Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, India.
Sci Rep. 2013;3:1357. doi: 10.1038/srep01357.
The persistence of a soft gap in the density of states above the superconducting transition temperature Tc, the pseudogap, has long been thought to be a hallmark of unconventional high-temperature superconductors. However, in the last few years this paradigm has been strongly revised by increasing experimental evidence for the emergence of a pseudogap state in strongly-disordered conventional superconductors. Nonetheless, the nature of this state, probed primarily through scanning tunneling spectroscopy (STS) measurements, remains partly elusive. Here we show that the dynamic response above Tc, obtained from the complex ac conductivity, is highly modified in the pseudogap regime of strongly disordered NbN films. Below the pseudogap temperature, T*, the superfluid stiffness acquires a strong frequency dependence associated with a marked slowing down of critical fluctuations. When translated into the length-scale of fluctuations, our results suggest a scenario of thermal phase fluctuations between superconducting domains in a strongly disordered s-wave superconductor.
在超导转变温度 Tc 以上，能态密度中持续存在的软隙，即赝能隙，长期以来一直被认为是非常规高温超导体的标志。然而，在过去的几年中，这一范式受到了强烈的修正，因为越来越多的实验证据表明，强无序常规超导体中出现了赝能隙态。尽管如此，通过扫描隧道光谱学（STS）测量探测到的这个状态的本质仍然部分难以捉摸。在这里，我们表明，在强无序 NbN 薄膜的赝能隙区，从复交流电导率获得的 Tc 以上的动力学响应发生了高度的改变。在赝能隙温度 T*以下，超流弹性获得了与临界涨落明显减慢相关的强烈频率依赖性。当将这些结果转化为涨落的长度尺度时，我们的结果表明在强无序 s 波超导体中，超导畴之间存在热相位涨落的情景。
