Abstract
The elastic constant $C$ of pure solid C${}_{60},$ measured at low frequencies with a dynamical mechanical analyzer, exhibits in the vicinity of the order-disorder phase transition at ${T}_{c}=260$ K a qualitatively different type of behavior. Besides the expected negative dip, an additional positive peak appears close to ${T}_{c}$ in the real part of $C$, resulting in a double anomaly. The experiments show that this hardening is due to a slow relaxational process. Within the macroscopic Landau theory we discuss possible origins of this unusual effect. We present a qualitative description based on the heat-diffusion central-peak model, which describes low-frequency dynamics near phase transitions. Extending this model with a critically temperature-dependent thermal diffusion time, we can explain the double anomaly by a crossover from isothermal to adiabatic behavior and back. The difference between the two limits arises due to a coupling between the fluctuations in order parameter and in temperature.
Published Version
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