Ultrasonic investigation of the transition at 6 K in the spin-liquid candidate $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}{\mathrm{Cu}}_{2}{\mathrm{(CN)}}_{3}$

Ultrasonic investigation of the transition at 6 K in the spin-liquid candidate $κ$ -(BEDT-TTF) $_{2} {Cu}_{2} {(CN)}_{3}$

Mario Poirier, Mathieu de Lafontaine, Kazuya Miyagawa, Kazushi Kanoda, and Yasuhiro Shimizu

Phys. Rev. B 89, 045138 – Published 29 January 2014

Abstract

An ultrasonic measurement has been performed on the spin-liquid organic candidate $κ$ -(BEDT-TTF) $_{2}$ Cu $_{2}$ (CN) $_{3}$ at low temperatures. A softening anomaly was observed on the longitudinal velocity along a direction perpendicular to the organic planes with a consistent one on the attenuation. The velocity anomaly appears as a frequency dependent softening peak below 20 K with a maximum value found near 5–6 K. A magnetic field affects predominantly the peak only below a temperature $T_{p} = 5.9$ K that is associated with the 6 K transition found on other physical properties. We suggest to relate the frequency dependent velocity softening at low temperatures to a spinon-phonon coupling, softening that is reduced below 5.9 K due to a pairing instability transition of the spinons.