Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Freitag
beobachtet, bei der Spannungssprünge in den I-U-Kennlinien aufgrund
von Larkin-Ovchinikov-Vortex Instabilitäten auftreten. Die experimentell
bestimmten Matching-Felder werden im Zusammenhang mit bestehenden
Theorien für dünne Schichten diskutiert.
TT 31.8 Fr 12:00 H20
Advanced manipulation of substrates for growth induced pinning
in YBCO thin films — •S. Brück 1 , J. Albrecht 1,2 , S. Leonhardt
2 , R. Spolenak 1 , G. Cristiani 2 , H.–U. Habermeier 2 , and G.
Schütz 1 — 1 Max-Planck-Institut für Metallforschung, Heisenbergstr.
3, D-70569 Stuttgart — 2 Max-Planck-Institut für Festkörperforschung,
Heisenbergstr. 1, D-70569 Stuttgart
One way to enhance flux pinning and thus the critical current density
in YBCO thin films is by modifying the substrates’ surface topography
[1,2]. Irradiation with a focused beam of gallium ions (FIB) creates dotlike
structures which induce pinning sites in the superconductor during
film growth. To enhance the density of these structures it is necessary to
prevent surface charging, which can be achieved by using substrates with
a higher conductivity. We show AFM images of the microstructures and
quantitative magnetooptical measurements of YBCO thin films grown on
such sophisticated substrates. It is found, that the critical current density
can be enhanced substantially by lateral size reduction of the created
structures.
[1] S. Leonhardt, J. Albrecht, R. Warthmann, H.-U. Habermeier and
H. Kronmüller, Physica C 341-348, 1979-1980 (2000)
[2] J. Albrecht, S. Leonhardt, H.-U. Habermeier, S. Brück, R. Spolenak
and H. Kronmüller, Physica C, in Press
TT 31.9 Fr 12:15 H20
Competitive localization of vortex lines or interacting bosons
by a defect — •Jan Kierfeld 1 and Valerii Vinokur 2 — 1 MPI
für Kolloid- und Grenzflächenforschung, 14424 Potsdam — 2 Materials
Science Division, Argonne National Laboratory
We present a theory describing the localization of N three-dimensional
vortex lines or two-dimensional bosons with short-ranged repulsive interaction
which are competing for a single columnar defect or potential
well. For N=2 we use a necklace model formulation to find a delocalization
transition between two different states with a single bound particle.
The transition is characterized by the onset of particle exchange on the
defect. For strong repulsive interaction between vortex lines a permutation
symmetry is broken and the potential well is occupied by the same
particle at all times. For decreasing repulsion or increasing temperature
a transition into a different bound state takes place in which the particles
occupy the potential well alternately. A generalization of this transition
to arbitrary vortex number N is proposed.
TT 31.10 Fr 12:30 H20
Enhanced Magnetic Flux Pinning of Nanocrystalline Mechanically
Alloyed Bulk MgB2 by Stoichiometry Variation and Doping
— •Olaf Perner 1 , Jürgen Eckert 2 , Günter Fuchs 1 , Konstantin
Nenkov 1 , Bernhard Holzapfel 1 , and Ludwig Schultz 1
— 1 Leibniz-Institut für Festkörper- und Werkstoffforschung Dresden, PF
270116, D-01171 Dresden — 2 TU Darmstadt, Petersenstr. 23, D-64287
Darmstadt
In order to overcome the poor intrinsic properties of MgB2 concerning
the low upper critical field and to obtain an enhancement of the magnetic
flux pinning a combination of microstructure optimization as well
as chemical variation of the pure MgB2 compound were chosen.
Starting from the elements Mg and B, the phase formation was realized
by the mechanical alloying technique receiving a partially reacted
MgB2 powder with grain sizes of about 10 nm. Short uniaxial hot pressing
under vacuum led to phase pure highly densified nanocrystalline bulk
samples with low impurity content. The variation of the stoichiometry by
increasing and reducing the Mg content by up to 10 wt.% as well as the
doping with various concentrations of SiO2 and MgO leads to increased
pinning forces Fp compared to the stoichiometric composition.
The MgB2 bulk sample with a Mg excess of 10 wt.% shows an improved
critical temperature Tc of 36.0 K and a critical current density jc
of about 10 6 A/cm 2 at 7.5 K in self field whereas Mg deficit of 10 wt.%
enhances the upper critical field Hc2 (9 T at 20 K). Doping with SiO2
which improves significantly Hc2 and jc reveals more effective than with
MgO. Microstructure investigations will be presented.
TT 32 Theoretische Modelierung von Materialien mit korrelierten Elektronen
Zeit: Freitag 10:15–13:00 Raum: H18
TT 32.1 Fr 10:15 H18
Quasiparticles from quantum Monte Carlo: Effective mass and
Wilson ratio for K3C60 — •Erik Koch and Olle Gunnarsson —
Max-Planck-Institut für Festkörperforschung, Stuttgart
Taking advantage of the fixed-node approximation to stabilize excited
states, we calculate quasiparticle energies by quantum Monte Carlo. We
apply this approach to a model describing K3C60 and show that it works
surprisingly well. We determine how the quasiparticle dispersion changes
with increasing correlation and compare to the many-body enhancement
of the Pauli susceptibility. While the self-energies for our model seem
to depend only weakly on the momentum (and band-index), we find a
Wilson ration that appears to behave qualitatively different from what
is expected from dynamical mean-field theory.
TT 32.2 Fr 10:30 H18
A wavefunction-based quantum chemical method for infinite
systems — •Beate Paulus — Max-Planck-Institut für Physik komplexer
Systeme, Nöthnitzer Straße 38, D-01187 Dresden
Wave-function based quantum chemical methods are applied to the
infinite system using an incremental scheme to partitioning the correlation
energy. After the success for a wide range of semiconductors [1] and
insulators even such weakly bound as rare gas crystals [2], the method
is extended to strongly correlated metallic systems [3]. As example we
present the cohesive energy of mercury, where the binding is entirely due
to correlations. [1] B. Paulus, P. Fulde and H. Stoll,Phys. Rev. B 54
(1996) 2556 [2] K. Rosciszewski, B. Paulus, P. Fulde, and H. Stoll,Phys.
Rev. B 60 (1999) 7905 [3] B. Paulus, Chem. Phys. Lett. 371 (2003) 7
TT 32.3 Fr 10:45 H18
TiOCl, an orbital-ordered system? — •Roser Valenti 1 , T. Saha-
Dasgupta 2 , Helge Rosner 3 , and Claudius Gros 4 — 1 Institut für
Theoretische Physik, Universität Frankfurt, D-60054 Frankfurt — 2 S.N.
Bose National Centre for Basic Sciences, Kolkata 700098, India — 3 Max-
Planck Institute for Chemical Physics of Solids, D-01187 Dresden —
4 Theoretische Physik, Universität des Saarlandes, D-66041 Saarbrücken
We present first principles density functional calculations and downfolding
studies of the electronic and magnetic properties of the layered
quantum spin system TiOCl. We attempt to clarify the concept of orbital
ordering in this material and discuss existing studies. An analysis
of the electronic structure of slightly distorted structures according to
the phononic modes allowed in this material suggests that this system is
subject to large orbital fluctuations driven by the electron-phonon coupling.
Based on these results, we propose a microscopic explanation of
the behavior of TiOCl at lower temperatures.
TT 32.4 Fr 11:00 H18
Hartree-Fock-Approximation und Berücksichtigung von
Abschirm-Effekten in Ab-Initio-Bandstruktur-Berechnungen
— •Olaf Peschel 1 , Ilan Schnell 2 und Gerd Czycholl 1 —
1 Universität Bremen, Institut für Theoretische Physik — 2 Los Alamos
National Laboratory, Theoretical Division
Wir gehen von den Bloch-Wellenfunktionen einer DFT-Hartree-
Rechnung (ohne Austausch-Korrelationspotential) aus, die wir zu
maximal lokalisierten Wannier-Funktionen Transformieren. Bezüglich
dieser Basis berechnen wir die Hopping- und Coulomb-Matrixelemente
des Hamilton-Operators in Zweiter Quantisierung und führen eine
selbstkonsistente Hartree-Fock-Rechnung durch. Alternativ können
wir eine Hartree-Fock-(HFA)-Rechnung mit bezüglich der Bloch-Basis
berechneten Matrixelementen durchführen.
Um über die HFA hinauszugehen und Abschirmeffekte zu berücksichtigen,
berechnen wir die Suszeptibilität in verallgemeinerter Lindhard-