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

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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.