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

通过测量频率相关的过量复合微波电导率（超导波动），研究了La2-xSrxCuO4 （LSCO）在宽Sr浓度范围内超导向正态转变的临界电荷动力学。通过对数据的动态标度分析，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，我们观察到过掺杂薄膜的二维未知临界电荷动力学（“二维未知”）。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2D- xy， （ii）最佳掺杂区3D-XY， （iii）过掺杂区2D-“U”。因此，空穴掺杂使临界电荷动力学的维数发生了两次变化。我们讨论了这种空穴掺杂的异常维度交叉的可能起源，包括基于在最佳掺杂区域附近可能存在隐藏量子临界点的解释。正确的高温超导理论模型应该能解释所有这些行为。(并不是说电子掺杂的铜酸盐被发现具有3D-XY特征。我们还在太赫兹区进行了电导率测量，发现超导波动的上限最多为零电阻温度的两倍。这与微波结果很好地吻合，表明能斯特信号是超导涨落表征的解释是错误的。我们发现了另一个散射时间变长的特征温度。这个温度与光发射研究中观察到的赝隙起始温度非常吻合。

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.