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CN 11-5639/O4
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All Express Letters

Direct loading of atoms from a macroscopic quadrupole magnetic trap into a microchip trap Hot!

Jun Cheng(程俊), Jing-fang Zhang(张敬芳), Xin-ping Xu(许忻平), Hai-chao Zhang(张海潮), Yu-zhu Wang(王育竹)
Chin. Phys. B, 2017, 26 (3): 033701 doi: 10.1088/1674-1056/26/3/033701
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We demonstrate the direct loading of cold atoms into a microchip 2-mm Z-trap, where the evaporative cooling can be performed efficiently, from a macroscopic quadrupole magnetic trap with a high loading efficiency. The macroscopic quadrupole magnetic trap potential is designed to be moveable by controlling the currents of the two pairs of anti-Helmholtz coils. The cold atoms are initially prepared in a standard six-beam magneto-optical trap and loaded into the macroscopic quadrupole magnetic trap, and then transported to the atom chip surface by moving the macroscopic trap potential. By means of a three-dimensional absorption imaging system, we are able to optimize the position alignment of the atom cloud in the macroscopic trap and the microchip Z-shaped wire. Consequently, with a proper magnetic transfer scheme, we load the cold atoms into the microchip Z-trap directly and efficiently. The loading efficiency is measured to be about 50%. This approach can be used to generate appropriate ultracold atoms sources, for example, for a magnetically guided atom interferometer based on atom chip.

Transparent conducting indium-tin-oxide (ITO) film as full front electrode in III-V compound solar cell Hot!

Pan Dai(代盼), Jianya Lu(卢建娅), Ming Tan(谭明), Qingsong Wang(王青松), Yuanyuan Wu(吴渊渊), Lian Ji(季莲), Lifeng Bian(边历峰), Shulong Lu(陆书龙), Hui Yang(杨辉)
Chin. Phys. B, 2017, 26 (3): 037305 doi: 10.1088/1674-1056/26/3/037305
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The application of transparent conducting indium-tin-oxide (ITO) film as full front electrode replacing the conventional bus-bar metal electrode in III-V compound GaInP solar cell was proposed. A high-quality, non-rectifying contact between ITO and 10 nm N+-GaAs contact layer was formed, which is benefiting from a high carrier concentration of the terrilium-doped N+-GaAs layer, up to 2×1019 cm-3. A good device performance of the GaInP solar cell with the ITO electrode was observed. This result indicates a great potential of transparent conducting films in the future fabrication of larger area flexible III-V solar cell.

Two-step quantum secure direct communication scheme with frequency coding Hot!

Xue-Liang Zhao(赵学亮), Jun-Lin Li(李俊林), Peng-Hao Niu(牛鹏皓), Hong-Yang Ma(马鸿洋), Dong Ruan(阮东)
Chin. Phys. B, 2017, 26 (3): 030302 doi: 10.1088/1674-1056/26/3/030302
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Quantum secure direct communication (QSDC) is an important branch of quantum cryptography. It can transmit secret information directly without establishing a key first, unlike quantum key distribution which requires this precursory event. Here we propose a QSDC scheme by applying the frequency coding technique to the two-step QSDC protocol, which enables the two-step QSDC protocol to work in a noisy environment. We have numerically simulated the performance of the protocol in a noisy channel, and the results show that the scheme is indeed robust against channel noise and loss. We also give an estimate of the channel noise upper bound.

Ballistic transport and quantum interference in InSb nanowire devices Hot!

Sen Li(李森), Guang-Yao Huang(黄光耀), Jing-Kun Guo(郭景琨), Ning Kang(康宁), Philippe Caroff, Hong-Qi Xu(徐洪起)
Chin. Phys. B, 2017, 26 (2): 027305 doi: 10.1088/1674-1056/26/2/027305
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An experimental realization of a ballistic superconductor proximitized semiconductor nanowire device is a necessary step towards engineering topological quantum electronics. Here, we report on ballistic transport in InSb nanowires grown by molecular-beam epitaxy contacted by superconductor electrodes. At an elevated temperature, clear conductance plateaus are observed at zero magnetic field and in agreement with calculations based on the Landauer formula. At lower temperature, we have observed characteristic Fabry-Pérot patterns which confirm the ballistic nature of charge transport. Furthermore, the magnetoconductance measurements in the ballistic regime reveal a periodic variation related to the Fabry-Pérot oscillations. The result can be reasonably explained by taking into account the impact of magnetic field on the phase of ballistic electron's wave function, which is further verified by our simulation. Our results pave the way for better understanding of the quantum interference effects on the transport properties of InSb nanowires in the ballistic regime as well as developing of novel device for topological quantum computations.

New progress on beam availability and reliability of PKU high intensity CW proton ECR ion source Hot!

Shi-Xiang Peng(彭士香), Ai-Lin Zhang(张艾霖), Hai-Tao Ren(任海涛), Yuan Xu(徐源), Tao Zhang(张滔), Jing-Feng Zhang(张景丰), Jia-Mei Wen(温佳美), Zhi-Yu Guo(郭之虞), Jia-Er Chen(陈佳洱)
Chin. Phys. B, 2017, 26 (2): 025206 doi: 10.1088/1674-1056/26/2/025206
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The stability and reliability of an ion source and its beam availability are extremely significant for any accelerator, especially for those high current long term CW operation ones like ADS. Although the first high quality 306-hours continuous wave (CW) operating curve at 50 mA@35 keV has been successfully obtained with a standard compact 2.45 GHz ECR ion source at Peking University (PKU), but the uncertainties that caused beam trips before are unacceptable during an accelerator real operation and should be eliminated. Meanwhile, no permission will be given when the beam power is upgraded from 50 mA@35 keV to 50 mA@50 keV. To improve the PKU CW proton source quality, several upgrades were done recently. After those improvements, a new long term CW proton beam experiment at 50 mA@50 keV was carried out in June 2016. The total running time is 300.5 hours, including near 6 hours ion source preparation and 294 hours non-disturb continuous operation. Within the continuous 13 days operation, no beam-off happened, no spark was observed, no beam drop appeared, no interrupting action was needed, and only a few beam fluctuations caused by the air conditional failure occurred. Beam availability and reliability within the 294 hours is 100%. The root-mean-square (RMS) emittance of this 50 mA@50 keV CW proton beam is about 0.186 π.mm.mrad. A careful inspection of the ion source was done after this long term operation and no obvious damage was found. The restart experimental results obtained after the ion source inspection prove the high repeatability of PKU PMECRIS. In addition, a 130-mA H+ beam was obtained at 50 kV with duty factor of 10% (100 Hz/1 ms) with this source. Details will be presented in this paper.

Highly conductive and transparent carbon nanotube-based electrodes for ultrathin and stretchable organic solar cells Hot!

Qingxia Fan(范庆霞), Qiang Zhang(张强), Wenbin Zhou(周文斌), Feng Yang(杨丰), Nan Zhang(张楠), Shiqi Xiao(肖仕奇), Xiaogang Gu(谷孝刚), Zhuojian Xiao(肖卓建), Huiliang Chen(陈辉亮), Yanchun Wang(王艳春), Huaping Liu(刘华平), Weiya Zhou(周维亚)
Chin. Phys. B, 2017, 26 (2): 028801 doi: 10.1088/1674-1056/26/2/028801
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In this work, we have presented a freestanding and flexible CNT-based film with sheet resistance of 60 Ω/□ and transmittance of 82% treated by nitric acid and chloroauric acid in sequence. Based on modified CNT film as a transparent electrode, we have demonstrated an ultrathin, flexible organic solar cell (OSC) fabricated on 2.5-μm PET substrate. The efficiency of OSC, combined with a composite film of poly (3-hexylthiophene) (P3HT) and phenyl-C61 butyric acid methyl ester (PCBM) as an active layer and with a thin layer of methanol soluble biuret inserted between the photoactive layer and the cathode, can be up to 2.74% which is approximate to that of the reference solar cell fabricated with ITO-coated glass (2.93%). Incorporating the as-fabricated ITO-free OSC with pre-stretched elastomer, 50% compressive deformation can apply to the solar cells. The results show that the as-prepared CNT-based hybrid film with outstanding electrical and optical properties could serve as a promising transparent electrode for low cost, flexible and stretchable OSCs, which will broaden the applications of OSC and generate more solar power than it now does.

Energy sharing induced by the nonlinear interaction Hot!

Yuan Liu(刘渊), Zhifang Feng(冯志芳), Weidong Li(李卫东)
Chin. Phys. B, 2017, 26 (1): 013401 doi: 10.1088/1674-1056/26/1/013401
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Strong energy sharing is shown by numerically investigating coupled multi-component Bose-Einstein condensates (BECs) with a harmonic trap to simulate the Fermi-Pasta-Ulam model (FPU). For two-component BECs, the energy exchanging between each part, from regular, quantum beating to complete energy sharing, is explored by simulating their Husimi distributions, the time evolution of energies and the statistical entropy. Meanwhile, in the three-component case, a more complex energy sharing behavior is reported and a strong energy sharing is found.

Implementation of LDA+Gutzwiller with Newton's method Hot!

Jian Zhang(张健), Ming-Feng Tian(田明锋), Guang-Xi Jin(金光希), Yuan-Feng Xu(徐远锋), Xi Dai(戴希)
Chin. Phys. B, 2017, 26 (1): 017103 doi: 10.1088/1674-1056/26/1/017103
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In order to calculate the electronic structure of correlated materials, we propose implementation of the LDA+Gutzwiller method with Newton's method. The self-consistence process, efficiency and convergence of calculation are improved dramatically by using Newton's method with golden section search and other improvement approaches. We compare the calculated results by applying the previous linear mix method and Newton's method. We have applied our code to study the electronic structure of several typical strong correlated materials, including SrVO3, LaCoO3, and La2O3Fe2Se2. Our results fit quite well with the previous studies.

Electron localization of linear symmetric molecular ion H32+ Hot!

Zheng-Mao Jia(贾正茂), Zhi-Nan Zeng(曾志男), Ru-Xin Li(李儒新)
Chin. Phys. B, 2017, 26 (1): 013203 doi: 10.1088/1674-1056/26/1/013203
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Electron localization in the dissociation of the symmetric linear molecular ion H32+ is investigated. The numerical simulation shows that the electron localization distribution is dependent on the central frequency and peak electric field amplitude of the external ultrashort ultraviolet laser pulse. When the electrons of the ground state are excited onto the 2pσ2Σu+ by a one-photon process, most electrons of the dissociation states are localized at the protons on both sides symmetrically. Almost no electron is stabilized at the middle proton due to the odd symmetry of the wave function. With the increase of the frequency of the external ultraviolet laser pulse, the electron localization ratio of the middle proton increases, for more electrons of the ground state are excited onto the higher 3pσ2Σu+ state. 50.9% electrons of all the dissociation events can be captured by the middle Coulomb potential well through optimizing the central frequency and peak electric field amplitude of the ultraviolet laser pulse. Besides, a direct current (DC) electric field can be utilized to control the electron motions of the dissociation states after the excitation of an ultraviolet laser pulse, and 68.8% electrons of the dissociation states can be controlled into the middle proton.

Modulation of intra- and inter-sheet interactions in short peptide self-assembly by acetonitrile in aqueous solution Hot!

Li Deng(邓礼), Yurong Zhao(赵玉荣), Peng Zhou(周鹏), Hai Xu(徐海), Yanting Wang(王延颋)
Chin. Phys. B, 2016, 25 (12): 128704 doi: 10.1088/1674-1056/25/12/128704
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Besides our previous experimental discovery (Zhao Y R, et al. 2015 Langmuir, 31, 12975) that acetonitrile (ACN) can tune the morphological features of nanostructures self-assembled by short peptides KⅢIK (KI4K) in aqueous solution, further experiments reported in this work demonstrate that ACN can also tune the mass of the self-assembled nanostructures. To understand the microscopic mechanism how ACN molecules interfere peptide self-assembly process, we conducted a series of molecular dynamics simulations on a monomer, a cross-β sheet structure, and a proto-fibril of KI4K in pure water, pure ACN, and ACN-water mixtures, respectively. The simulation results indicate that ACN enhances the intra-sheet interaction dominated by the hydrogen bonding (H-bonding) interactions between peptide backbones, but weakens the inter-sheet interaction dominated by the interactions between hydrophobic side chains. Through analyzing the correlations between different groups of solvent and peptides and the solvent behaviors around the proto-fibril, we have found that both the polar and nonpolar groups of ACN play significant roles in causing the opposite effects on intermolecular interactions among peptides. The weaker correlation of the polar group of ACN than water molecule with the peptide backbone enhances H-bonding interactions between peptides in the proto-fibril. The stronger correlation of the nonpolar group of ACN than water molecule with the peptide side chain leads to the accumulation of ACN molecules around the proto-fibril with their hydrophilic groups exposed to water, which in turn allows more water molecules close to the proto-fibril surface and weakens the inter-sheet interactions. The two opposite effects caused by ACN form a microscopic mechanism clearly explaining our experimental observations.

Tunable in-plane spin orientation in Fe/Si (557) film by step-induced competing magnetic anisotropies Hot!

Jin Tang(汤进), Wei He(何为), Yong-Sheng Zhang(张永圣), Yan Li(李岩), Wei Zhang(张伟), Syed Sheraz Ahmad, Xiang-Qun Zhang(张向群), Zhao-Hua Cheng(成昭华)
Chin. Phys. B, 2016, 25 (12): 127501 doi: 10.1088/1674-1056/25/12/127501
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Although the spin-reorientation transition from out-of-plane to in-plane in Fe/Si film is widely reported, the tuning of in-plane spin orientation is not yet well developed. Here, we report the thickness-, temperature- and Cu-adsorption-induced in-plane spin-reorientation transition processes in Fe/Si (557) film, which can be attributed to the coexistence of two competing step-induced uniaxial magnetic anisotropies, i.e., surface magnetic anisotropy with magnetization easy axis perpendicular to the step and volume magnetic anisotropy with magnetization easy axis parallel to the step. For Fe film thickness smaller than 32 monolayer (ML), the magnitudes of two effects under various temperatures are extracted from the thickness dependence of uniaxial magnetic anisotropy. For Fe film thickness larger than 32 ML, the deviation of experimental results from fitting results is understood by the strain-relief-induced reduction of volume magnetic anisotropy. Additionally, the surface and volume magnetic anisotropies are both greatly reduced after covering Cu capping layer on Fe/Si (557) film while no significant influence of NaCl capping layer on step-induced magnetic anisotropies is observed. The experimental results reported here provide various practical methods for manipulating in-plane spin orientation of Fe/Si films and improve the understanding of step-induced magnetic anisotropies.

Microscale vortex laser with controlled topological charge Hot!

Xing-Yuan Wang(王兴远), Hua-Zhou Chen(陈华洲), Ying Li(黎颖), Bo Li(李波), Ren-Min Ma(马仁敏)
Chin. Phys. B, 2016, 25 (12): 124211 doi: 10.1088/1674-1056/25/12/124211
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A microscale vortex laser is a new type of coherent light source with small footprint that can directly generate vector vortex beams. However, a microscale laser with controlled topological charge, which is crucial for virtually any of its application, is still unrevealed. Here we present a microscale vortex laser with controlled topological charge. The vortex laser eigenmode was synthesized in a metamaterial engineered non-Hermitian micro-ring cavity system at exceptional point. We also show that the vortex laser cavity can operate at exceptional point stably to lase under optical pumping. The microscale vortex laser with controlled topological charge can serve as a unique and general building block for next-generation photonic integrated circuits and coherent vortex beam sources. The method we used here can be employed to generate lasing eigenmode with other complex functionalities.

High performance photodetectors based on high quality InP nanowires Hot!

Yan-Kun Yang(杨燕琨), Tie-Feng Yang(杨铁锋), Hong-Lai Li(李洪来), Zhao-Yang Qi(祁朝阳), Xin-Liang Chen(陈新亮), Wen-Qiang Wu(吴文强), Xue-Lu Hu(胡学鹿), Peng-Bin He(贺鹏斌), Ying Jiang(蒋英), Wei Hu(胡伟), Qing-Lin Zhang(张清林), Xiu-Juan Zhuang(庄秀娟), Xiao-Li Zhu(朱小莉), An-Lian Pan(潘安练)
Chin. Phys. B, 2016, 25 (11): 118106 doi: 10.1088/1674-1056/25/11/118106
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In this paper, small diameter InP nanowires with high crystal quality were synthesized through a chemical vapor deposition method. Benefitting from the high crystallinity and large specific surface area of InP nanowires, the simply constructed photodetector demonstrates a high responsivity of up to 1170 A·W-1 and an external quantum efficiency of 2.8×105% with a fast rise time of 110 ms and a fall time of 130 ms, even at low bias of 0.1 V. The effect of back-gate voltage on photoresponse of the device was systematically investigated, confirming that the photocurrent dominates over thermionic and tunneling currents in the whole operation. A mechanism based on energy band theory at the junction between metal and semiconductor was proposed to explain the back-gate voltage dependent performance of the photodetectors. These convincing results indicate that fine InP nanowires will have a brilliant future in smart optoelectronics.

Spatially resolved gap closing in single Josephson junctions constructed on Bi2Te3 surface Hot!

Yuan Pang(庞远), Junhua Wang(王骏华), Zhaozheng Lyu(吕昭征), Guang Yang(杨光), Jie Fan(樊洁), Guangtong Liu(刘广同), Zhongqing Ji(姬忠庆), Xiunian Jing(景秀年), Changli Yang(杨昌黎), Li Lu(吕力)
Chin. Phys. B, 2016, 25 (11): 117402 doi: 10.1088/1674-1056/25/11/117402
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Full gap closing is a prerequisite for hosting Majorana zero modes in Josephson junctions on the surface of topological insulators. Previously, we have observed direct experimental evidence of gap closing in Josephson junctions constructed on Bi2Te3 surface. In this paper we report further investigations on the position dependence of gap closing as a function of magnetic flux in single Josephson junctions constructed on Bi2Te3 surface.

Large scale fabrication of nitrogen vacancy-embedded diamond nanostructures for single-photon source applications Hot!

Qianqing Jiang(姜倩晴), Wuxia Li(李无瑕), Chengchun Tang(唐成春), Yanchun Chang(常彦春), Tingting Hao(郝婷婷), Xinyu Pan(潘新宇), Haitao Ye(叶海涛), Junjie Li(李俊杰), Changzhi Gu(顾长志)
Chin. Phys. B, 2016, 25 (11): 118105 doi: 10.1088/1674-1056/25/11/118105
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Color centers in diamond are prominent candidates for generating and manipulating quantum states of light, even at room temperature. However, the photon collection efficiency of bulk diamond is greatly reduced by refraction at the diamond/air interface. To address this issue, we fabricated arrays of diamond nanostructures, differing in both diameter and top end shape, with HSQ, PMMA, and Cr as the etching mask materials, aiming toward large scale fabrication of single-photon sources with enhanced collection efficiency made of nitrogen vacancy (NV) embedded diamond. With a mixture of O2 and CHF3 gas plasma, diamond pillars with diameters down to 45 nm were obtained. The top end shape evolution has been represented with a simple model. The tests of size dependent single-photon properties confirmed an improved single-photon collection efficiency enhancement, larger than tenfold, and a mild decrease of decoherence time with decreasing pillar diameter was observed as expected. These results provide useful information for future applications of nanostructured diamond as a single-photon source.

Analysis of the blackbody-radiation shift in an ytterbium optical lattice clock Hot!

Yi-Lin Xu(徐艺琳), Xin-Ye Xu(徐信业)
Chin. Phys. B, 2016, 25 (10): 103202 doi: 10.1088/1674-1056/25/10/103202
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We accurately evaluate the blackbody-radiation shift in a 171Yb optical lattice clock by utilizing temperature measurement and numerical simulation. In this work. three main radiation sources are considered for the blackbody-radiation shift, including the heated atomic oven, the warm vacuum chamber, and the room-temperature vacuum windows. The temperatures on the outer surface of the vacuum chamber are measured during the clock operation period by utilizing seven calibrated temperature sensors. Then we infer the temperature distribution inside the vacuum chamber by numerical simulation according to the measured temperatures. Furthermore, we simulate the temperature variation around the cold atoms while the environmental temperature is fluctuating. Finally, we obtain that the total blackbody-radiation shift is -1.289(7) Hz with an uncertainty of 1.25×10-17 for our 171Yb optical lattice clock. The presented method is quite suitable for accurately evaluating the blackbody-radiation shift of the optical lattice clock in the case of lacking the sensors inside the vacuum chamber.

Large linear magnetoresistance in a new Dirac material BaMnBi2 Hot!

Yi-Yan Wang(王义炎), Qiao-He Yu(郁巧鹤), Tian-Long Xia(夏天龙)
Chin. Phys. B, 2016, 25 (10): 107503 doi: 10.1088/1674-1056/25/10/107503
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Dirac semimetal is a class of materials that host Dirac fermions as emergent quasi-particles. Dirac cone-type band structure can bring interesting properties such as quantum linear magnetoresistance and large mobility in the materials. In this paper, we report the synthesis of high quality single crystals of BaMnBi2 and investigate the transport properties of the samples. BaMnBi2 is a metal with an antiferromagnetic transition at TN=288 K. The temperature dependence of magnetization displays different behavior from CaMnBi2 and SrMnBi2, which suggests the possible different magnetic structure of BaMnBi2. The Hall data reveals electron-type carriers and a mobility μ(5 K)=1500 cm2/V·s. Angle-dependent magnetoresistance reveals the quasi-two-dimensional (2D) Fermi surface in BaMnBi2. A crossover from semiclassical MR~H2 dependence in low field to MR~H dependence in high field, which is attributed to the quantum limit of Dirac fermions, has been observed in magnetoresistance. Our results indicate the existence of Dirac fermions in BaMnBi2.

The spin Hall effect in single-crystalline gold thin films Hot!

Dai Tian(田岱), Caigan Chen(陈才干), Hua Wang(王华), Xiaofeng Jin(金晓峰)
Chin. Phys. B, 2016, 25 (10): 107201 doi: 10.1088/1674-1056/25/10/107201
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The spin Hall effect has been investigated in 10-nm-thick epitaxial Au (001) single crystal films via H-pattern devices, whose minimum characteristic dimension is about 40 nm. By improving the film quality and optimizing the in-plane geometry parameters of the devices, we explicitly extract the spin Hall effect contribution from the ballistic and bypass contribution which were previously reported to be dominating the non-local voltage. Furthermore, we calculate a lower limit of the spin Hall angle of 0.08 at room temperature. Our results indicate that the giant spin Hall effect in Au thin films is dominated not by the interior defects scattering, but by the surface scattering. Besides, our results also provide an additional experimental method to determine the magnitude of spin Hall angle unambiguously.

Carrier transport in III-V quantum-dot structures for solar cells or photodetectors Hot!

Wenqi Wang(王文奇), Lu Wang(王禄), Yang Jiang(江洋), Ziguang Ma(马紫光), Ling Sun(孙令), Jie Liu(刘洁), Qingling Sun(孙庆灵), Bin Zhao(赵斌), Wenxin Wang(王文新), Wuming Liu(刘伍明), Haiqiang Jia(贾海强), Hong Chen(陈弘)
Chin. Phys. B, 2016, 25 (9): 097307 doi: 10.1088/1674-1056/25/9/097307
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According to the well-established light-to-electricity conversion theory, resonant excited carriers in the quantum dots will relax to the ground states and cannot escape from the quantum dots to form photocurrent, which have been observed in quantum dots without a p-n junction at an external bias. Here, we experimentally observed more than 88% of the resonantly excited photo carriers escaping from InAs quantum dots embedded in a short-circuited p-n junction to form photocurrent. The phenomenon cannot be explained by thermionic emission, tunneling process, and intermediate-band theories. A new mechanism is suggested that the photo carriers escape directly from the quantum dots to form photocurrent rather than relax to the ground state of quantum dots induced by a p-n junction. The finding is important for understanding the low-dimensional semiconductor physics and applications in solar cells and photodiode detectors.

Unified semiclassical approach to electronic transport from diffusive to ballistic regimes Hot!

Hao Geng(耿浩), Wei-Yin Deng(邓伟胤), Yue-Jiao Ren(任月皎), Li Sheng(盛利), Ding-Yu Xing(邢定钰)
Chin. Phys. B, 2016, 25 (9): 097201 doi: 10.1088/1674-1056/25/9/097201
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We show that by integrating out the electric field and incorporating proper boundary conditions, a Boltzmann equation can describe electron transport properties, continuously from the diffusive to ballistic regimes. General analytical formulas of the conductance in D=1,2,3 dimensions are obtained, which recover the Boltzmann-Drude formula and Landauer-Büttiker formula in the diffusive and ballistic limits, respectively. This intuitive and efficient approach can be applied to investigate the interplay of system size and impurity scattering in various charge and spin transport phenomena, when the quantum interference effect is not important.

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