Andreev reflection at half-metal-superconductor interfaces with non-uniform magnetization

Andreev reflection at the interface between a half-metallic ferromagnet and a spin-singlet superconductor is possible only if it is accompanied by a spin flip. Here we calculate the Andreev reflection amplitudes for the case that the spin flip originates from a spatially non-uniform magnetization direction in the half metal. We calculate both the microscopic Andreev reflection amplitude for a single reflection event and an effective Andreev reflection amplitude describing the effect of multiple Andreev reflections in a ballistic thin film geometry. It is shown that the angle and energy dependence of the Andreev reflection amplitude strongly depends on the orientation of the gradient of the magnetization with respect to the interface. Establishing a connection between the scattering approach employed here and earlier work that employs the quasiclassical formalism, we connect the symmetry properties of the Andreev reflection amplitudes to the symmetry properties of the anomalous Green function in the half metal.

Role of interactions in time-of-flight expansion of atomic clouds from optical lattices

We calculate the effect of interactions on the expansion of ultracold atoms from a single site of an optical lattice. We use these results to predict how interactions influence the interference pattern observed in a time of flight experiment. We find that for typical interaction strengths their influence is negligible, yet that they reduce visibility near a scattering resonance.

Enhanced triplet Andreev reflection off a domain wall in a lateral geometry

We find that the triplet Andreev reflection amplitude at the interface between a half-metal and an s-wave superconductor in the presence of a domain wall is significantly enhanced if the half metal is a thin film, rather than an extended magnet. The enhancement is by a factor $l_{rm d}/d$, where $l_{rm d}$ is the width of the domain wall and $d$ the film thickness. We conclude that in a lateral geometry, domain walls can be an effective source of the triplet proximity effect.

Temperature and magnetic-field dependence of the quantum corrections to the conductance of a network of quantum dots

We calculate the magnetic-field and temperature dependence of all quantum corrections to the ensemble-averaged conductance of a network of quantum dots. We consider the limit that the dimensionless conductance of the network is large, so that the quantum corrections are small in comparison to the leading, classical contribution to the conductance. For a quantum dot network the conductance and its quantum corrections can be expressed solely in terms of the conductances and form factors of the contacts and the capacitances of the quantum dots. In particular, we calculate the temperature dependence of the weak localization correction and show that it is described by an effective dephasing rate proportional to temperature.

Interaction correction to the conductance of a ballistic conductor

In disordered metals, electron-electron interactions are the origin of a small correction to the conductivity, the Altshuler-Aronov correction. Here we investigate the Altshuler-Aronov correction of a conductor in which the electron motion is ballistic and chaotic. We consider the case of a double quantum dot, which is the simplest example of a ballistic conductor in which the Altshuler-Aronov correction is nonzero. The fact that the electron motion is ballistic leads to an exponential suppression of the correction if the Ehrenfest time is larger than the mean dwell time or the inverse temperature.