Triplet Waves in a Quantum Spin Liquid

TL;DR

This study investigates the excitation spectrum of a quantum spin liquid material using neutron scattering, revealing coherent triplet modes, their temperature-dependent damping, and providing a detailed theoretical description.

Contribution

It provides the first detailed neutron scattering analysis of triplet excitations in a quantum spin liquid, including temperature effects and a theoretical model for the observed phenomena.

Findings

Identification of a coherent singlet-triplet mode with sinusoidal dispersion

Observation of thermally activated band narrowing and damping

Quantitative description of the excitation spectrum using the Single Mode Approximation

Abstract

We report a neutron scattering study of the spin-1/2 alternating bond antiferromagnet Cu(NO_3)_2. 2.5D_2O for 0.06<k_BT/J_1<1.5. For k_BT/J_1 << 1 the excitation spectrum is dominated by a coherent singlet-triplet mode centered at J_1=0.442(2) meV with sinusoidal dispersion and a bandwidth of J_2=0.106(2) meV. A complete description of the zero temperature contribution to the scattering function from this mode is provided by the Single Mode Approximation. At finite temperatures we observe exponentially activated band narrowing and damping. The relaxation rate is thermally activated and wave vector dependent with the period icity of the reciprocal lattice.