Imaging superconducting vortex cores and lattices with a scanning tunneling microscope

Abstract

The observation of vortices in superconductors was a major breakthrough in\ndeveloping the conceptual background for superconducting applications. Each\nvortex carries a flux quantum, and the magnetic field radially decreases from\nthe center. Techniques used to make magnetic field maps, such as magnetic\ndecoration, give vortex lattice images in a variety of systems. However, strong\ntype II superconductors allow penetration of the magnetic field over large\ndistances, of order of the magnetic penetration depth \\lambda.\nSuperconductivity survives up to magnetic fields where, for imaging purposes,\nthere is nearly no magnetic contrast. Static and dynamic properties of vortices\nare largely unknown at such high magnetic fields. Reciprocal space studies\nusing neutron scattering give insight into the collective behavior. But the\nmicroscopic details of vortex arrangements and their motion remain difficult to\nobtain. Direct real space visualization can be made using scanning tunneling\nmicroscopy and spectroscopy (STM/S). Instead of using magnetic contrast, the\nelectronic density of states describes spatial variations of the quasiparticle\nand pair wavefunction properties. These are of order of the superconducting\ncoherence length \\xi, which is much smaller than \\lambda. In principle,\nindividual vortices can be imaged using STM up to the upper critical field\nwhere vortex cores, of size \\xi, overlap. In this review, we describe recent\nadvances in vortex imaging made with scanning tunneling microscopy and\nspectroscopy. We introduce the technique and discuss vortex images which reveal\nthe influence of the Fermi surface distribution of the superconducting gap on\nthe internal structure of vortices, the collective behavior of the lattice in\ndifferent materials and conditions, and the observation of vortex lattice\nmelting.\n