Characteristic features of the pseudogap and superconducting states of YBa$_2$Cu$_3$O$_{7-x}$

The relaxation dynamics of the lattice and low-energy quasiparticles in a YBa$_2$Cu$_3$O$_{7-x}$ superconductor are studied by the light reflection technique with femtosecond temporal resolution in a wide temperature range. It is shown that, for $T>T_c$, there exist two temperature regions with qualitatively and quantitatively different excitation dynamics, and the transition between these regions is of a hysteretic nature. It is also found that the character of changes observed in the charge carrier relaxation dynamics in the superconducting state testifies to the presence of an anisotropic gap with nodes at the Fermi surface.