc., are eco-friendly Pb-free TE materials with relatively large Seebeck coefficient and low thermal conductivity and have aroused intensive research as a popular theme in the TE field. In this review, we summarize the TE performance and device development of MDLCs. The features of crystalline and electronic structure are first analyzed, and then the strategies that have emerged to enhance the TE figure of merits of these materials are illustrated in detail. The final part of this review describes the advance in TE device research for MDLCs. In the outlook, the challenges and future directions are also discussed to promote the further development of MDLCs TE materials.

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Enhanced thermoelectric performance through homogenously dispersed MnTe nanoparticles in p-type Bi_0.52Sb_1.48Te₃ nanocomposites

陆天奇, 南鹏飞, 宋思龙, 朱欣悦, 赵怀周, 邓元

2018 (4): 47207-047207. doi: 10.1088/1674-1056/27/4/047207

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In this work, we report that the thermoelectric properties of Bi_0.52Sb_1.48Te₃ alloy can be enhanced by being composited with MnTe nano particles (NPs) through a combined ball milling and spark plasma sintering (SPS) process. The addition of MnTe into the host can synergistically reduce the lattice thermal conductivity by increasing the interface phonon scattering between Bi_0.52Sb_1.48Te₃ and MnTe NPs, and enhance the electrical transport properties by optimizing the hole concentration through partial Mn²⁺ acceptor doping on the Bi³⁺ sites of the host lattice. It is observed that the lattice thermal conductivity decreases with increasing the percentage of MnTe and milling time in a temperature range from 300 K to 500 K, which is consistent with the increasing of interfaces. Meanwhile, the bipolar effect is constrained to high temperatures, which results in the figure of merit zT peak shifting toward higher temperature and broadening the zT curves. The engineering zT is obtained to be 20% higher than that of the pristine sample for the 2-mol% MnTe-added composite at a temperature gradient of 200 K when the cold end temperature is set to be 300 K. This result indicates that the thermoelectric performance of Bi_0.52Sb_1.48Te₃ can be considerably enhanced by being composited with MnTe NPs.

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Effect of depositing PCBM on perovskite-based metal-oxide-semiconductor field effect transistors

栾苏珍, 汪钰成, 刘银涛, 贾仁需

2018 (4): 47208-047208. doi: 10.1088/1674-1056/27/4/047208

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In this manuscript, the perovskite-based metal-oxide-semiconductor field effect transistors (MOSFETs) with phenyl-C₆₁-butyric acid methylester (PCBM) layers are studied. The MOSFETs are fabricated on perovskites, and characterized by photoluminescence spectra (PL), x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS). With PCBM layers, the current-voltage hysteresis phenomenon is effetely inhibited, and both the transfer and output current values increase. The band energy diagrams are proposed, which indicate that the electrons are transferred into the PCBM layer, resulting in the increase of photocurrent. The electron mobility and hole mobility are extracted from the transfer curves, which are about one order of magnitude as large as those of PCBM deposited, which is the reason why the electrons are transferred into the PCBM layer and the holes are still in the perovskites, and the effects of ionized impurity scattering on carrier transport become smaller.

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Room-temperature operating extended short wavelength infrared photodetector based on interband transition of InAsSb/GaSb quantum well

孙令, 王禄, 鲁金蕾, 刘洁, 方俊, 谢莉莉, 郝智彪, 贾海强, 王文新, 陈弘

2018 (4): 47209-047209. doi: 10.1088/1674-1056/27/4/047209

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Here in this paper, we report a room-temperature operating infrared photodetector based on the interband transition of an InAsSb/GaSb quantum well. The interband transition energy of 5-nm thick InAs_0.91Sb_0.09 embedded in the GaSb barrier is calculated to be 0.53 eV (2.35 μm), which makes the absorption range of InAsSb cover an entire range from short-wavelength infrared to long-wavelength infrared spectrum. The fabricated photodetector exhibits a narrow response range from 2.0 μm to 2.3 μm with a peak around 2.1 μm at 300 K. The peak responsivity is 0.4 A/W under -500 -mV applied bias voltage, corresponding to a peak quantum efficiency of 23.8% in the case without any anti-reflection coating. At 300 K, the photodetector exhibits a dark current density of 6.05×10^-3 A/cm² under -400-mV applied bias voltage and 3.25×10^-5 A/cm² under zero, separately. The peak detectivity is 6.91×10¹⁰ cm·Hz^1/2/W under zero bias voltage at 300 K.

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An overview of thermoelectric films: Fabrication techniques, classification, and regulation methods

冯静静, 祝薇, 邓元

2018 (4): 47210-047210. doi: 10.1088/1674-1056/27/4/047210

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Thermoelectric materials have aroused widespread concern due to their unique ability to directly convert heat to electricity without any moving parts or noxious emissions. Taking advantages of two-dimensional structures of thermoelectric films, the potential applications of thermoelectric materials are diversified, particularly in microdevices. Well-controlled nanostructures in thermoelectric films are effective to optimize the electrical and thermal transport, which can significantly improve the performance of thermoelectric materials. In this paper, various physical and chemical approaches to fabricate thermoelectric films, including inorganic, organic, and inorganic-organic composites, are summarized, where more attentions are paid on the inorganic thermoelectric films for their excellent thermoelectric responses. Additionally, strategies for enhancing the performance of thermoelectric films are also discussed.

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Enhanced thermoelectric properties of p-type polycrystalline SnSe by regulating the anisotropic crystal growth and Sn vacancy

刘呈燕, 苗蕾, 王潇漾, 伍少海, 郑岩岩, 邓梓阳, 陈玉莲, 王桂文, 周小元

2018 (4): 47211-047211. doi: 10.1088/1674-1056/27/4/047211

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Thermoelectric selenides have attracted more and more attentions recently. Herein, p-type SnSe polycrystalline bulk materials with good thermoelectric properties are presented. By using the SnSe₂ nanostructures synthesized via a wet-chemistry route as the precursor, polycrystalline SnSe bulk materials were successfully obtained by a combined heat-treating process under reducing atmosphere and following spark plasma sintering procedure. As a reference, the SnSe nanostructures synthesized via a wet-chemistry route were also fabricated into polycrystalline bulk materials through the same process. The thermoelectric properties of the SnSe polycrystalline transformed from SnSe₂ nanostructures indicate that the increasing of heattreating temperature could effectively decrease the electrical resistivity, whereas the decrease in Seebeck coefficient is nearly invisible. As a result, the maximum power factor is enhanced from 5.06×10^-4 W/m·K² to 8.08×10^-4 W/m·K² at 612℃. On the other hand, the reference sample, which was obtained by using SnSe nanostructures as the precursor, displays very poor power factor of only 1.30×10^-4 W/m·K² at 537℃. The x-ray diffraction (XRD), scanning electron microscope (SEM), x-ray fluorescence (XRF), and Hall effect characterizations suggest that the anisotropic crystal growth and existing Sn vacancy might be responsible for the enhanced electrical transport in the polycrystalline SnSe prepared by using SnSe₂ precursor. On the other hand, the impact of heat-treating temperature on thermal conductivity is not obvious. Owing to the boosting of power factor, a high zT value of 1.07 at 612℃ is achieved. This study provides a new method to synthesize polycrystalline SnSe and pave a way to improve the thermoelectric properties of polycrystalline bulk materials with similar layered structure.

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Enhanced thermoelectric performance in p-type Mg₃Sb₂ via lithium doping

王浩, 陈进, 陆天奇, 朱坤杰, 李珊, 刘军, 赵怀周

2018 (4): 47212-047212. doi: 10.1088/1674-1056/27/4/047212

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The Zintl compound Mg₃Sb₂ has been recently identified as promising thermoelectric material owing to its high thermoelectric performance and cost-effective, nontoxicity and environment friendly characteristics. However, the intrinsically p-type Mg₃Sb₂ shows low figure of merit (zT=0.23 at 723 K) for its poor electrical conductivity. In this study, a series of Mg_3-xLi_xSb₂ bulk materials have been prepared by high-energy ball milling and spark plasma sintering (SPS) process. Electrical transport measurements on these materials revealed significant improvement on the power factor with respect to the undoped sample, which can be essentially attributed to the increased carrier concentration, leading to a maximum zT of 0.59 at 723 K with the optimum doping level x=0.01. Additionally, the engineering zT and energy conversion efficiency are calculated to be 0.235 and 4.89%, respectively. To our best knowledge, those are the highest values of all reported p-type Mg₃Sb₂-based compounds with single element doping.

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Conductivity and band alignment of LaCrO₃/SrTiO₃ (111) heterostructure

洪彦鹏, 王欣欣, 曲国良, 厉承剑, 薛红霞, 刘科践, 李永春, 熊昌民, 窦瑞芬, 何林, 聂家财

2018 (4): 47301-047301. doi: 10.1088/1674-1056/27/4/047301

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In this work, we investigate the electrical transport property and electronic structure of oxide heterostructure LaCrO₃/SrTiO₃ (111). The interface grown under relatively low oxygen partial pressure is found to be metallic with a conducting critical thickness of 11 unit cells of LaCrO₃. This criticality is also observed by x-ray photoelectron spectroscopy, in which the Ti³⁺ signal intensity at the spectrum edge of the Ti-2p_3/2 core level increases rapidly when the critical thickness is reached. The variations of the valence band offset and full width at half maximum of the core-level spectrum with LaCrO₃ thickness suggest that the built-in fields exist both in LaCrO₃ and in SrTiO₃. Two possible origins are proposed:the charge transfer from LaCrO₃ and the formation of a quantum well in SrTiO₃. Our results shed light on the understanding of the doping mechanism at the polar/non-polar oxide interface. Moreover, due to the interesting lattice and spin structure of LCO in the (111) direction, our work provides a basis for further exploring the novel topological quantum phenomena in this system.

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Gap plasmon-enhanced photoluminescence of monolayer MoS₂ in hybrid nanostructure

余乐, 刘頔, 祁晓卓, 熊霄, 冯兰天, 李明, 郭国平, 郭光灿, 任希锋

2018 (4): 47302-047302. doi: 10.1088/1674-1056/27/4/047302

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Monolayer transition-metal dichalcogenides (TMDs) have attracted a lot of attention for their applications in optics and optoelectronics. Molybdenum disulfide (MoS₂), as one of those important materials, has been widely investigated due to its direct band gap and photoluminescence (PL) in visible range. Owing to the fact that the monolayer MoS₂ suffers low light absorption and emission, surface plasmon polaritons (SPPs) are used to enhance both the excitation and emission efficiencies. Here, we demonstrate that the PL of MoS₂ sandwiched between 200-nm-diameter gold nanoparticle (AuNP) and 150-nm-thick gold film is improved by more than 4 times compared with bare MoS₂ sample. This study shows that gap plasmons can possess more optical and optoelectronic applications incorporating with many other emerging two-dimensional materials.

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Double band-inversions of bilayer phosphorene under strain and their effects on optical absorption

何诗, 杨谋, 王瑞强

2018 (4): 47303-047303. doi: 10.1088/1674-1056/27/4/047303

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Strain is a powerful tool to engineer the band structure of bilayer phosphorene. The band gap can be decreased by vertical tensile strain or in-plane compressive strain. At a critical strain, the gap is closed and the bilayer phosphorene is turn to be a semi-Dirac semimetal material. If the strain is stronger than the criterion, a band-inversion occurs and it re-happens when the strain is larger than another certain value. For the zigzag bilayer phosphorene ribbon, there are two edge band dispersions and each dispersion curve represents two degenerate edge bands. When the first band-inversion happens, one of the edge band dispersion disappears between the band-cross points while the other survives, and the latter will be eliminated between another pair of band-cross points of the second band-inversion. The optical absorption of bilayer phosphorene is highly polarized along armchair direction. When the strain is turn on, the optical absorption edge changes. The absorption rate for armchair polarized light is decreased by gap shrinking, while that for zigzag polarized light increases. The band-touch and band-inversion respectively result in the sublinear and linear of absorption curve versus light frequency in low frequency limit.

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Bias polarity-dependent unipolar switching behavior in NiO/SrTiO₃ stacked layer

孙献文, 贾彩虹, 刘献省, 李国强, 张伟风

2018 (4): 47304-047304. doi: 10.1088/1674-1056/27/4/047304

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The identification of the switching location has been a key technology to tune the physical properties of unipolar resistive switching (RS) cells. Here we report the RS properties of Au/NiO/SrTiO₃(STO)/Pt memory cells. The switching repeatability is closely related to the applied bias polarity, which is different from the scenario of the Au/STO/Pt cells reported in our previous researches. The high resistance in positive bias is greater than that in negative bias. The R(T)-R₀I²R(T) plot of the on-state I-V curve shows a regular shape only with a slight bend and an abnormal shape with an abrupt increase and decrease under negative and positive bias, respectively. These comparative experimental results reveal that the conductance filament consisting of oxygen vacancies grows from the cathode to the anode. The spatial RS location is identified with the weaker part along the conductance filament length direction, which should be near the NiO/STO interface and STO/Pt interface under positive and negative bias, respectively.

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Low specific on-resistance GaN-based vertical heterostructure field effect transistors with nonuniform doping superjunctions

毛维, 王海永, 石朋毫, 王晓飞, 杜鸣, 郑雪峰, 王冲, 马晓华, 张进成, 郝跃

2018 (4): 47305-047305. doi: 10.1088/1674-1056/27/4/047305

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A novel GaN-based vertical heterostructure field effect transistor (HFET) with nonuniform doping superjunctions (non-SJ HFET) is proposed and studied by Silvaco-ATLAS, for minimizing the specific on-resistance (R_onA) at no expense of breakdown voltage (BV). The feature of non-SJ HFET lies in the nonuniform doping concentration from top to bottom in the n-and p-pillars, which is different from that of the conventional GaN-based vertical HFET with uniform doping superjunctions (un-SJ HFET). A physically intrinsic mechanism for the nonuniform doping superjunction (non-SJ) to further reduce R_onA at no expense of BV is investigated and revealed in detail. The design, related to the structure parameters of non-SJ, is optimized to minimize the R_onA on the basis of the same BV as that of un-SJ HFET. Optimized simulation results show that the reduction in R_onA depends on the doping concentrations and thickness values of the light and heavy doping parts in non-SJ. The maximum reduction of more than 51% in R_onA could be achieved with a BV of 1890 V. These results could demonstrate the superiority of non-SJ HFET in minimizing R_onA and provide a useful reference for further developing the GaN-based vertical HFETs.

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Strategies for optimizing the thermoelectricity of PbTe alloys

翟近泽, 王腾, 王洪超, 苏文斌, 王雪, 陈婷婷, 王春雷

2018 (4): 47306-047306. doi: 10.1088/1674-1056/27/4/047306

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The thermoelectric materials have been considered as a potential candidate for the new power generation technology based on their reversible heat and electricity conversion. Lead telluride (PbTe) is regarded as an excellent mid-temperature thermoelectric material due to its suitable intrinsic thermoelectric properties. So tremendous efforts have been done to improve the thermoelectric performance of PbTe, and figures of merit, zT > 2.0, have been reported. Main strategies for optimizing the thermoelectric performance have been focused as the main line of this review. The band engineering and phonon scattering engineering as two main effective strategies are systemically summarized here. The band engineering, like band convergence, resonant levels, and band flatting have been addressed in improving the power factor. Additionally, phonon scattering engineerings, such as atomic-scale, nano-scale, meso-scale, and multi-scale phonon scatterings have been applied to reduce the thermal conductivity. Besides, some successful synergistic effects based on band engineerings and phonon scatterings are illustrated as a simultaneous way to optimize both the power factor and thermal conductivity. Summarizing the above three main parts, we point out that the synergistic effects should be effectively exploited, and these may further boost the thermoelectric performance of PbTe alloys and can be extended to other thermoelectric materials.

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Influence of fin architectures on linearity characteristics of AlGaN/GaNFinFETs

刘婷婷, 张凯, 朱广润, 周建军, 孔月婵, 郁鑫鑫, 陈堂胜

2018 (4): 47307-047307. doi: 10.1088/1674-1056/27/4/047307

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We investigate the influence of fin architecture on linearity characteristics of AlGaN/GaNFinFET. It is found that the FinFET with scaled fin dimensions exhibits much flatter G_m characteristics than the one with long fins as well as planar HEMT. According to the comparative study, we provide direct proof that source resistance rather than tri-gate structure itself dominates the G_m behavior. Furthermore, power measurements show that the optimized FinFET is capable of delivering a much higher output power density along with significant improvement in linearity characteristics than conventional planar HEMT. This study also highlights the importance of fin design in GaN-based FinFET for microwave power application, especially high-linearity applications.

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Topological phase diagrams and Majorana zero modes of the Kitaev ladder and tube

王一鸣, 李志聃, 韩强

2018 (4): 47401-047401. doi: 10.1088/1674-1056/27/4/047401

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In this paper, we study two quasi-one-dimensional (1D) Kitaev models with ladder-like and tube-like spatial structures, respectively. Our results provide the phase diagrams and explicit expressions of the Majorana zero modes. The topological phase diagrams are obtained by decomposing the topological invariants and the topological conditions for topologically nontrivial phases are given precisely. For systems which belongs to topological class BDI, we obtain the regions in the phase diagrams where the topological numbers show even-odd effect. For the Kitaev tube model a phase factor induced by the magnetic flux in the axial direction of the tube is introduced to alter the classification of the tube Hamiltonian from class BDI to D. The Kitaev tube of class D is characterized by the Z₂ index when the number of chains is odd while 0, 1, 2 when the number of chains is even. The phase diagrams show periodic behaviors with respect to the magnetic flux. The bulk-boundary correspondence is demonstrated by the observations that the topological conditions for the bulk topological invariant to take nontrivial values are precisely those for the existence of the Majorana zero modes.

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The structure and elasticity of phase B silicates under high pressure by first principles simulation

刘雷, 易丽, 刘红, 李营, 庄春强, 杨龙星, 刘桂平

2018 (4): 47402-047402. doi: 10.1088/1674-1056/27/4/047402

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The structures and elasticities of phase B silicates with different water and iron (Fe) content are obtained by first-principles simulation to understand the effects of water and Fe on their properties under high pressure. The lattice constants a and b decrease with increasing water content. On the contrary, c increases with increasing water content. On the other hand, the b and c decrease with increasing Fe content while a increases with increasing Fe content. The decrease of M (metal)-O octahedral volume is greater than the decrease of SiO polyhedral volume over the same pressure range. The density, bulk modulus and shear modulus of phase B increase with increasing Fe content and decrease with increasing water content. The compressional wave velocity (V_p) and shear wave velocity (V_s) of phase B decrease with increasing water and Fe content. The comparisons of density and wave velocity between phase B silicate and the Earth typical structure provide the evidence for understanding the formation of the X-discontinuity zone of the mantle.

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Distinction between critical current effects and intrinsic anomalies in the point-contact Andreev reflection spectra of unconventional superconductors

何格, 魏忠旭, Jérémy Brisbois, 贾艳丽, 黄裕龙, 周花雪, 倪顺利, Alejandro V Silhanek, 单磊, 朱北沂, 袁洁, 董晓莉, 周放, 赵忠贤, 金魁

2018 (4): 47403-047403. doi: 10.1088/1674-1056/27/4/047403

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In this work, we discuss the origin of several anomalies present in the point-contact Andreev reflection spectra of (Li_1-xFe_x)OHFeSe, LiTi₂O₄, and La_2-xCe_xCuO₄. While these features are similar to those stemming from intrinsic superconducting properties, such as Andreev reflection, electron-boson coupling, multigap superconductivity, d-wave and p-wave pairing symmetry, they cannot be accounted for by the modified Blonder-Tinkham-Klapwijk (BTK) model, but require to consider critical current effects arising from the junction geometry. Our results point to the importance of tracking the evolution of the dips and peaks in the differential conductance as a function of the bias voltage, in order to correctly deduce the properties of the superconducting state.

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Composition design for (PrNd-La–Ce)₂Fe₁₄B melt-spun magnets by machine learning technique

李锐, 刘瑶, 左淑兰, 赵同云, 胡凤霞, 孙继荣, 沈保根

2018 (4): 47501-047501. doi: 10.1088/1674-1056/27/4/047501

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Data-driven technique is a powerful and efficient tool for guiding materials design, which could supply as an alternative to trial-and-error experiments. In order to accelerate composition design for low-cost rare-earth permanent magnets, an approach using composition to estimate coercivity (H_cj) and maximum magnetic energy product ((BH)_max) via machine learning has been applied to (PrNd-La-Ce)₂Fe₁₄B melt-spun magnets. A set of machine learning algorithms are employed to build property prediction models, in which the algorithm of Gradient Boosted Regression Trees is the best for predicting both H_cj and (BH)_max, with high accuracies of R²=0.88 and 0.89, respectively. Using the best models, predicted datasets of H_cj or (BH)_max in high-dimensional composition space can be constructed. Exploring these virtual datasets could provide efficient guidance for materials design, and facilitate the composition optimization of 2:14:1 structure melt-spun magnets. Combined with magnets' cost performance, the candidate cost-effective magnets with targeted properties can also be accurately and rapidly identified. Such data analytics, which involves property prediction and composition design, is of great time-saving and economical significance for the development and application of LaCe-containing melt-spun magnets.

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Investigation of magnetization reversal process in pinned CoFeB thin film by in-situ Lorentz TEM

裴科, 夏卫星, 王保敏, 文兴成, 盛萍, 刘家平, 刘新才, 李润伟

2018 (4): 47502-047502. doi: 10.1088/1674-1056/27/4/047502

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Exchange bias effect has been widely employed for various magnetic devices. The experimentally reported magnitude of exchange bias field is often smaller than that predicted theoretically, which is considered to be due to the partly pinned spins of ferromagnetic layer by antiferromagnetic layer. However, mapping the distribution of pinned spins is challenging. In this work, we directly image the reverse domain nucleation and domain wall movement process in the exchange biased CoFeB/IrMn bilayers by Lorentz transmission electron microscopy. From the in-situ experiments, we obtain the distribution mapping of the pinning strength, showing that only 1/6 of the ferromagnetic layer at the interface is strongly pinned by the antiferromagnetic layer. Our results prove the existence of an inhomogeneous pinning effect in exchange bias systems.

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First-order reversal curve investigated magnetization switching in Pd/Co/Pd wedge film

李岩, 何为, 孙瑞, 弓子召, 李娜, Qeemat Gul, 张向群, 成昭华

2018 (4): 47503-047503. doi: 10.1088/1674-1056/27/4/047503

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The magnetization switching plays an essential role in spintronic devices. In this study, a Pd(3 nm)/Co(0.14-1.68 nm)/Pd(5 nm) wedge film is deposited on an MgO (111) substrate by molecular beam epitaxy. We investigate the polar magneto-optical Kerr effect (MOKE) and carry out the first-order reversal curve (FORC) measurements. For the wedge system, it is observed that the Co thickness could drive the spin reorientation transition (SRT) from out-of-plane to in-plane. Meanwhile, we find the different types of magnetization switchings in the continuous SRT process, which can originate from the formation of different magnetic compositions. Our work provides the possibility of tuning the interfacial effect, and paves the way to analyzing magnetization switching.

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Linear and nonlinear optical analysis on semiorganic _L-proline cadmium chloride single crystal

Mohd Anis,M I Baig,S S Hussaini,M D Shirsat,Mohd Shkir,H A Ghramh

2018 (4): 47801-047801. doi: 10.1088/1674-1056/27/4/047801

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In the current investigation, _L-proline cadmium chloride monohydrate (LPCC) single crystal is grown by a slow solvent evaporation technique to identify its credibility for nonlinear optical device applications. The constituent elements of LPCC crystal are determined by the energy dispersive spectroscopic (EDS) technique. The single crystal x-ray diffraction technique is used to determine the structural dimensions of LPCC crystal. The UV-visible studies are carried out within a wavelength range of 200 nm-1100 nm to determine the optical transmittance of LPCC crystal. The linear optical parameters of LPCC crystal are evaluated using the transmittance data to discuss its importance for distinct optical devices. The Nd:YAG laser assisted Kurtz-Perry test is carried out to determine the enhancement in second harmonic generation efficiency of LPCC crystal with reference to KDP crystal. The Z-scan technique is employed to assess the third order nonlinear optical (TONLO) properties of LPCC crystal at 632.8 nm. The Z-scan data are utilized to evaluate the TONLO refraction, absorption and susceptibility of LPCC crystal. The color oriented luminescence behavior of LPCC crystal is investigated within a spectral range of 350 nm-700 nm. The dependence of dielectric constant and dielectric loss on temperature and frequency is evaluated through the dielectric measurement studies.

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Structural, electronic, vibrational, and thermodynamic properties of Zr_1-xHf_xCo: A first-principles-based study

刘俊超, 袁志红, 李世长, 孔祥刚, 虞游, 马生贵, 桑革, 高涛

2018 (4): 47802-047802. doi: 10.1088/1674-1056/27/4/047802

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The physical properties including structural, electronic, vibrational and thermodynamic properties of Zr_1-xHf_xCo (x is the concentration of constituent element Hf, and changes from 0 to 1) are investigated in terms of the ABINIT program. The results reveal that all of Zr_1-xHf_xCo have similar physical properties. When Hf concentration x gradually increases from 0.0 to 1.0, the lattice constant decreases from 3.217 Å to 3.195 Å very slowly. The calculated density of states (DOS) indicates that the metallic nature is enhanced and the electrical conductivity turns better with the increase of Hf. Moreover, as Hf concentration increases from 0 to 1, the Fermi energy gradually increases from -6.96 eV to -6.21 eV, and the electronic density of states at the Fermi level (N(E_f)) decreases from 2.795 electrons/eV f.u. down to 2.594 electrons/eV f.u., both of which imply the decrease of chemical stability. The calculated vibrational properties show that the increase of Hf concentration from 0 to 1 causes the maximum vibrational frequency to decrease gradually from about 223 cm^-1 to 186 cm^-1, which suggests a lower dispersion gradient and lower phonon group velocities for these modes. Finally, the phonon related thermodynamic properties are obtained and discussed.

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Characteristic improvements of thin film AlGaInP red light emitting diodes on a metallic substrate

赵斌, 胡巍, 唐先胜, 霍雯雪, 韩丽丽, 赵明龙, 马紫光, 王文新, 贾海强, 陈弘

2018 (4): 47803-047803. doi: 10.1088/1674-1056/27/4/047803

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We report a type of thin film AlGaInP red light emitting diode (RLED) on a metallic substrate by electroplating copper (Cu) to eliminate the absorption of GaAs grown substrate. The fabrication of the thin film RLED is presented in detail. Almost no degradations of epilayers properties are observed after this substrate transferred process. Photoluminescence and electroluminescence are measured to investigate the luminous characteristics. The thin film RLED shows a significant enhancement of light output power (LOP) by improving the injection efficiency and light extraction efficiency compared with the reference RLED on the GaAs parent substrate. The LOPs are specifically enhanced by 73.5% and 142% at typical injections of 2 A/cm² and 35 A/cm² respectively from electroluminescence. Moreover, reduced forward voltages, stable peak wavelengths and full widths at half maximum are obtained with the injected current increasing. These characteristic improvements are due to the Cu substrate with great current spreading and the back reflection by bottom electrodes. The substrate transferred technology based on electroplating provides an optional way to prepare high-performance optoelectronic devices, especially for thin film types.

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Hot spots enriched plasmonic nanostructure-induced random lasing of quantum dots thin film

单锋, 张晓阳, 吴静远, 张彤

2018 (4): 47804-047804. doi: 10.1088/1674-1056/27/4/047804

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Here, a plasmon-enhanced random laser was achieved by incorporating gold nanostars (NS) into disordered polymer and CdSe/ZnS quantum dots (QDs) gain medium films, in which the surface plasmon resonance of gold NS can greatly enhance the scattering cross section and bring a large gain volume. The random distribution of gold NS in the gain medium film formed a laser-mode resonator. Under a single-pulse pumping, the scattering center of gold NS-based random laser exhibits enhanced performance of a lasing threshold of 0.8 mJ/cm² and a full width as narrow as 6 nm at half maximum. By utilizing the local enhancement characteristic of the electric field at the sharp apexes of the gold NS, the emission intensity of the random laser was increased. In addition, the gold NS showed higher thermal stability than the silver nanoparticles, withstanding high temperature heating up to 200℃. The results of metal nanostructures with enriched hot spots and excellent temperature stability have tremendous potential applications in the fields of biological identification, medical diagnostics, lighting, and display devices.

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Off-stoichiometry indexation of BiFeO₃ thin film on silicon by Rutherford backscattering spectrometry

王泽松, 肖仁政, 邹长伟, 谢伟, 田灿鑫, 薛书文, 刘贵昂, 付德君

2018 (4): 47901-047901. doi: 10.1088/1674-1056/27/4/047901

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BiFeO₃ is a multiferroic material with physical properties very sensitive to its stoichiometry. BiFeO₃ thin films on silicon substrate are prepared by the sol-gel method combined with layer-by-layer annealing and final annealing schemes. X-ray diffraction and scanning electron microscopy are employed to probe the phase structures and surface morphologies. Using Rutherford backscattering spectrometry to quantify the nonstoichiometries of BiFeO₃ thin films annealed at 100℃-650℃. The results indicate that Bi and Fe cations are close to the stoichiometry of BiFeO₃, whereas the deficiency of O anions possibly plays a key role in contributing to the leakage current of 10^-5 A/cm² in a wide range of applied voltage rather than the ferroelectric polarizations of BiFeO₃ thin films annealed at high temperature.

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Effects of growth conditions on optical quality and surface morphology of InGaAsBi

李家恺, 艾立鹍, 齐鸣, 徐安怀, 王庶民

2018 (4): 48101-048101. doi: 10.1088/1674-1056/27/4/048101

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The effects of Bi flux and pressure of AsH₃ on Bi incorporation, surface morphology and optical properties of InGaAsBi grown by gas source molecular beam epitaxy are studied. It is found that using relatively low pressure of AsH₃ and high Bi flux can strengthen the effect on the incorporation of Bi and increase its content linearly with Bi flux until it nearly reaches a saturation value. The result from Rutherford backscattering spectroscopy (RBS) confirms that the Bi incorporation can increase up to 1.13%. By adjusting Bi and As flux, we could improve the surface morphology of InGaAsBi sample. Room temperature photoluminescence shows strong and broad light emission at energy levels much smaller than the InGaAs bandgap.

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Recent advances in non-Pb-based group-IV chalcogenides for environmentally-friendly thermoelectric materials

杜炳生, 简基康, 刘海涛, 刘骄, 邱磊

2018 (4): 48102-048102. doi: 10.1088/1674-1056/27/4/048102

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Pb-based group-IV chalcogenides including PbTe and PbSe have been extensively studied as high performance thermoelectric materials during the past few decades. However, the toxicity of Pb inhibits their applications in vast fields due to the serious harm to the environment. Recently the Pb-free group-IV chalcogenides have become an extensive research subject as promising thermoelectric materials because of their unique thermal and electronic transport properties as well as the enviromentally friendly advantage. This paper briefly summarizes the recent research advances in Sn-, Ge-, and Si-chalcogenides thermoelectrics, showing the unexceptionally high thermoelectric performance in SnSe single crystal, and the significant improvement in thermoelectric performance for those polycrystalline materials by successfully modulating the electronic and thermal transport through using some well-developed strategies including band engineering, nanostructuring and defect engineering. In addition, some important issues for future device applications, including N-type doping and mechanical and chemical stabilities of the new thermoelectrics, are also discussed.

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Structural and electrical properties of reactive magnetron sputtered yttrium-doped HfO₂ films

张昱, 徐军, 周大雨, 王行行, 陆文琪, Chi-Kyu Choi

2018 (4): 48103-048103. doi: 10.1088/1674-1056/27/4/048103

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Hafnium oxide thin films doped with different concentrations of yttrium are prepared on Si (100) substrates at room temperature using a reactive magnetron sputtering system. The effects of Y content on the bonding structure, crystallographic structure, and electrical properties of Y-doped HfO₂ films are investigated. The x-ray photoelectron spectrum (XPS) indicates that the core level peak positions of Hf 4f and O 1s shift toward lower energy due to the structure change after Y doping. The depth profiling of XPS shows that the surface of the film is completely oxidized while the oxygen deficiency emerges after the stripping depths have increased. The x-ray diffraction and high resolution transmission electron microscopy (HRTEM) analyses reveal the evolution from monoclinic HfO₂ phase towards stabilized cubic HfO₂ phase and the preferred orientation of (111) appears with increasing Y content, while pure HfO₂ shows the monoclinic phase only. The leakage current and permittivity are determined as a function of the Y content. The best combination of low leakage current of 10^-7 A/cm² at 1 V and a highest permittivity value of 29 is achieved when the doping ratio of Y increases to 9 mol%. A correlation among Y content, phase evolution and electrical properties of Y-doped HfO₂ ultra-thin film is investigated.

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Low-temperature synthesis of apatite-type La_9.33Ge₆O₂₆ as electrolytes with high conductivity

尹广超, 赵国栋, 殷红, 贾福超, 景强, 付圣贵, 孙美玲, 高伟

2018 (4): 48201-048201. doi: 10.1088/1674-1056/27/4/048201

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In the present study, high-quality apatite-type La_9.33Ge₆O₂₆ powders are successfully synthesized by a facile molten-salt synthesis method (MSSM) at low temperatures, using LiCl, LiCl/NaCl mixture (mass ratio 1:1) as molten salt, respectively. Experimental results indicate that the optimal mass ratio between reactant and molten salt is 1:2, and LiCl/NaCl mixed molten-salt is more beneficial for forming high-quality La_9.33Ge₆O₂₆ powders than LiCl individual molten-salt. Comparing with the conventional solid-state reaction method (SSRM), the synthesis temperature of apatite-type La_9.33Ge₆O₂₆ powders using the MSSM decreases more than 350℃, which can effectively avoid Ge loss in the preparation process of precursor powders. Furthermore, the powders obtained by the MSSM are homogeneous, non-agglomerated and well crystallized, which are very favorable for gaining dense pellets in the premise of avoiding Ge loss. On the basis of high-quality precursor powders, the dense and pure ceramic pellets of La_9.33Ge₆O₂₆ are gained at a low temperature of 1100℃ for 2 h, which exhibit higher conductivities (σ _850℃(LiCl)=2.3×10^-2 S·cm^-1, σ _{850℃(LiCl/NaCl)}=4.9×10^-2 S·cm^-1) and lower activation energies (E_a(LiCl)=1.02 eV, E_a(LiCl/NaCl)=0.99 eV) than that synthesized by the SSRM.

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Enhancement of thermoelectric properties of SrTiO₃/LaNb-SrTiO₃ composite by different doping levels

王柯鲜, 王俊, 李艳, 邹涛, 王晓欢, 李建波, 曹正, 师文静, 新巴雅尔

2018 (4): 48401-048401. doi: 10.1088/1674-1056/27/4/048401

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Strontium titanate (STO) is an n-type oxide thermoelectric material, which has shown great prospects in recent years. The doping of La and Nb into STO can improve its power factor, whereas its thermal conductivity is still very high. Thus, in order to obtain a high thermoelectric figure-of-merit zT, it is very important to reduce its thermal conductivity. In this paper, using a combination of a hydrothermal method and a high-efficiency sintering method, we succeed in preparing a composite of pure STO and LaNb-doped STO, which simultaneously realizes lower thermal conductivity and higher Seebeck coefficient, therefore, the thermoelectric properties of STO are significantly improved. In the SrTiO₃/LaNb-SrTiO₃ bulk samples, the lowest thermal conductivity is 2.57 W·m^-1·K^-1 and the highest zT is 0.35 at 1000 K for the STO/La10Nb20-STO sample.

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Graphene-enhanced thermoelectric properties of p-type skutterudites

秦丹丹, 刘嫄, 孟宪福, 崔博, 祁亚亚, 蔡伟, 隋解和

2018 (4): 48402-048402. doi: 10.1088/1674-1056/27/4/048402

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Nanocomposite is proved to be an effective method to improve thermoelectric performance. In the present study, graphene is introduced into p-type skutterudite La_0.8Ti_0.1Ga_0.1Fe₃CoSb₁₂ by plasma-enhanced chemical vapor deposition (PECVD) method to form skutterudite/graphene nanocomposites. It is demonstrated that the graphene has no obvious effect on the electrical conductivity of La_0.8Ti_0.1Ga_0.1Fe₃CoSb₁₂, but the Seebeck coefficient is slightly improved at high temperature, thereby leading to high power factor. Furthermore, due to the enhancement of phonon scattering by the graphene, the lattice thermal conductivity is reduced significantly. Ultimately, the maximum zT value of La_0.8Ti_0.1Ga_0.1Fe₃CoSb₁₂/graphene is higher than that of graphene-free alloy and reaches to 1.0 at 723 K. Such an approach raised by us enriches prospects for future practical application.

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Fundamental and progress of Bi₂Te₃-based thermoelectric materials

洪敏, 陈志刚, 邹进

2018 (4): 48403-048403. doi: 10.1088/1674-1056/27/4/048403

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Thermoelectric materials, enabling the directing conversion between heat and electricity, are one of the promising candidates for overcoming environmental pollution and the upcoming energy shortage caused by the over-consumption of fossil fuels. Bi₂Te₃-based alloys are the classical thermoelectric materials working near room temperature. Due to the intensive theoretical investigations and experimental demonstrations, significant progress has been achieved to enhance the thermoelectric performance of Bi₂Te₃-based thermoelectric materials. In this review, we first explored the fundamentals of thermoelectric effect and derived the equations for thermoelectric properties. On this basis, we studied the effect of material parameters on thermoelectric properties. Then, we analyzed the features of Bi₂Te₃-based thermoelectric materials, including the lattice defects, anisotropic behavior and the strong bipolar conduction at relatively high temperature. Then we accordingly summarized the strategies for enhancing the thermoelectric performance, including point defect engineering, texture alignment, and band gap enlargement. Moreover, we highlighted the progress in decreasing thermal conductivity using nanostructures fabricated by solution grown method, ball milling, and melt spinning. Lastly, we employed modeling analysis to uncover the principles of anisotropy behavior and the achieved enhancement in Bi₂Te₃, which will enlighten the enhancement of thermoelectric performance in broader materials

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Scalability of dark current in silicon PIN photodiode

丰亚洁, 李冲, 刘巧莉, 王华强, 胡安琪, 何晓颖, 郭霞

2018 (4): 48501-048501. doi: 10.1088/1674-1056/27/4/048501

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The mechanism for electrical conduction is investigated by the dark temperature-dependent current-voltage characteristics of Si PIN photodiodes with different photosensitive areas. The characteristic tunneling energy E₀₀ can be obtained to be 1.40 meV, 1.53 meV, 1.74 meV, 1.87 meV, and 2.01 meV, respectively, for the photodiodes with L=0.25 mm, 0.5 mm, 1 mm, 1.5 mm, and 2 mm by fitting the ideality factor n versus temperature curves according to the tunneling-enhanced recombination mechanism. The trap-assisted tunneling-enhanced recombination in the i-layer plays an important role in our device, which is consistent with the experimental result that area-dependent leakage current is dominant with the side length larger than 1 mm of the photosensitive area. Our results reveal that the quality of the bulk material plays an important role in the electrical conduction mechanism of the devices with the side length larger than 1 mm of the photosensitive area.

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Closed-form breakdown voltage/specific on-resistance model using charge superposition technique for vertical power double-diffused metal-oxide-semiconductor device with high-κ insulator

陈雪, 汪志刚, 王喜, James B Kuo

2018 (4): 48502-048502. doi: 10.1088/1674-1056/27/4/048502

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An improved vertical power double-diffused metal-oxide-semiconductor (DMOS) device with a p-region(P1) and high-κ insulator vertical double-diffusion metal-oxide-semiconductor (HKP-VDMOS) is proposed to achieve a better performance on breakdown voltage (BV)/specific on-resistance (R_on,sp) than conventional VDMOS with a high-κ insulator (CHK-VDMOS). The main mechanism is that with the introduction of the P-region, an extra electric field peak is generated in the drift region of HKP-VDMOS to enhance the breakdown voltage. Due to the assisted depletion effect of this p-region, the specific on-resistance of the device could be reduced because of the high doping density of the N-type drift region. Meanwhile, based on the superposition of the depleted charges, a closed-form model for electric field/breakdown voltage is generally derived, which is in good agreement with the simulation result within 10% of error. An HKP-VDMOS device with a breakdown voltage of 600 V, a reduced specific on-resistance of 11.5 mΩ·cm² and a figure of merit (FOM) (BV²/R_on,sp) of 31.2 MW·cm^-2 shows a substantial improvement compared with the CHK-VDMOS device.

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Research on the radiation hardened SOI devices with single-step Si ion implantation

戴丽华, 毕大炜, 胡志远, 刘小年, 张梦映, 张正选, 邹世昌

2018 (4): 48503-048503. doi: 10.1088/1674-1056/27/4/048503

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Silicon-on-insulator (SOI) devices are sensitive to the total ionizing dose effect due to the existence of buried oxide. In this paper, an extra single-step Si ion implantation into buried oxide layer prior to the normal complementary metal-oxide-semiconductor transistor (CMOS) process is used to harden the SOI wafer. The top-Si quality of the hardened SOI wafer is confirmed to be good enough for device manufacturing through various characterization methods. The radiation experiments show that the total ionizing dose tolerance of the Si implanted SOI device is improved significantly. The metastable electron traps introduced by Si implantation is also investigated by electrical stress. The results show that these traps are very instable, and electrons will tunnel into or out of the metastable electron traps quickly after hot-electron-injection or hot-hole-injection.

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Water-based processed and alkoxide-based processed indium oxide thin-film transistors at different annealing temperatures

栗旭阳, 喻志农, 程锦, 陈永华, 薛建设, 郭建, 薛唯

2018 (4): 48504-048504. doi: 10.1088/1674-1056/27/4/048504

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In this study, indium oxide (In₂O₃) thin-film transistors (TFTs) are fabricated by two kinds of low temperature solution-processed technologies (T_a ≤ 300℃), i.e., water-based (DIW-based) process and alkoxide-based (2-ME-based) process. The thickness values, crystallization properties, chemical structures, surface roughness values, and optical properties of In₂O₃ thin-films and the electrical characteristics of In₂O₃ TFTs are studied at different annealing temperatures. Thermal annealing at higher temperature leads to an increase in the saturation mobility (μ_sat) and a negative shift in the threshold voltage (V_TH). The DIW-based processed In₂O₃-TFT annealed at 300℃ exhibits excellent device performance, and one annealed at 200℃ exhibits an acceptable μ_sat of 0.86 cm²/V·s comparable to that of a-Si:H TFTs, whereas the 2-ME-based TFT annealed at 300℃ exhibits an abundant μ_sat of 1.65 cm²/Vs and one annealed at 200℃ is inactive. The results are attributed to the fact that the DIW-based process induces a higher degree of oxidation and less defect states than the 2-ME-based process at the same temperature. The DIW-based process for fabricating the In₂O₃ TFT opens the way for the development of nontoxic, low-cost, and low-temperature oxide electronics.

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Analytically determining frequency and amplitude of spontaneous alpha oscillation in Jansen's neural mass model using the describing function method

徐瑶, 张春会, Ernst Niebur, 王俊松

2018 (4): 48701-048701. doi: 10.1088/1674-1056/27/4/048701

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Spontaneous alpha oscillations are a ubiquitous phenomenon in the brain and play a key role in neural information processing and various cognitive functions. Jansen's neural mass model (NMM) was initially proposed to study the origin of alpha oscillations. Most of previous studies of the spontaneous alpha oscillations in the NMM were conducted using numerical methods. In this study, we aim to propose an analytical approach using the describing function method to elucidate the spontaneous alpha oscillation mechanism in the NMM. First, the sigmoid nonlinear function in the NMM is approximated by its describing function, allowing us to reformulate the NMM and derive its standard form composed of one nonlinear part and one linear part. Second, by conducting a theoretical analysis, we can assess whether or not the spontaneous alpha oscillation would occur in the NMM and, furthermore, accurately determine its amplitude and frequency. The results reveal analytically that the interaction between linearity and nonlinearity of the NMM plays a key role in generating the spontaneous alpha oscillations. Furthermore, strong nonlinearity and large linear strength are required to generate the spontaneous alpha oscillations.

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