Nonequilibrium quasiparticle relaxation in the vortex state of La _2-xSr_xCuO₄

We have measured the charge dynamics in the vortex state of La _2-xSr_xCuO₄ by femtosecond time-resolved reflectance, which we demonstrate to be a direct probe of low-energy quasiparticle states. Application of a c-axis magnetic field induces regions surrounding vortex cores that display pseudogap charge dynamics. We determine the characteristic width ∼130 Å in optimally doped material and we show that it increases with decreasing doping. These results confirm a new experimental method of probing the microscopic properties of vortices in the cuprates.