Chin. Phys. B

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Perpendicular magnetization switching by large spin—orbit torques from sputtered Bi₂Te₃ ^Hot!

Zhenyi Zheng(郑臻益), Yue Zhang(张悦), Daoqian Zhu(朱道乾), Kun Zhang(张昆), Xueqiang Feng(冯学强), Yu He(何宇), Lei Chen(陈磊), Zhizhong Zhang(张志仲), Dijun Liu(刘迪军), Youguang Zhang(张有光), Pedram Khalili Amiri, Weisheng Zhao(赵巍胜)

Chin. Phys. B, 2020, 29 (7): 078505 doi: 10.1088/1674-1056/ab9439
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Spin-orbit torque (SOT) effect is considered as an efficient way to switch the magnetization and can inspire various high-performance spintronic devices. Recently, topological insulators (TIs) have gained extensive attention, as they are demonstrated to maintain a large effective spin Hall angle (θ_SH^eff), even at room temperature. However, molecular beam epitaxy (MBE), as a precise deposition method, is required to guarantee favorable surface states of TIs, which hinders the prospect of TIs towards industrial application. In this paper, we demonstrate that Bi₂Te₃ films grown by magnetron sputtering can provide a notable SOT effect in the heterostructure with perpendicular magnetic anisotropy CoTb ferrimagnetic alloy. By harmonic Hall measurement, a high SOT efficiency (8.7±0.9 Oe/(10⁹ A/m²)) and a large θ_SH^eff (3.3±0.3) are obtained at room temperature. Besides, we also observe an ultra-low critical switching current density (9.7×10⁹ A/m²). Moreover, the low-power characteristic of the sputtered Bi₂Te₃ film is investigated by drawing a comparison with different sputtered SOT sources. Our work may provide an alternative to leverage chalcogenides as a realistic and efficient SOT source in future spintronic devices.

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Dependence of mechanical properties on the site occupancy of ternary alloying elements in γ'-Ni₃Al: Ab initio description for shear and tensile deformation ^Hot!

Minru Wen(文敏儒), Xing Xie(谢兴), Huafeng Dong(董华锋), Fugen Wu(吴福根), Chong-Yu Wang(王崇愚)

Chin. Phys. B, 2020, 29 (7): 078103 doi: 10.1088/1674-1056/ab8a38
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The site occupancy behavior of ternary alloying elements in γ'-Ni₃Al (a key strengthening phase of commercial Ni-based single-crystal superalloys) can change with temperature and alloy composition owing to the effect of entropy. Using a total-energy method based on density functional theory, the dependence of tensile and shear behaviors on the site preference of alloying elements in γ'-Ni₃Al were investigated in detail. Our results demonstrate that Fe, Ru, and Ir can significantly improve the ideal tensile and shear strength of the γ' phase when occupying the Al site, with Ru resulting in the strongest enhancement. In contrast, elements with fully filled d orbitals (i.e., Cu, Zn, Ag, and Cd) are expected to reduce the ideal tensile and shear strength. The calculated stress-strain relationships of Ni₃Al alloys indicate that none of the alloying elements can simultaneously increase the ideal strength of the γ' phase for both Ni1-site and Ni2-site substitutions. In addition, the charge redistribution and the bond length of the alloying elements and host atoms during the tensile and shear processes are analyzed to unveil the underlying electronic mechanisms.

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Modulation of carrier lifetime in MoS₂ monolayer by uniaxial strain ^Hot!

Hao Hong(洪浩), Yang Cheng(程阳), Chunchun Wu(吴春春), Chen Huang(黄琛), Can Liu(刘灿), Wentao Yu(于文韬), Xu Zhou(周旭), Chaojie Ma(马超杰), Jinhuan Wang(王金焕), Zhihong Zhang(张智宏), Yun Zhao(赵芸), Jie Xiong(熊杰), Kaihui Liu(刘开辉)

Chin. Phys. B, 2020, 29 (7): 077201 doi: 10.1088/1674-1056/ab99ba
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Carrier lifetime is one of the most fundamental physical parameters that characterizes the average time of carrier recombination in any material. The control of carrier lifetime is the key to optimizing the device function by tuning the electro-optical conversion quantum yield, carrier diffusion length, carrier collection process, etc. Till now, the prevailing modulation methods are mainly by defect engineering and temperature control, which have limitations in the modulation direction and amplitude of the carrier lifetime. Here, we report an effective modulation on the ultrafast dynamics of photoexcited carriers in two-dimensional (2D) MoS₂ monolayer by uniaxial tensile strain. The combination of optical ultrafast pump-probe technique and time-resolved photoluminescence (PL) spectroscopy reveals that the carrier dynamics through Auger scattering, carrier-phonon scattering, and radiative recombination keep immune to the strain. But strikingly, the uniaxial tensile strain weakens the trapping of photoexcited carriers by defects and therefore prolongs the corresponding carrier lifetime up to 440% per percent applied strain. Our results open a new avenue to enlarge the carrier lifetime of 2D MoS₂, which will facilitate its applications in high-efficient optoelectronic and photovoltaic devices.

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Irradiation study of liquid crystal variable retarder for Full-disk Magneto-Graph payload onboard ASO-S mission ^Hot!

Jun-Feng Hou(侯俊峰), Hai-Feng Wang(王海峰), Gang Wang(王刚), Yong-Quan Luo(骆永全), Hong-Wei Li(李宏伟), Zhen-Long Zhang(张振龙), Dong-Guang Wang(王东光), Yuan-Yong Deng(邓元勇)

Chin. Phys. B, 2020, 29 (7): 074208 doi: 10.1088/1674-1056/ab8c40
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The Advanced Space-based Solar Observatory (ASO-S) is a mission proposed by the Chinese Solar Physics Community. As one of the three payloads of ASO-S, the Full-disc Magneto-Graph (FMG) will measure the photospheric magnetic fields of the entire solar disk with high spatial and temporal resolution, and high magnetic sensitivity, where liquid crystal variable retarder (LCVR) is the key to whether FMG can achieve its scientific goal. So far, there is no space flight experience for LCVR. Therefore, irradiation study for LCVRs becomes more important and urgent in order to make sure their safety and reliability in space application. In this paper, γ irradiation, proton irradiation, and ultra-violet (UV) irradiation are tested for LCVRs respectively. The optical and chemical properties during irradiation tests are measured and analyzed. For optical properties, there is no significant change in those parameters FMG payload concerned except the retardation. Although there is no drastic degradation in the retardation versus voltage during irradiations, the amount of retardation variation is much higher than the instrument requirements. Thus, an in-flight retardation versus voltage should be added in FMG payload, reducing or even avoiding the impact of retardation change. For chemical properties, the clearing point and birefringence of the LC materials almost have no change; the ion density dose not change below 60 krad[Si], but begin to increase dramatically above 60 krad[Si].

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Effect of weak disorder in multi-Weyl semimetals ^Hot!

Zhen Ning(宁震), Bo Fu(付博), Qinwei Shi(石勤伟), Xiaoping Wang(王晓平)

Chin. Phys. B, 2020, 29 (7): 077202 doi: 10.1088/1674-1056/ab9612
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We study the behaviors of three-dimensional double and triple Weyl fermions in the presence of weak random potential. By performing the Wilsonian renormalization group (RG) analysis, we reveal that the quasiparticle experiences strong renormalization which leads to the modification of the density of states and quasiparticle residue. We further utilize the RG analysis to calculate the classical conductivity and show that the diffusive transport is substantially corrected due to the novel behavior of the quasiparticle and can be directly measured by experiments.

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Improvement of valley splitting and valley injection efficiency for graphene/ferromagnet heterostructure ^Hot!

Longxiang Xu(徐龙翔), Wengang Lu(吕文刚), Chen Hu(胡晨), Qixun Guo(郭奇勋), Shuai Shang(尚帅), Xiulan Xu(徐秀兰), Guanghua Yu(于广华), Yu Yan(岩雨), Lihua Wang(王立华), Jiao Teng(滕蛟)

Chin. Phys. B, 2020, 29 (7): 077304 doi: 10.1088/1674-1056/ab8db2
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The valley splitting has been realized in the graphene/Ni heterostructure with the splitting value of 14 meV, and the obtained valley injecting efficiency from the heterostructure into graphene was 6.18% [Phys. Rev. B 92 115404 (2015)]. In this paper, we report a way to improve the valley splitting and the valley injecting efficiency of the graphene/Ni heterostructure. By intercalating an Au monolayer between the graphene and the Ni, the split can be increased up to 50 meV. However, the valley injecting efficiency is not improved because the splitted valley area of graphene moves away from the Fermi level. Then, we mend the deviation by covering a monolayer of Cu on the graphene. As a result, the valley injecting efficiency of the Cu/graphene/Au/Ni heterostructure reaches 10%, which is more than 60% improvement compared to the simple graphene/Ni heterostructure. Then we theoretically design a valley-injection device based on the Cu/graphene/Au/Ni heterostructure and demonstrate that the valley injection can be easily switched solely by changing the magnetization direction of Ni, which can be used to generate and control the valley-polarized current.

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Facile and fast growth of high mobility nanoribbons of ZrTe₅ ^Hot!

Jingyue Wang(王璟岳), Jingjing Niu(牛晶晶), Xinqi Li(李新祺), Xiumei Ma(马秀梅), Yuan Yao(姚湲), Xiaosong Wu(吴孝松)

Chin. Phys. B, 2020, 29 (6): 068102 doi: 10.1088/1674-1056/ab889a
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Recently, ZrTe₅ has received a lot of attention as it exhibits various topological phases, such as weak and strong topological insulators, a Dirac semimetal, a three-dimensional quantum Hall state, and a quantum spin Hall insulator in the monolayer limit. While most of studies have been focused on the three-dimensional bulk material, it is highly desired to obtain nanostructured materials due to their advantages in device applications. We report the synthesis and characterizations of ZrTe₅ nanoribbons. Via a silicon-assisted chemical vapor transport method, long nanoribbons with thickness as thin as 20 nm can be grown. The growth rate is over an order of magnitude faster than the previous method for the bulk crystals. Moreover, transport studies show that the nanoribbons are of low unintentional doping and high carrier mobility, over 30000 cm²/V·s, which enable reliable determination of the Berry phase of π in the ac plane from quantum oscillations. Our method holds great potential in growth of high quality ultra-thin nanostructures of ZrTe₅.

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High-resolution angle-resolved photoemission study of oxygen adsorbed Fe/MgO(001) ^Hot!

Mingtian Zheng, Eike F. Schwier, Hideaki Iwasawa, Kenya Shimada

Chin. Phys. B, 2020, 29 (6): 067901 doi: 10.1088/1674-1056/ab9196
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We have investigated the electronic states of clean Fe(001) and oxygen adsorbed Fe(001)-p(1×1)-O films epitaxially grown on MgO(001) substrates by means of polarization-dependent angle-resolved photoemission spectroscopy (ARPES) and extensive density-functional theory (DFT) calculations. The observed Fermi surfaces and band dispersions of pure Fe near the Fermi level were modified upon oxygen adsorption. By the detailed comparison of ARPES and DFT results of the oxygen adsorbed Fe surface, we have clarified the orbital-dependent p-d hybridization in the topmost and second Fe layers. Furthermore, the observed energy levels and Fermi wave numbers for the oxygen adsorbed Fe surface were deviated from the DFT calculations depending on the orbital characters and momentum directions, indicating an anisotropic interplay of the electron correlation and p-d hybridization effects in the surface region.

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Optical spin-to-orbital angular momentum conversion instructured optical fields ^Hot!

Yang Zhao(赵阳), Cheng-Xi Yang(阳成熙), Jia-Xi Zhu(朱家玺), Feng Lin(林峰), Zhe-Yu Fang(方哲宇), Xing Zhu(朱星)

Chin. Phys. B, 2020, 29 (6): 067301 doi: 10.1088/1674-1056/ab862a
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We investigate the dynamic quantities: momentum, spin and orbital angular momenta (SAM and OAM), and their conversion relationship in the structured optical fields at subwavelength scales, where the spin-orbit interaction (SOI) plays a key role and determines the behaviors of light. Specifically, we examine a nanostructure of a Ag nanoparticle (Ag NP) attached on a cylindrical Ag nanowire (Ag NW) under illumination of elliptically polarized light. These dynamic quantities obey the Noether theorem, i.e., for the Ag nanoparticle with spherical symmetry, the total angular momentum consisting of SAM and OAM conserves; for the Ag NW with translational symmetry, the orbital momentum conserves. Meanwhile, the spin-to-orbital angular momentum conversion is mediated by SOI arising from the spatial variation of the optical potential. In this nanostructure, the conservation of momentum imposes a strict restriction on the propagation direction of the surface plasmon polaritons along the Ag NW. Meanwhile, the orbital momentum is determined by the polarized properties of the excitation light and the topography of the Ag NP. Our work offers insights to comprehend the light behaviors in the structured optical fields in terms of the dynamic quantities and benefits to the design of optical nano-devices based on interactions between spin and orbital degrees of freedom.

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Scalable preparation of water-soluble ink of few-layered WSe₂ nanosheets for large-area electronics ^Hot!

Guoyu Xian(冼国裕), Jianshuo Zhang(张建烁), Li Liu(刘丽), Jun Zhou(周俊), Hongtao Liu(刘洪涛), Lihong Bao(鲍丽宏), Chengmin Shen(申承民), Yongfeng Li(李永峰), Zhihui Qin(秦志辉), Haitao Yang(杨海涛)

Chin. Phys. B, 2020, 29 (6): 066802 doi: 10.1088/1674-1056/ab889e
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Few-layer two-dimensional (2D) semiconductor nanosheets with a layer-dependent band gap are attractive building blocks for large-area thin-film electronics. A general approach is developed to fast prepare uniform and phase-pure 2H-WSe₂ semiconducting nanosheets at a large scale, which involves the supercritical carbon dioxide (SC-CO₂) treatment and a mild sonication-assisted exfoliation process in aqueous solution. The as-prepared 2H-WSe₂ nanosheets preserve the intrinsic physical properties and intact crystal structures, as confirmed by Raman, x-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscope (STEM). The uniform 2H-WSe₂ nanosheets can disperse well in water for over six months. Such good dispersivity and uniformity enable these nanosheets to self-assembly into thickness-controlled thin films for scalable fabrication of large-area arrays of thin-film electronics. The electronic transport and photoelectronic properties of the field-effect transistor based on the self-assembly 2H-WSe₂ thin film have also been explored.

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Fast achievement of quantum state transfer and distributed quantum entanglement by dressed states ^Hot!

Liang Tian(田亮), Li-Li Sun(孙立莉), Xiao-Yu Zhu(朱小瑜), Xue-Ke Song(宋学科), Lei-Lei Yan(闫磊磊), Er-Jun Liang(梁二军), Shi-Lei Su(苏石磊), Mang Feng(冯芒)

Chin. Phys. B, 2020, 29 (5): 050306 doi: 10.1088/1674-1056/ab7e9a
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We propose schemes to realize quantum state transfer and prepare quantum entanglement in coupled cavity and cavity-fiber-cavity systems, respectively, by using the dressed state method. We first give the expression of pulses shape by using dressed states and then find a group of Gaussian pulses that are easy to realize in experiment to replace the ideal pulses by curve fitting. We also study the influence of some parameters fluctuation, atomic spontaneous emission, and photon leakage on fidelity. The results show that our schemes have good robustness. Because the atoms are trapped in different cavities, it is easy to perform different operations on different atoms. The proposed schemes have the potential applications in dressed states for distributed quantum information processing tasks.

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Tunable deconfined quantum criticality and interplay of different valence-bond solid phases ^Hot!

Bowen Zhao(赵博文), Jun Takahashi, Anders W. Sandvik

Chin. Phys. B, 2020, 29 (5): 057506 doi: 10.1088/1674-1056/ab889b
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We use quantum Monte Carlo simulations to study an S=1/2 spin model with competing multi-spin interactions. We find a quantum phase transition between a columnar valence-bond solid (cVBS) and a Néel antiferromagnet (AFM), as in the scenario of deconfined quantum-critical points, as well as a transition between the AFM and a staggered valence-bond solid (sVBS). By continuously varying a parameter, the sVBS-AFM and AFM-cVBS boundaries merge into a direct sVBS-cVBS transition. Unlike previous models with putative deconfined AFM-cVBS transitions, e.g., the standard J-Q model, in our extended J-Q model with competing cVBS and sVBS inducing terms the transition can be tuned from continuous to first-order. We find the expected emergent U(1) symmetry of the microscopically Z₄ symmetric cVBS order parameter when the transition is continuous. In contrast, when the transition changes to first-order, the clock-like Z₄ fluctuations are absent and there is no emergent higher symmetry. We argue that the confined spinons in the sVBS phase are fracton-like. We also present results for an SU(3) symmetric model with a similar phase diagram. The new family of models can serve as a useful tool for further investigating open questions related to deconfined quantum criticality and its associated emergent symmetries.

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Microstructure and ferromagnetism ofheavily Mn doped SiGe thin flims ^Hot!

Huanming Wang(王焕明), Sen Sun(孙森), Jiayin Xu(徐家胤), Xiaowei Lv(吕晓伟), Yuan Wang(汪渊), Yong Peng(彭勇), Xi Zhang(张析), Gang Xiang(向钢)

Chin. Phys. B, 2020, 29 (5): 057504 doi: 10.1088/1674-1056/ab8219
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Heavily Mn-doped SiGe thin films were grown by radio frequency magnetron sputtering and then treated by post-growth thermal annealing. Structural characterizations reveal the coexistence of Mn-diluted SiGe crystals and Mn-rich nanoclusters in the annealed films. Magnetic measurements indicate the ferromagnetic ordering of the annealed samples above room temperature . The data suggest that the ferromagnetism is probably mainly contributed by the Ge-rich nanoclusters and partially contributed by the tensile-strained Mn-diluted SiGe crystals. The results may be useful for room temperature spintronic applications based on group IV semiconductors.

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Role of the spin anisotropy of the interchain interaction in weakly coupled antiferromagnetic Heisenberg chains ^Hot!

Yuchen Fan(樊宇辰), Rong Yu(俞榕)

Chin. Phys. B, 2020, 29 (5): 057505 doi: 10.1088/1674-1056/ab820b
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In quasi-one-dimensional (q1D) quantum antiferromagnets, the complicated interplay of intrachain and interchain exchange couplings may give rise to rich phenomena. Motivated by recent progress on field-induced phase transitions in the q1D antiferromagnetic (AFM) compound YbAlO₃, we study the phase diagram of spin-1/2 Heisenberg chains with Ising anisotropic interchain couplings under a longitudinal magnetic field via large-scale quantum Monte Carlo simulations, and investigate the role of the spin anisotropy of the interchain coupling on the ground state of the system. We find that the Ising anisotropy of the interchain coupling can significantly enhance the longitudinal spin correlations and drive the system to an incommensurate AFM phase at intermediate magnetic fields, which is understood as a longitudinal spin density wave (LSDW). With increasing field, the ground state changes to a canted AFM order with transverse spin correlations. We further provide a global phase diagram showing how the competition between the LSDW and the canted AFM states is tuned by the Ising anisotropy of the interchain coupling.

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High pressure synthesis and characterization of the pyrochlore Dy₂Pt₂O₇: A new spin ice material ^Hot!

Qi Cui(崔琦), Yun-Qi Cai(蔡云麒), Xiang Li(李翔), Zhi-Ling Dun(顿志凌), Pei-Jie Sun(孙培杰), Jian-Shi Zhou(周建十), Hai-Dong Zhou(周海东), Jin-Guang Cheng(程金光)

Chin. Phys. B, 2020, 29 (4): 047502 doi: 10.1088/1674-1056/ab7b58
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The cubic pyrochlore Dy₂Pt₂O₇ was synthesized under 4 GPa and 1000℃ and its magnetic and thermodynamic properties were characterized by DC and AC magnetic susceptibility and specific heat down to 0.1 K. We found that Dy₂Pt₂O₇ does not form long-range magnetic order, but displays characteristics of canonical spin ice such as Dy₂Ti₂O₇, including (1) a large effective moment 9.64 μ_B close to the theoretical value and a small positive Curie-Weiss temperature θ_CW=+0.77 K signaling a dominant ferromagnetic interaction among the Ising spins; (2) a saturation moment ～4.5 μ_B being half of the total moment due to the local <111> Ising anisotropy; (3) thermally activated spin relaxation behaviors in the low (～1 K) and high (～20 K) temperature regions with different energy barriers and characteristic relaxation time; and most importantly, (4) the presence of a residual entropy close to Pauling's estimation for water ice.

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Electronic structure and spatial inhomogeneity of iron-based superconductor FeS ^Hot!

Chengwei Wang(王成玮), Meixiao Wang(王美晓), Juan Jiang(姜娟), Haifeng Yang(杨海峰), Lexian Yang(杨乐仙), Wujun Shi(史武军), Xiaofang Lai(赖晓芳), Sung-Kwan Mo, Alexei Barinov, Binghai Yan(颜丙海), Zhi Liu(刘志), Fuqiang Huang(黄富强), Jinfeng Jia(贾金峰), Zhongkai Liu(柳仲楷), Yulin Chen(陈宇林)

Chin. Phys. B, 2020, 29 (4): 047401 doi: 10.1088/1674-1056/ab75d4
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Iron-based superconductor family FeX (X =S, Se, Te) has been one of the research foci in physics and material science due to their record-breaking superconducting temperature (FeSe film) and rich physical phenomena. Recently, FeS, the least studied FeX compound (due to the difficulty in synthesizing high quality macroscopic crystals) attracted much attention because of its puzzling superconducting pairing symmetry. In this work, combining scanning tunneling microscopy and angle resolved photoemission spectroscopy (ARPES) with sub-micron spatial resolution, we investigate the intrinsic electronic structures of superconducting FeS from individual single crystalline domains. Unlike FeTe or FeSe, FeS remains identical tetragonal structure from room temperature down to 5 K, and the band structures observed can be well reproduced by our ab-initio calculations. Remarkably, mixed with the 1×1 tetragonal metallic phase, we also observe the coexistence of √5×√5 reconstructed insulating phase in the crystal, which not only helps explain the unusual properties of FeS, but also demonstrates the importance of using spatially resolved experimental tools in the study of this compound.

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Moisture-sensitive torsional cotton artificial muscle and textile ^Hot!

Yuanyuan Li(李媛媛), Xueqi Leng(冷雪琪), Jinkun Sun(孙进坤), Xiang Zhou(周湘), Wei Wu(兀伟), Hong Chen(陈洪), Zunfeng Liu(刘遵峰)

Chin. Phys. B, 2020, 29 (4): 048103 doi: 10.1088/1674-1056/ab7745
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Developing moisture-sensitive artificial muscles from industrialized natural fibers with large abundance is highly desired for smart textiles that can respond to humidity or temperature change. However, currently most of fiber artificial muscles are based on non-common industrial textile materials or of a small portion of global textile fiber market. In this paper, we developed moisture-sensitive torsional artificial muscles and textiles based on cotton yarns. It was prepared by twisting the cotton yarn followed by folding in the middle point to form a self-balanced structure. The cotton yarn muscle showed a torsional stroke of 42.55°/mm and a rotational speed of 720 rpm upon exposure to water moisture. Good reversibility and retention of stroke during cyclic exposure and removal of water moisture were obtained. A moisture-sensitive smart window that can close when it rains was demonstrated based on the torsional cotton yarn muscles. This twist-based technique combining natural textile fibers provides a new insight for construction of smart textile materials.

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Nanofabrication of 50 nm zone plates through e-beam lithography with local proximity effect correction for x-ray imaging ^Hot!

Jingyuan Zhu(朱静远), Sichao Zhang(张思超), Shanshan Xie(谢珊珊), Chen Xu(徐晨), Lijuan Zhang(张丽娟), Xulei Tao(陶旭磊), Yuqi Ren(任玉琦), Yudan Wang(王玉丹), Biao Deng(邓彪), Renzhong Tai(邰仁忠), Yifang Chen(陈宜方)

Chin. Phys. B, 2020, 29 (4): 047501 doi: 10.1088/1674-1056/ab7800
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High resolution Fresnel zone plates for nanoscale three-dimensional imaging of materials by both soft and hard x-rays are increasingly needed by the broad applications in nanoscience and nanotechnology. When the outmost zone-width is shrinking down to 50 nm or even below, patterning the zone plates with high aspect ratio by electron beam lithography still remains a challenge because of the proximity effect. The uneven charge distribution in the exposed resist is still frequently observed even after standard proximity effect correction (PEC), because of the large variety in the line width. This work develops a new strategy, nicknamed as local proximity effect correction (LPEC), efficiently modifying the deposited energy over the whole zone plate on the top of proximity effect correction. By this way, 50 nm zone plates with the aspect ratio from 4:1 up to 15:1 and the duty cycle close to 0.5 have been fabricated. Their imaging capability in soft (1.3 keV) and hard (9 keV) x-ray, respectively, has been demonstrated in Shanghai Synchrotron Radiation Facility (SSRF) with the resolution of 50 nm. The local proximity effect correction developed in this work should also be generally significant for the generation of zone plates with high resolutions beyond 50 nm.

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Fundamental band gap and alignment of two-dimensional semiconductors explored by machine learning ^Hot!

Zhen Zhu(朱震), Baojuan Dong(董宝娟), Huaihong Guo(郭怀红), Teng Yang(杨腾), Zhidong Zhang(张志东)

Chin. Phys. B, 2020, 29 (4): 046101 doi: 10.1088/1674-1056/ab75d5
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Two-dimensional (2D) semiconductors isoelectronic to phosphorene have been drawing much attention recently due to their promising applications for next-generation (opt)electronics. This family of 2D materials contains more than 400 members, including (a) elemental group-V materials, (b) binary III-VII and IV-VI compounds, (c) ternary III-VI-VII and IV-V-VII compounds, making materials design with targeted functionality unprecedentedly rich and extremely challenging. To shed light on rational functionality design with this family of materials, we systemically explore their fundamental band gaps and alignments using hybrid density functional theory (DFT) in combination with machine learning. First, calculations are performed using both the Perdew-Burke-Ernzerhof exchange-correlation functional within the general-gradient-density approximation (GGA-PBE) and Heyd-Scuseria-Ernzerhof hybrid functional (HSE) as a reference. We find this family of materials share similar crystalline structures, but possess largely distributed band-gap values ranging approximately from 0 eV to 8 eV. Then, we apply machine learning methods, including linear regression (LR), random forest regression (RFR), and support vector machine regression (SVR), to build models for the prediction of electronic properties. Among these models, SVR is found to have the best performance, yielding the root mean square error (RMSE) less than 0.15 eV for the predicted band gaps, valence-band maximums (VBMs), and conduction-band minimums (CBMs) when both PBE results and elemental information are used as features. Thus, we demonstrate that the machine learning models are universally suitable for screening 2D isoelectronic systems with targeted functionality, and especially valuable for the design of alloys and heterogeneous systems.

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General principles to high-throughput constructing two-dimensional carbon allotropes ^Hot!

Qing Xie(谢庆), Lei Wang(王磊), Jiangxu Li(李江旭), Ronghan Li(李荣汉), Xing-Qiu Chen(陈星秋)

Chin. Phys. B, 2020, 29 (3): 037306 doi: 10.1088/1674-1056/ab6c4b
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We propose general principles to construct two-dimensional (2D) single-atom-thick carbon allotropes. They can be viewed as the generalization of patterning Stone-Walse defects (SWDs) by manipulating bond rotation and of patterning inverse SWDs by adding (or removing) carbon pairs on the pristine graphene, respectively. With these principles, numerous 2D allotropes of carbon can be systematically constructed. Using 20 constructed 2D allotropes as prototypical and benchmark examples, besides nicely reproducing all well-known ones, such as pentaheptites, T-graphene, OPGs, etc, we still discover 13 new allotropes. Their structural, thermodynamic, dynamical, and electronic properties are calculated by means of first-principles calculations. All these allotropes are metastable in energy compared with that of graphene and, except for OPG-A and C3-10-H allotropes, the other phonon spectra of 18 selected allotropes are dynamically stable. In particular, the proposed C3-11 allotrope is energetically favorable than graphene when the temperature is increased up to 1043 K according to the derived free energies. The electronic band structures demonstrate that (i) the C3-8 allotrope is a semiconductor with an indirect DFT band gap of 1.04 eV, (ii) another unusual allotrope is C3-12 which exhibits a highly flat band just crossing the Fermi level, (iii) four allotropes are Dirac semimetals with the appearance of Dirac cones at the Fermi level in the lattices without hexagonal symmetry, and (vi) without the spin-orbit coupling (SOC) effect, the hexagonal C3-11 allotrope exhibits two Dirac cones at K and K' points in its Brillouin zone in similarity with graphene.

																
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