Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Dienstag
of the system are represented by the conduction electron spin conserved
by the electron-impurity coupling. A perturbative RG scheme is used to
investigate the intermediate energy region of the model. We show that,
in analogy to previous work on Kondo impurities out of equilibrium [1],
the running coupling constants acquire a parametric dependence on the
energy of the scattering electrons, induced by the thermal excitations of
the impurity system.
[1] A. Rosch, J. Paaske, J. Kroha, and P Wölfle, Phys. Rev. Lett. 90,
076804 (2003).
TT 16.6 Di 18:00 H19
Optical fingerprints of Kondo-correlations — •Rolf Helmes,
Michael Sindel, Laszlo Borda, and Jan von Delft — Sektion
Physik und CeNS, LMU, Theresienstr. 37, 80333 Muenchen
We calculate the recombination spectra of trapped excitons in selfassembled,
optically excited, charge-tunable quantum dots of the type
recently investigated experimentally by R. J. Warburton et al. (Nature
405, 926). If the experimental situation is such that the initial state decays
via exciton recombination into a final state which is a Kondo-state
with strong correlations between the electrons in the dot and those in the
semiconductor, the Kondo-effect should be visible in the recombination
spectrum. We use Wilson’s numerical renormalization group method to
calculate this spectrum. Depending on the numbers of electrons in the
charge-tunable quantum dot, we find an unusual dip in the Kondo-peak
of the spectrum.
TT 16.7 Di 18:15 H19
NRG study of the Kondo effect in the presence of itinerantelectron
ferromagnetism — •M. Sindel 1 , J. Martinek 2 , L.
Borda 1 , J. Barnas 3 , J. König 2 , G. Schön 2 , and J. von Delft 1
— 1 LMU Muenchen — 2 U Karlsruhe — 3 U Poznan, Polen
The successful observation of the Kondo effect in molecular quantum
dots (QDs) like carbon nanotubes and single molecules attached to metallic
electrodes opened the possibility to study the influence of many-body
correlations in the leads (superconductivity or ferromagnetism) on the
Kondo effect. The Kondo effect in QDs - artificial magnetic impurities
- attached to ferromagnetic leads is studied with the numerical renormalization
group (NRG) method. Here we adapt this method to the
case of a QD coupled to ferromagnetic leads with parallel magnetization
directions. It is shown that the QD level is spin-split due to presence
of ferromagnetic electrodes, leading to a suppression of the Kondo effect.
The resulting splitting of the Kondo resonance is similar to the
usual magnetic-field-induced splitting. We find that this splitting can
be fully compensated by an appropriately tuned external magnetic field
and thus the Kondo effect can be restored, confirming a recent prediction.
Although the resulting Kondo resonance then has an unusual spin
asymmetry with a reduced Kondo temperature, the ground state is still
a locally-screened state, describable by Fermi liquid theory and a generalized
Friedel sum rule, and transport in the unitary limit is not spin
dependent.
TT 16.8 Di 18:30 H19
Exact ground states of Anderson models with assisted hopping
— •Marcus Kollar and Florian Schütz — Institut für Theoretische
Physik, Universität Frankfurt, Robert-Mayer-Str. 8, 60054 Frankfurt/Main
Due to the Coulomb interaction the hopping amplitude for an electron
into and out of a quantum dot depends on the presence of an oppositespin
electron in the dot, leading to a description in terms of an Anderson
impurity model with assisted hopping [1]. Here we show that the application
of a local correlator to an uncorrelated state yields the exact ground
state for certain parametrizations of this model. We discuss possible valence
regimes and calculate the impurity spectral function using a set of
variational excited states.
[1] F. Guinea, Phys. Rev. B 67, 195104 (2003).
TT 16.9 Di 18:45 H19
Finite-size effects in an ultrasmall Kondo quantum box —
•Thomas Hand, Hans Kroha, and Hartmut Monien — Physikalisches
Institut, Universität Bonn
We investigate the correlation effects induced by a Kondo impurity
embedded in such a quantum box, using as a model a one-dimensional
tight-binding band with a finite number of lattice sites which is coupled
to an Anderson impurity. The model is solved by means of the density
matrix renormalization group (DMRG) including the Lanczos method as
well as the correction vector method. We examined systems with more
than 500 sites and kept 2600 states. We define the Kondo temperature
TK for the case of a discrete conduction electron density of states (level
spacing ∆E). The strength of the correlations, indicated by the weight of
the Kondo resonance, W, is determined by the local conduction density
of states at the impurity site, x0. Hence, in addition to the even/odd
effect predicted in [1], W depends both on the symmetry of the conduction
electron wave function at the Fermi edge, Ψ0(x), and on the impurity
position x0. We also investigate the crossover to the continuum limit.
[1] W. B. Thimm, J. Kroha, and J. v. Delft, Phys. Rev. Lett. 82, 2143
(1999).
TT 17 Postersitzung II: Transport, Quantenkohärenz, Quantenflüssigkeiten
Zeit: Dienstag 14:30–19:00 Raum: Poster A
TT 17.1 Di 14:30 Poster A
Charge transport through a two-site system driven by a highfrequency
field — •Michael Straß, Jörg Lehmann, Sébastien
Camalet, Sigmund Kohler, Gerd-Ludwig Ingold, and Peter
Hänggi — Institut für Physik, Universität Augsburg
We study the transport properties of a two-site system driven by an
external oscillating field. Possible experimental realizations are molecular
wires in laser fields or coupled quantum dots exposed to microwave
radiation. The current and the associated noise are computed within
a high-frequency approximation in which the time-dependent system is
mapped to a static one described by renormalized, effective parameters
that depend on the driving frequency and amplitude [1]. This allows for
an illustrative explanation of the characteristic dips in the noise characterized
by the Fano factor.
We compare our analytic results of the Fano factor with an exact numeric
solution obtained by a Floquet approach [2]. The external field
allows to switch between three qualitatively different situations: either
transport through a tunnel contact, a two-barrier setup or a open channel.
Therefore tunning the field permits a control of the transport properties
of two-site systems in molecular electronic devices.
[1] S. Kohler, S. Camalet, M. Strass, J. Lehmann, G.-L. Ingold, and
P. Hänggi, Chem. Phys. (in press), cond-mat/0306704.
[2] S. Camalet, J. Lehmann, S. Kohler, and P. Hänggi,
Phys. Rev. Lett. 90, 210602 (2003).
TT 17.2 Di 14:30 Poster A
Laser driven electronic transport through molecular wires
in the presence of electron-phonon interaction — •Jörg
Lehmann 1 , Sigmund Kohler 1 , Volkhard May 2 , and Peter
Hänggi 1 — 1 Institut für Physik, Universität Augsburg — 2 Institut für
Physik, Humboldt Universität zu Berlin
Recently, possibilities of laser control of electric currents through
molecular wires have been investigated [1-2]. So far, these theoretical
studies have neglected the influence of a coupling of the wire electrons
to vibrational modes of the molecule itself and its surroundings, thereby
assuming a fully coherent transport through the molecular wire.
Here, we put forward a quantum Boltzmann equation approach in the
Floquet basis. This approach allows to take into account exactly the coherent
dynamics in the presence of an arbitrarily strong time-periodic
field, while describing the influence of a weak coupling of the wire to
its environment comprised of electronic leads and phononic degrees of
freedom within a mean-field approximation.
As exemplary application of our formalism, we study the influence of
electron-phonon interaction on the generation of a directed current in the
absence of a mean external bias, i.e. a molecular electron pump.
[1] J. Lehmann, S. Kohler, P. Hänggi, and A. Nitzan, Phys. Rev. Lett. 88,
228305 (2002).
[2] J. Lehmann, S. Camalet, S. Kohler und P. Hänggi, Chem. Phys.
Lett. 368, 282 (2003).