# Nature of the spin liquid ground state in a breathing kagome compound studied by NMR and series expansion

## TL;DR

This study investigates the magnetic properties of a breathing kagome lattice in a vanadium oxyfluoride compound using NMR and series expansion, revealing a likely gapless spin liquid state with specific interaction ratios.

## Contribution

It provides the first detailed NMR analysis of a breathing kagome compound, quantifying interaction ratios and suggesting a gapless spin liquid ground state.

## Key findings

- The ratio of interactions within the breathing kagome plane is approximately 0.55.
- The mean antiferromagnetic interaction is around 60 K.
- The excitation spectrum is essentially gapless with a very small maximum gap.

## Abstract

In the vanadium oxyfluoride compound (NH$_4$)$_2$[C$_7$H$_{14}$N][V$_7$O$_6$F$_{18}$] (DQVOF), the V$^{4+}$ (3d$^1$, $S=1/2$) ions realize a unique, highly frustrated breathing kagome lattice composed of alternately-sized, corner-sharing equilateral triangles. Here we present an $^{17}$O NMR study of DQVOF, which isolates the local susceptibility of the breathing kagome network. By a fit to series expansion we extract the ratio of the interactions within the breathing kagome plane, $J_\triangledown / J_\vartriangle = 0.55(4)$, and the mean antiferromagnetic interaction $\bar{J}=60(7)$~K. Spin lattice, $T_1$, measurements reveal an essentially gapless excitation spectrum with a maximum gap $\Delta / \bar{J}=0.007(7)$. Our study provides new impetus for further theoretical investigations in order to establish whether the gapless spin liquid behavior displayed by DQVOF is intrinsic to its breathing kagome lattice or whether it is due to perturbations to this model, such as a residual coupling of the V$^{4+}$ ions in the breathing kagome planes to the interlayer V$^{3+}$ ($S=1$) spins.