NMR studies of the spin liquid candidate material, κ-(BEDT-TTF)₂Cu₂(CN)₃, with varying magnetic field and pressure

An organic triangular-lattice Mott insulator, κ-(BEDT-TTF)₂Cu₂(CN)₃, is the first candidate for a quantum spin liquid material. Even 20 years after the first report of this phenomenon, the nature of spin liquid state has still been debated. One of key phenomena to this issue is the so-called 6K-anomaly, which refers to the anomalies that appear in magnetic, thermodynamic, dielectric and lattice properties around 6K. We performed the in-depth study of the 6K-anomaly, combining ¹H and ¹³C NMR measurements in two ways; one is to examine the magnetic-field dependence of the anomaly and the other is to trace the fate of the anomaly upon the Mott transition to a metallic phase by pressure.

At ambient pressure, ¹³C NMR spectra show a significant decrease in the spin shift and a simultaneous line broadening suggestive of an inhomogeneous state below 6K. Moreover, we found that the 6K-anomaly is robust against the magnetic field up to 15Tesla. The 1/T₁ at ¹H sites, where the hyperfine coupling with conduction electrons is smaller than that at ¹³C sites, obeys similar temperature dependence to that at ¹³C sites down to 6K; however, below that, 1/T₁ shows quite different temperature variations at the ¹H and ¹³C sites, indicative of different relaxation mechanisms at the ¹³C and ¹H sites at low temperatures. This can be a key to understanding the anomalous state below 6K.

With ¹H NMR measurements under pressure variation, we found that the 6K-anomaly suddenly and completely disappear when the system enters into the metallic.