Spin dynamics in the high-field phases of volborthite

TL;DR

This study uses 51V NMR to explore spin dynamics in volborthite, revealing magnon bound states in high magnetic fields and complex magnetic behavior below the plateau, suggesting exotic spin states.

Contribution

First detailed NMR investigation of spin excitations and magnetic states in volborthite under high magnetic fields, highlighting magnon bound states and potential exotic phases.

Findings

Magnon bound states indicated by large g-factor in the 1/3 magnetization plateau.

Small internal fields and power-law relaxation suggest complex magnetic ordering.

Evidence of exotic spin states possibly caused by magnon condensation.

Abstract

We report single-crystal 51V NMR studies on volborthite Cu3V2O7(OH)2 2H2O, which is regarded as a quasi-two-dimensional frustrated magnet with competing ferromagnetic and antiferromagnetic interactions. In the 1/3 magnetization plateau above 28 T, the nuclear spin-lattice relaxation rate 1/T1 indicates an excitation gap with a large effective g factor in the range of 4.6-5.9, pointing to magnon bound states. Below 26 T where the gap has closed, the NMR spectra indicate small internal fields with a Gaussian-like distribution, whereas 1/T1 shows a power-law-like temperature dependence in the paramagnetic state, which resembles a slowing down of spin fluctuations associated with magnetic order. We discuss the possibility of an exotic spin state caused by the condensation of magnon bound states below the magnetization plateau.