Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Mittwoch
TT 23.2 Mi 17:45 H19
Persistent spin currents in mesoscopic Heisenberg rings —
•Florian Schütz, Marcus Kollar, and Peter Kopietz — Institut
für Theoretische Physik, Universität Frankfurt, Robert-Mayer-Str.
8, 60054 Frankfurt/Main
We discuss spin transport in magnetic insulators and show that a spatially
inhomogeneous magnetic field gives rise to a persistent spin current
around a mesoscopic Heisenberg ring [1]. This is a mesoscopic quantum
interference effect and is analogous to persistent charge currents in normal
metal rings threaded by a magnetic flux. The spin current is carried
by magnons and is driven by a geometric flux. For a ferromagnetic ring
spin waves are thermally activated while for an antiferromagnetic ring
quantum fluctuations lead to a magnetization current even in the ground
state. Since moving magnetic dipoles produce electric fields, the magnetization
current leads to an electric dipole moment perpendicular to the
ring plane, which may allow the experimental detection of this effect.
[1] F. Schütz, M. Kollar, and P. Kopietz, Phys. Rev. Lett. 91, 017205
(2003); cond-mat/0308230.
TT 23.3 Mi 18:00 H19
Spin-dependent transport through quantum dots attached to
ferromagnetic leads — •Jürgen König 1,2 , Jan Martinek 1 , and
Gerd Schön 1 — 1 Institut für Theoretische Festkörperphysik, Universität
Karlsruhe — 2 Institut für Theoretische Physik III, Ruhr-Universität
Bochum
We study spin-dependent transport through quantum dots coupled to
ferromagnetic leads. First, we consider strong dot-lead coupling and ana-
lyze how the spin polarization of the leads affects the Kondo effect. Based
on a scaling approach we predict [1] that an effective Zeeman splitting
and hence a splitting of the zero-bias anomaly is induced. With an additional
magnetic field, this splitting can be compensated, and the strongcoupling
limit of the Kondo effect is recovered. These results are backed
up by rigorous numerical-renormalization-group calculations [2]. Second,
weak dot-lead coupling is considered. We determine the linear conductance
as a function of the relative angle between the leads’ magnetization
directions [3]. An interaction-induced rotation of the quantum-dot spin
is found, which yields a reduction of the spin-valve effect.
[1] J. Martinek et al., Phys. Rev. Lett. 91, 127203 (2003).
[2] J. Martinek, M. Sindel, L. Borda, J. Barnas, J. König, G. Schön, and
J. v. Delft, accepted for Phys. Rev. Lett.
[3] J. König and J. Martinek, Phys. Rev. Lett. 90, 166602 (2003).
TT 23.4 Mi 18:15 H19
Semiclassical theory of weak antilocalization in chaotic systems.
— •Oleg Zaitsev 1 , Diego Frustaglia 2 , and Klaus Richter 1 —
1 Universität Regensburg, Theoretische Physik, D-93040 Regensburg —
2 Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
We propose a semiclassical theory of weak antilocalization in clean
ballistic systems with chaotic dynamics. Our approach is based on the
semiclassical Landauer formula that we extended to include spin-orbit interaction.
The general theory is applied to a chaotic billiard with Rashba
interaction. We find that the spin relaxation in chaotic systems is considerably
slower than in the diffusive systems. As a consequence, one expects
different scales of the weak antilocalization in these cases.
TT 24 Postersitzung III: Korrelierte Elektronen, ”Orbital Physics”
Zeit: Mittwoch 14:30–19:00 Raum: Poster A
TT 24.1 Mi 14:30 Poster A
LDA+DMFT results for ARPES spectra of SrVO3 — V.
I. Anisimov 1 , K. Held 2 , •G. Keller 3 , D. E. Kondakov 1 , I.
Nekrasov 1 , T. Pruschke 4 , and D. Vollhardt 3 — 1 Institut für
Metallphysik, Ekaterinburg GSP-170, Russland — 2 Max Planck Institut
für Festkörperforschung, 70569 Stuttgart — 3 Theoretische Physik III,
Universität Augsburg, 86135 Augsburg — 4 Institut für Theoretische
Physik, Universität Göttingen, 37077 Göttingen
We report angle resolved photoemission spectra of SrVO3 calculated
by LDA+DMFT(QMC). Starting from LDA band structure calculations
in the LMTO basis set [1], we use a projection method [2,3] to extract a
Hamiltonian for the three-fold degenerate t2g bands from the total band
structure. With this reduced LDA-Hamiltonian as input to DMFT(QMC)
calculations [4], we obtain (via maximum entropy) angle resolved spectral
functions A(k, ω) on the real axis. The spectra exhibit featureless upper
and lower Hubbard bands with a quasi-particle peak which is narrowed
by correlations. In view of the good agreement of our previous (not angle
resolved) LDA+DMFT results [5] with high-resolution PES for SrVO3
[6], it will be interesting to see how our predictions compare with future
ARPES experiments.
[1] O. K. Andersen, Phys. Rev. B 12, 3060 (1975)
[2] N. Marzari and D. Vanderbilt, Phys. Rev. B 56, 12847 (1997)
[3] W. Ku et al., Phys. Rev. Lett. 89, 167204 (2002)
[4] K. Held et al., Psi-k Newsletter #56 (April 2003), p. 65-103
[5] I. Nekrasov et al., cond-mat/0211508
[6] A. Sekiyama et al., cond-mat/0206471
TT 24.2 Mi 14:30 Poster A
Charge and orbital order in Fe3O4 — •I. Leonov 1,2 , A. N
Yaresko 3 , V. N. Antonov 4 , M. A. Korotin 2 , V. I. Anisimov 2 ,
and D. Vollhardt 1 — 1 Theoretical Physics III, University of
Augsburg, 86135 Augsburg — 2 Institute of Metal Physics, Ekaterinburg
GSP-170, Russia — 3 Max Planck Institute for Physics of Complex
Systems, 01187 Dresden — 4 Institute of Metal Physics, 03142 Kiev,
Ukraine
The issue of charge and orbital ordering in the low-temperature monoclinic
(P2/c) structure of magnetite (Fe3O4)[1] is investigated using
LSDA+U. The ground state is found to display both charge order (CO)
and orbital order (OO) [2]. The CO is described by a [001] charge density
wave with a minor [001 ] modulation, and is incompatible with the An-
2
derson criterion. The OO agrees with the Kugel-Khomskii theory. The
system is described by two order parameters: (i) the difference between
the t2g minority occupancies of 2+ and 3+ Fe cations, and (ii) the total
3d charge difference. While the former is large (0.5), the screening of
charge disproportion is so effective that the latter is rather small (0.23).
This agrees well with the results of bond valence sum analysis (0.2).
The LSDA+U results are also in reasonably good with measured optical
spectra.
[1] J. P. Wright, J. P. Attfield and P. G. Radaelli, Phys. Rev. Let. 87,
266401 (2001)
[2] I. Leonov, A. N. Yaresko, V. N. Antonov, M. A. Korotin, V. I. Anisimov
and D. Vollhardt (preprint)
TT 24.3 Mi 14:30 Poster A
DFT-Gutzwiller Calculations for ferromagnetic bcc Nickel
— •Torsten Ohm 1 , Stefan Weiser 1 , Werner Weber 1 , and
Jörg Bünemann 2 — 1 Institut für Physik, Universität Dortmund —
2 Fachbereich Physik, Phillips Universität Marburg
Thin films of bcc Nickel can be produced on appropriate substrates and
can be studied experimentally; e.g., by photoemission spectroscopy. This
has stimulated us to carry out a multi-band Gutzwiller calculation for
the quasi-particle energy bands. Our single particle Hamiltonian is based
on results of spin-density functional theory ground state calculations using
the LAPW-Wien2k code. In the DFT calculations, some problems
with the k-mesh integration had to be overcome in order to establish
well converged total energy values. The spin-only magnetic moment is
found to be about 20 percent smaller than in fcc Nickel. For the multiband
Gutzwiller calculations we use the DFT lattice constant and the
same atomic Racah interaction parameters A,B, and C as in our previous
calculations for fcc Nickel. The Gutzwiller quasi-particle bands and the
Fermi surfaces are compared with the corresponding DFT results.
TT 24.4 Mi 14:30 Poster A
Quantum Monte Carlo approach to the Holstein model —
•Martin Hohenadler, Hans Gerd Evertz, and Wolfgang von
der Linden — Institute for Theoretical and Computational Physics,
Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria
Based on the canonical Lang-Firsov transformation of the Hamiltonian,
we develop a novel quantum Monte Carlo algorithm for the Holstein
model with one electron. Separation of the fermionic degrees of freedom
by a reweighting of the probability distribution leads to a dramatic reduction
in computational effort. A principle component representation
of the phonon degrees of freedom allows us to sample completely uncorrelated
phonon configurations. Despite a minus-sign problem, which