Magnetic skyrmion bundles join the family of topological solitons

Magnetic solitons are localized magnetic textures that are similar to ordinary particles-they have defined size and can move and interact with each other. Topological solitons [1],in addition, are characterized by the so-called topological charge or topological index, which allows classifying magnetic solitons and has the following meaning.     

Quantum spin Hall and quantum valley Hall effects in trilayer graphene and their topological structures

The present study pertains to the trilayer graphene in the presence of spin orbit coupling to probe the quantum spin/valley Hall effect. The spin Chern-number C_s for energy-bands of trilayer graphene having the essence of intrinsic spin–orbit coupling is analytically calculated. We find that for each valley and spin, C_s is three times larger in trilayer graphene as compared to single layer graphene. Since the spin Chern-number corresponds to the number of edge states,consequently the trilayer graphene has edge states, three times more in comparison to single layer graphene. We also study the trilayer graphene in the presence of both electric-field and intrinsic spin–orbit coupling and investigate that the trilayer graphene goes through a phase transition from a quantum spin Hall state to a quantum valley Hall state when the strength of the electric field exceeds the intrinsic spin coupling strength. The robustness of the associated topological bulk-state of the trilayer graphene is evaluated by adding various perturbations such as Rashba spin–orbit(RSO) interaction αR, and exchange-magnetization M. In addition, we consider a theoretical model, where only one of the outer layers in trilayer graphene has the essence of intrinsic spin–orbit coupling, while the other two layers have zero intrinsic spin–orbit coupling.Although the first Chern number is non-zero for individual valleys of trilayer graphene in this model, however, we find that the system cannot be regarded as a topological insulator because the system as a whole is not gaped.     

Compass-anisotropy-modulated helical states and skyrmion crystals in chiral magnets

The compass-type anisotropy appears naturally in chiral magnets with strong spin-orbit coupling. In this work, we investigate the critical roles of compass anisotropy in modulating various spin textures of chiral magnets, by Monte Carlo simulations. The simulated results reveal a gradual helical reorientation and varying symmetry of skyrmion crystal structures as a function of compass anisotropy. Furthermore, an extended continuum spin model with the lattice discretization anisotropy is proposed to interpret the dependences of helical and skyrmion crystal structures on the compass anisotropy. It is demonstrated that specific helical propagating directions are favored by the high-order lattice anisotropy arising from spin interactions in discretized lattice. Besides that, some threshold values for the helical structures are identified by analytical approach.     

Experimental observation of topological Hall effects in compensated ferrimagnet-heavy metal layered structures

The topological Hall effect(THE) as a powerful probe for the experimental observation of topological spin textures, such as magnetic skyrmions, has been observed in a wide variety of distinct material systems. However, limited experimental observations have been reported for antiferromagnetic(AFM) materials. Here, the THE signals in the AFM state were observed in compensated ferrimagnetic thin films interfaced with heavy metals at the magnetization compensation temperature(T_M).Ferrimagnetic CoTb thin films grown on Pt thin films were used in the experiments. The Co Tb films exhibited a magnetization compensation point at which the moments of Co and Tb sublattices canceled each other, giving rise to the AFM state. The temperature(T)-dependent Hall measurements showed anomalous Hall effect(AHE) and THE responses at T≠T_M but pure THE responses at T=T_M. Control measurements and analyses suggest that the THE responses are associated with interfacial Dzyaloshinskii-Moriya interaction(DMI) rather than the overlapping of different AHE signals in the structure. This work presents the first-ever observation of interfacial DMI-induced THE in AFM metal trilayered systems and demonstrates a new approach for electrical reading of chiral spin textures in AFM thin film-based heterostructures.     

拓扑绝缘体电子态的电场调控

文章简要介绍了对拓扑绝缘体性质的电场控制,主要包括三维拓扑绝缘体表面磁性的电场控制、电子在p-n结中的类光输运行为以及拓扑绝缘体量子点的特性.     

拓扑绝缘体电子态的电场调控

文章简要介绍了对拓扑绝缘体性质的电场控制,主要包括三维拓扑绝缘体表面磁性的电场控制、电子在p-n结中的类光输运行为以及拓扑绝缘体量子点的特性.     

Orientation effects accompanying the propagation of ultrarelativistic electrons through crystals

The results obtained by experimentally investigating the dynamics of the propagation of 1.2-GeV electrons through a thin silicon single crystal are discussed. The orientation dependences of electron scattering into a small solid angle, which are measured at various scattering angles, under conditions where the effects of crystallographic axes and planes manifest themselves are interpreted. It is shown that there are such electron-scattering directions for which the orientation “independence” of the scattering intensity with respect to a specific crystallographic plane is observed and that there also exists a crystal-axis orientation for which the region of a uniform angular distribution of the intensity is observed.     

Hall mobility of hot electrons in CdS

Photo-Hall effect of hot electrons in a pure single crystal of CdS was observed, for the first time. The measurement was carried out for electric fields up to 4,300 V/cm in magnetic fields up to 40 kOe at 4.2 K. The saturation of the drift velocity V_d of electrons due to optical phonon emission was observed. The saturated value of V_d is found to be about 1.7 × 10⁷ cm/sec.     

Hall effect of hot electrons in n-PbTe

Measurements of the Hall-coefficient were performed on n-PbTe at 80K in different crystallographic directions as a function of electric field up to 1.1kV/cm and magnetic fields up to 0.6T. The observed behaviour gives evidence for the presence of strong equivalent intervalley transfer. As the crystallographic orientation had no significant influence on the threshold and the appearance of an instability, equivalent intervalley scattering was excluded as a possible mechanism for this instability.     

The stability of the fractional quantum Hall effect in topological insulators

With the recent observation of graphene-like Landau levels at the surface of topological insulators, the possibility of fractional quantum Hall effect, which is a fundamental signature of strong correlations, has become of interest. Some experiments have reported intra-Landau level structure that is suggestive of fractional quantum Hall effect. This paper discusses the feasibility of fractional quantum Hall effect from a theoretical perspective, and argues that while this effect should occur, ideally, in the n=0 and |n|=1 Landau levels, it is ruled out in higher |n| Landau levels. Unlike graphene, the fractional quantum Hall effect in topological insulators is predicted to show an interesting asymmetry between n=1 and n=−1 Landau levels due to spin-orbit coupling.     

The Nernst-Ettingshausen, Seebeck, and Hall effects in Sb2Te3 single crystals

This paper reports on measurement of the temperature dependences of the following transport coefficients: electrical conductivity in the σ₁₁ cleavage plane, the Seebeck coefficients S ₁₁ and S ₃₃ (axis 3 is along the trigonal crystal axis), the Hall coefficients R ₁₂₃ and R ₃₂₁, and the Nernst-Ettingshausen constant Q ₁₂₃; all measurements were made on high-quality Czochralski-grown Sb₂Te₃ single crystals. The results obtained are analyzed in terms of phenomenological theory. It is shown that the main features of the experimental data, including the anisotropy of the Hall and Seebeck effects, can be explained within a two-band model with notice-ably different anisotropy of the mobilities of holes of two types in the cleavage-plane and trigonal-axis directions. Estimates are made of the band-gap width (?_g?0.3 eV), as well as of the energy gap between the main and additional valence-band extrema (Δ?_v～0.1 eV).     

声子晶体中的多重拓扑相

研究了圆环型波导依照蜂窝结构排列的声子晶体系统中的拓扑相变.利用晶格结构的点群对称性实现赝自旋,并在圆环中引入旋转气流来打破时间反演对称性.通过紧束缚近似模型计算的解析结果表明,没有引入气流时,调节几何参数,系统存在普通绝缘体和量子自旋霍尔效应绝缘体两个相;引入气流后,可以实现新的时间反演对称性破缺的量子自旋霍尔效应相,而增大气流强度,则可以实现量子反常霍尔效应相.这三个拓扑相可以通过自旋陈数来分类.通过有限元软件模拟了多个系统中边界态的传播,发现不同于量子自旋霍尔效应相,量子反常霍尔相系统的表面只支持一种自旋的边界态,并且它无需时间反演对称性保护.     

Spectroscopy of non-equilibrium electrons in quantum Hall conductors

Spectroscopy of local cyclotron emission from the hot spots is carried out on a GaAs/AlGaAs heterostructure two-dimensional electron gas system at B=6 T (ν=2.5) by applying a terahertz scanning microscope. The spectra of CE at the current entry and exit corners (hot spots) are remarkably broadened towards lower frequencies with increasing I up to 300 μA, indicating substantial relevance of non-equilibrium electrons generated in higher-level LLs; in terms of effective electron temperature, T_E reaching as high as 300 K is suggested while T_E=25–30 K on an average in the surrounding region (within a distance of 50 μm) about the hot point.     

Non-abelian quantum Hall effect in topological flat bands

Inspired by the recent theoretical discovery of robust fractional topological phases without a magnetic field, we search for the non-abelian quantum Hall effect in lattice models with topological flat bands. Through extensive numerical studies on the Haldane model with three-body hard-core bosons loaded into a topological flat band, we find convincing numerical evidence of a stable ν=1 bosonic non-abelian quantum Hall effect, with the characteristic threefold quasidegeneracy of ground states on a torus, a quantized Chern number, and a robust spectrum gap. Moreover, the spectrum for two-quasihole states also shows a finite energy gap, with the number of states in the lower-energy sector satisfying the same counting rule as the Moore-Read pfaffian state.     

磁性拓扑绝缘体中的量子化反常霍尔效应——无需外磁场的量子化霍尔效应

文章从平常霍尔效应出发,介绍了反常霍尔效应及其内秉物理机制,并在此基础上介绍了其量子化版本——量子化反常霍尔效应.然后从拓扑有序态的角度,重点讨论了量子化反常霍尔效应与量子霍尔效应、量子自旋霍尔效应、拓扑绝缘体等之间的区别与内在联系.最后介绍了通过在拓扑绝缘体(Bi2Se3,Bi2Te3和Sb2Te3)薄膜中掺杂过渡金属元素(Cr或Fe)实现量子化反常霍尔效应的方法.