Induction, investigation, and stabilization of unusual superconducting states in Bi_0.5Sb_1.5Te₃ under pressure and at ambient pressure

Bi₂Te₃, Bi₂Se₃, and related alloys are semiconductors with notable topological characteristics adjacent to electronic instabilities, like Fermi surface topology changes. It has been suggested that doped Cu_xBi₂Se₃ possesses a topological superconducting state, which is crucial for quantum technology but has not yet been experimentally confirmed. To avoid the complexity resulting from doping, we used high pressure to induce this state in the topological semiconductor Bi_0.5Sb_1.5Te₃ (BST). Under pressure three superconducting phases (BST-I, -II, and -III) were observed. Using the pressure-quench protocol (PQP) recently developed by us, we successfully retained the pressure-induced phase in BST-I at ambient pressure and showed the bulk nature of this state. Other superconducting phases were also retained in BST-II and -III at ambient pressure and subjected to thermal and temporal stability testing. While PQP maintains superconducting phases in BST at ambient pressure, both depressurization and PQP enhance its T_c, possibly due to microstructures formed during these processes, offering another avenue to raise T_c. These findings are supported by our density-functional theory calculations.