Interaction induced staggered spin-orbit order in two-dimensional electron gas.

We propose and formulate an interaction induced staggered spin-orbit order as a new emergent phase of two-dimensional Fermi gases. We show that when some form of inherent spin splitting via Rashba-type spin-orbit coupling renders two helical Fermi surfaces to become significantly "nested," a Fermi surface instability arises. To lift this degeneracy, a spontaneous symmetry-breaking spin-orbit density wave develops, causing a surprisingly large quasiparticle gapping with chiral electronic states. Since the staggered spin-orbit order is associated with a condensation energy, quantified by the gap value, destroying such spin-orbit interaction costs sufficiently large perturbation field or temperature or dephasing time. The BiAg(2) surface state is shown to be a representative system for realizing such novel spin-orbit interaction with tunable and large strength, and the spin splitting is decoupled from charge excitations. These functional properties are relevant for spin electronics, spin caloritronics, and spin-Hall effect applications.