Vidar Gudmundsson,
Science Institute, University of Iceland,
Dunhaga 3, IS107 Reykjavik, Iceland
As a basis for the first two projects we designed a numerical model of a twodimensional electron gas (2DEG) with or without an external magnetic field. The 2DEG is confined by a general confinement potential that does allow us to consider quantum rings, dots with various geometries, or short quantum wires. The numerical model is implemented using a gridfree approach to DFT theory to describe the Coulomb interaction between several electrons [1]. The use of a functional basis instead of a spatial grid results in compact matrices which are ideal for a parallel calculation where the construction of the CPUintensive elements is distributed between the nodes of a cluster.
In addition to the evaluation of the ground state properties we extended the model to follow the time evolution of a system after an initial short radiation pulse in the TeraHertz regime is used to excite it. The direct integration of the time evolution operator for the system within a gridfree DFT formalism allows us to subject it to strong external perturbation and observe nonlinear effects.
The first project centered on the effects of a short TeraHertz pulse on the persistent current in a 2DEG in a quantum ring.


The second project did focus on the effects of an impurity on the FIRspectroscopy of a short quantum wire.

The third project is a longterm project with the aim to describe a transport through the timedependent nanosystem. We have already taken the first steps to investigate a transport through a quantum wire in a magnetic field. In the middle of the wire is a simple scattering potential, that will later be replaced by the potential of the nanosystem of interest, while the wire plays the role of the contacts.
We are now building the description of the static scattering using the LippmannSchwinger equation together with the Greens function of the wire in the magnetic field. There are still several steps that we have to formulate carefully in the problem. Most likely we will also allow this model and research to branch off in several directions reflecting our interests and connection to interesting experiments.Generally, the time at the NCTS has been very valuable to me. I have had the time to explore a new formalism with respect to a gridfree implementation of DFTtheory and apply it to a confined 2DEG under strong time dependent excitation. I have had time to learn FORTRAN 95, and most important of all, I have had very good discussions and cooperation with Prof. ChiShung Tang on interesting new physical phenomena.
The office staff of the center has been extremely helpful to the whole family in organizing our environment to perfection, and we are deeply touched by the open Taiwanese culture that has accepted us wherever we have ventured during these 11 excellent months we will never forget.