Magnetic-field modulation of topological electronic state and emergent magneto-transport in a magnetic Weyl semimetal

The modulation of topological electronic state by an external magnetic field is highly desired for condensed-matter physics.Schemes to achieve this have been proposed theoretically,but few can be realized experimentally.Here,combining transverse transport,theoretical calculations,and scanning tunneling microscopy/spectroscopy(STM/S)investigations,we provide an observation that the topological electronic state,accompanied by an emergent magneto-transport phenomenon,was modulated by applying magnetic field through induced non-collinear magnetism in the magnetic Weyl semimetal EuB6.A giant unconventional anomalous Hall effect(UAHE)is found during the magnetization re-orientation from easy axes to hard ones in magnetic field,with a UAHE peak around the low field of 5 kOe.

非常规材料:电子分布不在原子上

在拓扑量子化学中,研究人员发展了空间群下完备的基本能带表示,既对应于实空间的局域轨道,又对应于倒空间的一组不可约表示.通过计算实际材料的高对称k点的不可约表示,再利用基本能带表示分解来判断材料的拓扑性质.本研究发现拓扑平庸的材料可以分为电子电荷中心在原子上的常规材料和电荷中心不在原子上的“非常规材料”.这种非常规的电子分布特征在特定晶体对称群下可以方便地由能带表示分解来判断:占据态是基础能带表示的组合,但不是原子轨道对应的能带表示的组合.同时作者引入空位上的(必要)能带表示和实空间不变量来标定这一非常规电子分布特性.利用这一基于对称性本征值的能带表示分解方法,本文通过高通量搜索找到了423个非常规材料,并讨论了这些材料热电、储氢、催化、超导等特殊的化学性质与非常规电荷分布的相关性.

S_(4)对称性体系中外尔半金属的高通量搜索

基于对称性的不可约表示和相容性关系,人们可以将材料分为拓扑(或平庸)绝缘体(满足相容性关系),或是拓扑半金属(不满足相容性关系).但对于外尔半金属的判断通常超出了这种基于对称性方法的能力.换句话说,不论材料中占据态能带满不满足相容性关系,或者对称性指标是否为零,外尔点都可能出现在该材料中.本文针对具有S4对称性的体系提出了一种新型的拓扑不变量χ,并以此来检测体系中是否有外尔点的存在.χ可以通过一维Wilson-loop方法计算出来.将这种方法应用到高通量第一性原理计算中,作者在非磁和磁性体系中预测了大量新的外尔半金属材料.这些新发现的拓扑半金属材料具有许多独特的性质,如磁阻挫效应、超导和自旋玻璃态等.这些材料为外尔费米子和各种奇异物态之间相互作用研究提供了平台.

Evolution of Weyl nodes in Ni-doped thallium niobate pyrochlore Tl_(2-x)Ni_(x)Nb_(2)O_(7)

The magnetic Weyl semimetal(WSM)is important for fundamental physics and potential applications due to its spontaneous magnetism,robust band topology,and enhanced Berry curvature.It possesses many unique quantum effects,including a large intrinsic anomalous Hall effect,Fermi arcs,and chiral anomaly.In this work,using ab initio calculations,we propose that Nidoped pyrochlore Tl2Nb2O7is a magnetic WSM caused by the exchange field splitting on bands around its quadratic band crossing point.The exchange field tuned by Ni 3d on-site Coulomb interaction parameter U drives the evolution of Weyl nodes and the resulting topological phase transition.As Weyl nodes can exist at generic points in the Brillouin zone and are hard to identify exactly,their creation and annihilation,i.e.,the change in their number,chirality,and distribution,have been consistently confirmed with a combined theoretical approach,which employs parity criterion,symmetry indicator analysis,and the Wilson loop of the Wannier center.We find that Weyl nodes remain in a large range of U and are close to the Fermi level,which makes the experimental observation very possible.We think that this method and our proposal of magnetic WSM will be useful in finding more WSMs and add to the understanding of the topological phase transition.

Spin excitations and spin wave gap in the ferromagnetic Weyl semimetal Co3Sn2S2

We report a comprehensive neutron scattering study on the spin excitations in the magnetic Weyl semimetal Co3Sn2S2 with a quasi-two-dimensional structure.Both in-plane and out-of-plane dispersions of the spin waves were revealed in the ferromagnetic state.Similarly,dispersive but damped spin excitations were found in the paramagnetic state.The effective exchange interactions were estimated using a semi-classical Heisenberg model to consistently reproduce the experimental TCand spin stiffness.However,a full spin wave gap below Eg=2.3 meV was observed at T=4 K.This value was considerably larger than the estimated magnetic anisotropy energy(~0.6 meV),and its temperature dependence indicated a significant contribution from the Weyl fermions.These results suggest that Co3Sn2S2 is a three-dimensional correlated system with a large spin stiffness,and the low-energy spin dynamics can interplay with the topological electron states.

Layer construction of topological crystalline insulator LaSbTe

Topological crystalline insulator(TCI)is one of the symmetry-protected topological states.Any TCI can be deformed into a simple product state of several decoupled two-dimensional(2 D)topologically nontrivial layers in its lattice respecting its crystalline symmetries called the layer construction(LC)limit.In this work,based on first-principles calculations we have revealed that both tetragonal LaSbTe(t-LaSbTe)and orthorhombic LaSbTe(o-LaSbTe)can be interpreted as stacking of 2 D topological insulators in each lattice space.The structural phase transition from t-LaSbTe to o-LaSbTe due to soft phonon modes demonstrates how the real space change can lead to the modification of topological states.Their symmetry-based indicators and topological invariants have been analyzed based on LC.We propose that LaSbTe is an ideal example demonstrating the LC paradigm,which bridges the crystal structures in real space to the band topology in momentum space.

Unprotected quadratic band crossing points and quantum anomalous Hall effect in FeB_(2) monolayer

Quadratic band crossing points(QBCPs)and quantum anomalous Hall effect(QAHE)have attracted the attention of both theoretical and experimental researchers in recent years.Based on first-principle calculations,we find that the FeB_(2) monolayer is a nonmagnetic semimetal with QBCPs at K.Through symmetry analysis and k·p invariant theory,we find that the QBCP is not protected by rotation symmetry and consists of two Dirac points with the same chirality(Berry phase of 2π).Once introducing Coulomb interactions,we find that there is a spontaneous-time-reversal-breaking instability of the spinful QBCPs,which gives rise to a C=2 QAH insulator with orbital moment ordering.

Photoelectrochemical performance of La^(3+)-doped TiO_2

La-doped TiO_2 thin films on titanium substrates were prepared by the sol–gel method with titanium tetrachloride as a precursor and La_2O_3 as a source of lanthanum. The heat-treatment temperature dependence of the photoelectrochemical performance of the La-doped TiO_2 film in 0.2 mol/L Na_2SO_4 was investigated by the MottSchottky equation, electrochemical impedance spectroscopy, and the open-circuit potential test. The results from the Mott-Schottky curves show that the obtained films all were n-type semiconductors, and the film at 300 °C had the highest conduction band position and the widest space charge layer. The electrochemical impendence spectroscopy（EIS） tests of the 300 °C film decreased most during the change from illuminated to dark. The potential of the La–TiO_2 thin film electrode was the lowest after the 300 °C heat treatment. The open-circuit potential indicated that the photoelectrical performance of the La-TiO_2 films was enhanced with the addition of the La element and the largest decline（837.8 mV） in the electrode potential was achieved with the 300 °C heat treatment.