Discovery of BKT correlations in the quantum kagome compound Cs${}_2$Cu${}_3$SnF${}_{12}$

Abstract

We investigate the microscopic properties of the kagome compound Cs${}_2$Cu${}_3$SnF${}_{12}$ using ${}^{63,65}$Cu nuclear quadrupolar resonance (NQR). Analysis of the local hyperfine fields below the Néel temperature $T_N=20$ K indicates a spin structure consistent with $P{2}_1/n$ symmetry of negative vector chirality. Measurements of the spin-lattice relaxation rate $T_1^{-1}$ reveal signatures of a gapless ground state and two-dimensional Berezinskii-Kosterlitz-Thouless (BKT)-type correlations above $T_N$, extending over a broad temperature range of approximately 130 K in zero magnetic field. Within the same temperature range, the observed increase in the NQR linewidth is consistent with short-range chiral order recently identified by neutron scattering. Our results establish Cs${}_2$Cu${}_3$SnF${}_{12}$ as a unique quantum kagome system exhibiting BKT behavior.

6 pages, 3 figures