Strong One-Dimensional Characteristics of Hole-Carriers in ReS₂ and ReSe₂

Each plane of layered ReS₂ and ReSe₂ materials has 1D chain structure, from which intriguing properties such as 1D character of the exciton states and linearly polarized photoluminescence originate. However, systematic studies on the 1D character of charge carriers have not been done yet. Here, we report on systematic and comparative studies on the energy-momentum dispersion relationships of layered transition metal dichalcogenides ReS₂ and ReSe₂ by angle resolved photoemission. We found that the valence band maximum or the minimum energy for holes is located at the high symmetric Z-point for both materials. However, the out-of-plane ([Formula: see text]) dispersion for ReSe₂ (20 meV) is found to be much smaller than that of ReS₂ (150 meV). We observe that the effective mass of the hole carriers along the direction perpendicular to the chain is about 4 times larger than that along the chain direction for both ReS₂ and ReSe₂. Remarkably, the experimentally measured hole effective mass is about twice heavier than that from first principles calculation for ReS₂ although the in-plane anisotropy values from the experiment and calculations are comparable. These observation indicate that bulk ReS₂ and ReSe₂ are unique semiconducting transition metal dichalcogenides having strong one-dimensional characters.