Search for quantum criticality and its control by precise microwave broadband spectroscopy

通过测量随频率变化的过剩复微波电导率（超导涨落），研究了Sr浓度变化范围较宽的La 2-xSrxCuO 4（LSCO）超导态向正常态转变的临界电荷动力学。通过对实验数据的动力学标度分析，我们发现了欠掺杂LSCO的二维（2D）-XY临界动力学和最佳掺杂LSCO的三维（3D-XY）临界动力学。此外，我们观察到一个二维未知的临界电荷动力学（“二维未知”）过掺杂薄膜。因此，发现LSCO相图中的临界行为分为以下三种类型：（i）欠掺杂区的2D-XY，（ii）最佳掺杂区的3D-XY，和（iii）过掺杂区的2D-“U”。因此，在临界电荷动力学的维数改变了两次与空穴掺杂。我们讨论了这种异常的尺寸交叉与空穴掺杂的可能起源，包括基于最佳掺杂区域附近可能存在的隐藏量子临界点的解释。正确的高温超导理论模型可以解释所有这些行为。(Not发现电子掺杂的铜氧化物显示出3D-XY特征。我们还在THz区进行了电导率测量，发现超导涨落的上限最多为零电阻温度的两倍。这与微波测量结果相一致，表明能斯特信号是超导涨落的代表的解释是错误的。我们发现了另一个特征温度，散射时间变得更长。这个温度与光电子能谱研究中观察到的赝能隙起始温度吻合得很好。

We studied the critical charge dynamics of the superconducting to the normal-state transition for La2-xSrxCuO4 (LSCO) with a wide range of the Sr concentration by measuring the frequency dependent excess complex microwave conductivity (superconducting fluctuations). By a dynamic scaling analysis of the data, we found the two-dimensional (2D)-XY critical dynamics for underdoped LSCO and the three dimensional (3D-XY) critical dynamics for optimally doped LSCO. Furthermore, we observed a two-dimensional unknown critical charge dynamics ("2D unknown") for overdoped thin films. Thus, it was found that the critical behavior in the phase diagram of LSCO is classified into the following three types: (i) 2D-XY for underdoped region, (ii) 3D-XY for optimally doped region, and (iii) 2D-"U" for overdoped region. Thus, the dimensionality in the critical charge dynamics is changed twice with hole doping. We discuss possible origins of such anomalous dimensional crossovers with hole doping, including an interpretation based on the possible existence of a hidden quantum critical point near the optimally doped region. Correct theoretical model for high-Tc superconductivity should explain all of these behaviors. (Not that electron doped cuprates were found to show 3D-XY features. for all carrier dopings)We also performed conductivity measurements in THz region, and found that the upper limit of superconductivity fluctuation is at most twice the zero resistance temperature. This is in good agreement with the microwave results, and suggests that the interpretation that the Nernst signal is the representation of superconductivity fluctuation is wrong. We found another characteristic temperature where scattering time becomes longer. This temperature agrees well with the pseudogap onset temperature observed in photoemission studies.