Theory of NMR 1/T-1 relaxation in a quantum spin nematic in an applied magnetic field
There is now strong theoretical evidence that a wide range of frustrated magnets should support quantum spin-nematic order in an applied magnetic field. Nonetheless, the fact that spin-nematic order does not break time-reversal symmetry makes it very difficult to detect in experiment. In this article, we continue the theme begun in Phys. Rev. B 88, 184430 (2013), of exploring how spin-nematic order reveals itself in the spectrum of spin excitations. Building on an earlier analysis of inelastic neutron scattering [Phys. Rev. B 91, 174402 (2015)], we show how the NMR 1/T-1 relaxation rate could be used to identify a spin-nematic state in an applied magnetic field. We emphasize the characteristic universal features of 1/T-1 using a symmetry-based description of the spin-nematic order parameter and its fluctuations. Turning to the specific case of spin-1/2 frustrated ferromagnets, we show that the signal from competing spin-wave excitations can be suppressed through a judicious choice of nuclear site and field direction. As a worked example, we show how P-31 NMR in the square lattice frustrated ferromagnet BaCdVO(PO4)(2) is sensitive to spin-nematic order.