Emergence of inhomogeneous moments from spin liquid in the triangular-lattice Mott insulator $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$

TL;DR

This study uses NMR spectroscopy to reveal that a spin liquid in a triangular-lattice Mott insulator develops inhomogeneous magnetic moments under magnetic fields, with persistent spin fluctuations down to very low temperatures.

Contribution

It demonstrates the emergence of spatially nonuniform magnetizations in a spin liquid without critical fluctuations, providing new insights into its magnetic properties.

Findings

Field-dependent spectral broadening observed

Magnetization levels off below 1 K

Low-lying spin fluctuations persist at low temperatures

Abstract

The static and dynamic local spin susceptibility of the organic Mott insulator $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$, a model material of the spin- 1/2 triangular lattice, is studied by $^{13}$C NMR spectroscopy from room temperature down to 20 mK. We observe an anomalous field-dependent spectral broadening with the continuous and bipolar shift distribution, appearing without the critical spin fluctuations. It is attributable to spatially nonuniform magnetizations induced in the spin liquid under magnetic fields. The amplitude of the magnetization levels off below 1 K, while the low-lying spin fluctuations survive toward the ground state, as indicated by the temperature profile of the relaxation rates.