Helicity-dependent photocurrents in the chiral Weyl semimetal RhSi

Weyl semimetals are crystals in which electron bands cross at isolated points in momentum space. Associated with each crossing point (or Weyl node) is an integer topological invariant known as the Berry monopole charge. The discovery of new classes of Weyl materials is driving the search for novel properties that derive directly from the Berry charge. The circular photogalvanic effect (CPGE), whereby circular polarized light generates a current whose direction depends on the helicity of the absorbed photons, is a striking example of a macroscopic property that emerges from Weyl topology. Recently, it was predicted that the rate of current generation associated with optical transitions near a Weyl node is proportional to its monopole charge. In Weyl semimetals that retain mirror symmetry the current is strongly suppressed by contributions from energy equivalent nodes of opposite charge. However, when all mirror symmetries are broken, as in chiral Weyl systems, nodes with opposite topological charge are no longer degenerate, opening a window of photon energies where the CPGE derived from the topological band structure can emerge. In this work we report the photon-energy dependence of the CPGE in the chiral Weyl semimetal RhSi. The spectrum reveals a helicity-sensitive response in a low-energy window that closes at 0.65 eV, in quantitative agreement with the bandstucture recently predicted from DFT calculations.

外尔半金属（Weyl semimetals）是一类电子能带在动量空间（momentum space）中于孤立点处发生交叉的晶体。每个交叉点（或称外尔节点，Weyl node）均对应一个被称为贝里单极子荷（Berry monopole charge）的整数拓扑不变量。新型外尔材料的发现推动了对源自贝里单极子荷的新奇物性的探索。圆光电效应（circular photogalvanic effect, CPGE）指圆偏振光可产生电流，且电流方向取决于所吸收光子的螺旋度，这是源自外尔拓扑的宏观物性的典型例证。近期有研究预测，外尔节点附近的光学跃迁（optical transitions）所对应的电流产生速率与其单极子荷成正比。在保留镜面对称性（mirror symmetry）的外尔半金属中，带有相反拓扑荷的能量简并节点的贡献会强烈抑制该电流。然而，当所有镜面对称性均被破缺时（如在手性外尔体系（chiral Weyl systems）中），带有相反拓扑荷的节点不再简并，从而开辟出一个光子能量窗口，在此窗口内源自拓扑能带结构（bandstructure）的圆光电效应得以显现。本研究报道了手性外尔半金属RhSi中圆光电效应的光子能量依赖特性。该光谱在0.65 eV处闭合的低能窗口中展现出螺旋度敏感响应，与近期通过密度泛函理论（Density Functional Theory, DFT）计算预测的能带结构（bandstructure）定量吻合。