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

6 Introduction
Figure 1: Scanning electron micrographs of nanoelectromechanical systems.
Left: a suspended quantum dot cavity and Hall-bar formed in a 130 nm thin
GaAs/GaAlAs membrane. Right: a 4 µm long, 130 nm thick free-standing beam
that contains a fully tuneable low-dimensional electron system. Five equally suspended
Au electrodes can be used to operate the device as two-dimensional electron
gas, quantum point contact, single or double dot. Taken from [15] by kind
permission of E. Weig.
freedom can be understood as a novel compound spin angular-momentum qubit.
In part II, we consider a further qubit candidate, the two-level system which is
built by two coupled quantum dots. In such a device, even at zero temperatures,
the dephasing time is limited by the e-p interaction via coupling to low-energy
phonons as bosonic excitations of the environment. In chapter 5, we give an introduction
to e-p interaction. Since phonons are quantised lattice vibrations, the
acousto-mechanical properties of the nanostructure are expected to influence the
e-p interaction. Recently, the fabrication of nanomechanical semiconductor resonators
(tiny, freely suspended membranes, bars and strings) which contain a layer
of conducting electrons was achieved [13] (see also Fig. 1). The vibrational properties
of these nanostructures differ drastically from bulk material. For instance,
the phonon spectrum is split into several subbands, leading to quantisation effects
of e.g. the thermal conductance [14]. In chapter 6, we study the effects of phonon
confinement on the electron transport through two coupled quantum dots. We
show that typical peculiarities of confined phonon systems, like van–Hove singularities
in the phonon density of states, manifest themselves in the non-linear
electron transport, acting as clear fingerprints of phonon confinement. In addition,
the confinement is shown to be an excellent tool to control phonon induced
decoherence in double quantum dots. We demonstrate that the use of “phonon
cavities” enables one to either strongly suppress or drastically enhance phonon
induced dissipation in such systems.