Singlet and triplet supercurrents in disordered mesoscopic systems

Singlet and triplet supercurrents in disordered mesoscopic systems

Keizer, R.S.

Klapwijk, T.M. (promotor)

Applied Sciences

2007-06-13

This thesis describes a series of experiments and a theoretical study in order to understand and control the behavior of electrons in many-body systems. In particular, the experiments concentrate on competition between the - antagonistic - electronic states of superconductivity and ferromagnetism, and are strongly motivated by the recent proposal of a new type of superconductivity which appears to defy the Pauli principle, if defined in the usual sense. As the central ingredient of our experiments, we use high-quality thin films of half-metallic Chromiumdioxide (or CrO2), best known for its large scale application in the magnetic recording industry, with an experimentally verified close to 100% spin-polarization. We start the experiments with a magnetotransport study of the magnetic properties of these films, and find a Planar Hall effect which demonstrates that the films, which are ordinarily supposed to have a single easy axis in the plane of the film, instead possess a biaxial magnetic anisotropy, which is analyzed in detail. We then contact the CrO2 films to a conventional (s-wave singlet) superconductor, using microfabrication techniques, and show that superconducting correlations originating in the superconductor, enter the CrO2 and persist over distances incompatible with spin singlet and ballistic transport, thereby providing strong evidence for the presence of s-wave triplet correlations. Moreover, from an application point of view, we show that the superconductor-half-metal-superconductor devices we have used to measure this effect are superconducting transistors that can be switched "on" and "off" with the magnetic field. The theoretical part focusses on the influence of voltages, rather than currents or temperature, on the collapse of the superconducting state in a short mesoscopic superconducting wire between normal metal contacts. It is shown that the collapse of the superconducting state is driven by the applied voltage, and not by the resulting current

superconductivity

http://resolver.tudelft.nl/uuid:2533256c-507d-4222-a289-e3cd263a9f60

978-90-8593-028-0

Institutional Repository

doctoral thesis

(c) 2007 R.S. Keizer