Pressure induced insulator to metal crossover in van der Waals ferromagnet CrGeTe₃

The van der Waals material CrGeTe₃ has attracted tremendous research interest because of the realisation of Heisenberg type ferromagnetism (FM) in the two-dimensional limit as well as their potential in fabrication of heterostructures with other layered materials. In this work, we have constructed a pressure-temperature electronic phase diagram of CrGeTe₃ based on the magnetic susceptibility, χ_ab, resistivity ρ_ab, and Raman spectroscopy data. With increasing pressure up to 11.0 GPa, the ρ_ab decreases by several orders of magnitude, and near a critical pressure P_c ~ 6.5 GPa, ρ_ab displays a correlated Fermi-liquid state which widens with increasing pressure. Below P_c, ρ_ab increases with a -log(T) dependence with decreasing temperature suggesting a 2D weak localization behaviour or Kondo scattering. On the metallic side A-coefficient of T²-term, which represents the electron-electron scattering, diverges on approaching P_c from high pressure side. Furthermore, the χ_ab data reveals that above 3.5 GPa, the long-range FM order disappears and possibly changes either into a short-range type order or a completely different state. The pressure dependence of Raman modes at low temperature clearly suggest deviation of Cr-Te-Cr bonding angle from 90° above 4.0 GPa and rules out any structural transitions in this pressure range. These evidences clearly indicate that the pressure induced insulator to metal transition in CrGeTe₃ is driven by electronic correlation without being assisted by structural degree freedom.