Research

One of the most attractive aspects of being a theoretical physicist is the enormous number of fascinating open problems one could study. Another one is the flexibility to actually study them. We are not bound by expensive lab equipment that can be used only for certain experiments. We are limited only by our own imagitation, or lack thereof. Members of our group are interested in various aspects of condensed matter theory:

Molecular electronics

Contact: C. Timm

Electronic transport through single molecules and molecular thin films is interesting both for possible applications and from a fundamental physics point of view. So far, we have mainly studied molecule-based transistors containing magnetic molecules. Here, the strong coupling between the tunneling electrons and the local spins in the molecule, e.g., in transition-metal ions, leads to characteristic signatures in the current-voltage characteristics. We have predicted new effects such as giant spin amplification: The total magnetic moment deposited in the electrodes due to spin-dependent tunneling through a magnetic molecule can be orders of magnitude larger than the molecular magnetic moment. We are working both on methodological progress in the description of transport far from equilibrium and on quantitative modeling of specific devices in collaboration with leading experimental groups. Our method of choice is the quantum master equation.

Pnictides

Contact: P. Brydon, C. Timm

The discovery of superconductivity in layered iron-oxygen-arsenic compounds in early 2008 has led to tremendous experimental and theoretical efforts to understand these compounds. By now, superconductivity has been observed in many related pnictides (i.e., compounds containing group-V anions), in several cases with transition temperatures above 50 K. This makes them the class with the second highest transition temperatures after the cuprates. Like in the cuprates, it is though that the superconductivity in doped compounds is intimately related to the magnetic order in the undoped parent compounds. However, the type of magnetic order is quite different from the cuprates. The pnictides are spin-density-wave metals, not antiferromagnetic Mott insulators. We are mainly interested in the spin-density-wave phase.

Topological superconductors

Contact: C. Timm, P. Brydon

Another hot topic that has recently come to the forefront of physics is topological insulators and superconductors. These compounds can be described in terms of weakly interacting quasiparticles, the single-particle Hamiltonian of which has non-trivial topological properties in reciprocal space. In simple cases they have an energy gap in the bulk but a gapless spectrum at their edge or surface. The gapless spectrum is very robust against disorder. The quantum Hall effect is an example but it has only recently been realized that the concepts are much more general. We are specifically interested in superconductors with non-trivial topological invariants. We study states at their surfaces, which include perfectly flat energy bands at the Fermi energy formed by Majorana particles. Majorana particles are fermions which are their own antiparticles.

Josephson junctions with unconventional superconductors

Contact: P. Brydon

The coherent tunneling of Cooper pairs between two superconductors separated by a tunnel barrier leads to the fascinating Josephson effects, e.g., to a alternating current for an applied static voltage. Josephson effects become even more interesting when the Cooper pairs in the superconductors in question have unconventional symmetries or if the barrier is magnetic. We consider the Josephson effect for two triplet superconductors, which have spin-1 Cooper pairs, separated by a ferromagnetic barrier. The three additional directions in the problem, given by the magnetization in the barrier and the triplet order parameters, lead to a rich variety of tunneling phenomena.

Diluted magnetic semiconductors

Contact: C. Timm

Diluted magnetic semiconductors are created by doping standard semiconducting compounds such as GaAs with magnetic ions like manganese. This introduces both carriers (holes in this case) and local magnetic moments. The magnetic interaction between the local moments is mediated by the carriers, leading to ferromagnetism with Curie temperatures in excess of 180 K in (Ga,Mn)As. These materials are promising for spintronics applications, in particular since other compounds have Curie temperatures above room temperature, and also interesting from the theorists point of view. They are difficult to understand, since they combine strong correlations with strong disorder. Important questions regarding the mechanism (or mechanisms) of ferromagnetic order and the nature of electronic states are still open. More, but not quite up-to-date, information can be found in the Heß lecture C. Timm has given at the University of Regensburg.