Imaging superconducting vortex cores and lattices with a scanning tunneling microscope

Abstract

Abstract. The observation of vortices in superconductors was a major breakthrough in developing the conceptual background for superconducting applications. Each vortex carries a flux quantum, and the magnetic field radially decreases from the center. Techniques used to make magnetic field maps, such as magnetic decoration, give vortex lattice images in a variety of systems. However, strong type II superconductors allow penetration of the magnetic field over large distances, of order of the magnetic penetration depth λ. Superconductivity survives up to magnetic fields where, for imaging purposes, there is no magnetic contrast at all. Static and dynamic properties of vortices are largely unknown at such high magnetic fields. Reciprocal space studies using neutron scattering have been employed to obtain insight into the collective behavior. But the microscopic details of vortex arrangements and their motion remain difficult to obtain. Direct real space visualization can be made using scanning tunneling microscopy and spectroscopy (STM/S). Instead of using magnetic contrast, the electronic density of states describes spatial variations of the quasiparticle and pair wavefunction properties. These are of order of the superconducting coherence length ξ, which is much smaller than λ. In principle, individual vortices can be imaged using STM up to the upper critical field where vortex cores, of size ξ, overlap. In this review, we describe recent advances in vortex imaging made with scanning tunneling microscopy and spectroscopy. We introduce the technique and discuss vortex images which reveal the influence of the Fermi surface distribution of the superconducting gap on the internal structure of vortices, the collective behavior of the lattice in different materials and conditions, and the observation of vortex lattice melting. We consider challenging lines of work, which include imaging vortices in nanostructures, multiband and heavy fermion superconductors, single layers and van-der-Waals crystals, studying current driven dynamics and the liquid vortex phases.

Charge density wave in layeredLa1−xCexSb2

“…[35,36,[44][45][46][47][48][49]. Pinning of the CDW bears some similarities to pinning of the vortex lattice in superconductors [50][51][52], and is discussed using the Fukuyama-Lee-Rice theory [53,54]. As in the vortex lattice, there are two competing energies [35,55].…”

Nodeless multiband superconductivity in stoichiometric single-crystalline CaKFe4As4

“…This is a common observation in the effective two-gap Fe based superconductors (e.g., in BaK122 [24]). Different surface terminations lead to different matrix elements for the tunneling between the tip and the sample [31,32]. This, in turn, leads to spatially varying contributions to the density of states from different parts of the Fermi surface.…”

“…This, in turn, leads to spatially varying contributions to the density of states from different parts of the Fermi surface. When the gap is not uniquely defined, as in a simple s−wave BCS superconductor, the [23,31,33,34]. The lines in Fig.…”

Three axis vector magnet set-up for cryogenic scanning probe microscopy

“…6,7 Scanning probe microscopes are, however, very sensitive to mechanical vibrations and require a mechanically stable platform. [8][9][10][11][12][13] Thus, to control the direction of the magnetic field, rotating the microscope is not a good solution, and it is better to use superconducting three axis coil systems. Hess et al used a three axis magnet and a dilution refrigerator to study the tilted vortex lattice in superconductors in a pioneering experiment.…”