Strain-induced partially flat band, helical snake states and interface superconductivity in topological crystalline insulators

Topological crystalline insulators in IV–VI compounds host novel topological surface states consisting of multi-valley massless Dirac fermions at low energy. Here we show that strain generically acts as an effective gauge field on these Dirac fermions and creates pseudo-Landau orbitals without breaking time-reversal symmetry. We predict the realization of this phenomenon in IV–VI semiconductor heterostructures, due to a naturally occurring misfit dislocation array at the interface that produces a periodically varying strain field. Remarkably, the zero-energy Landau orbitals form a flat band in the vicinity of the Dirac point, and coexist with a network of snake states at higher energy. We propose that the high density of states of this flat band gives rise to interface superconductivity observed in IV–VI semiconductor multilayers at unusually high temperatures, with non-Bardeen–Cooper–Schrieffer behaviour. Our work demonstrates a new route to altering macroscopic electronic properties to achieve a partially flat band, and provides a starting point for realizing novel correlated states of matter. In topological crystalline insulators, crystal symmetries give rise to particular electronic structures. As now shown, strain further induces pseudo-Landau states in IV–VI heterostructures—a mechanism possibly responsible for the superconductivity observed in such systems.