Perfect flat band with chirality and charge ordering out of strong spin-orbit interaction

Abstract

Spin-orbit interaction established itself as a major role player for emergent phenomena in modern condensed matter including a topological insulator, spin liquid and spin-dependent transports. However, its function is rather limited to adding topological nature to each phases of matter. We prove by our spinor line graph theory that a very strong spin-orbit interaction realized in 5d pyrochlore electronic systems generates multiply degenerate perfect flat bands. Unlike any of the previous flat bands, the electrons living there localize in real space by destructively interfering with each other in a spin selective manner ruled by the SU(2) gauge field. These electrons avoid the Coulomb interaction by self-organizing their localized wave functions. This gives rise to the trimerized charge ordering hand in hand with a stiff spin chirality, which may explain the recently found exotic low-temperature insulating phase of CsW2O6.