The superconducting gap Δc, the pseudogap Δp and pair fluctuations above Tc in overdoped YCaBaCuO from femtosecond time-domain spectroscopy
J. Demsar, B. Podobnik, V. V. Kabanov, D. Mihailovic, Th. Wolf
Abstract
The low-energy electronic excitation spectrum and gap structure in optimally doped and overdoped Y1-xCaxBa2Cu3O7-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 Δp and the other is a T-dependent collective gap Δc(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 Δc(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.
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