Spectral evidence of a ghostly transition: Theory of NMR 1/T_1 relaxation in a quantum spin nematic in applied magnetic field

TL;DR

This paper proposes using NMR 1/T_1 relaxation rates as a universal and practical method to detect elusive spin-nematic order in frustrated quantum magnets, supported by symmetry analysis and a specific material example.

Contribution

It introduces a novel approach to identify spin-nematic order via NMR 1/T_1 relaxation, building on previous neutron scattering studies and emphasizing universal spectral features.

Findings

NMR 1/T_1 relaxation reveals spin-nematic order signatures.

Optimal nuclear site and field orientation can suppress competing signals.

Application to BaCdVO(PO_4)_2 demonstrates experimental feasibility.

Abstract

There is now strong theoretical evidence that a wide range of frustrated magnets should support quantum spin-nematic order in 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. We emphasise 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 NMR in the square-lattice frustrated ferromagnet BaCdVO(PO_4)_2 is sensitive to spin-nematic order.