Effects of intense optical phonon pumping on the structure and electronic properties of yttrium barium copper oxide

We investigate the structural modulations induced by optical excitation of a polar phonon mode in YBa$_2$Cu$_3$O$_7$, using first-principles calculations based on density functional theory. We focus on the intense-excitation regime in which we expect that fourth-order phonon-phonon coupling terms dominate, and model the structural modulations induced by pulses of such intensity. Our calculations of the phonon-phonon anharmonicities confirm that the cubic coupling between modes, shown in earlier work to cause a quasi-static change in the apical O - Cu distance and a buckling of the CuO$_2$ planes, is the leading contribution at moderate pump strengths. At higher pump strengths ($\sim$10 MV/cm) the previously neglected quartic couplings become relevant and produce an additional shearing of the CuO$_2$ planes. Finally, we analyze the changes in the electronic and magnetic properties associated with the induced structural changes.