On the Origin of the Double Superconducting Transition in Overdoped YBa2Cu3Ox

TL;DR

This study investigates the origin of the double superconducting transition in overdoped YBa2Cu3Ox, revealing that the bulk transition occurs at 88 K while the 92 K transition is due to filamentary surface effects.

Contribution

It provides evidence that the higher temperature transition is from a minor surface phase, clarifying the nature of the double transition in overdoped YBCO.

Findings

Bulk transition at 88 K from fully oxygenated YBCO

Higher temperature transition at 92 K from filamentary surface phase

Transitions shift oppositely under hydrostatic pressure

Abstract

The superconducting transition in a single overdoped, detwinned YBa2Cu3Ox (YBCO) crystal is studied using four different probes. Whereas the AC and DC magnetic susceptibilities find a dominant transition at 88 K with a smaller effect near 92 K, the specific heat and electrical resistivity reveal only a single transition at 88 K and 92 K, respectively. Under hydrostatic pressures to 0.60 GPa these two transitions shift in opposite directions, their separation increasing. The present experiments clearly show that the bulk transition lies at 88 K and originates from fully oxygenated YBCO; the 92 K transition likely arises from filamentary superconductivity in a minority optimally doped phase (< 1 %) of YBCO located at or near the crystal surface.