Wide critical fluctuations of the field-induced phase transition in graphite

TL;DR

This study reveals wide critical fluctuations near the phase transition in graphite under high magnetic fields, showing deviations from mean-field behavior and indicating a 3DXY universality class for the fluctuations.

Contribution

It demonstrates that the phase transition in graphite exhibits wide critical fluctuations and belongs to the 3DXY universality class, challenging the typical narrow fluctuation window.

Findings

Observation of a BCS-like specific heat jump at the transition

Wide fluctuation tail spreading to twice T_c with field variation

Departure from mean-field behavior due to large Ginzburg number

Abstract

In the immediate vicinity of the critical temperature (T$_c$) of a phase transition, there are fluctuations of the order parameter, which reside beyond the mean-field approximation. Such critical fluctuations usually occur in a very narrow temperature window in contrast to Gaussian fluctuations. Here, we report on a study of specific heat in graphite subject to high magnetic field when all carriers are confined in the lowest Landau levels. The observation of a BCS-like specific heat jump in both temperature and field sweeps establishes that the phase transition discovered decades ago in graphite is of the second-order. The jump is preceded by a steady field-induced enhancement of the electronic specific heat. A modest (20 percent) reduction in the amplitude of the magnetic field (from 33 T to 27 T) leads to a threefold decrease of T$_c$ and a drastic widening of the specific heat anomaly, which acquires a tail spreading to two times T$_c$. We argue that the steady departure from the mean-field BCS behavior is the consequence of an exceptionally large Ginzburg number in this dilute metal, which grows steadily as the field lowers. Our fit of the critical fluctuations indicates that they belong to the $3DXY$ universality class, similar to the case of $^4$He superfluid transition.