Buckled honeycomb lattice and unconventional magnetic response

We study the magnetic response of buckled honeycomb-lattice materials. The buckling breaks the sublattice symmetry, enhances the spin-orbit coupling, and allows the tuning of a topological quantum phase transition. As a result, there are two doubly degenerate spin-valley coupled massive Dirac bands, which exhibit an unconventional Hall plateau sequence under strong magnetic fields. We show how to externally control the splitting of anomalous zeroth Landau levels, the prominent Landau level crossing effects, and the polarizations of spin, valley, and sublattice degrees of freedom. In particular, we reveal that in a p-n junction, spin-resolved fractionally quantized conductance appears in a two-terminal measurement with a spin-polarized current propagating along the interface. In the low-field regime where the Landau quantization is not applicable, we provide a semiclassical description for the anomalous Hall transport. We comment briefly on the effects of electron-electron interactions and Zeeman couplings to electron spins and to atomic orbitals.