Two-dimensional XSe2（X = Mn, V） based magnetic tunneling junctions with high Curie temperature

Two-dimensional(2D) magnetic crystals have attracted great attention due to their emerging new physical phenomena. They provide ideal platforms to study the fundamental physics of magnetism in low dimensions. In this research,magnetic tunneling junctions(MTJs) based on XSe2(X = Mn, V) with room-temperature ferromagnetism were studied using first-principles calculations. A large tunneling magnetoresistance(TMR) of 725.07% was obtained in the MTJs based on monolayer MnSe2. Several schemes were proposed to improve the TMR of these devices. Moreover, the results of our non-equilibrium transport calculations showed that the large TMR was maintained in these devices under a finite bias.The transmission spectrum was analyzed according to the orbital components and the electronic structure of the monolayer XSe2(X = Mn, V). The results in this paper demonstrated that the MTJs based on a 2D ferromagnet with room-temperature ferromagnetism exhibited reliable performance. Therefore, such devices show the possibility for potential applications in spintronics.

Origin of the anomalous trends in band alignment of GaX/ZnGeX_2(X = N, P, As, Sb) heterojunctions

Utilizing first-principles band structure method, we studied the trends of electronic structures and band offsets of the common-anion heterojunctions GaX/ZnGeX_2(X = N, P, As, Sb). Here, ZnGeX_2 can be derived by atomic transmutation of two Ga atoms in GaX into one Zn atom and one Ge atom. The calculated results show that the valence band maximums(VBMs) of GaX are always lower in energy than that of ZnGeX_2, and the band offset decreases when the anion atomic number increases. The conduction band minimums(CBMs) of ZnGeX_2 are lower than that of GaX for X = P, As, and Sb, as expected. However, surprisingly, for ZnGeN2, its CBM is higher than GaN. We found that the coupling between anion p and cation d states plays a decisive role in determining the position of the valence band maximum, and the increased electronegativity of Ge relative to Ga explains the lower CBMs of ZnGeX_2 for X = P, As, and Sb. Meanwhile, due to the high ionicity, the strong coulomb interaction is the origin of the anomalous behavior for nitrides.

Atomic-Ordering-Induced Quantum Phase Transition between Topological Crystalline Insulator and Z_2 Topological Insulator

Topological phase transition in a single material usually refers to transitions between a trivial band insulator and a topological Dirac phase, and the transition may also occur between different classes of topological Dirac phases.It is a fundamental challenge to realize quantum transition between Z_2 nontrivial topological insulator（TI） and topological crystalline insulator（TCI） in one material because Z_2 TI and TCI have different requirements on the number of band inversions. The Z_2 TIs must have an odd number of band inversions over all the time-reversal invariant momenta, whereas the newly discovered TCIs, as a distinct class of the topological Dirac materials protected by the underlying crystalline symmetry, owns an even number of band inversions. Taking PbSnTe_2 alloy as an example, here we demonstrate that the atomic-ordering is an effective way to tune the symmetry of the alloy so that we can electrically switch between TCI phase and Z_2 TI phase in a single material. Our results suggest that the atomic-ordering provides a new platform towards the realization of reversibly switching between different topological phases to explore novel applications.

Group velocity matters for accurate prediction of phonon-limited carrier mobility

First-principles approaches have recently been developed to replace the phenomenological modeling approaches with adjustable parameters for calculating carrier mobilities in semiconductors.However,in addition to the high computational cost,it is still a challenge to obtain accurate mobility for carriers with a complex band structure,e.g.,hole mobility in common semiconductors.Here,we present a computationally efficient approach using isotropic and parabolic bands to approximate the anisotropy valence bands for evaluating group velocities in the first-principles calculations.This treatment greatly reduces the computational cost in two ways:relieves the requirement of an extremely denseκmesh to obtain a smooth change in group velocity,and reduces the 5-dimensional integral to 3-dimensional integral.Taking Si and SiC as two examples,we find that this simplified approach reproduces the full first-principles calculation for mobility.If we use experimental effective masses to evaluate the group velocity,we can obtain hole mobility in excellent agreement with experimental data over a wide temperature range.These findings shed light on how to improve the first-principles calculations towards predictive carrier mobility in high accuracy.

Theoretical understanding of correlation between magnetic phase transition and the superconducting dome in high-Tc cuprates

Many issues concerning the origin of high-temperature superconductivity(HTS)are still under debate.For example,how the magnetic order varies with doping and its relationship with the superconducting temperature(Tc);and why Tcalways peaks near the quantum critical point.In this paper,taking hole-doped La_(2)CuO_(4)as a classical example,we employ the first-principles band structure and total energy calculations with Monte Carlo simulations to explore how the symmetry-breaking magnetic ground state evolves with hole doping and the origin of a dome-shaped superconductivity region in the phase diagram.We demonstrate that the local antiferromagnetic order and doping play key roles in determining the electron-phonon coupling,thus Tc.Initially,the La_(2)CuO_(4)possesses a checkerboard local antiferromagnetic ground state.As the hole doping increases,Tcincreases with the enhanced electron-phonon coupling strength.But as the doping increases further,the strength of the antiferromagnetic interaction weakens and spin fluctuation increases.At the critical doping level,a magnetic phase transition occurs that reduces the local antiferromagnetism-assisted electron-phonon coupling,thus diminishing the Tc.The superconductivity disappears in the heavily overdoped region when the ferromagnetic order dominates.These observations could account for why cuprates have a dome-shaped superconductivity region in the phase diagram.Our study,thus,contributes to a fundamental understanding of the correlation between doping,local magnetic order,and superconductivity of HTS.

低对称性的亚波长阵列增强型的GeSe偏振探测器

偏振敏感光电探测器可通过探测光偏振态的变化从而分辨纹理、应力和粗糙度等多种信息,在安全、医疗诊断和航空航天领域具有独特的优势.基于面内各向异性的二维材料的偏振光探测器可简化传统探测器的复杂透镜系统,促进了光电器件的集成化与小型化.然而,基于面内各向异性的二维材料的偏振光探测器的光电流各向异性比不足(通常小于10),限制了偏振信号的检测分辨率.本文设计了基于二维硒化亚锗(GeSe)的亚波长阵列(SWA)结构,进一步提高其各向异性灵敏度,并将光电流各向异性从1.6提高到18.本研究发现,SWA结构可以进一步增强电荷分布的不对称性,提高GeSe材料在扶手椅方向上的光吸收和光电跃迁概率,从而提高了偏振灵敏度.此外,基于GeSe SWA的光电探测器在808 nm的近红外波长下表现出宽的功率范围(40 dB)和弱光探测能力(0.1 LUX).该工作为提高二维材料的偏振灵敏度提供了可行性指导,将有利于促进高分辨率偏振成像传感器的发展.

Electronic origin of the unusual thermal properties of copper-based semiconductors:The s-d coupling-induced large phonon anharmonicity

Copper(Cu)-based materials(such as cuprates,Cu chalcogenides,and Cu halides)often exhibit unusual properties such as superconductivity,ultralow thermal conductivity,and superionicity.However,the electronic origin of these unusual behaviors remains elusive.In this study,we demonstrate that the high-lying occupied 3d orbital of Cu causes a strong s-d coupling with its unoccupied 4s state when local symmetry is reduced.This leads to strong phonon anharmonicity and is responsible for these intriguing properties.For example,during thermal transport,symmetry-controlled s-d coupling can substantially lower the lattice potential barrier,thereby enhancing the anharmonicity and scattering between phonons and ultimately significantly reducing lattice thermal conductivity.We confirmed this understanding with Raman spectra measurements,which demonstrated a remarkable red shift in the phonon vibrational frequency with an increase in the temperature of Cu-based semiconductors.Our study shows that the cause of phonon anharmonicity is related to the fundamental electronic structures,which can also explain other unusual physical properties of the Cu compounds.

Effectively tuning the stability and optoelectronic properties of halide perovskites by B-site alloying

Halide perovskites(ABX_(3)), as promising candidates for solar cell photovoltaic devices, have recently attracted increasing research interest [1-3], but many issues remain unsolved for their practical applications. One such issue is their poor stability, which largely reduces device lifetime and efficiency [4]. In recent decades, using inorganic ions or organic molecules for replacement or alloying the A-site atoms in halide perovskites has improved thermodynamic stability to some extent.

Machine learning in materials science

Traditional methods of discovering new materials,such as the empirical trial and error method and the density functional theory(DFT)-based method,are unable to keep pace with the development of materials science today due to their long development cycles,low efficiency,and high costs.Accordingly,due to its low computational cost and short development cycle,machine learning is coupled with powerful data processing and high prediction performance and is being widely used in material detection,material analysis,and material design.In this article,we discuss the basic operational procedures in analyzing material properties via machine learning,summarize recent applications of machine learning algorithms to several mature fields in materials science,and discuss the improvements that are required for wide-ranging application.

Quantum engineering of non-equilibrium efficient p-doping in ultra-wide band-gap nitrides

Ultra-wide band-gap nitrides have huge potential in micro-and optoelectronics due to their tunable wide band-gap,high breakdown field and energy density,excellent chemical and thermal stability.However,their application has been severely hindered by the low p-doping efficiency,which is ascribed to the ultrahigh acceptor activation energy originated from the low valance band maximum.Here,a valance band modulation mode is proposed and a quantum engineering doping method is conducted to achieve high-efficient p-type ultra-wide band-gap nitrides,in which GaN quantum-dots are buried in nitride matrix to produce a new band edge and thus to tune the dopant activation energy.By non-equilibrium doping techniques,quantum engineering doped AIGaN:Mg with Al content of 60%is successfully fabricated.The Mg activation energy has been reduced to about 21 meV,and the hole concentration reaches higher than 10^(18)cm^(-3)at room temperature.Also,similar activation energies are obtained in AIGaN with other Al contents such as 50%and 70%;indicating the universality of the quantum engineering doping method.Moreover,deep-ultraviolet light-emission diodes are fabricated and the improved performance further demonstrates the validity and merit of the method.With the quantum material growth techniques developing,this method would be prevalently available and tremendously stimulate the promotion of ultra-wide band-gap semiconductor-based devices.

Reviewing and understanding the stability mechanism of halide perovskite solar cells

Finding sustainable and renewable energy to replace traditional fossil fuel is critical for reducing greenhouse gas emission and avoiding environment pollution.Solar cells that convert energy of sunlight into electricity offer a viable route for solving this issue.At present,halide perovskites are the most potential candidate materials for solar cell with considerable power conversion efficiency,whereas their stability remains a challenge.In this work,we summarize four different key factors that influence the stability of halide perovskites:(a)effect of environmental moisture on the degradation of halide perovskites.The performance of halide perovskite solar cells is reduced due to hydrated crystal hinders the diffusion of photo-generated carriers,which can be solved by materials encapsulation technique;(b)photoinduced instability.Through uncovering the underlying physical mechanism,we note that materials engineering or novel device structure can extend the working life of halide perovskites under continuous light exposure;(c)thermal stability.Halide perovskites are rapidly degraded into PbI2 and volatile substances as heating due to lower formation energy,whereas hybrid perovskite is little changed;(d)electric field effect in the degradation of halide perovskites.The electric field impacts significantly on the carrier separation,changes direction of photo-induced currents and generates switchable photovoltaic effect.For each key factor,we have shown in detail the underlying physical mechanisms and discussed the strategies to overcome this stability difficulty.We expect this review from both theoretical and experimental points of view can be beneficial for development of perovskite solar cell materials and promotes practical applications.

High-performance phosphorene electromechanical actuators

Phosphorene,a two-dimensional material that can be exfoliated from black phosphorus,exhibits remarkable mechanical,thermal,electronic,and optical properties.In this work,we demonstrate that the unique structure of pristine phosphorene endows this material with exceptional quantum-mechanical performance by using first-principles calculations.

Ⅳ族半导体Ge_(1-x)Sn_(x)中的自发磁化及磁阻回滞现象

以硅基半导体为代表的Ⅳ族半导体在现代工业中扮演着举足轻重的作用.然而,Ⅳ族半导体因其缺乏非常强的交换相互作用难以产生铁磁序,从而限制了蓬勃发展的自旋电子学器件在Ⅳ族半导体领域的应用.本文通过分子束外延生长Ⅳ族半导体Ge_(1-x)Sn_(x),引入原子排列的无序效应,从而打破反演对称性,在实验上成功观测到了自发铁磁序的产生.通过低温输运测量,可以清晰观测到电阻的回滞现象.通过磁化特性测量,进一步可以观测到自发铁磁序对应的磁滞回线,从而清晰地证明了Ⅳ族半导体Ge_(1-x)Sn_(x)中自发铁磁序的存在.通过第一性原理计算,作者指出此自发铁磁序源于临近布里渊区中心的范霍夫奇点引起的自旋极化的半金属态(half-metallicity).该工作成功地在非磁性的Ⅳ族半导体中诱导出来自发铁磁序,对自旋电子学应用于Ⅳ族半导体进而和现行的硅基工业标准兼容具有重要意义.

高效钙钛矿太阳能电池中缓冲层界面工程的深入理解:第一性原理研究

近年来钙钛矿太阳能电池的能量转换效率迅速提高.界面工程是进一步改善钙钛矿太阳能电池性能的有前途的途径.本文中,我们进行第一性原理计算,以探索四种候选缓冲材料(MACl,MAI,PbCl2与PbI2)对MAPbI3吸收层与TiO2之间界面电子结构的影响.我们发现MAX(X=Cl,I)作为缓冲层将引入高电子势垒并增强电子-空穴复合.此外,MAX不能很好地钝化表面状态.PbI2的导带最小值远低于MAPbI3吸收层的导带最小值,这极大地限制了吸收层的能带弯曲和太阳能电池的开路电压.另一方面,合适的带边能级位置,与TiO2表面的小的晶格失配以及出色的表面钝化性能使得PbCl2成为钙钛矿太阳能电池吸收层/电子传输层界面工程的有希望的缓冲材料.因此,我们在这项工作中获得的结果可以使人们对具有缓冲层的界面工程的效果有更深入的理解,这有利于改善钙钛矿太阳能电池和相关光电器件的性能.

First-principles study of defect control in thin-film solar cell materials

A solar cell is a photovoltaic device that converts solar radiation energy to electrical energy, which plays a leading role in alleviating global energy shortages and decreasing air pollution levels typical of conventional fossil fuels. To render solar cells more efficient, high visible-light absorption rates and excellent carrier transport properties are required to generate high carrier levels and high output voltage. Hence, the core material, i.e., the absorption layer, should have an appropriate direct band gap and be effectively doped by both p-and n-types with minimal carrier traps and recombination centers. Consequently, defect properties of absorbers are critical in determining solar cell efficiency. In this work, we review recent first-principles studies of defect properties and engineering in four representative thin-film solar cells, namely CdTe, Cu(In,Ga)Se2, Cu2ZnSnS4, and halide perovskites. The focal points include basic electronic and defect properties, existing problems, and possible solutions in engineering defect properties of those materials to optimize solar cell efficiency.

Nitrogen-vacancy centers promote super-radiant maser performance

The technological application of masers,i.e.,the so-called microwave amplification by stimulated emission of radiation(Fig.1),is restricted by insatiable operating conditions,including deep-freezing and high-vacuum environments.The masers that can be operated at room temperature are enticing but difficult to achieve[1-6].

The origin of electronic band structure anomaly in topological crystalline insulator group-Ⅳ tellurides

Group-Ⅳ tellurides have exhibited exotic band structures.Specifically,despite the fact that Sn sits between Ge and Pb in the same column of the periodic table,cubic SnTe is a topological crystalline insulator with band inversion,but both isovalent GeTe and PbTe are trivial semiconductors with normal band order.By performing first-principles band structure calculations,we unravel the origin of this abnormal behaviour by using symmetry analysis and the atomic orbital energy levels and atomic sizes of these elements.In group-Ⅳ tellurides,the s lone pair band of the group-Ⅳ element is allowed by symmetry to couple with the anion valence p band at the L-point,and such s–p coupling leads to the occurrence of bandgap at the L-point.We find that such s–p coupling is so strong in SnTe that it inverts the band order near the bandgap;however,it is not strong enough in both GeTe and PbTe,so they remain normal semiconductors.The reason for this is the incomplete screening of the core of the relatively tight-binding Ge 4s orbital by its 3d orbitals and the large atomic size and strong relativistic effect in Pb,respectively.Interestingly,we also find that the rhombohedral distortion removes the inversion symmetry and the reduced s–p coupling transforms theα-SnTe back to a normal semiconductor.Our study demonstrates that,in addition to spin–orbital coupling,strain and interface dipole fields,inter-orbital coupling is another effective way to engineer the topological insulators.