Low-energy electronic structure in Y[sub 1−x]Ca[sub x]Ba[sub 2]Cu[sub 3]O[sub 7−δ]: comparison of time-resolved optical spectroscopy, NMR, neutron and tunneling data

Abstract

Time-resolved optical measurements give information on the quasiparticle relaxation dynamics in YBCO, from which the evolution of the gap with doping and temperature can be systematically deduced. In this paper these optical charge-channel “pseudogap” data are compared with the “pseudogap” obtained from the NMR Knight shift Ks, spin polarized neutron scattering (SPNS) and single particle tunneling measurements. A simple energy level diagram is proposed to explain the different “gap” magnitudes observed by different spectroscopies in YBa2Cu3O7−δ, whereby the spin gap Δs in NMR and SPNS corresponds to a triplet local pair state, while Δp in the charge excitation spectrum corresponds to the pair dissociation energy. At optimum doping and in the overdoped state, an additional T-dependent gap becomes evident, which closes at Tc, suggesting a cross-over to a more conventional BCS-like superconductivity scenario.