Phonon signatures of multiple topological quantum phase transitions in compressed TlBiS2
Phonon signatures of multiple topological quantum phase transitions in compressed TlBiS2
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.
Pressure dependence of the frequency of (a) Eg and (b) A1g modes of TlBiS2. (c) P vs FWHM of Eg mode and (d). P vs FWHM of A1g mode.