Cascade of pressure-driven phase transitions in the topological nodal-line superconductor PbTaSe2

We report a succession of pressure-tuned structural transitions in the topological nodal-line superconductor PbTaSe2, evidenced from synchrotron x-ray diffraction, elastic neutron scattering, Raman spectroscopy, and electrical transport measurements up to 56 GPa, accompanied by first-principles calculations to uncover the evolution of the underlying electronic structure. In contrast to the previously proposed small shift of the Pb Wyckoff coordinate in the sub-GPa regime, our study reveals that it is rather a transition from P (6) over bar m2 (alpha phase) to P6(3)mc (beta phase), subsequently followed by a transition to P6/mmm (gamma phase) at similar to 7.5 GPa and to Pmmm (delta phase) at similar to 44 GPa. In addition, the first-principles calculations unambiguously demonstrate the multiple types of topological fermions associated with these different pressure-driven structures. Collectively, our results not only present the intriguing structural transitions in this topological PbTaSe2 superconductor, they also provide the impetus to study topological phase transitions and their physical consequences in a broader class of topological materials.