1. Ultrafast excitations and strongly driven systems
2. Spin-density excitations
3. Transport in diluted magnetic semiconductors

1. Ultrafast excitations and strongly driven systems
We study the intersubband dynamics in semiconductor quantum wells:
The electrons in a doped quantum well couple to electromagnetic radiation through a collective excitation, the so-called intersubband plasmon, whose frequency typically lies in the terahertz range. We study the properties of these plasmons, with an emphasis on nonlinear properties and dissipation.

For example,  in a strongly driven quantum well, the photoabsorption peak changes its shape, and a strong intensities, a bistability develops. We have shown that this intersubband bistability can be optically controlled: using short terahertz control pulses, the state of the system can be rapidly switched, within a few picoseconds. This effect may find applications for modulators or optical switches. [H.O. Wijewardane and C.A. Ullrich, Appl. Phys. Lett. 84, 3984 (2004)]

On the more fundamental side, we have done some recent work that is concerned with nonadiabatic effects in the exchange-correlation potential of TDDFT. We use quantum wells as simple model systems to study the interplay of elastic and dissipative behavior of the electron liquid.
[H.O. Wijewardane and C.A. Ullrich, Phys. Rev. Lett. 95, 086401 (2005); C.A. Ullrich and I.V. Tokatly, Phys. Rev. B (2006)].


2. Spin-density excitations
If we include the spin degree of freedom, we see that there are actually two kinds of collective modes in a quantum well: the charge and the spin plasmon, where the spin-up and spin-down densities move in and out of phase.


Spin plasmons are of interest, since they offer the opportunity to study fundamental concepts of electron spin dynamics that are important for the field of spintronics. In particular, we study how the spin plasmon is affected by spin-orbit coupling, and by the spin Coulomb drag effect.

In spintronics and quantum computing, one is interested in the characteristic times that determine the decay of a majority spin population, and the dephasing of a transverse spin excitation. It is known that spin-orbit coupling in semiconductors dominates both lifetimes (through the so-called D'yakonov-Perel' mechanism).
In the case of intersubband spin plasmons,  one can define similar lifetimes:
However, the physics which govers the intersubband lifetimes is now much different. It turns out that spin-orbit coupling effects do not lead to dephasing of intersubband spin plasmons at all! The reason is a compensation through collective many-body effects.
[C.A. Ullrich and M.E. Flatte, Phys. Rev. B 66, 205305 (2002) and Phys. Rev. B 68, 235310 (2003)]

Instead, electronic many-body effects cause dissipation in spin dynamics an a different way, namely through the spin Coulomb drag effect. This is an intrinsic effect, which cannot be avoided even in a perfectly clean device, and happens whenever electrons of opposite spin move with different velocity. We have recently proposed an experiment which would measure the spin Coulomb drag effect through the plasmon linewidth of a parabolic quantum well.
[I. D'Amico and C.A. Ullrich, Phys. Rev. B 74, 121303(R) (2006)]

3. Transport in diluted magnetic semiconductors
In this project, we consider charge and spin transport in bulk semiconductors in the presence of magnetic impurities, such as (Ga,Mn)As. Such materials are the basis for many proposals to construct spintronics devices such as diodes and transistors. We have developed an approach which allows us to calculate conductivities and dielectric functions of dilute magnetic semiconductors even when the system is strongly disordered.
[F. V. Kyrychenko and C.A. Ullrich, cond-mat/0607177]