V. A. Schweigert

Non-quantized penetration of magnetic field in the vortex state of superconductors

As first pointed out by Bardeen and Ginzburg in the early sixties, the amount of magnetic flux carried by vortices in superconducting materials depends on their distance from the sample edge, and can be smaller than one flux quantum, φ0 = h/2e (where h is Plancks constant and e is the electronic charge). In bulk superconductors, this reduction of flux becomes negligible at sub-micrometre distances from the edge, but in thin films the effect may survive much farther into the material. But the effect has not been observed experimentally, and it is often assumed that magnetic field enters type II superconductors in units of φ0. Here we measure the amount of flux introduced by individual vortices in a superconducting film, finding that the flux always differs substantially from φ0. We have observed vortices that carry as little as 0.001φ0, as well as ‘negative vortices’, whose penetration leads to the expulsion of magnetic field. We distinguish two phenomena responsible for non-quantized flux penetration: the finite-size effect and a nonlinear screening of the magnetic field due to the presence of a surface barrier. The latter effect has not been considered previously, but is likely to cause non-quantized penetration in most cases.

Magnetic field dependence of the exciton energy in a quantum disk

The groundstate energy and binding energy of an exciton, confined in a^M quantum disk, are calculated as a function of an external magnetic field. The confinement potential is a hard wall of finite height. The diamagnetic shift is investigated for magnetic fields up to 40

Influence of the confinement geometry on surface superconductivity

T

Flux Penetration and Expulsion in Thin Superconducting Disks

. Our results are applied to

Saddle-point states and energy barriers for vortex entrance and exit in superconducting disks and rings

In_{y}Al_{1-y}As/Al_{x}Ga_{1-x}As

Vortex states in superconducting rings

self-assembled quantum dots and very good agreement with experiments is obtained. Furthermore, we investigated the influence of the dot size on the diamagnetic shift by changing the disk radius. The exciton excited states are found as a function of the magnetic field. The relative angular momentum is not a quantum number and changes with the magnetic field strength.

Off-center D − centers in a quantum well in the presence of a perpendicular magnetic field: Angular-momentum transitions and magnetic evaporation

The nucleation field for surface superconductivity,

Classical Double-Layer Atoms: Artificial Molecules

H_{c3}

Hysteresis in mesoscopic superconducting disks: The Bean-Livingston barrier

, depends on the geometrical shape of the mesoscopic superconducting sample and is substantially enhanced with decreasing sample size. As an example we studied circular, square, triangular and wedge shaped disks. For the wedge the nucleation field diverges as

Vortex matter in mesoscopic superconducting disks and rings

H_{c3}/H_{c2}=\sqrt{3}/\alpha