Is the `Finite Bias Anomaly' in planar GaAs-Superconductor junctons caused by point-contact like structures?

We correlate transmission electron microscope (TEM) pictures of superconducting In contacts to an AlGaAs/GaAs heterojunction with differential conductance spectroscopy performed on the same heterojunction. Metals deposited onto a (100) AlGaAs/GaAs heterostructure do not form planar contacts but, during thermal annealing, grow down into the heterostructure along crystallographic planes in pyramid-like `point contacts'. Random surface nucleation and growth gives rise to a different interface transmission for each superconducting point contact. Samples annealed for different times, and therefore having different contact geometry, show variations in $dI/dV$ characteristic of ballistic transport of Cooper pairs, wave interference between different point emitters, and different types of weak localization corrections to Giaever tunneling. We give a possible mechanism whereby the `finite bias anomaly' of Poirier et al. (Phys. Rev. Lett., {\bf 79}, 2105 (1997)), also observed in these samples, can arise by adding the conductance of independent superconducting point emitters in parallel

Magneto-optical Studies of Spin Phenomena in CdMnTe Doped with Co and Cr

We investigate the magneto-optical and magnetic properties of two quaternary diluted magnetic semiconductor alloys, Cd1&#8722;x&#8722;yMnxCryTe and Cd1&#8722;x&#8722;yMnxCoyTe, with fixed Mn concentration x ??? 0.37 and, respectively, with concentrations of Cr in the range 0 < y < 0.07 and Co in the range&#9251;0 < y < 0.009. The introduction of Cr and Co leads to very different behaviors, including the occurrence of ferromagnetic order in the case of Cd1&#8722;x&#8722;yMnxCryTe. We discuss the possible origins leading to the observed behaviors.ope