What happens to geometric phase when spin-orbit interactions lift band degeneracy?

Abstract

Spin-orbit interaction lifts accidental band degeneracy. The geometric phase gamma(C)=pi for circuits surrounding a line of such degeneracy cannot survive completely unchanged. The change depends on how the spin is fixed during adiabatic evolution. For spin fixed along the internal spin-orbit field, gamma(C) decreases to zero as the circuit collapses around the line of lifted degeneracy. For spin fixed along a perpendicular axis, the conical intersection persists and gamma(C)=pi is unchanged.