Charge Density Wave transient states induced by ultrashort hard X-ray pulses
The interaction of quantum systems with ultrashort laser pulses may lead to transient states far from equilibrium. Among these, Charge Density Wave (CDW) systems offer very rich non-equilibrium physics when submitted to ultrashort optical excitations in the infrared regime. In this Letter, we show that CDW systems pumped by ultrashort x-ray pulses excite the system in very different way. The x-ray pump, 6000 times more energetic than a laser pulse, pumping and probing the whole bulk system simultaneously, drives CDW systems in a novel reversible out of equilibrium state. The successive CDW states observed in the NbSe$_3$ system with respect to the arrival of x-ray pulses follow a non-thermal kinetic path. It is characterized by a change in 2k$_F$ wavevector, correlation lengths and electronic gap, as well as a change of dimensionality, all of this takes place within a very long recovery time ranging from 450$ns$ to 0.01$s$. Several scenarios are proposed, including transient nucleation and recombination of charged topological objects.