Pressure-induced metallization and robust superconductivity in pristine 1T-HfSe2

The two-dimensional semiconductor 1T-HfSe2 is found to have highly tunable transport properties under pressure including metallization and pressure-driven superconductivity. The temperature-dependent resistivity of the sample suggests that a charge-density wave (CDW) state exists at low pressures in HfSe2, but it is suppressed below 20 GPa. It is further found that metallization takes place at ∼24 GPa followed by the appearance of a superconducting state at 26 GPa with a Tc of 6.7 K. Upon further compression to 33 GPa, the Tc increases monotonically to 7.5 K. Raman spectra, x-ray diffraction, transport measurements, and density-functional theory calculations suggest that the occurrence of the pressure-induced metallization and superconducting transition are intimately linked to a structural phase transition from the trigonal (P 3‾ m1) to a hexagonal structure (P63/mmc). Because of the phase transition, a massive structural reconstruction, and substantial band-structure changes around the Fermi level take place, which are due to the modification of weak van der Waals forces. The pressure-induced manipulation of the transport properties of 1T-HfSe2 could provide crucial information towards its practical applications.