Pressure-induced semiconductor to metal transition and its impact on the electronic, optical, and thermodynamic properties of the new Kitaev spin liquid candidate CoI2

Abstract

We investigate the recently identified Kitaev candidate, the unconventional two-dimensional (2D) crystalline material van der Waals CoI2. Our approach involves employing ab-initio calculations to study the effects of applied pressure on its physical properties. CoI2 was found to be a semiconductor with a band gap energy of 2.1 eV. By applying pressure up to 13 GPa, a transition from a semiconductor to a metallic state takes place. This transition was explained by the displacement of the initially filled 5p-iodide band, overlapping with partially filled Co-3d states, leading to the formation of a hybridized conduction band and the emergence of a metallic ground state. Furthermore, by subjecting the CoI2 to pressure, a significant increase of the optical conductivity from 9900 to 11600 (Ω cm)−1, and the refractive index increases from 2.75 to 3.5. Our thermodynamic calculations reveal a captivating tale of CoI2; detecting a peak at 55 K in the temperature-normalized heat capacity which suggests the presence of entropy variation in the material, indicating a shift in the spin configuration within CoI2. In addition, a low thermal conductivity (ranging from 5 to 0.1 W m−1 K−1) underscores the strong interactions among quantum spins.