Institute of Theoretical Physics University of Cologne
Zülpicher Str. 77
D-50937 Cologne
Germany
Phone: +49-(221)-470 4994
Fax: +49-(221)-470-2189
Room: 211
email: rosch _@_ thp.uni-koeln.de
Bachelor-, Masters-, Diplom- und Doktorarbeiten zu vergeben,
bachelor, master's, diploma and PhD projects available
Recent Research Highlights: (updated 2/09)
Skyrmion Lattice in a Chiral Magnet (2/09)
In the early 1960th the British physicist J. Skyrme showed that a certain
knot or whirl in pion fields could be interpreted as a neutron or proton.
Since then, the idea of topological solitons, i.e. of
stable non-linear field configurations, has been a powerful paradigm in many
fields of physics. For example, corresponding whirls in the spin configuration
of electrons (now called "skyrmions"), play an important role for
field-effect transistors
in large magnetic field showing the quantum hall effect.
However, these skyrmions have only been observed indirectly.
Our collegues from the group of C. Pfleiderer and P. Böni from the TU
Munich have now observed with neutron scattering
a peculiar magnetic structure in the metallic magnet MnSi. Our theoretical
calculations show rather unambiguously
that this structure can be interpreted as a lattice of skyrmion tubes. To stabilize this strange state of matter, an important element is that MnSi crystals are chiral, i.e. the crystal and its mirror image are different. Therefore
magnetic structures tend to get twisted, for example, more to the right than to the left which helps to form the skyrmions,
see
Science 323, 915 (2009) or
arXiv:0902.1968 for more details
[see also Perspectives article by J. Zaanen,
Science 323, 888 (2009) , C. Day in
Physics Today 62, 12 (2009),
Spektrum der Wissenschaften , Deutschlandfunk (German national radio) in Forschung aktuell, 24.02.09,
audio ]
An experimental proof of the topological nature of the skyrmions and the corresponding Berry phase is possible by measuring the Hall effect, see our
arXiv:0902.1933.
Quantum simulator for Mott transition (12/08) In many materials
an interesting effect can be observed: when the interactions are
increased the system switches from a metallic state to an insulating
state. The group of Immanuel Bloch in Mainz succeeded to simulate this
physics by placing ultracold K atoms in an artifical lattice made of
standing waves of laser light. To detect this state they observed how
the radius of the atomar cloud changes when it is compressed by an
external potential. The measurements where compared to our
calculations (in a collaboration with T. Costi from Jülich) using
the dynamical mean field theory. To our knowledge, this is also the
first direct experimental text of dynamical mean field theory without
using any fitting parameters. The cold atoms in the optical lattice
can be described with high precision with the Hubbard model with known
hopping rates and local repulsions. We expect that in future it will
be possible to simulate more and more solid state phenomena (including magnetism, high-temperature superconductivity,...) using cold atoms in optical lattices.
Article: Metallic and Insulating Phases of Repulsively Interacting Fermions in a 3D Optical Lattice
U. Schneider, L. Hackermuller, S. Will, Th. Best, I. Bloch, T. A. Costi, R. W. Helmes, D. Rasch, A. Rosch,
Science 322, 1520 (2008) or
arXiv:0809.1464 (see also Perspectives article by L. Fallani and M. Inguscio
Science 322, 1480 (2008))
see also Deutschlandfunk (German national radio) in Forschung aktuell, 5.12.08, audio ,
S. Wessel in Physik Journal 3, 20 (2009)).
for an example, how novel states of matter can be realized with atoms in optical lattices see
Metastable superfluidity of repulsive fermionic atoms in optical lattices
A. Rosch, D. Rasch, B. Binz, M. Vojta, Phys. Rev. Lett. 101, 265301 (2008) or
arXiv:0809.0505