  Two-dimensional topological insulators and two-dimensional materials with
ferroelastic characteristics are intriguing materials and many examples have
been reported both experimentally and theoretically. Here, we present the
combination of both features - a two-dimensional ferroelastic topological
insulator that simultaneously possesses ferroelastic and quantum spin Hall
characteristics. Using first-principles calculations, we demonstrate Janus
single-layer MSSe (M=Mo, W) stable two-dimensional crystals that show the
long-sought ferroelastic topological insulator properties. The material
features low switching barriers and strong ferroelastic signals, beneficial for
applications in nonvolatile memory devices. Moreover, their topological phases
harbor sizeable nontrivial band gaps, which supports the quantum spin Hall
effect. The unique coexistence of excellent ferroelastic and quantum spin Hall
phases in single-layer MSSe provides extraordinary platforms for realizing
multi-purpose and controllable devices.
