STM spectroscopy of vortex cores and the flux lattice

The low-temperature scanning tunneling microscope, STM, is capable of measuring the local density of states across a surface with sub-nanometer resolution. This allows vortices and the flux lattice to be imaged by measuring the spatial variations in the local tunneling spectra on the surface of a superconductor. Bound core states or scattering quasiparticle states are highlighted by measuring the conductance at tip-sample bias setting below or above the superconducting gap, respectively. The tunneling conductance spectra display several subgap peaks whose energy and amplitude vary not only with the radial distance from the core but also with the angular position and strength of applied magnetic field. Details, such as the notion of local velocity of the circulating superfluid current, the influence of neighboring vortices, and anisotropy from the crystalline band structure are inferred from these spectra. (orig.)