Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Mittwoch
TT 24.20 Mi 14:30 Poster A
The physics of chiral helices in a metal — •Inga Fischer and
Achim Rosch — Institut für Theoretische Physik, Universität zu Köln
At high pressures, MnSi exhibits non-Fermi liquid behaviour over a
wide temperature and pressure range [1,2]. This may point to the existence
of a genuine non-Fermi liquid phase in an extremely pure cubic
system. Recent neutron scattering measurements [3] suggest the presence
of partial helical order in this phase, motivating us to study the physics
of chiral helices theoretically.
Helical order with a large pitch is stabilized by weak spin-orbit
(Dzyaloshinskii–Moriya) interactions in the non-centrosymmetric
crystal structure of MnSi. The interplay of static helical order and
further spin-orbit interactions in the conduction electron bands leads
to the formation of surprisingly large band-gaps in certain regions of
the Brillouin zone. We discuss their experimental consequences and
furthermore investigate the nature of magnetic fluctuations in MnSi.
[1] C. Pfleiderer et al., Nature 414 427–430 (2001).
[2] N. Doiron-Leyraud et al., Nature 425 595–599 (2003).
[3] C. Pfleiderer et al., to be published.
TT 24.21 Mi 14:30 Poster A
Die stark-Kopplungs-Phase des ionischen Hubbard Modells —
•Salvatore Manmana 1,2 , Volker Meden 3 , Reinhard Noack 2
und Kurt Schönhammer 3 — 1 Institut f. Theoretische Physik III,
Universität Stuttgart, Pfaffenwaldring 57/V, 70550 Stuttgart — 2 AG
Vielteilchennumerik, Philipps-Universität Marburg, Renthof 6, 35032
Marburg — 3 Institut f. Theoretische Physik, Georg-August-Universität
Göttingen, Tammannstr. 1, 37077 Göttingen
Das Phasendiagramm des ionischen Hubbard-Modells war ein in der
Literatur viel diskutiertes Thema. In einer numerischen Arbeit mit
Hilfe der DMRG-Methode (Dichtematrix-Renormierungsgruppe) ist es
uns gelungen, ein konsistentes Bild des Phasendiagramms zu erstellen.
Bei der Untersuchung der Eigenschaften der Phasen findet man
überraschenderweise eine Korrelierte-Isolator-Phase mit einer divergierenden
elektrischen Suszeptibilität und einem Potenzgesetzabfall in der
Dichte-Dichte-Korrelationsfunktion. In diesem Poster sollen diese ungewoehnlichen
Eigenschaften und die bisherigen Erklärungsversuche vorgestellt
werden.
TT 24.22 Mi 14:30 Poster A
Incommensurate charge-density wave in the stripe phase of
the two-dimensional Hubbard-Holstein model — •Serguei Varlamov
and Götz Seibold — Institut für Physik, BTU Cottbus, PBox
101344, 03013 Cottbus, Germany
We investigate the structure of the stripe phase in the two-dimensional
infinite-U Hubbard-Holstein model by means of the slave-boson technique.
We find in the mean-field approximation that the incommensurate
charge density waves (ICDW) appear in the region of the g vs δ phase
diagram where an ICDW is expected to form in the frustrated phaseseparation
scenario. Our main finding is that due to the competition
between strong on-site correlations and CDW ordering instability the incommensurability
of CDW depends from the number of carriers leading
to the reduction of the incommensurability with increasing of hole doping.
The influence of long-range Coulomb interaction on the structure of
ICDW is discussed.
TT 24.23 Mi 14:30 Poster A
Electronic structure, magnetism, and superconductivity in
pseudo-ladder compounds — •S.-L. Drechsler 1 , H. Rosner 1 ,
N.M. Plakida 2 , A. Vladimirov 3 , H. Eschrig 1 , and M. Knupfer 1
— 1 IFW-Dresden, P.O. Box 270116, 01171 Dresden, Germany —
2 MPI-CPfS Dresden, Germany — 3 J. Inst. f. Nuclear Res., Dubna,
Russia
The electronic structure of ACu2O3 compounds [A=Ca,Mg] has been
calculated within the local density approximation and compared with
x-ray absorption spectroscopy and magnetic neutron scattering data [1].
Both compounds deviate markedly from the usual pdσ cuprate picture.
Significant interlayer exchange in the stacking c-direction is found to be
responsible for the missing spin gap (generic for ideal two-leg ladders)
and the finite Néel-temperature. Hence, they can be modelled as weakly
coupled anisotropic Heisenberg (Hubbard) bi-layers. Within a derived effective
t-J-model in mean-field approximation nodeless superconductivity
below 50 K is predicted for Ca0.88Na0.12Cu2O3 (optimal hole doping).
[1]V. Kiryukhin et al., PRB 64, 1444418 (2001).
TT 24.24 Mi 14:30 Poster A
Time-dependent Gutzwiller theory of magnetic excitations in
the Hubbard model — •Goetz Seibold 1 , Federico Becca 2 , and
Jose Lorenzana 3 — 1 Lehrstuhl Theoretische Physik, BTU Cottbus,
03013 Cottbus, Germany — 2 SISSA, 34014 Trieste, Italy — 3 Dip. di
Fisica, Universita di Roma ’La Sapienza’, 00185 Roma, Italy
We use a spin-rotational invariant Gutzwiller energy functional to compute
random-phase-approximation-like (RPA) fluctuations on top of the
Gutzwiller approximation (GA). Since no restrictions are imposed on
the symmetry of the underlying saddle-point solution, our approach is
suitable for the evaluation of the magnetic susceptibility and dynamical
structure factor in strongly correlated inhomogeneous systems. We
present a detailed study of the quality of our approach by comparing
with exact diagonalization results and show its much higher accuracy
compared to the conventional Hartree-Fock+RPA theory. In infinite dimensions,
where the GA becomes exact for the Gutzwiller variational energy,
we evaluate ferromagnetic and antiferromagnetic instabilities from
the transverse magnetic susceptibility. The resulting phase diagram is in
complete agreement with previous variational computations.
TT 24.25 Mi 14:30 Poster A
Stability of metallic stripes in the extended one-band Hubbard
model — •Goetz Seibold 1 and Jose Lorenzana 2 — 1 Lehrstuhl
Theoretische Physik, BTU Cottbus, 03013 Cottbus, Germany — 2 Dip.
di Fisica, Universita di Roma ’La Sapienza’, 00185 Roma, Italy
Based on an unrestricted Gutzwiller approximation (GA) we investigate
the stripe orientation and periodicity in an extended one-band Hubbard
model. A negative ratio between next-nearest and nearest neighbor
hopping t ′ /t, as appropriate for cuprates, favors partially filled (metallic)
stripes for both vertical and diagonal configurations. At around optimal
doping diagonal stripes, site centered (SC) and bond centered (BC) vertical
stripes become degenerate suggesting strong lateral and orientational
fluctuations. We find that within the GA the resulting phase diagram
is in agreement with experiment whereas it is not in the Hartree-Fock
approximation due to a strong overestimation of the stripe filling. Results
are in agreement with previous calculations within the three-band
Hubbard model but with the role of SC and BC stripes interchanged.
TT 24.26 Mi 14:30 Poster A
Ionic Hubbard Model in High Dimension — •Krunoslav Pozgajcic
and Claudius Gros — Universität des Saarlandes, Fakultät 7,
Theoretische Physik, Postfach 15 11 50, D-66041 Saarbrücken
Ionic Hubbard model was proposed for the description of organic
mixed-stack charge-transfer crystals and also for the description of the
ferroelectric transition in perovskite materials. Theoretical efforts have
mostly been directed to the one dimensional case, which has be thoroughly
investigated by various methods on disposal for the one dimensional
systems. Recently, the self-energy functional approximation (SFA)
has been proposed (M. Potthoff, Eur. Phys. J. B 32, 429(2003)) as a
unifying scheme for a few cluster methods. We will present results for
the one dimensional and the higher dimensional ionic Hubbard model
calculated in the SFA framework.
TT 24.27 Mi 14:30 Poster A
Self-energy effects in the two-dimensional Hubbard-Model from
Functional Renormalisation
— •Daniel Rohe — Max-Planck-Institut FKF, Heisenbergstr. 1, 70569
Stuttgart
We present a numerical study of the two-dimensional Hubbard Model
in the normal state at low temperatures by means of a functional renormalisation
group scheme. It is found that anomalous features arise in
the self-energy slightly above a mean-field transition temperature T*.
This offers information on possible mechanisms responsible for unusual
properties of the model in the normal (non-symmetry-broken) state.
TT 24.28 Mi 14:30 Poster A
Druckeinfluss auf die magnetische Ordnung in CeNiGa2 —
•Andreas Eichler 1 , Ernst Bauer 2 , Robert Hauser 2 , Heinrich
Weiss 1 und Torsten Burghardt 1 — 1 Institut für Technische Physik,
TU Braunschweig, Mendelssohnstr. 2, D-38106 Braunschweig — 2 Institut
für Festkörperphysik, TU Wien, A-1040 Wien
Die Verbindung CeNiGa2 ist bekannt als antiferromagnetisches Kondogitter
mit einer Néel-Temperatur von ca. 4 K und relativ hoher elektronischer
spezifischer Wärme von 490 mJ/molK 2 . Messungen des elektrischen
Widerstandes unter Druck hatten ergeben, dass TN schnell abnimmt und