石墨中场致相变的宽临界涨落
Wide critical fluctuations of the field-induced phase transition in graphite
摘要：
在相变的临界温度（T $_c$）附近，存在序参量的涨落，这超出了平均场近似。这种临界涨落通常发生在一个非常窄的温度窗口中，与高斯涨落相反。在这里，我们报告的比热在石墨受到高磁场的研究时，所有的载流子被限制在最低的朗道水平。在温度和场扫描中观察到的BCS样比热跃变确定了几十年前在石墨中发现的相变是二级的。跳跃之前是一个稳定的场诱导的电子比热增强。磁场振幅的适度（20%）降低（从33 T到27 T）会导致T $_c$降低三倍，比热异常急剧扩大，从而获得扩展到T $_c$两倍的尾部。我们认为，从平均场BCS行为的稳定偏离是一个非常大的金兹伯格数在这个稀金属，它稳步增长的字段降低的后果。我们对临界涨落的拟合表明它们属于$3DXY$普适类，类似于$^4$ He超流跃迁的情况。
英文摘要：
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.
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