ORAL SESSION 5D7: Hybrid Systems

Friday, Feb. 25, 9:00 a.m. – 11:00 a.m., Arboretum 3-4 (Hyatt)

Chairs: A. Ignatiev (UH/TCSUH)

5D7.3 Nonvolatile Switchable Josephson Junctions with Application as Memory for Superconducting Electronics

Mark B. Johnson, Naval Research Laboratory, 4555 Overlook Av SW, Washington DC, 20375, USA

Presenting Author: M.B. Johnson

There is a history of three decades of interest in superconducting weak links that can be switched on and off, or whose parameters can be controlled over a continuous range. Our novel superconducting switch uses a hybrid materials system: a microstructured ferromagnetic film spans a superconducting bridge in a simple bilayer geometry. Locally strong magnetic fields at the edge of the ferromagnet "quench" superconductivity and create a weak link. The strength of the weak link can be varied between ON and OFF by controlling the in-plane magnetization of the ferromagnet. In micron sized, low temperature superconductor (Pb and Nb) prototypes, the critical current is suppressed by one to two orders of magnitude over a large range of reduced temperature. The observation of Shapiro steps demonstrates the ac Josephson effect in the quenched state. As a switchable, nonvolatile weak link, energy is required only to change device states, which are thereafter maintained in thermodynamic equilibrium. Applications as a controllable weak link and as a nonvolatile storage cell in a high density superconducting random access memory will be presented. Prospects for device fabrication using oxide superconductors will be discussed.

5D7.6 Transport in Mesoscopic Superconductor-Normal Metal Heterostructures

Frank K. Wilhelm, Quantum Transport Group, Department of Applied Physics, Delft University of Technology, P.O. Box 5046, 2600 GA Delft, The Netherlands

Presenting Author: F.K. Wilhelm

If a normal metal is put into good metallic contact with a superconductor, it can acquire superconducting properties. This proximity effect is mediated by Andreev reflections at the interface.

We will discuss charge transport of such proximity systems using the quasiclassical Green's function technique. We find several mechanisms:

· The supercurrent across an SNS-junction, which can be controlled and even reversed by an external voltage, turning it into a pi-contact.

· The dissipative diffusion current, which profits from the superconducting correlations and can be separately measured. This component displays several surprising features, most prominent the "reentrance'' to the normal state conductance at the Fermi edge and the "anomalous proximity effect'', the reduction of the conductance although the local conductivity is increased. Both phenomena reflect the spatial structure of the propagation of the superconducting correlations on mesoscopic scales.

· Tunneling into an induced minigap through a barrier attached to the system

The strength of the proximity effect and hence the change in conductance with respect to the normal state can be externally controlled by an applied phase difference. We discuss several physical realizations, in which these phenomena as well as their particular interplay give rise to new effects.