本征磁性拓扑绝缘体MnBi2Te4电子结构的压力应变调控

由于MnBi2Te4电子结构具有对晶格常数的改变相当敏感的特性,本文采用基于密度泛函理论的第一性原理方法对MnBi2Te4反铁磁块体的电子结构施加等体积应变调控.研究发现体系能带结构在材料等体积拉伸和压缩作用下变化灵敏,体系出现绝缘体-金属相变.特别地,当施加特定应变后导带和价带在Γ处出现交叉,体系呈零带隙状态.在此应...     

Electronic Structure of Magnetic Topological Insulators Mn(Bi1 – xSbx)2Te4 with Various Concentration of Sb Atoms

JETP Letters - Intrinsic magnetic topological insulator MnBi2Te4 provides a promising platform to implement the quantum anomalous Hall effect at increased temperatures and other unique topological...     

Lattice Dynamics of Bi2Те3 and Vibrational Modes in Raman Scattering of Topological Insulators MnBi2Te4·n(Bi2Te3)

JETP Letters - This work is devoted to the experimental study and symmetry analysis of the Raman-active vibration modes in MnBi2Te4·n(Bi2Te3) van der Waals topological insulators, where n is...     

Grain size of MnBi films

MnBi films were prepared by annealing Bi-Mn composite films. The grain size of the MnBi films was strongly influenced by the temperatureT _s of the film substrate during evaporation of Bi and Mn. For 55-nm-thick films, the numberN of MnBi grains per mm² decreased from 4×10⁶ per mm² forT _s =+90°C to 1 per mm² forT _s =−40°C. The diameters of the largest grains observed were about 5 mm. For interpretation of this result, the formation process of MnBi films was observed magneto-optically and electronmicroscopically. It was found thatN was determined by the structure of Bi in the Bi-Mn composite films and that this strongly depended onT _s .     

第一性原理研究闪锌矿结构MnSb和MnBi半金属性和磁性的稳定性

采用基于第一性原理的全势能线性缀加平面波方法对闪锌矿结构MnSb和MnBi的电子结构进行自旋极化计算。闪锌矿结构MnSb和MnBi处于晶格平衡时都是半金属性的,并且它们自旋向下电子能带带隙分别是1.32eV 和1.27eV。闪锌矿结构MnSb和MnBi的自旋总磁矩都为4.00μB/formula,总磁矩主要来源于Mn的原子磁矩,Sb和Bi的原子磁矩对总磁矩的贡献很小而且为负值,它们具有明显的铁磁性特征. 使晶体晶格在±10%的范围内发生各向同性形变,对闪锌矿结构MnSb和MnBi的电子结构进行计算. 计算结果表明,当晶格各向同性形变分别为-1 % ~ 10 %和-4 % ~10 %时,闪锌矿结构MnSb和MnBi仍然保持半金属铁磁性,并且总磁矩都稳定于4.00μB/formula.     

第一性原理研究闪锌矿结构MnSb和MnBi半金属性和磁性的稳定性

采用基于第一性原理的全势能线性缀加平面波方法对闪锌矿结构MnSb和MnBi的电子结构进行自旋极化计算。闪锌矿结构MnSb和MnBi处于晶格平衡时都是半金属性的,并且它们自旋向下电子能带带隙分别是1.32eV 和1.27eV。闪锌矿结构MnSb和MnBi的自旋总磁矩都为4.00μB/formula,总磁矩主要来源于Mn的原子磁矩,Sb和Bi的原子磁矩对总磁矩的贡献很小而且为负值,它们具有明显的铁磁性特征. 使晶体晶格在±10%的范围内发生各向同性形变,对闪锌矿结构MnSb和MnBi的电子结构进行计算. 计算结果表明,当晶格各向同性形变分别为-1 % ~ 10 %和-4 % ~10 %时,闪锌矿结构MnSb和MnBi仍然保持半金属铁磁性,并且总磁矩都稳定于4.00μB/formula.     

掺Nd的MnBi薄膜的磁性和磁光特性

本文讨论掺Nd的MnBi薄膜的结构,磁性和磁光性能。由X射线衍射和Auger谱分析表明,经适当热处理后,形成以MnBi为主的晶格结构,发现Nd掺入后可能存在MnBiNd合金的衍射线,并按NiAs型结构计算其晶格常数:a=4.14?,c=5.80?。转矩曲线和磁滞廻线结果给出薄膜具有很好的垂直膜面各向异性,当投料量Mn/Bi的值在2左右时,可得到合适的σ_s,适当的H_c值(1—4kOe),以及较大的Kerr转角为θ_K(1.5—2°)。磁光谱表明 关键词：     

MnBi磁性多层膜磁光科尔效应的数值模拟

基于各向异性的平面电磁波传输理论和4×4矩阵法，计算了MnBi，Mn_0.53Bi_0.47，Mn_0.52Bi_0.44Sb_0.04磁性多层膜系列的磁光科尔旋转角、椭圆率随波长、入射角、磁性层厚度变化曲线.计算结果表明，模拟的科尔旋转角、椭圆率随波长变化规律与实验结果吻合很好；在一定波长的垂直入射下，模拟的科尔旋转角、椭圆率随磁性层厚度变化曲线出现科尔谱极大；在磁性层厚度一定的情况下，当入射光以某一角度 关键词： 4×4矩阵法 磁性多层膜 磁光科尔谱 介电张量     

Topological surface states in lead-based ternary telluride Pb(Bi(1-x)Sb(x))2Te4

We have performed angle-resolved photoemission spectroscopy on Pb(Bi(1-x)Sb(x))2Te4, which is a member of lead-based ternary tellurides and has been theoretically proposed as a candidate for a new class of three-dimensional topological insulators. In PbBi2Te4, we found a topological surface state with a hexagonally deformed Dirac-cone band dispersion, indicating that this material is a strong topological insulator with a single topological surface state at the Brillouin-zone center. Partial replacement of Bi with Sb causes a marked change in the Dirac carrier concentration, leading to the sign change of Dirac carriers from n type to p type. The Pb(Bi(1-x)Sb(x))2Te4 system with tunable Dirac carriers thus provides a new platform for investigating exotic topological phenomena.     

In-Situ Alignment of MnBi Crystals Induced by High Magnetic Field above Curie Temperature

Above Curie temperature, MnBi crystals are aligned in situ along the c-axis in a Bi matrix by a high fabrication magnetic field H f of 10 T. Magnetic testing shows a pronounced anisotropy in magnetization in directions normal and parallel to the fabrication field, resulting from the alignment. The successful alignment m v result from the fact that the easy magnetization direction is along the c-axis of MnBi and the high fabrication field of 10 T is large enough to rotate the ）MnBi crystal to this direction even though the temperature is above the Curie temperature.     

Magnetic alignment of MnBi crystals and magnetic properties of MnBi–Bi composites

Below Curie temperature T_C, MnBi crystals are aligned along c-axis in a Bi matrix under a fabrication field H_f of 0.5 T. Above T_C, this alignment is also accomplished by quenching under a high H_f of 10 T. Such a method has a prominent feature that MnBi crystals grow preferentially and congregate along the H_f direction. Magnetic testing shows a pronounced anisotropy in magnetization in directions normal and parallel to H_f, resulting from the alignment. In the case of the alignment below T_C, H_f increases the transition temperature of spin-reorientation and the change in magnetization.     

Robustness of topologically protected surface states in layering of Bi2Te3 thin films

Bulk Bi2Te3 is known to be a topological insulator. We investigate surface states of Bi2Te3(111) thin films of one to six quintuple layers using density-functional theory including spin-orbit coupling. We construct a method to identify topologically protected surface states of thin film topological insulators. Applying this method to Bi2Te3 thin films, we find that the topological nature of the surface states remains robust with the film thickness and that the films of three or more quintuple layers have topologically nontrivial surface states, which agrees with experiments.     

Effect of magnetic field on the TC and magnetic properties for the aligned MnBi compound

In the compound MnBi, a first-order transition from the paramagnetic to the ferromagnetic state can be triggered by an applied magnetic field and the Curie temperature increases nearly linearly with an increase in magnetic field by ∼2 K/T. Under a field of 10 T, T_C increases by 20 and 22 K during heating and cooling, respectively. Under certain conditions a reversible magnetic field or temperature induced transition between the paramagnetic and ferromagnetic states can occur. A magnetic and crystallographic H-T phase diagram for MnBi is given. Magnetic properties of MnBi compound aligned in a Bi matrix have been investigated. In the low temperature phase MnBi, a spin-reorientation takes place during which the magnetic moments rotate from being parallel to the c-axis towards the basal plane at ∼90 K. A measuring Dc magnetic field applied parallel to the c-axis of MnBi suppresses partly the spin-reorientation transition. Interestingly, the fabricated magnetic field increases the temperature of spin-reorientation transition T_s and the change in magnetization for MnBi. For the sample solidified under 0.5 T, the change in magnetization is ∼70% and T_s is ∼91 K.     

Robust surface state of intrinsic topological insulator Bi2Te2Se thin films: a first-principles study

Bi(2)Te(2)Se, a ternary tetradymite compound, has recently been identified to be a three-dimensional topological insulator. In this paper, we theoretically study the electronic structures of bulk and thin films of Bi(2)Te(2)Se employing spin-orbit coupling (SOC) self-consistently with density-functional theory. It is found that SOC plays an important role in determining the electronic properties of Bi(2)Te(2)Se. A finite bandgap opens up in the surface states of Bi(2)Te(2)Se thin films due to the hybridization of the top and bottom surface states of films. The intrinsic Bi(2)Te(2)Se thin films of three or more quintuple layers exhibit a robust topological nature of electronic structure with the Fermi energy intersecting the Dirac cone of the surface states only once between time-reversal-invariant momenta. These characteristics of Bi(2)Te(2)Se are similar to the topological behavior of Bi(2)Te(3), promising a variety of potential applications in nanoelectronics and spintronics.     

Magneto-optic spectrum and electronic structure of single-crystal MnBi

Single crystals of MnBi were used for magneto-optic Kerr-angle spectroscopy and photoelectron spectroscopy. The Kerr-angle peak at 3.35 eV was observed for our nearly oxygen-free bulk samples, but the role of the oxide overlayer, demonstrated by Auger spectroscopy, as a possible origin of this peak cannot fully be assessed. The energy bands from photoelectron spectroscopy were in general agreement with calculated band structures, but were not extensive enough, due to sample cleavage properties, to distinguish reliably between band-structure calculations that disagree.     

Landau quantization and the thickness limit of topological insulator thin films of Sb2Te3

We report the experimental observation of Landau quantization of molecular beam epitaxy grown Sb{2}Te{3} thin films by a low-temperature scanning tunneling microscope. Different from all the reported systems, the Landau quantization in a Sb{2}Te{3} topological insulator is not sensitive to the intrinsic substitutional defects in the films. As a result, a nearly perfect linear energy dispersion of surface states as a 2D massless Dirac fermion system is achieved. We demonstrate that four quintuple layers are the thickness limit for a Sb{2}Te{3} thin film being a 3D topological insulator. The mechanism of the Landau-level broadening is discussed in terms of enhanced quasiparticle lifetime.     

反铁磁半金属 EuCd2Pn2（Pn = As,Sb）中磁交换诱导的外尔态

由于丰富的拓扑量子效应及巨大的潜在应用价值,拓扑材料逐渐成为凝聚态物理前沿的研究材料体系。其中,作为与石墨烯具有相似电子结构的材料,三维拓扑半金属吸引了越来越多的研究兴趣。目前已知的拓扑半金属大多为非磁性的,而磁性拓扑半金属数量有限,与非磁性拓扑半金属相比较,研究开展的还比较少。磁性与拓扑之间的相互作用能够导致非常规的物理性质,如反常霍尔效应甚至量子反常霍尔效应等。此外,在一些具有特殊磁结构的拓扑半金属中,施加外磁场能够调制其自旋结构,从而影响其拓扑能带结构。在该综述中,笔者将详细介绍利用外磁场在 EuCd2Pn2 (Pn = As, Sb) 反铁磁半金属材料中通过调制自旋结构从而改变晶体结构对称性来诱导拓扑相变。此外,笔者也将简单介绍包括 GdPtBi 和 MnBi2Te4 在内的几个相关材料。该综述中讨论的外磁场调控的磁交换诱导的拓扑相变不仅有望应用于拓扑器件,也有助于为理解磁性与拓扑态之间的紧密关联提供新的线索,对于设计新的磁性拓扑材料有启发意义。综述最后,笔者对发展磁性拓扑半金属做了一些简单展望。     

Impact of Co Atoms on the Electronic Structure of Bi2Te3 and MnBi2Te4 Topological Insulators

Journal of Experimental and Theoretical Physics - This work is devoted to an experimental investigation of the electronic structure of the surface of topological insulators of various stoichiometry...     

Effect of a high magnetic field on the morphology and magnetic properties of the MnBi compound during the Mn1.08Bi-MnBi phase transformation process

Effect of a 10 T high magnetic field on the morphology and magnetic properties of the MnBi compounds during the Mn_1.08Bi-MnBi phase transformation has been investigated. Results indicate that the field has split the MnBi crystal along the (0 0 1)-crystal plane during the Mn_1.08Bi-MnBi phase transformation process and the split MnBi crystals align and aggregate along the magnetic field direction. Along with the change of the MnBi phase morphology, the magnetic property changes greatly. Indeed, with the alignment and aggregation of the spit MnBi phases, the saturation magnetization M_s and the magnetic susceptibility χ increase, and the coercive field H_c and the remnant magnetization M_r decrease. This implies that a high magnetic field may have caused the magnetic property of the MnBi phase to transform towards soft magnetism. Above results may be attributed to the enhancement of the magnetization and the Mn_1.08Bi-MnBi phase transformation in a high magnetic field.     

Substitutional alloy of Bi and Te at high pressure

Being a best known thermoelectric material and a topological insulator at ambient condition, magic bismuth telluride (Bi2Te3) under pressure transforms into several superconducting phases, whose structures remain unsolved for decades. Here, we have solved the two long-puzzling low high-pressure phases as seven- and eightfold monoclinic structures, respectively, through particle-swarm optimization technique on crystal structure prediction. Above 14.4 GPa, we experimentally discovered that Bi2Te3 unexpectedly develops into a Bi-Te substitutional alloy by adopting a body-centered cubic disordered structure stable at least up to 52.1 GPa. The continuously monoclinic distortion leads to the ultimate formation of the Bi-Te alloy, which is attributed to the Bi→Te charge transfer under pressure. Our research provides a route to find alloys made of nonmetallic elements for a variety of applications.