Unlocking of time reversal, space-time inversion and rotation invariants in magnetic materials

Abstract

Time reversal ($T$) and space inversion are symmetries of our universe in the low-energy limit. Fundamental theorems relate their corresponding quantum numbers to the spin for elementary particles: ${\hat{T}}^2=(\hat{P}\hat{T}{)}^2=-1$ for half-odd-integer spins and ${\hat{T}}^2=(\hat{P}\hat{T}{)}^2=+1$ for integer spins. Here we show that for elementary excitations in magnetic materials, this "locking" between quantum numbers is lifted: ${\hat{T}}^2$ and $(\hat{P}\hat{T}{)}^2$ take all four combinations of $+1$ and $-1$ regardless of the value of the spin, where $T$ now represents the composite symmetry of time reversal and lattice translation. Unlocked quantum numbers lead to new forms of minimal coupling between these excitations and external fields, enabling novel physical phenomena such as the "cross-Lamor precession", indirectly observable in a proposed light-absorption experiment. We list the magnetic space groups with certain high-symmetry momenta where such excitations may be found.