Pressure-induced phase transitions and superconductivity in a quasi–1-dimensional topological crystalline insulator α-Bi ₄ Br ₄

Abstract

Significance The quasi–1-dimensional bismuth bromide, α-Bi 4 Br 4 , has been predicted to be a rotational symmetry-protected topological crystalline insulator. The structural study under high pressure indicates that the α-Bi 4 Br 4 phase is stable up to 4.3 GPa. There is a rich phase diagram of physical properties under high pressure in the α-Bi 4 Br 4 phase (i.e., a pressure-induced insulator–metal transition and, most importantly, a superconductive phase near the boundary of the insulator–metal transition). These findings help to answer questions, such as whether it is possible for the symmetry-protected electrons to form Cooper pairs. The α-Bi 4 Br 4 undergoes a pressure-induced structural transition above 4.3 GPa to a triclinic P- 1 phase, which is another superconductive phase.