Phonon signatures of multiple topological quantum phase transitions in compressed TlBiS₂: A combined experimental and theoretical study

V. Rajaji^1,2, Raagya Arora^1,2, Saurav Ch. Sarma^3,2, B. Joseph⁴, Sebastian C. Peter^3,2, Umesh V. Waghmare^5,2, and Chandrabhas Narayana^1,2,*

¹Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560 064, India

²School of Advance Materials, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560 064, India

³New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560 064, India

⁴Elettra Sincrotrone Trieste, S.S. 14, Km 163.5 in Area Science Park, Basovizza, Trieste 34012, Italy

⁵Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560 064, India

Abstract

We report the hydrostatic pressure induced two topological phase transitions in strong spin-orbit coupled material TlBiS₂ at room temperature. Frequencies of the A_1g and E_g phonons are observed to increase monotonically up to ~4.0 GPa, but clear slope change in A_1g mode at ~1.8 GPa. Interestingly, there are two clear anomalies noticed in phonon linewidths of E_g mode at pressures ~0.5 GPa and ~1.8 GPa. 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 n_M  reveal that the phonon anomalies at ~0.5 GPa and ~1.8 GPa 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 ~1.8 GPa results in  n_M = 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 GPa and ~1.8 GPa respectively, before undergoing a structural phase transition at ~4 GPa.