The superconducting gap \Delta_{c}, the pseudogap \Delta_{p} and pair fluctuations above Tc in overdoped YCaBaCuO from femtosecond time-domain spectroscopy

TL;DR

This study uses femtosecond spectroscopy to reveal two coexisting gaps in overdoped YCaBaCuO, identifying a pseudogap and a collective gap with distinct temperature dependencies and fluctuation behaviors.

Contribution

It provides direct real-time evidence of two distinct gaps in overdoped cuprates and characterizes their temperature dependence and fluctuation regimes.

Findings

Identification of a temperature-independent pseudogap $_{p}$ present above Tc.

Observation of a T-dependent collective gap _{c}(T) closing at Tc.

Fluctuations of _{c}(T) are limited to a few Kelvin, consistent with Ginzburg-Landau theory.

Abstract

The low-energy electronic excitation spectrum and gap structure in optimally doped and overdoped Y_{1-x}Ca_{x}Ba_{2}Cu_{3}O_{7-\delta} single crystals are investigated by real-time measurements of the quasiparticle relaxation dynamics with femtosecond optical spectroscopy. From the amplitude of the photoinduced reflectivity as a function of time, temperature and doping x we find clear evidence for the coexistence of two distinct gaps in the entire overdoped phase. One is a temperature-independent pseudogap $\Delta_{p}$ and the other is a T-dependent collective gap $\Delta_{c}(T)$ which has a BCS-like T-dependence closing at Tc. From quasiparticle relaxation time measurements above Tc we ascertain that fluctuations associated with the collective gap $\Delta_{c}(T)$ are limited to a few K, consistent with time-dependent Ginzburg-Landau theory and are distinct from the pseudogap whose presence is apparent well above Tc for all x.