Highly anisotropic characters of hole carriers in ReS₂ and ReSe₂

Two-dimensional (2D) materials have greatly drawn attentions in condensed matter physics, since it shows intriguing electronic structure such as the Dirac cone in graphene and valley degeneracy in MoS₂, and also often shows strong electron-electron correlation effect due to low electronic dimensionality such as in charge density wave formation in NbSe₂. Group 7 transition metal dichalcogenides, ReS₂ and ReSe₂ are recently highlighted, since layer-layer interaction is much weaker than the other 2D materials and their highly anisotropic features. Here, we perform systematic directional dependence of effective mass of hole carriers in ReS₂ and ReSe₂ using angle resolved photoemission spectroscopy (ARPES). Remarkably, observed effective masses are heavier than those from first principles calculations and the effective masses along Re chain direction show seven times lighter than the other directions which is much stronger anisotropy than predicted by first principles calculations.