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Temperature dependence of impurity bound states in d-wave superconductors

We study the evolution with temperature of quasiparticle bound states around non-magnetic impurities in d-wave superconductors. The associated local density of states has a fourfold symmetry which has recently been observed in Zn-doped Bi2212 using scanning tunneling microscopy (STM). From the corresponding Bogoliubov-de Gennes equation we find that with increasing temperature the magnitude of the bound state energy increases and the amplitude of the fourfold contribution in the spinor wave functions decreases. In the pseudogap regime above T_c the fourfold angular dependence of the local tunneling conductance persists as long as the superconducting fluctuations are sufficiently strong to support a finite local order parameter. Once the gap function vanishes completely, the angular structure of the bound state wave function becomes featureless. These effects should be observable in STM studies of impurity doped high-temperature superconductors.