Microscopic investigation of the flux dynamics of type-II superconducting films

Type-II superconducting Pb and Pb-In alloy films have been deposited on top of the two-dimensional electron gas (2DEG) in a GaAs/${\mathrm{Al}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ga}}_{\mathit{x}}$As heterostructure in a Hall bar geometry with 10 μm characteristic dimensions. The superconductor was sufficiently close to the 2DEG for the inhomogeneous field of the flux vortices to be projected almost intact through the electronic system below. Measurements of the Hall voltage and magnetoconductivity at the 2DEG in an applied magnetic field show abrupt correlated changes that reflect the motion of very small bundles of flux (typically 4–10 flux quanta). Consequently we have been able to study vortex-bundle nucleation, flux pinning, and vortex-antivortex annihilation on this microscopic scale. In the low-field region where the magnetoconductivity is expected to be linear in an applied field, a simple model has been used to calculate the density of vortices and antivortices present at a given applied field.