A gap-protected zero-Hall effect state in the quantum limit of the non-symmorphic metal KHgSb

A recurring theme in topological matter is the protection of unusual electronic states by symmetry, for example, protection of the surface states in Z₂ topological insulators by time-reversal symmetry^1-3. Recently, interest has turned to unusual surface states in the large class of non-symmorphic materials^4-12. In particular, KHgSb is predicted to exhibit double quantum spin Hall states¹⁰. Here we report measurements of the Hall conductivity in KHgSb in a strong magnetic field B. In the quantum limit, the Hall conductivity is observed to fall exponentially to zero, but the diagonal conductivity is finite. A large gap protects this unusual zero-Hall state. We theoretically propose that, in this quantum limit, the chemical potential drops into the bulk gap, intersecting equal numbers of right- and left-moving quantum spin Hall surface modes to produce the zero-Hall state. The zero-Hall state illustrates how topological protection in a non-symmorphic material with glide symmetry may lead to highly unusual transport phenomena.