  Through either elements intercalation or application of pressure, transition
metal dichalcogenide 1T-TiSe2 exhibits superconductivity in proximity to a
charge density wave (CDW) quantum critical point (QCP), thus providing an ideal
avenue to study the correlation between the two symmetry-breaking exotic
quantum electronic states. We report herein that, in addition to the well-known
superconducting dome that emerges within the low pressure range of 2 - 4 GPa
and peaks with the maximal Tc of about 1.8 K, the pressure induces another
separate superconducting transition starting around 15 GPa with a substantially
higher Tc that reaches 5.6 K at about 21.5 GPa. The high-pressure X-ray
diffraction and Raman spectroscopy measurements unveil that the
superconductivity reentrant is caused by a first-order structural phase
transition (from P-3m1 space group to Pnma space group), which is also
supported by the density functional theory calculation. A comparative
theoretical calculation also reveals that the conventional phonon-mediated
mechanism can account for the superconductivity of 1T-TiSe2 under low pressure,
while the electron-phonon coupling of 4O-TiSe2 under high pressure is too weak
to induce the superconductivity with a Tc as high as 5.6 K. This implies that
the emergent superconductivity in the 4O-TiSe2 may have an unconventional
origin. Our finding would open a new window toward the discovery of more exotic
quantum states in transition metal dichalcogenides via high pressure.
