Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Donnerstag
In a long Josephson junction (LJJ) made of alternating regions with
positive and negative critical currents (so-called 0 and π regions) halfinteger
vortices (semifluxons) may spontaneously appear at the boundaries
between 0 and π parts.
The semifluxons have unique properties: they are pinned at the 0-πboundary
but may have two different polarities corresponding to the half
integer flux +Φ0/2 and −Φ0/2. In comparison with the freely moving
fluxon, the semifluxon represents the ground state of the system and
therefore is extremely stable. Semifluxons may be obtained and investigated
in 0-π-junctions made of d-wave superconductors, in junctions with
ferromagnetic barrier or in conventional LJJs with specially designed injectors.
In this talk I discuss the properties of semifluxons and their interaction
with integer fluxons (solitons) in view of possible applications for
information storage and processing in classical and quantum domains.
Hauptvortrag TT 25.6 Do 12:15 H20
π-superconductivity in S/F nano-structures — •Marco
Aprili 1,2 , T. Kontos 1 , W. Guichard 3 , ML. Della Rocca 1,2 , J.
Lesueur 2 , and P. Gandit 3 — 1 CSNSM-CNRS, Bat.108 Université
Paris-Sud, 91400 Orsay, France — 2 Laboratoire de Physique Quantique
ESPCI, 10 rue Vauquelin, 75005 Paris, France — 3 CRTBT-CNRS, 25
Avenue des Martyrs, 38000 Grenoble, France
TT 26 Korrelierte Elektronen: Theorie II
Hybrid nanostructures not only open new routes for studying the coexistence
between superconductivity and ferromagnetism, but they also
provide new insights to fundamentals of quantum electronics. We have
investigated the change in the superconducting wavefunction resulting
from the interaction between the spins of a Cooper pair and the exchange
field. Tunnelling spectroscopy reveals an inhomogeneous superconducting
state induced in a ferromagnetic thin film by the proximity effect.
As a consequence, critical current and macroscopic quantum interference
experiments show π-coupling when the ferromagnetic thin film is coupled
with a second superconductor. π-coupling originates a spontaneous half
quantum flux in a superconducting ring. This phase transition has been
recently observed by SQUID and Hall magnetometry (Regensburg-Orsay
experiment).
Zeit: Donnerstag 09:30–13:00 Raum: H18
TT 26.1 Do 09:30 H18
Itineranzeffekte in Spinkorrelationen eindimensionaler Mott
Isolatoren — •Anja Grage 1 , M. Joseph Bhaseen 2 , Fabian H. L.
Essler 2 und Florian Gebhard 1 — 1 Fachbereich Physik, Philipps-
Universität Marburg, 35032 Marburg, Germany — 2 Department of
Physics, Brookhaven National Laboratory, Upton, NY 11973-5000
Mott-Isolatoren werden häufig mit Hilfe des Spin-1/2 Heisenberg-
Modells analysiert, das die Spinfreiheitsgrade im Hubbard-Modell bei
starker lokaler Coulomb-Abstoßung U/t charakterisiert. Neutronenstreuexperimente
[1,2] an zweidimensionalen Spin-1/2 Antiferromagneten zeigen
jedoch, daß die Dispersion erheblich von der des Heisenberg-Modells
abweicht. Dies unterstreicht die Bedeutung der Itineranz der Elektronen
auch in Mott Isolatoren.
Wir untersuchen magnetische Eigenschaften quasi-eindimensionaler
Mott-Isolatoren, zum Beispiel von Strontium-Kupraten oder Bechgaard-
Salzen. Wir berechnen den dynamischen magnetischen Strukturfaktor in
der Nähe der antiferromagnetischen Wellenzahl k = π fuer kleines U/t
im Rahmen der Feldtheorie exakt und diskutieren dessen Verhalten als
Funktion von U/t. Wir finden signifikante Effekte für U/t ≈ 2-3.
[1] R. Coldea et al., Phys. Rev. Lett. 86, 5377 (2001).
[2] H.M. Rønnow et al., Phys. Rev. Lett. 87, 037202 (2001).
TT 26.2 Do 09:45 H18
Phase diagram of the one-dimensional t − J model — •Jochen
Hub, Catia Lavalle, and Alejandro Muramatsu — Universität
Stuttgart, Pfaffenwaldring 57, 70550 Stuttgart
Using a recently developed hybrid-loop algorithm [1] we determine the
phase diagram of the t − J model in one-dimension, where a Luttingerliquid
phase (with Kρ < 1 and Kρ > 1), a spin-gap phase and a region
of phase-separation are determined on the basis of a detailed finite-size
scaling.
[1] C. Lavalle, M. Arikawa, S. Capponi, F. F. Assaad and M. Muramatsu
Phys. Rev. Lett. 90, 216401 (2003)
TT 26.3 Do 10:00 H18
Electron-phonon interaction in the t-J model — •Oliver
Rösch and Olle Gunnarsson — Max-Planck-Institut für
Festkörperforschung, 70569 Stuttgart
Starting from the three-band model of a CuO2 layer we derive a t − J
model with electron-phonon interaction taking into account the modulation
of both hopping and Coulomb integrals by phonons. The former
is found to be dominant, but the modulation of the Coulomb integrals
cannot be neglected.
The model is studied on finite clusters using exact diagonalization. It
explains the anomalous softening of the half-breathing mode upon doping
and a weaker renormalization of the breathing mode as observed exper-
imentally in many cuprates. The coupling to other modes is not strong
in this model.
TT 26.4 Do 10:15 H18
Effective Models by Self–Similar Continuous Unitary Transformations:
the Hubbard Model at Strong Coupling —
•Alexander Reischl, Erwin Müller-Hartmann, and Götz
Silvester Uhrig — Institut für theoretische Physik, Universität zu
Köln, Zülpicher Str. 77, D-50937 Köln, Germany
An effective model conserving the number of double occupancies is
derived for the half–filled Hubbard model at strong coupling.
The mapping to the effective model is performed by a self–similar continuous
unitary transformation (CUT). The sectors of different double
occupancy are decoupled. Therefore the effective model is a generalized
t–J–model. A truncation scheme that selects operators according to their
locality is used to close the flow equations.
The convergence and accuracy of the method are discussed in the light
of the developing itinerant character of the system for increasing bandwidth.
The apparent energy–gap for a single hole or a double occupancy
is calculated within the effective model.
Our main result is that the mapping of the Hubbard–Model to the
t–J–model is possible and very robust as long as the apparent gap is
positive. In this regime relatively small clusters suffice to determine the
parameters of the effective model reliably. Besides correlated hopping and
interaction of charges the most prominent interactions are the two–spin
nearest neighbor exchange and the four–spin ring–exchange.
TT 26.5 Do 10:30 H18
Ferroelektrische Phasen im erweiterten Falicov-Kimball-Modell
— •Claudia Schneider und Gerd Czycholl — Institut für Theoretische
Physik, Universität Bremen
Ferroelektrische Phasenübergänge und Ordnungs-Unordnungs-
Übergänge in der Ladungsdichte stellen zwei konkurrierende Phänomene
dar, die in der Behandlung des Falicov-Kimball-Modells (FKM) auftreten.
Damit läßt sich das FKM nicht nur auf Systeme anwenden, in denen
Valenzübergänge eine Rolle spielen (z.B. YbInCu4), sondern auch auf
Ferroelektrika wie SmB6 und vergleichbare Verbindungen. In der vorliegenden
Untersuchung wird das Modell um eine lokale Hybridisierung
zwischen Leitungsband und Valenzband sowie um eine endliche Valenzbandbreite
erweitert. Es werden Ergebnisse zu der Frage vorgestellt,
in welchen Parameter- und Temperaturbereichen spontane Polarisation
und Ladungsdichtewellen (CDW-Lösungen) auftreten, wenn man den
Grenzfall schwacher Wechselwirkung in Hartree-Fock-Näherung betrachtet.
Ferner wird geprüft, inwieweit die gefundenen Lösungen in höheren
Ordnungen Störungsrechnung stabil bleiben.