Dissertation/Thesis Abstract

Fluctuations of superconductivity in LSCO measured with terahertz time-domain spectroscopy
by Bilbro, Lucas S., Ph.D., The Johns Hopkins University, 2012, 207; 3532405
Abstract (Summary)

In the underdoped pseudogap regime of the high-temperature superconductors, one expects that due to low superfluid densities and short correlation lengths, superconducting fluctuations will be significant for transport and thermodynamic properties. However, there has been disagreement about how high in temperature they may persist, their role in the phenomenology of the pseudogap regime, and their significance for understanding high-temperature superconductivity. We use THz time-domain spectroscopy (TTDS) to probe the temporal fluctuations of superconductivity above the critical temperature (Tc) in La2−xSrxCuO 4 (LSCO) thin films over a large doping range. Signatures of the fluctuations persist in the conductivity in a narrow temperature range above the critical temperature Tc. Our measurements show that superconducting correlations do not make an appreciable contribution to the charge transport anomalies of the pseudogap in LSCO at temperatures well above T c. I compare our results for an underdoped (x=0.095) sample with measurements of diamagnetism in a similarly doped crystal of La 1.91Sr0.09CuO4. I show, through a vortex-plasma model, that if the fluctuation diamagnetism originates solely in vortices, then these vortices must exhibit an anomalously large vortex diffusion constant, more than two orders of magnitude larger than the Bardeen-Stephen estimate. This points to either the extremely unusual properties of vortices in the underdoped d-wave cuprates or a contribution to the diamagnetic response that is not superconducting in origin. Finally, I introduce preliminary results of THz conductivity measurements of very underdoped LSCO films, near where superconductivity is fully suppressed, which may provide evidence of quantum fluctuations.

Indexing (document details)
Advisor: Armitage, N. Peter
Commitee: Foster, Mark, Khurgin, Jacob, Melnikov, Kirill, Tchernyshyov, Oleg
School: The Johns Hopkins University
Department: Physics and Astronomy
School Location: United States -- Maryland
Source: DAI-B 74/03(E), Dissertation Abstracts International
Subjects: Condensed matter physics
Keywords: Critical temperature, Superconductivity, Superfluid densities, Terahertz, Time-domain
Publication Number: 3532405
ISBN: 978-1-267-75240-6
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