Anomalous magnetotransport phenomena in θ-(BEDT-TTF)2I3

Anomalous magnetotransport phenomena have been observed in θ-(BEDT-TTF)₂I₃ crystals at temperatures below 15 K. The magnetoresistance M : (1) is a linear function of the magnetic field H, (2) is not affected by the angle between the electric current and the magnetic field, (3) but depends on the magnetic field orientation with respect to the crystal axis. Magnetoresistance is expressed as M = (aH²_a + bH²_b + cH²_c)₀^-3/2/H in terms of H = (H_a, H_b, H_c), the zero field resistivity ₀, and parameters a, b, and c which are independent of temperature and magnetic field. We have found that b a > c. Magnetoresistance up to 40 is observed for H = 7T along the b-axis at T = 1.5K.     

BiTiO3电子结构及光学性质的第一性原理研究

采用基于第一性原理的赝势平面波方法, 对BiTiO₃的多种结构进行了计算. 计算结果表明, C1C1结构最为稳定, 对应晶格参数为a=b=5.606 Å, c=9.954 Å; α=β=105.1°, γ=61.2°. 进一步对C1C1结构的BiTiO₃的能带结构、电子性质和光学性质进行了研究, 发现BiTiO₃是间接带隙半导体, 其费米面附近的能带主要由Ti-3d和O-2p层的电子态构成. 通过介电函数、复折射率和反射率等的研究, 发现BiTiO₃的光学性质为近各向同性.     

Anisotropic magnetic susceptibility of single crystal Ca2Y2Cu5O10 with edge-sharing CuO2 chain

We report a successful crystal growth of Ca₂Y₂Cu₅O₁₀ and anisotropic behavior of magnetic susceptibility associated with an antiferromagnetic phase transition. Single crystals were obtained by the floating-zone method using an infrared radiation furnace. Composition and structural characterization shows that the grown crystals are qualified for physical measurements. The magnetic susceptibility along each crystallographic axis shows a typical behavior of antiferromagnetic phase transition at T_N=31 K, and the ordered magnetic moments are found to lie along the b-axis.     

核磁共振碳谱研究:烷烃化学位移和(CSS)与分子路径指数矢量(VPM)

系统研究了核磁共振碳谱和化学位移规律及其定量构谱关系(QSSR).本文研究了一组十元素分子路径指数矢量VPM,并发现它与烷烃化学位移和CCS有良好线性相关性.采用多元线性回归进行准确估计与预测,结果优良.     

Strain-induced magnetism in ReS_2 monolayer with defects

We investigate the effects of strain on the electronic and magnetic properties of ReS_2 monolayer with sulfur vacancies using density functional theory.Unstrained ReS_2 monolayer with monosulfur vacancy(V_s) and disulfur vacancy(V_(2S))both are nonmagnetic.However,as strain increases to 8%,V_S-doped ReS_2 monolayer appears a magnetic half-metal behavior with zero total magnetic moment.In particular,for V_(2S)-doped ReS_2 monolayer,the system becomes a magnetic semiconductor under 6%strain,in which Re atoms at vicinity of vacancy couple anti-ferromagnetically with each other,and continues to show a ferromagnetic metal characteristic with total magnetic moment of 1.60μb under 7%strain.Our results imply that the strain-manipulated ReS_2 monolayer with V_S and V_(2S) can be a possible candidate for new spintronic applications.     

Hamiltonian circle actions on symplectic manifolds and the signature

Let M be a symplectic manifold with a Hamiltonian circle action with isolated fixed points. We prove that σ (M) = b₀(M) − b₂(M) + b₄(M) − b₆(M) + … where σ (M) is the signature of M and b_i(M) is the ith Betti number of M.     

定向凝固单晶冰的取向确定与选晶

基于六方冰晶偏振光学特性,定义了用于确定冰晶晶体取向的三个参数:光轴倾角α,消光角β和与冰晶基面(0001)面内晶体学择优方向1120与温度梯度的夹角γ,提出了定量判定冰晶晶体取向的理论基础,并在定向凝固平台上采用偏光显微镜成功实现了冰晶晶体取向的精确主动控制,获得了任意取向的单晶冰.本文成功解决了冰晶的定向凝固晶体取向确定和选择的难题,为冰晶生长过程中相关理论问题的研究提供了有效的途径.     

Anisotropic thermoelectric transport properties in polycrystalline SnSe_2

It is reported that SnSe_2 consisting of the same elements as SnSe, is a new promising thermoelectric material with advantageous layered structure. In this work, the thermoelectric performance of polycrystalline SnSe_2 is improved through introducing SnSe phase and electron doping(Cl doped in Se sites). The anisotropic transport properties of SnSe_2 are investigated. A great reduction of the thermal conductivity is achieved in SnSe_2 through introducing SnSe phase, which mainly results from the strong SnSe_2–SnSe inter-phase scattering. Then the carrier concentration is optimized via Cl doping, leading to a great enhancement of the electrical transport properties, thus an extraordinary power factor of ～5.12 μW·cm~(-1)·K~(-2) is achieved along the direction parallel to the spark plasma sintering(SPS) pressure direction( P). Through the comprehensive consideration on the anisotropic thermoelectric transport properties, an enhanced figure of merit ZT is attained and reaches to ～ 0.6 at 773 K in SnSe_2-2% SnSe after 5% Cl doping along the P direction, which is much higher than ～ 0.13 and ～ 0.09 obtained in SnSe_2-2% SnSe and pristine SnSe_2 samples, respectively.     

Conductivity anisotropy in directionally solidified CaZrO3---CaSZ and MgO-MgSZ eutectics

The results of the impedance spectroscopy measurements on eutectic samples based on zirconium oxide are presented here. Samples of CaZrO₃---ZrO₂(cubic) and MgO---ZrO₂(cubic) have been grown by a directional solidification procedure such that the different phases appear nearly oriented along the growth direction (lamellae in the system of CaZrO₃-ZrO₂(cubic) and fibers of MgO in a ZrO₂ matrix in the other system). The DC electrical conductivity has been measured by impedance spectroscopy along and across the growth axis. For CaZrO₃---ZrO₂ the coductivity is clearly anisotropic. The following values for σT have been obtained: the conductivity at 600 °C equals 2.0 × 10⁻⁶ Ω⁻¹ cm⁻¹ perpendicular to the fiber axis and 1.4 × 10⁻⁵ Ω⁻¹ cm⁻¹ parallel to it and with an activation energy of 1.3 eV for σT. For MgO---ZrO₂(cubic) the isotropic value of the conductivity at 600 °C is 10⁻⁴ Ω⁻¹ cm⁻¹ with activation energy for σT of 1.5 eV. The anisotropic conductivity in the CaZrO₃---ZrO₂ (cubic) system has been explained by a model of an ordered stacking of oxygen conducting (cubic ZrO₂) and non-conducting (CaZrO₃ or MgO) phases.     

Entropy of planar random surfaces on the lattice

We study planar random surfaces on a hypercubic lattice in two and three dimensions by Monte Carlo techniques. Our data are consistent with the formula n₀(A;C) A^b₀A, where n₀(A;C) is the number of planar random surfaces with area A and boundary C. We find b₀ = −1.4 ± 0.2, = 5.31 ± 0.03 (for d = 2) and b₀ = −1.5 ± 0.2, = 7.13 ± 0.05 (for d = 3). The values of b₀ disagree with those obtained from the Polyakov string model.     

Observation of large in-plane anisotropic transport in van der Waals semiconductor Nb2SiTe4

Two-dimensional (2D) van der Waals material is a focus of research for its widespread application in optoelectronics, memories, and spintronics. The ternary compound Nb₂SiTe₄ is a van der Waals semiconductor with excellent air stability and small cleavage energy, which is suitable for preparing a few layers counterpart to explore novel properties. Here, properties of bulk Nb₂SiTe₄ with large in-plane electrical anisotropy are demonstrated. It is found that hole carriers dominate at a temperature above 45 K with a carrier active energy of 31.3 meV. The carrier mobility measured at 100 K is about 213 cm²·V^-1·s^-1 in bulk Nb₂SiTe₄, higher than the reported results. In a thin flake Nb₂SiTe₄, the resistivity ratio between the crystalline axes of a and b is reaching about 47.3 at 2.5 K, indicating that there exists a large anisotropic transport behavior in their basal plane. These novel transport properties provide accurate information for modulating or utilizing Nb₂SiTe₄ for electronic device applications.     

Stability,electronic structure,and optical properties of lead-free perovskite monolayer Cs3B2X9(B=Sb,Bi;X=Cl,Br,I)and bilayer vertical heterostructure Cs3B2X9/Cs3B’2X9(B,B’=Sb,Bi;X=Cl,Br,I)

The lead-free perovskites Cs₃B₂X₉(B=Sb,Bi;X=Cl,Br,I)as the popular photoelectric materials have excellent optical properties with lower toxicity.In this study,we systematically investigate the stable monolayer Cs₃B₂X₉and bilayer vertical heterostructure Cs₃B₂X₉/Cs3B02X9(B,B0=Sb,Bi;X=Cl,Br,I)via first-principles simulations.By exploring the electrical structures and band edge positions,we find the band gap reduction and the band type transition in the heterostructure Cs₃B₂X₉/Cs3B02X9 due to the charge transfer between layers.Furthermore,the results of optical properties reveal light absorption from the visible light to UV region,especially monolayer Cs3Sb2I9 and heterostructure Cs3Sb2I9/Cs3Bi2I9,which have absorption peaks in the visible light region,leading to the possibility of photocatalytic water splitting.These results provide insights for more two-dimensional semiconductors applied in the optoelectronic and photocatalytic fields.     

Hierarchical dimensional crossover of an optically-trapped quantum gas with disorder

Dimensionality serves as an indispensable ingredient in any attempt to formulate low-dimensional physics, and studying the dimensional crossover at a fundamental level is challenging. The purpose of this work is to study the hierarchical dimensional crossovers, namely the crossover from three dimensions (3D) to quasi-2D and then to 1D. Our system consists of a 3D Bose–Einstein condensate trapped in an anisotropic 2D optical lattice characterized by the lattice depths V₁ along the x direction and V₂ along the y direction, respectively, where the hierarchical dimensional crossover is controlled via V₁ and V₂. We analytically derive the ground-state energy, quantum depletion and the superfluid density of the system. Our results demonstrate the 3D-quasi-2D-1D dimensional crossovers in the behavior of quantum fluctuations. Conditions for possible experimental realization of our scenario are also discussed.     

ZnAl2O4:Cr3+晶体4A2基态ZFS及其三角晶格畸变研究

用对角化哈密顿矩阵的方法,借助New man晶场叠加模型,研究了ZnAl₂O₄:Cr³⁺晶体的基态零场分裂(ZFS)及其电子光谱,理论结果与实验一致.定量研究表明,掺杂晶体ZnAl₂O₄:Cr³⁺中,络离子(CrO₆)^9-局域结构应有压缩的三角畸变(△θ=3.06°).同时指出,自旋二重态对⁴A₂基态ZFS参量b₂⁰的贡献不可忽略,而对g因子的贡献甚微.     

基于AsP/MoS2异质结的偏振光电探测器

线偏振光的探测能力是评价偏振光电探测器件的重要指标。黑砷磷(AsP)是一种较为稳定的平面内各向异性材料,由于其面内结构各向异性,其对线偏振光较为敏感,在偏振探测领域有着重要的应用潜力。本文介绍了一种基于AsP/MoS2的高度偏振敏感光电探测器。由于AsP各向异性的光吸收、MoS2有效的载流子收集和输运能力以及范德华异质结对暗电流的抑制作用,该光电探测器实现了大于300的电流开关比,0.27 A/W的电流光响应度以及2×10^10 Jones的比探测率。更重要的是,此类光电探测器在638 nm波段实现了高达3.06二向色性比的偏振特性。这些实验结果表明AsP/MoS2异质结构在偏振光电探测领域有着广阔的应用前景。     

Exact monodromy group of N = 2 heterotic superstring

We describe an N = 2 heterotic superstring model of rank 3 which is dual to the type II string compactified on a Calabi-Yau manifold with Betti numbers b_1,1 = 2 and b_1,2 = 86. We show that the exact duality symmetry found from the type II realization contains the perturbative duality group of the heterotic model, as well as the exact quantum monodromies of the rigid SU(2) super-Yang-Mills theory. Moreover, it contains a non-perturbative monodromy which is stringy in origin and corresponds roughly to an exchange of the string coupling with the compactification radius.     

Specific heat and anisotropic superconducting and normal-state magnetization of HoNi2B2C

The magnetization of single-crystal HoNi₂B₂C has been measured as a function of applied field (H) and temperature in order to probe the interplay between superconductivity and magnetism in this complex layered system. The normal-state magnetic susceptibility of HoNi₂B₂C is highly anisotropic with a Curie-Weiss-like temperature dependence for H applied perpendicular to the c-axis and with a much weaker temperature dependence for H applied parallel to the c-axis, indicating that the Ho⁺³ magnetic moments lie predominately in the tetragonal a−b plane below 20 K. High-field magnetization (2000 Oe), low-field magnetization (20 Oe) and zero-field specific heat all give an antiferromagnetic ordering temperature of T_N=5.0 K. Remarkably, in 20 Oe applied field both superconductivity (T_c=8.0 K) and antiferromagnetism (T_N=5.0 K) clearly make themselves manifest in the magnetization data. From these magnetization data a phase diagram in the H−T plane was constructed for both directions of applied field. This phase diagram shows a non-monotonic temperature dependence of H_c2 with a deep minimum at T_N=5 K. The high-field magnetization data for H applied perpendicular to the c-axis also reveal a cascade of three phase transitions for T < 5 K and H < 15 000 Oe, contributing to the rich H versus T phase diagram for HoNi₂B₂C at low temperatures.     

Aspects of “low field” magnetotransport in epitaxial thin films of the ferromagnetic metallic oxide SrRuO3

Epitaxial thin films of the conductive ferromagnetic oxide SrRuO₃ were grown on an (0 0 1) SrTiO₃ (STO) substrate by using DC sputtering technique. The magnetic and magnetoresistive properties of the films were measured by applying the magnetic field both perpendicular (out-of-plane) and parallel (in-plane) to the film plane and ever maintaining the direction of the applied field perpendicular to that of the transport current. The films grown on an (0 0 1) STO substrate showed identical magnetization properties in two orthogonal crystallographic directions of the substrate, [1 0 0]_S and [0 0 1]_S (in-plane and out-of-plane geometry), which suggests the presence of a multi domain structure within the plane of the film. For such samples, no anisotropic field (hard axis) along de [0 0 1]s direction, i.e., perpendicular to the film-plane could be detected. Nevertheless, a distinguishable temperature dependent out-of-plane anisotropic magnetoresistance (MR) along with strong temperature dependent low field hysteretic MR(H) behavior was detected for the studied films. A negative MR ratio MR(T)=[ρ(μ₀H=9 T; T)−ρ( μ₀H=0 T; T)]/ρ( μ₀H=0 T; T) on the order of a few percent, with maximums of 6% and 4% (right at the Curie temperature, T_C 160 K) was calculated for an in-plane and out-of plane measuring geometry, respectively. In addition there is an equally strong MR effect at low temperatures, which might be related to the temperature dependence of the magnetocrystalline anisotropy together with a magnetization rotation. Both the MR(T) behavior and the achieved values (except for T<30 K) are similar to those obtained on SrRuO₃ films grown on 2° miscut (0 0 1) STO substrates with the current parallel to the field and parallel to the direction, which was identified as the easier axis for magnetization.     

应用分子子图指数估计与预测烷烃核磁共振碳谱位移和

系统地研究了核磁共振碳谱(¹³C NMR)与化学位移规律及其定量构谱关系(QSSR).本文研究了一组多元素分子子图指数矢量(VMSG),并发现它与烷烃化学位移和(CCSA)有很好线性相关性.采用多元线性回归(MVLR)进行准确估计与预测,结果良好.