STM differential conductance of a disordered extreme type-II superconductor at high magnetic fields

Abstract

We calculate the tunneling conductance between a scanning tunneling microscope tip and the surface of an extreme type-II superconductor in a high magnetic field and at zero temperature. In a clean system, and with electrons tunneling only along vortex lines, we find that the spatially averaged differential conductance $\ensuremath{\sigma}(V)$ has a V-shape dependence on the bias voltage $V$, reflecting the presence of gapless points and near gapless regions in the quasiparticle excitation spectrum of a superconductor in high magnetic fields. Within a $T$-matrix approximation for a homogeneous superconductor, we investigate the influence of nonmagnetic impurities on the differential conductance. We find that in the presence of disorder the differential conductance becomes finite at zero bias and develops a nonlinear dependence on the bias voltage. We apply our theory to calculate the differential conductance $\ensuremath{\sigma}(V)$ of the superconductor ${\text{LuNi}}_{2}{\text{B}}_{2}\text{C}$ in the mixed state.