SYMMETRY-BREAKING IN THE TUNNELING CONDUCTANCE OF A SUPERCONDUCTING VORTEX

Recent STM experiments on c-axis tunnelling into a vortex core of YBCO have yielded a sub-gap differential conductance which is not invarian t under a reversal of the applied bias. By computing the differential conductance G(E) of a normal-superconducting contact to all orders in the tunnelling strength, we predict that in the absence of open channe ls in the superconductor, G(E) is necessarily an even function of E an d that an asymmetric current-voltage characteristic implies the existe nce of propagating bound-states within the vortex core.