Quantum Transport in Hybrid Structures
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- Institutt for fysikk 
This thesis presents the findings of four research papers on two topics in condensed-matter physics. Namely: i) The interplay between superconductivity and magnetism in Josephson junctions, and ii) current-driven domain wall motion. First and foremost, this thesis is devoted to the theoretical study of hybrid structures from a fundamental physics point of view. Three of the papers belong to the first category, while the last paper belongs to the latter. The interplay between the dissipationless currents offered by superconductors and the spin-polarization from ferromagnets gives rise to exciting new phenomena which show great potential for use in low-temperature nanoelectronics. We study Josephson junctions where the weak link shows some magnetic behavior. In paper I we consider the possibility of ferromagnetism originating from a mass renormalization of carriers with opposite spin, i.e., a spin bandwidth asymmetry, and compare with the standard Stoner model. We find that junctions with spin bandwidth asymmetry yield a larger number of 0 − π- transitions than junctions with Stoner ferromagnetism. In paper II we study a Josephson junction where the weak link is a half-metal, i.e., a completely polarized ferromagnet, and the interfaces are spin-active. We compare s- and d-wave pairing and find that the temperature dependence of dxy-wave junctions differs qualitatively from that of s- and dx2−y2-wave junctions. Moreover, we find that we can control the groundstate phase of the junction by controlling the magnetic misalignment at the interfaces. In paper III we consider an antiferromagnetic Josephson junction and find that, in contrast to previous results, the even-odd effect can only be observed above a threshold value of the staggered order parameter. Below this value, the current decays monotonically with increasing junction widths. We also explain the physics behind this effect in terms of the phase-shifts picked up by the quasiparticles in the antiferromagnetic region. The concept of spin-transport in magnetic structures is important both in applications and fundamental physics. Controllable domain wall motion via spin-transfer torque in textured ferromagnets may be used to represent information. One aspect that must be addressed is that the required current density to induce high-speed domain wall motion must be lowered as much as possible to prevent Joule heating. In paper IV we solve the Landau- Lifshitz-Gilbert-equations with a newly proposed torque term and show that it reduces the required current density with up to three orders of magnitude.