ONLINE: New Surprises in High Tc Tunnel Junctions

Who: Douglas Natelson, Rice University
Monday, April 13, 2020 - 4:00pm

Abstract: High temperature superconductivity remains one of the major open problems in physics.  It also is connected to another open question:  what is the right way to think about “strange metals”, for which our standard fermi liquid model of metals doesn’t seem to apply?  Atomically precise, epitaxial tunnel junctions are a means to examine these materials in a nominally simple configuration that minimizes disorder and surface effects.  We use c-axis epitaxial junctions to examine tunneling between La(2-x)Sr(x)CuO4 source and drain electrodes separated by a 2 nm barrier of the undoped, Mott-insulating parent compound La2CuO4, over a broad range of doping, temperature, and bias.  Measurements of shot noise in the tunneling current reveal evidence for pairing of charges at temperatures above the superconducting transition, and at bias scales large compared to the superconducting gap [Nature 572, 493-498 (2019)].  Despite their high degree of structural perfection, tunneling spectroscopy of these junctions reveals marked deviations from expectations of the Bardeen-Cooper-Schrieffer (BCS) picture, including complete suppression of the Josephson current, strong residual conductance at zero bias in the T=0 limit, strongly suppressed coherence peaks, and non-conservation of transverse momentum in tunneling.  I will discuss these results, their implications, and next steps in these and related systems.

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