project title: Quasiclassical Descrition of Coherent Quantum System
project leader: Prov. Doz. Matthias Eschrig
access given (in days): 31
access used (in days): 9
local host: Dr. Juha Kopu
home institution: Institut für Theoretische Festkörperphysik, Universität Karlsruhe
country of institution: Germany
starting date (yyyy-mm-dd):
Dr. Eschrig is a junior scientist from University of Karlsruhe. He is interested in the studies on the equilibrium supercurrent in a heterostructure containing a piece of strongly ferromagnetic material sandwiched between two singlet superconductors (an SFS structure). Supercurrent flow in such a system can arise because of the proximity-induced pair correlations in the ferromagnet. However, because of the exchange splitting between the two spin bands, conventional spin-singlet proximity effect is confined to a very thin layer (thickness of the order of interatomic distances) inside the ferromagnet. In particular, if the ferromagnet is fully polarized (only one spin band metallic and the other insulating), a half metal, the spin-singlet proximity effect should be entirely absent. However, even in this case there remains the possibility of supercurrent being mediated by (equal-spin) triplet correlations in the ferromagnet, if a mechanism for singlet-triplet conversion exists. Such a situation has been theoretically studied in the dirty limit [Bergeret et al., PRL 86, 4096 (2001)], as well as in the clean limit [Eschrig et al., PRL 90, 137003 (2003)], the latter work dealing especially with a half-metallic ferromagnet. Interestingly, the symmetries of the relevant triplet correlations in these two cases are entirely different: in the dirty limit they are even in the momentum space, and odd in energy, whereas in the clean limit the dominating correlations are odd in momentum and even in energy (these being the two possibilities allowed by the Pauli principle). Dr. Eschrig aims to investigate, together with Dr Juha Kopu from the LTL, more thoroughly the role of these two classes of triplet correlations at arbitrary impurity density - in the intermediate regime between the clean and dirty limit. Their collaborative work is motivated by the recent experimental discovery of supercurrent in a superconductor-half metal-superconductor structure [R. S. Keizer et al., Nature 439, 825 (2006)].
During the reporting period this project was completed. A manuscript about the proximity effect was prepared and published [26].