Staggered Spin-Orbit Order: A New Paradigm of Broken Symmetry Phase of Matter

Abstract

We propose a novel spin-orbit density wave order, which can arise in a variety of materials classes. In systems where the noninteracting wavefunctions are defined by an exotic quantum number such as total angular momentum, pseudospin, or helical quantum number owing to spin-orbit coupling of various natures, interaction can induce an emergent spin-orbit density wave even when time-reversal symmetry is intact. This density wave order is different from standard time-reversal breaking spin or orbital density wave. We apply this idea to explain the enigmatic “hidden order” phase in heavy fermion URu2Si2 as well as an unknown gapped quasiparticle state observed in two-dimensional electron gases, such as the surface state of BiAg2.