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

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-δ} 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 $Δ_{p}$ and the other is a T-dependent collective gap $Δ_{c}(T)$ 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 $Δ_{c}(T)$ 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.

4 pages, 4 figures, accepted for publication in Phys.Rev.Lett