Decoherence effects in nonequilibrium transport through mesoscopic systems

Nonequilibrium electron transport through mesoscopic systems has recently attracted a great amount of attention in both experimental and theoretical physics. Of particular interest are systems where the spin degree of freedom of the electron plays an important role because their investigation might lead to the development of a new class of electronic devices in the future. Relaxation and decoherence of the electron spin, which result from the coupling of the spin to its environment, are inevitable. A thorough understanding of their origin and their impact on transport properties of the system is not only of crucial importance for technological applications, but also a very interesting topic in fundamental research. In this thesis, diagrammatic methods for nonequilibrium transport are developed and applied to two different model systems, and the results are analysed with a special focus on the role of relaxation and decoherence rates. First, transport through a quantum wire which is coupled to a ferromagnetic spin chain is investigated using a calculation within the Keldysh Green's function formalism. The electrons in the wire are coupled to spin waves, the so-called magnons, in the spin chain. The self-energy contribution which is due to this coupling is calculated in self-consistent Born approximation. The differential conductance is calculated for different setups, and the states which contribute to transport are identified. One would expect that the electron-magnon coupling leads to relaxation and decoherence of the spin. However, it is found that the coupling can lead to the formation of the magnetic polaron, a state which stands out due to its long life time and which can clearly be identified in the nonlinear differential conductance. Second, analytical renormalisation group (RG) approaches are developed to study nonequilibrium transport through a quantum dot in the Kondo regime. Interactions within the quantum dot, which are essential for the occurrence of the Kondo effect, are taken into account exactly, and a perturbative expansion is performed in the renormalised coupling between the quantum dot and the leads. In contrast to other perturbative RG methods which have been applied to the Kondo model in nonequilibrium, the approach used here ensures that relaxation and decoherence rates emerge during the RG flow even in the leading-order RG equations. The Real Time RG in time space is used to calculate the nonequilibrium differential conductance for a single molecular magnet in a magnetic field, which can be mapped to a fully anisotropic Kondo model. It turns out that varying the anisotropy constants of the molecular magnet can induce a quantum phase transition between a Kondo effect, which manifests itself in a resonance in the differential conductance, and a situation where only a step that is due to inelastic cotunnelling occurs in the conductance. The Real Time RG in time space is also applied to the calculation of finite frequency noise for transport in the isotropic Kondo model. The renormalisation of the frequency-dependent couplings leads to the evolution of a dip in the noise. The line shape in the vicinity of the dip is sensitive to the decoherence rate. Finally, the Real Time RG in frequency space is applied to the anisotropic Kondo model. It has several advantages compared to the Real Time RG in time space, in particular that the RG flow is always cut off by relaxation and decoherence rates, no matter up to which order in the coupling the RG equations are expanded. Results for the renormalised g-factor, the relaxation and decoherence rates, the magnetic susceptibility, and the differential conductance areobtained, including all terms in leading order and logarithmically enhanced terms in next-to-leading order. In particular, the precise line shape at resonance and for small magnetic fields, where logarithmic enhancements or suppressions are found, its dependence on the relaxation and decoherence rates, and the influence of the anisotropy of the couplings is discussed.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2009

Author dc:creator

• Reininghaus, Frank

Contributors dc:contributor

• Schoeller, Herbert

• info:eu-repo/classification/ddc/530
• Nichtgleichgewicht
• Renormierungsgruppe
• Kondo-Modell
• Physik
• Dekohärenz
• magnetisches Polaron
• nonequilibrium
• decoherence
• renormalisation group
• Kondo effect
• magnetic polaron

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-113592%22

OAI record oai:identifier

https://publications.rwth-aachen.de/oai2d?verb=GetRecord&metadataPrefix=oai_dc&identifier=oai%3Apublications.rwth-aachen.de%3A51285