Dichroism as a probe for parity-breaking phases of spin-orbit coupled metals

Abstract

Recently, a general formalism was presented for gyrotropic, ferroelectric, and multipolar order in spin-orbit coupled metals induced by spin-spin interactions. Here, I point out that the resulting order parameters are equivalent to expectation values of operators that determine natural circular dichroic signals in optical and x-ray absorption. Some general properties of these operator equivalents and the resulting dichroisms are mentioned, and I list several material examples in this connection, including Weyl semimetals. The particular case of the tensor order in the pyrochlore superconductor Cd2Re2O7 is treated in more detail, including calculations of the x-ray absorption and circular dichroism at the O K edge.

Nematic order on the surface of a three-dimensional topological insulator

“…In the undoped limit µ = 0, we define k A ≡ Λ and the order parameter is local in space. This order parameter was first introduced by one of us in the context of nematic instabilities of the Majorana surface state of superfluid 3 He-B [73], and its 3D analog was proposed as an order parameter for parity-breaking phases of spin-orbit coupled bulk metals [74,75]. In the doped limit, defined as µ v F Λ, only (angular) degrees of freedom on the Fermi surface are relevant and we define k A ≡ |∂| [76].…”