Magneto-acoustic study near the quantum critical point of the frustrated quantum antiferromagnet Cs2CuCl4

TL;DR

This study uses magneto-acoustic techniques to explore the quantum critical behavior of Cs2CuCl4, revealing how sound velocity and attenuation relate to magnetic phase transitions and spin-liquid properties near the quantum critical point.

Contribution

It provides the first detailed magneto-acoustic analysis of Cs2CuCl4 near its quantum critical point, demonstrating the applicability of a classical Landau model and identifying a small magnetic excitation gap.

Findings

Observation of sound velocity softening and anomalies near the QCP.

Identification of two features in sound attenuation linked to magnetic ordering and spin-liquid behavior.

Evidence of a small magnetic excitation gap from elastic constant analysis.

Abstract

Magneto-acoustic investigations of the frustrated triangular-lattice antiferromagnet Cs2CuCl4 were performed for the longitudinal modes c11 and c33 in magnetic fields along the a-axis. The temperature dependence of the sound velocity at zero field shows a mild softening at low temperature and displays a small kink-like anomaly at TN. Isothermal measurements at T < TN of the sound attenuation reveal two closely-spaced features of different character on approaching the materials quantum-critical point (QCP) around Bs = 8.5 T for B // a. The peak at slightly lower fields remains sharp down to the lowest temperature and can be attributed to the ordering temperature TN(B). The second anomaly which is rounded and which becomes reduced in size upon cooling is assigned to the materials spin-liquid properties preceding the long-range antiferromagnetic ordering. These two features merge upon cooling suggesting a coincidence at the QCP. The elastic constant at lowest temperatures of our experiment at 32 mK can be well described by a Landau free energy model with a very small magnetoelastic coupling constant G/kB = 2.8 K. The applicability of this classical model indicates the existence of a small gap in the magnetic excitation spectrum which drives the system away from quantum criticality.