Impurity resonances in the mixed state of high-Tc superconductors

We study the quasiparticle resonance states near strong impurities in the mixed state of a d-wave superconductor. These states give rise to zero-bias peaks in the local density of states, observed in scanning tunneling microscopy experiments. The field dependence of the peaks is obtained by averaging with respect to the spatially non-uniform Doppler shifts in the energy of excitations. The hybridization of the magnetic field-induced nodal quasiparticles with the impurity resonances results in the suppression and broadening of the zero-bias peaks. The height of the peaks is found to scale as \sqrt{Hc2/H}. The magnetic field response of the peaks is shown to be strongly dependent on the field orientation.