Pressure-Induced Re-Entrant Superconductivity in Transition Metal Dichalcogenide TiSe₂

Transition metal dichalcogenide TiSe₂ exhibits a superconducting dome within a low pressure range of 2-4 GPa, which peaks with the maximal transition temperature T_c of ≈1.8 K. Here it is reported that applying high pressure induces a new superconducting state in TiSe₂, which starts at ≈16 GPa with a substantially higher T_c that reaches 5.6 K at ≈21.5 GPa with no sign of decline. Combining high-throughput first-principles structure search, X-ray diffraction, and Raman spectroscopy measurements up to 30 GPa, It is found that TiSe₂ undergoes a first-order structural transition from the 1T phase under ambient pressure to a new 4O phase under high pressure. Comparative ab initio calculations reveal that while the conventional phonon-mediated pairing mechanism may account for the superconductivity observed in 1T-TiSe₂ under low pressure, the electron-phonon coupling of 4O-TiSe₂ is too weak to induce a superconducting state whose transition temperature is as high as 5.6 K under high pressure. The new superconducting state found in pressurized TiSe₂ requires further study on its underlying mechanism.