Spin-orbit coupling and electron-phonon scattering - Fachbereich ...

Abstract
iii
Abstract
It is the purpose of this work to study the interplay of interaction and confinement
in nanostructures using two examples.
In part I, we investigate the effects of spin-orbit interaction in parabolically
confined ballistic quantum wires and few-electron quantum dots. In general, spinorbit
interaction couples the spin of a particle to its orbital motion. In nanostructures,
the latter can easily be manipulated by means of confining potentials. In
the first part for this work, we answer the question how the spatial confinement
influences spectral and spin properties of electrons in nanostructures with substantial
spin-orbit coupling. The latter is assumed to originate from the structure
inversion asymmetry at an interface. Thus, the spin-orbit interaction is given by
the Rashba model.
For a quantum wire, we show that one-electron spectral and spin properties
are governed by a combined spin orbital-parity symmetry of wire. The breaking
of this spin parity by a perpendicular magnetic field leads to the emergence of a
significant energy splitting at k = 0 and hybridisation effects in the spin density.
Both effects are expected to be experimentally accessible by means of optical
or transport measurements. In general, the spin-orbit induced modifications of the
subband structure are very sensitive to weak magnetic fields. Because of magnetic
stray fields, this implies several consequences for future spintronic devices, which
depend on ferromagnetic leads.
For the spin-orbit interaction in a quantum dot, we derive a model, inspired by
an analogy with quantum optics. This model illuminates most clearly the dominant
features of spin-orbit coupling in quantum dots. The model is used to discuss
an experiment for observing coherent oscillations in a single quantum dot with
the oscillations driven by spin-orbit coupling. The oscillating degree of freedom
represents a novel, composite spin-angular momentum qubit.
In part II, the interplay of mechanical confinement and electron-phonon interaction
is investigated in the transport through two coupled quantum dots. Phonons
are quantised modes of lattice vibration. Geometrical confinement in nanomechanical
resonators strongly alters the properties of the phonon system. We study
a free-standing quantum well as a model for a nano-size planar phonon cavity. We
show that coupled quantum dots are a promising tool to detect phonon quantum
size effects in the electron transport. For particular values of the dot level splitting
∆, piezo-electric or deformation potential scattering is either drastically reduced
as compared to the bulk case, or strongly enhanced due to van–Hove singularities
in the phonon density of states. By tuning ∆ via gate voltages, one can either control
dephasing in double quantum dot qubit systems, or strongly increase emission
of phonon modes with characteristic angular distributions.