Ultrafast mid-infrared response of YRa2Cu3O7-delta

Optical spectra of high-transition-temperature superconductors in the midin frared display a gap of in-plane conductivity whose role for superconductiv ity remains unresolved. Femtosecond measurements of the mid-infrared reflec tivity of YBa2Cu3O7-delta after nonequilibrium optical excitation are used to demonstrate the ultrafast fill-in of this gap and reveal two gap constit uents: a picosecond recovery of the superconducting condensate in underdope d and optimally doped material and, in underdoped YBa2Cu3O7-delta, an addit ional subpicosecond component related to pseudogap correlations. The temper ature-dependent amplitudes of both contributions correlate with the antifer romagnetic 41-millielectronvolt peak in neutron scattering, supporting the coupling between charges and spin excitations.