Phonon effects and ESR in NH 4 CuCl 3

Abstract
We perform ultrasonic and ESR experiments up to 50 T in NH4CuCl3. As a function of temperature we find anomalies for the elastic constants c22 and c66 around the structural transition at 70 K. As a function of magnetic field we observe pronounced effects for these modes at the beginning and end of each magnetisation plateau. A linear relation of the c66 mode and the magnetic susceptibility is observed. The ESR modes follow a linear field dependence within the magnetic plateaus.

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