Spectroscopic and spatial characterization of superconducting vortex core states with a scanning tunneling microscope

The Abrikosov flux lattice and individual superconducting vortices are imaged with a low temperature scanning tunneling microscope. Differential conductance measurements taken while scanning allow one to image the quasiparticle wavefunctions. Different quasiparticle energy states are highlighted by tunneling at different dc voltages. States which are bound to the vortex core are imaged by tunneling with a voltage below the superconducting gap energy. The lowest energy bound wavefunctions which exist at the fermi level show a radial extent of about a coherence length, while higher energy bound states display a more extended wavefunction. Likewise scattering states can be imaged by tunneling with voltages above the superconducting energy gap. The results are interpreted within the context of Bardeen–Cooper–Schrieffer theory.