Observation of two distinct pairs fluctuation lifetimes and supercurrents in the pseudogap regime of cuprate junctions

Abstract

Pairs fluctuation supercurrents and inverse lifetimes in the pseudogap regime are reported. These were measured on epitaxial c-axis junctions of the cuprates, with a PrBa2Cu3O7−δ barrier sandwiched in between two YBa2Cu3O7−δ or doped YBa2Cu3Oy electrodes, with or without magnetic fields parallel to the a-b planes. All junctions had a Tc(high)≈85–90 K and a Tc(low)≈50–55 K electrodes, allowing us to study pairs fluctuation supercurrents and inverse lifetimes in between these two temperatures. In junctions with a pseudogap electrode under zero field, an excess current due to pair fluctuations was observed which persisted at temperatures above Tc(low), in the pseudogap regime, and up to about Tc(high). No such excess current was observed in junctions without an electrode with a pseudogap. The measured conductance spectra at temperatures above Tc(low) were fitted using a modified fluctuations model by Scalapino [Phys. Rev. Lett. 24, 1052 (1970)] of a junction with a serial resistance. We found that in the pseudogap regime, the conductance vs voltage consists of a narrow peak sitting on top of a very broad peak. This yielded two distinct pairs fluctuation lifetimes in the pseudogap electrode which differ by an order of magnitude up to about Tc(high). Under in-plane fields, these two lifetime values remain separated in two distinct groups, which varied with increasing field moderately. We also found that detection of Amperian pairing [Phys. Rev. X 4, 031017 (2014)] in our cuprate junctions is not feasible, due to Josephson vortices penetration into the superconducting electrodes which drove the necessary field above the depairing field.

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