Nonequilibrium ``melting'' of a charge density wave insulator via an ultrafast infrared laser pulse*

In equilibrium, electrons interacting with lattice vibrations have a transition either to a charge density wave phase (a static modulation of the electronic charge) or to a superconductor (electron pairs move without resistance). If the coupling is weak, the system orders in the Bardeen-Cooper-Schrieffer scenario, where the ordering occurs at some transition temperature Tc and a gap simultaneously forms in the density of states. In strong-coupling, preformed pairs bind at a high temperature (forming a gap in the density of states) and the ordering only occurs at a lower temperature. We employ an exact solution of a model for pump-probe time-resolved photoemission spectroscopy to show how, in nonequilibrium, a third scenario arises: the gap disappears in the presence of a nonzero order parameter, and then reforms well after the pulse has passed. This nonequilibrium ``phase transition'' scenario qualitatively describes all of the available experiments on the ultrafast melting of a charge density wave.