Quasiparticle excitations and ballistic transport in the mixed state of mesoscopic superconductors

As the size of the superconducting sample with a few fluxoids is less than the dephasing length new physics comes into play. The quasiparticle excitations in vortices form coherent quantum-mechanical states providing thus a possibility to control the phase-coherent transport through the sample by changing the number of fluxoids and their configuration. Thus, mesoscopic samples with a few vortices realize a new type of magnetically tunable Andreev waveguides. The sample conductance measured in the direction of the applied magnetic field is determined by the transparency of different multivortex configurations (giant multiquanta vortices and vortex molecules) which form a set of quantum channels. The transmission coefficient for each channel is controlled by multiple Andreev reflections within the vortex cores and at the sample edge. These interference processes result in a stepwise and/or oscillating behavior of the conductance as a function of the applied magnetic field. This is a vortex-based switch with the magnetic field playing the role of the gate voltage.