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Fluctuations versus preformed pairing in cuprate superconductors: How high is high temperature superconductivity.
  Condensed Matter seminars

Tuesday 07 March 2017
from 11:00 to 12:00
at 112:028 (Nordita South)
Speaker : Vladimir Krasnov (Stockholm University)
Abstract : How high can the superconducting transition temperature be? This principle question has been discussed for many decades. The debate is fueled by an obscure border between superconducting and normal states in cuprate superconductors, caused by the presence of a normal-state pseudogap. The pseudogap bears some similarities with the superconducting gap, which has led to a suggestion that the pseudogap is a precursor of superconductivity. In this view the onset of Cooper pairing occurs at a temperature Tc0, significantly higher than Tc, but the macroscopic phase coherence is suppressed by strong fluctuations of the phase, but not the amplitude, of the order parameter. Such precursor correlations are different from ordinary superconducting fluctuations in a sense that the locus of ordinary fluctuations coincides with Tc and both amplitude and phase fluctuations occur simultaneously at T >Tc . Thus, in order to answer on the question posted above it is necessary to distinguish superconducting and non-superconducting contributions to the pseudogap in cuprates. In this presentation I will review our recent development of a novel angular-dependent magnetotunneling technique, which facilitates unambiguous separation of superconducting (supporting circulating screening currents) and nonsuperconducting (not supporting screening currents) contributions to the pseudogap phenomenon in layered Bi2Sr2CaCu2O8 cuprates. Our data indicate persistence of superconducting correlations at temperatures up to 1.5Tc in a form of both phase and amplitude fluctuations of the superconducting order parameter. However, despite a profound fluctuations region, only a small fraction of the pseudogap spectrum is caused by superconducting correlations, while the dominating part comes from a competing nonsuperconducting order, which does not support circulating orbital currents.

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