Evidence for crossover from a Bose-Einstein condensate to a BCS-like superconductor with doping in YBa2Cu3O(7-x) from quasiparticle relaxation dynamics experiments

TL;DR

This study investigates the evolution of superconducting properties in YBa2Cu3O(7-x) across different doping levels using femtosecond quasiparticle relaxation experiments, revealing a crossover from a Bose-Einstein condensate to a BCS-like superconductor.

Contribution

It provides experimental evidence of a doping-induced crossover in the nature of superconductivity in YBa2Cu3O(7-x) through ultrafast quasiparticle dynamics measurements.

Findings

Underdoped samples show no gap change at Tc, indicating pre-formed pairs.

Near optimal doping, a divergence in quasiparticle recombination time at Tc suggests a BCS-like gap.

Evidence of a crossover from Bose-Einstein to BCS superconductivity with doping.

Abstract

Time resolved measurements of quasiparticle (QP) relaxation dynamics on the femtosecond timescale are reported as a function of temperature and doping in YBa2Cu3O(7-x) for (0.48 < x < 0.1). In the underdoped state (x > 0.15), there is no evidence for any changes in the low-energy gap structure at Tc from either photoinduced QP absorption or QP relaxation time data. In combination with the sum rule, this implies the existence of pre-formed pairs up to a much higher temperature T*. Near x = 0.1 a rapid cross-over is observed, to a state where the QP recombination time diverges and the photoinduced QP density falls to zero at Tc, indicating the existence of a temperature-dependent superconducting gap which closes at Tc.