Proximity effects and Josephson currents in ferromagnet : spin-triplet superconductors junctions

by Damien Terrade

Institution: University of Stuttgart
Department: Fakultät Mathematik und Physik
Degree: PhD
Year: 2015
Record ID: 1107408
Full text PDF: http://elib.uni-stuttgart.de/opus/volltexte/2015/9994/


Spin-triplet superconductivity, first attached to the description of He3, is now generally considered to also occur in heavy-fermions compounds and in perovskite ruthenium oxide Sr2RuO4. The latter material is especially interesting since many experiments show strong evidences for a unitary chiral spin-triplet state. Moreover, the recent fabrication of thin heterostructures made of ferromagnetic SrRuO3 on the top of Sr2RuO4 strongly encourages new theoretical studies on the interplay between spin-triplet superconductor and ferromagnet in similar fashion to spin-singlet superconductors. Using an extended tight-binding Hamiltonian to model the superconductor, we discuss in this thesis the specific proximity effects of such interface by solving self-consistently the Bogoliubov-De Gennes equations on two- and three-dimensional lattices in the ballistic limit. We obtain the spatial profile of the superconducting order parameters at the interface as well as the spin-polarisation and the current across the Josephson junctions. In contrast to heterostructures made of spin-singlet superconductor, we show that the physical properties at the interface are not only controlled by the strength of the magnetization inside the ferromagnet but also by its orientation due to the existence of a finite pair spin projection of the spin-triplet Cooper pairs. We analyse in the first part the spin-polarisation and the Gibbs free energy at the three-dimensional ferromagnet-chiral spin-triplet superconductor interface. Then, the second part of the thesis is dedicated to the study of the Josephson junctions made of a chiral spin-triplet superconductor and a ferromagnetic barrier. More precisely, we analyse the existence of 0-Pi state transitions in two- and three-dimensional junctions with respect to the strength and the orientation of the magnetization. Finally, we study the proximity effects at the interface of helical spin-triplet superconductors. They differ from the chiral superconductor by the direction of the pair spin polarisation of the Cooper pairs and by the properties of the edge states, present at the boundaries, which can sustain dissipationless spin-current. Spin-Triplett Supraleitung, welche zuerst in flüssigem He3 beobachtet wurde, wird inzwischen auch den Schwere-Fermionen-Systemen und dem Perowskit Rutheniumoxid Sr2RuO4 zugeschrieben. Rutheniumoxid ist dabei von besonderem Interesse, da viele Experimente auf einen unitären, chiralen Spin-Triplett Zustand hinweisen. Zudem ist es seit kurzer Zeit möglich, dünne Heterostrukturen mit ferromagnetischem SrRuO3 auf einem Substrat von Sr2RuO4 herzustellen. Damit wird eine Untersuchung der Grenzflächeneffekte zwischen Spin-Triplett Supraleiter und Ferromagnet möglich, ganz ähnlich wie für den bekannten Fall der Spin-Singlett Supraleiter. In dieser Dissertation diskutieren wir theoretisch den spezifischen Nahfeldeffekt an einer solchen Grenzfläche. Dabei verwenden wir einen erweiterten tight-binding Hamiltonian für den Supraleiter und lösen die Bogoliubov-De Gennes Gleichungen in…