Unconventional superconductivity in different material classes is one of the most relevant issues in modern solid state physics. Since the microscopic theory of conductivity (Bardeen, Cooper, Schrieffer 1957) was introduced it is common knowledge that a pairing interaction of mobile charge carriers is mandatory for the formation of the superconducting state. For the vast majority of superconductors the pairing interaction is induced by the electron-phonon coupling mechanism. However, starting with the discovery of superconductivity in the heavy fermion compound CeCu2Si2, and later in organic charge transfer salts and the high temperature superconductors it became evident that the pairing interaction may well be induced by alternative mechanisms. To identify such mechanisms is the ultimate goal of our research in this field.

Heavy-Fermion Materials

Heavy fermion systems are intermetallic compounds containing non-complete 4f- or 5f-electronic shells. The orbital overlap with ligand atoms in the lattice leads to correlation effects in the the system of delocalized electrons. As a result, the effective mass of the electrons can increase by orders of magnitude as compared to the free electron mass. Heavy fermion materials exhibit very interesting ground states - such as unconventional superconductivity, small-moment band magnetism and non Fermi liquid behavior. We employ the technique of tunneling spectroscopy on planar tunnel junctions with one electrode being a heavy-fermion superconductor thin film. By using this technique the electronic density of states of the heavy-fermion material can be measured. This gives the most direct information on the superconducting state.

Organic Thin Films - Charge Transfer Salts

Embedded in the activities of the transregional collaborative research center 49 Condensed matter systems with variable many-body interactions we study the growth of single crystals and thin films of (new) organic charge transfer systems. Our research focuses on the class of neutral-ionic phase transition systems in which a change of the donor-acceptor charge transfer degree occurs at a critical temperature. This is associated with pronounced dielectric anomalies and a change of the charge transfer gap with an associated color change. We study the influence of electronic correlation effects on the charge transport in theses effectively one-dimensional materials.

Vortex guiding effects in nano-structured superconductors

Under the main responsibility of Dr. Oleksandr Dobrovolskiy and in collaboration with Prof. Valerij Shklovskij (Kharkov) we study single-vortex motion in epitaxial Nb thin films with periodic surface structures on the nanometer scale which provide symmetric or asymmetric pinning potentials for vortices in the mixed state. The periodic nanostructures are prepared by focused ion beam etching or Co-C deposition using focused electron beam induced deposition techniques. In particular, our aim is to analyze the dc- and ac-response of the vortex movement on external stimuli with a view to both, basic research on vortex dynamics and superconductor electronics of the fluxonics type.

On the national level, the project is funded by the DFG (Non-linear vortex dynamics in superconductors with anisotropic pinning nano structures). On the international level, our research team takes part in the Nanoscale Superconductivity COST Action MP1201 - Nanoscale Superconductivity: Novel Functionalities through Optimized Confinement of Condensate and Fields (NanoSC -COST).


geändert am 23. Mai 2014  E-Mail: Prof. Dr. Michael


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Druckversion: 23. Mai 2014, 08:54