Fields of interests
Electronic Phase Transitions
Superconducting Thin-Film Nanostructures
Materials |
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Electronic Phase Transitions |
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Phase transitions in conventional and unconventional superconductors
In conventional type-II superconductors that are in the superconducting state and exposed to an external magnetic
field H between the lower und the upper critical fields Hc1 and Hc2,
respectively, magnetic-flux quanta form in most cases a regular hexagonal lattice that resembles an ordinary crystal
lattice in a solid.
In copper-oxygen high-temperature superconductors this lattice can liquefy already below Hc2, to
form a fluid-like phase. This first-order melting transition Hm is a further phase transition,
while Hc2 is not well defined in these materials.
We are studying the thermodynamics of the magnetic-phase diagrams in various systems of conventional and unconventional
superconductors to understand the influence of microscopic material parameters on the different vortex phases and the
phase transitions between them.
A research program supported by SNF.
In collaboration with G. Ravikumar, Bhabha Atomic Research Center, Mumbay, India and
H. Berger, Ecole Polytechnique Fédérale de Lausanne, Switzerland, and
P. Canfield, Iowa State University, United States, and
Th. Wolf, Forschungszentrum Karlsruhe, Germany.
Bose-Einstein condensation in quantum spin systems.
Quantum spin systems have become a subject of intense research, both theoretically and experimentally. A number of such systems
show magnetic-field induced phase transitions at zero temperature that have been interpreted as a Bose-Einstein condensation (BEC)
of dilute magnons. A Bose-Einstein condensate corresponds to a state with a macroscopic occupation of a single coherent quantum
mechanical state. Already known Bose-Einstein condensates include dilute atomic gas clouds at low temperatures and superfluid
4He. While coherence on a macroscopic scale has been demonstrated in these well established systems (e.g., by
interference experiments), it has not yet been observed in quantum magnets such as TlCuCl3. In our research group we
have implemented a number of experimental approaches to the problem of finding a direct evidence for a macroscopic coherent state
in a quantum magnet.
A research program supported by SNF. In collaboration with K. Krämer, University of
Bern, Switzerland.
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Superconducting Thin-Film-Nanostructures |
“Seeing Light with Superconductors”,
a report written for laypersons (in German) |
Fast Superconducting Single-Photon Detectors
In many areas of research and technology progress depends on fast and
sensitive detectors for electromagnetic radiation that can detect single
photons over a wide spectral range. Nanometer-sized superconducting meanders
are very promising detectors for many future applications such as quantum communication and encryption.
In the last few years we have obtained the expertise to fabricate the necessary
high-quality nanostructures at the FIRST Center for Micro- and Nanoscience at ETH Zürich.
We are focusing on the material properties of the underlying superconducting thin films and on the question
how these properties, together with finite-size effects influence the performance of the detectors.
One of our long-term goals is to extend the spectral sensitivity from the visible and
near-infrared range to longer and/or shorter wavelengths.
In collaboration with Dr.H.-W. Hübers, Institute for Planetary Research, German Aerospace Center (DLR),
Berlin, Germany and Prof. M. Siegel, Institute for Micro- und Nanoelectronic
Systems, University of Karlsruhe, Germany. |
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Materials |
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Metallic transition-metal oxides
We are interested in the question why some transition-metal oxides
that have similar crystallographic features as the well known copper-oxygen
high-temperature superconductors, have nevertheless very different electronic
properties and do not show any signs of superconductivity.
We have therefore established a small preparation lab for synthesising oxides by
powder metallurgy, using mainly wet chemical synthesis routes. We characterise
the samples using X-ray diffraction, magnetic and transport measurements.
In collaboration with
J. Karpinski, ETH Zürich, Switzerland.
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