Marcenat Christophe, Klein Thierry, LeBoeuf David, Jaoui Alexandre, Seyfarth Gabriel, Kačmarčík Jozef, Kohama Yoshimitsu, Cercellier Hervé, Aubin Hervé, Behnia Kamran, Fauqué Benoît
Université Grenoble Alpes, CEA, IRIG, PHELIQS, LATEQS, F-38000 Grenoble, France.
Université Grenoble Alpes, CNRS, Grenoble INP, Institut Néel, F-38000 Grenoble, France.
Phys Rev Lett. 2021 Mar 12;126(10):106801. doi: 10.1103/PhysRevLett.126.106801.
In the immediate vicinity of the critical temperature (T_{c}) of a phase transition, there are fluctuations of the order parameter that 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 a 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%) reduction in the amplitude of the magnetic field (from 33 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 as in the case of the ^{4}He superfluid transition.
在相变的临界温度（(T_{c})）附近，存在超出平均场近似的序参量涨落。与高斯涨落不同，这种临界涨落通常发生在非常窄的温度窗口内。在此，我们报告一项关于在高磁场下石墨比热的研究，此时所有载流子都被限制在最低朗道能级。在温度和磁场扫描中观察到类似BCS的比热跃变，证实了几十年前在石墨中发现的相变是二级相变。在跃变之前，电子比热会因磁场诱导而稳定增强。磁场幅度适度降低（从33 T降至27 T）会导致(T_{c})降低三倍，比热异常急剧变宽，其尾部扩展到两倍(T_{c})。我们认为，与平均场BCS行为的稳定偏离是这种稀金属中异常大的金兹堡数的结果，该金兹堡数随着磁场降低而稳定增长。我们对临界涨落的拟合表明，它们与(^{4}He)超流相变一样，属于三维XY普适类。
