Phonon signatures of multiple topological quantum phase transitions in compressed TlBi S2: A combined experimental and theoretical study

We report the hydrostatic pressure induced two topological phase transitions in strong spin-orbit coupled material TlBiS2 at room temperature. Frequencies of the A1g and Eg phonons are observed to increase monotonically up to ∼4.0GPa, but with a clear slope change in A1g mode at ∼1.8GPa. Interestingly, there are two clear anomalies noticed in phonon linewidths of Eg mode at pressures ∼0.5 and ∼1.8GPa. Such anomalies are evidence of isostructural electronic transitions associated with unusual electron-phonon coupling. The high-pressure synchrotron powder diffraction and Raman show a first-order phase transition above 4 GPa. First-principles density functional theory-based calculations of electronic band structure, topological invariant Z2 and mirror Chern number nM reveal that the phonon anomalies at ∼0.5 and ∼1.8GPa are linked to the band inversions at Γ and F points of the Brillouin zone respectively. The first band inversion at Γ point at ∼0.5 GPa changes the Z2 from 0 to 1 leading to the transition of TlBiS2 system into a topological insulator. The second band inversion at F point at ∼1.8GPa results in nM=2, revealing a transition to a topological crystalline insulating state. Therefore the applied pressure systematically tunes the electronic states of TlBiS2 from a normal semiconductor to a topological insulator and finally into a topological crystalline insulator at two distinct pressures of ∼0.5 and ∼1.8GPa respectively, before undergoing a structural phase transition at ∼4GPa.