二维VOBr2单层的结构畸变及其磁性和铁电性

借助第一性原理计算,对VOBr₂单层的结构、磁性以及铁电性进行了系统研究.计算结果表明低温下VOBr₂会产生自发铁电极化,从高对称顺电相转变为铁电相结构.与同族姊妹材料VOI₂不同的是,V的二聚化现象不仅无法在VOBr₂中稳定存在,还会导致局域磁_矩淬灭.此外,VOBr₂易磁化轴在面内a轴方向,面内a,b轴上近邻磁矩均为反铁磁耦合.VOBr₂中的铁电极化主要由V在a轴方向V—O—V链上的铁电位移产生,大小约为40μC/cm~2.与铁电同步翻转相比,通过分步翻转不同链上的铁电极化,可以有效降低铁电翻转能垒,从而有望通过低能外场实现部分或个别链上的铁电翻转,为低能耗高密度铁电存储器件设计提供新的思路和方向.     

Non-Stoichiometry Effects on the Extreme Magnetoresistance in Weyl Semimetal WTe_2

Non-stoichiometry effect on the extreme magnetoresistance is systematically investigated for the Weyl semimetal WTe_2. Magnetoresistance and Hall resistivity are measured for the as-grown samples with a slight difference in Te vacancies and the annealed samples with increased Te vacancies. The fits to a two-band model show that the magnetoresistance is strongly dependent on the residual resistivity ratio(i.e., the degree of non-stoichiometry),which is eventually understood in terms of electron doping that not only breaks the balance between electron-type and hole-type carrier densities, but also reduces the average carrier mobility. Thus the compensation effect and ultrahigh mobility are probably the main driving force of the extreme magnetoresistance in WTe_2.     

Dynamically Characterizing the Structures of Dirac Points via Wave Packets

Topological non-trivial band structures are the core problem in the field of topological materials. We investigate the topological band structure in a system with controllable Dirac points from the perspective of wave packet dynamics. By adding a third-nearest-neighboring coupling to the graphene model, additional pairs of Dirac points emerge. The emergence and annihilation of Dirac points result in hybrid and parabolic points, and we show that these band structures can be revealed by the dynami...