Two crossovers in the pseudogap regime of YBa$_2$Cu$_3$O$_{7-x}$ superconductors observed by ultrafast spectroscopy

We have investigated the temperature dependence of the optical reflectivity on
a femtosecond scale in a near-optimally doped YBa$_2$Cu$_3$O$_{7-x}$
superconductor. The combined study of the lattice and carrier dynamics at
temperatures above T$_c$ 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.