Observation of quantum vortices and supercurrent distribution by scanning SQUID microscope

Evgeni V. Il’ichev, Hans E. Hoenig, H.‐G. Meyer +5 more

V. M. Kaurov

Mark Sheldon Wistrom

This thesis presents the direct observations of magnetic flux vortices in superconducting weakly coupled arrays. Real space images of vortices were obtained for various values of f, the fractional flux per unit cell in the array, and , the strength of the coupling between the islands of the superconducting array. Images and results were obtained for a single vortex in a triangular array, demonstrating the effect of changing , which increased the penetration depth of the magnetic field of the vortex in the sample. The ground state of a triangular array is measured and discussed as a function of f with emphasis for f = 1/4, 1/3 where there are several degenerate ground states corresponding to different vortex patterns. Annealing the arrays at a particular f is also discussed. In particular, for a square array at f = 1/2, the domain walls are the dominant defects, and they move through the sample as the temperature increases resulting in a single domain. The instrument used to perform the experiments, a Scanning SQUID Microscope (SSM), is discussed. The vortices in the superconducting arrays have weak magnetic fields that change over small length scales, requiring certain techniques for measurement. Also presented are the numerical calculations relating the magnetic field output from the SSM to the underlying phases of the array.

Andrey F. Vilesov

Nils Schopohl

Jun‐Yi Ge, V. N. Gladilin, Jacques Tempere +2 more

Quantized vortices, as topological defects, play an important role in both physics and technological applications of superconductors. Normally, the nucleation of vortices requires the presence of a high magnetic field or current density, which allow the vortices to enter from the sample boundaries. At the same time, the controllable generation of individual vortices inside a superconductor is still challenging. Here, we report the controllable creation of single quantum vortices and antivortices at any desirable position inside a superconductor. We exploit the local heating effect of a scanning tunneling microscope (STM) tip: superconductivity is locally suppressed by the tip and vortex-antivortex pairs are generated when supercurrent flows around the hot spot. The experimental results are well-explained by theoretical simulations within the Ginzburg-Landau approach.