الفهرس 
1 INTRODUCTIONOnly 14 pages are availabe for public view
 |  | from | 108 |  |  |
| | | from | 108 | | |
Abstract
Recent advance of nanotechnology has stimulated much interest in the study
of quantum transport in mesoscopic structures. The present thesis is divided
into two parts. In the first part of the thesis, the spin current through a
quantum dot system is calculated using a quantum master equation approach
in the weak-coupling regime. To be able to efficiently calculate, also at low
temperatures, the time evolution of the reduced density matrix in the present
approach which includes a sum over Matsubara terms, a high-temperature
approximation was derived which proves to be rather accurate in comparison
to the exact results. In the present model it is assumed that the energy levels
of the dot are split by a constant magnetic field. An additional external (laser)
field is used to control the currents of the two spin polarizations. This is
either done using the phenomenon of coherent destruction of tunneling or
optimal control theory. Scenarios are studied in which the spin current is
reversed while the charge current is kept constant.
The aim of the second part is to study the quantum transport properties of
a mesoscopic device in the presence of an external microwave field. A model
for such mesoscopic device is proposed and it is formed of a superconductor
quantum dot coupled to two ferromagnetic reservoirs via two quantum point
contacts.
An expression for the conductance was derived using Landauer-Büttiker
i
formula. The effect of an external magnetic field was taken into consideration.
Also, the spin polarization is expressed in terms of both Andreev-reflection
probabilities for spin-up and spin-down. Numerical calculations are performed
for the present proposed nanoscale device. This device operates in the
mesoscopic regime as indicated from the dependence of the conductance on
the temperature. from the results, two peaks appeared due to the Zeeman
splitting of the quasiparticle density of states. The dependence of spin
polarization on the considered parameters confirms that the spin flip of
electrons when Andreev-reflection tunneling occurs through the junction. The
spin polarization of the tunneled electrons through the junction gives rise
to a non-equilibrium spin density in the superconductor and also due to
Zeeman splitting of the quasiparticle density of states. Both equations for
the conductance and spin polarization show a dependence on the magnetic
field, the geometrical dimensions of the device, temperature, bias voltage and
charging energy of the quantum dot.
Supervisors
Prof. Mohamed Abd Allah Semary........................Prof. Bernhard Kramer
Signuture................................................................Signature
Prof. Adel Helmy Phillips......................................Dr. Ayman Saleh Atallah
Signature................................................................Signature
Prof. Gamal Abd El Nasser
Signature:
Chairman of physics department