Two Crossovers in the Pseudogap Regime of $\mathrmY\mathrmB\mathrma_2\mathrmC\mathrmu_3\mathrmO_7-$δ$$ Superconductors Observed by Ultrafast Spectroscopy

We have investigated the temperature dependence of the optical reflectivity on a femtosecond scale in a near optimally doped YBa2Cu3O7-δ superconductor. The combined study of the lattice and carrier dynamics at temperatures above Tc allows us to identify two crossover temperatures in the normal state, giving evidence for an inhomogeneity of the pseudogap regime. These crossovers exhibit a clear hysteresis behavior depending on the direction of temperature change. The carrier and lattice dynamics within the crossover regimes show distinct differences from and similarities to the superconducting state, which may help in choosing between the competing theories for the pseudogap state.