Isosymmetric phase transitions, ultrahigh ductility, and topological nodal lines in $α−Ag_{2}S$

Physical review / B 102(14), 140101 (2020). doi:10.1103/PhysRevB.102.140101
We report two reversible pressure-induced isosymmetric phase transitions in $α−Ag_{2}S$ that are accompanied by two compressive anomalies at 7.5 and 16 GPa, respectively. The first transition arises from a sudden and drastic puckering of the wrinkled Ag-S layers, which leads to an anomalous structural softening at high pressure and gives rise to the ultrahigh compressive ductility in $α−Ag_{2}S$. The second transition stems from a pressure-driven electronic state crossover from a conventional semiconductor to a topological metal. The band-crossing points near the Fermi energy form a nodal-line structure due to the preservation of the time-reversal and space-inversion symmetries under pressure. Our findings not only reveal the underlying mechanism responsible for the ultrahigh ductility in this class of inorganic semiconductors, but also provide a distinctive member to the growing family of topological metals and semimetals.
Published by Inst., Woodbury, NY