Coherent and Correlated Spin Transport in Nanoscale Superconductors
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- Institutt for fysikk 
Motivated by the desire for better understanding of nanoelectronic systems, we theoretically study the conductance and noise characteristics of current flow between superconductors, ferromagnets, and normal-metals. Such nanostructures can reveal information about superconductor proximity effects, spin-relaxation processes, and spintronic effects with potential applications for different areas of mesoscopic physics. We employ the quasiclassical theory of superconductivity in the Keldysh formalism, and calculate the nonequilibrium transport of spin and charge using various approaches like the circuit theory of quantum transport and full counting statistics. For two of the studied structures, we have been able to compare our theory to experimental data and obtain good agreement. Transport and relaxation of spin polarized current in superconductors is governed by energy-dependent transport coefficients and spin-flip rates which are determined by quantum interference effects. We calculate the resulting temperature-dependent spin flow in ferromagnet-superconductor devices. Experimental data for spin accumulation and spin relaxation in a superconducting nanowire is in agreement with the theory, and allows for a spin-flip spectroscopy that determines the dominant mechanism for spin-flip relaxation in the studied samples. A ferromagnet precessing under resonance conditions can give rise to pure spin current injection into superconductors. We find that the absorbed spin current is measurable as a temperature dependent Gilbert damping, which we calculate and compare to experimental data. Crossed Andreev reflection denotes superconducting pairing of electrons flowing from different normal-metal or ferromagnet terminals into a superconductor. We calculate the nonlocal currents resulting from this process in competition with direct electron transport between the normal-metal terminals. We take dephasing into account, and study the nonlocal current when the types of contact in the system varies from e.g. ballistic conductors or tunnel barriers. In the tunneling case, we calculate the magnetization-dependent full counting statistics, which determines all noise properties including the cross-correlations that can resolve the contributions due to crossed Andreev reflection and direct electron transport. We evaluate the magnetization-dependent two-particle probability that the constituents of spin-entangled pairs from crossed Andreev reflection flow into different ferromagnetic contacts. This probability implies violation of a Bell inequality, and determines the performance of a superconductor-ferromagnet entangler.
Has partsMorten, Jan Petter; Brataas, Arne; Belzig, Wolfgang. Spin transport in diffusive superconductors. Phys. Rev. B. 70(21): 212508, 2004.
Morten, Jan Petter; Brataas, Arne; Belzig, Wolfgang. Spin transport and magnetoresistance in ferromagnet/superconductor/ferromagnet spin valves. Phys. Rev. B. 72: 014510, 2005.
Poli, Ninos; Morten, Jan Petter; Urech, Mattias; Brataas, Arne; Haviland, David B.; Korenivski, Vladislav. Spin Injection and Relaxation in a Mesoscopic Superconductor. Phys. Rev. Lett. 100: 136601, 2008.
Morten, Jan Petter; Brataas, Arne; Bauer, Gerrit; Belzig, Wolfgang; Tserkovnyak, Yaroslav. Proximity effect-assisted absorption of spin currents in superconductors. .
Morten, Jan Petter; Brataas, Arne; Belzig, Wolfgang. Circuit theory of crossed Andreev reflection. Phys. Rev. B. 74: 214510, 2006.
Morten, Jan Petter; Brataas, Arne; Belzig, Wolfgang. Circuit theory for crossed Andreev reflection and nonlocal conductance. Appl. Phys. A. 89(3): 609-612, 2007.
Morten, Jan Petter. Quantum physics and the boundaries of space and time. Complexity, 2008.
Morten, Jan Petter; Huertas-Hernando, Daniel; Belzig, Wolfgang; Brataas, Arne. Elementary charge transfer processes in a superconductor-ferromagnet entangler. Europhys. Lett. 81: 40002, 2008.
Morten, Jan Petter; Huertas-Hernando, Daniel; Belzig, Wolfgang; Brataas, Arne. Full counting statistics of crossed Andreev reflection. .