Dynamics of broken symmetry nodal and anti-nodal excitations in Bi_{2} Sr_{2} CaCu_{2} O_{8+\delta} probed by polarized femtosecond spectroscopy

The dynamics of excitations with different symmetry is investigated in the superconducting (SC) and normal state of the high-temperature superconductor Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+\delta }$ (Bi2212) using optical pump-probe (Pp) experiments with different light polarizations at different doping levels. The observation of distinct selection rules for SC excitations, present in A${{\rm 1g}}$andB${{\rm 1g}}$ symmetries, and for the PG excitations, present in A${}_{1\mathrm{g}}$ and B${}_{2\mathrm{g}}$ symmetries, by the probe and absence of any dependence on the pump beam polarization leads to the unequivocal conclusion of the existence of a spontaneous spatial symmetry breaking in the pseudogap (PG) state.