Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Montag
ticity. As a model it is suggested that the turbulent state breaks down
when a fluctuation crosses a threshold level. Applying Rice’s formula for
the average level crossing per unit time of a stationary normal process
gives information on the probability density function and the spectral
bandwidth of the fluctuations. As a result a normal distribution is found
and the standard deviation as well as the spectral bandwidth are determined
quantitatively. In addition to superfluid turbulence this model
may be applicable also to other instabilities which result from random
fluctuations exceeding a certain threshold.
TT 4.3 Mo 10:45 H19
Ground state properties and non-equilibrium dynamics of
hard-core bosons confined on optical lattices — •Marcos
Rigol Madrazo and Alejandro Muramatsu — Institut fuer
Theoretische Physik III, Universitaet Stuttgart, Pfaffenwaldring 57,
D-70550 Stuttgart, Germany
We study by means of an exact numerical approach, a gas of hard core
bosons (HCB) confined on optical lattices. The ground state properties
of such systems are analyzed. Local incompressible phases appear in the
system, like in the case of interacting soft-core bosons [1] and fermions
[2,3]. The changes in momentum distribution function and in the natural
orbitals (effective single particle states) introduced by the formation
of such phases are analyzed. We also study non-equilibrium properties
for those systems, which within our numerical approach can be obtained
exactly for systems with ∼ 200 particles on lattices with ∼ 3000 sites.
In particular we analyze the free expansion of the gas when it is released
from the trap turning off the confining potential. We show that the expansion
is non-trivial (as opposed to the fermionic case) and new features
to be observed in the experiments are analyzed.
[1] G. G. Batrouni, V. Rousseau, R. T. Scalettar, M. Rigol, A. Muramatsu,P.
J. H. Denteneer, and M. Troyer, Phys. Rev. Lett. 89, 117203
(2002).
[2] M. Rigol, A. Muramatsu, G. G. Batrouni, and R. T. Scalettar, Phys.
Rev. Lett. 91, 130403 (2003).
[3] M. Rigol and A. Muramatsu, cond-mat/0309670 (2003).
TT 4.4 Mo 11:00 H19
Ultracold fermions and the SU(N) Hubbard model — •Carsten
Honerkamp 1 and Walter Hofstetter 2 — 1 MPI Solid State Research,
Stuttgart — 2 MIT, Cambridge, USA
We investigate the fermionic SU(N) Hubbard model on the twodimensional
square lattice for weak to moderate interaction strengths
using one-loop renormalization group and mean-field methods. For the
repulsive case U > 0 at half filling and small N the dominant tendency
is towards breaking of the SU(N) symmetry. For N > 6 staggered flux
order takes over as the dominant instability, in agreement with the large-
N limit. Away from half filling for N = 3 the system rearranges the
particle densities such that two flavors remain half filled by cannibalizing
the third flavor. In the attractive case and odd N a full Fermi surface
coexists with a superconductor in the ground state. These results may
be relevant to future experiments with cold fermionic atoms in optical
lattices.
TT 4.5 Mo 11:15 H19
Self-energy and critical temperature of weakly interacting
bosons — •Peter Kopietz, Sascha Ledowski, and Nils Hasselmann
— Institut für Theoretische Physik, Universität Frankfurt,
Robert-Mayer-Str. 8, 60054 Frankfurt/Main
Using the exact renormalization group we calculate the momentumdependent
self-energy Σ(k) at zero frequency of weakly interacting bosons
at the critical temperature Tc of Bose-Einstein condensation in dimensions
3 ≤ D < 4. We obtain the complete crossover function interpolating
between the critical regime k ≪ kc, where Σ(k) ∝ k 2−η , and the
short-wavelength regime k ≫ kc, where Σ(k) ∝ k 2(D−3) in D > 3 and
Σ(k) ∝ ln(k/kc) in D = 3. From our Σ(k) we find for the interaction-
induced shift of Tc in three dimensions ∆Tc/Tc ≈ 1.23 an 1/3 , where a is
the s-wave scattering length and n is the density [1].
[1] S. Ledowski, N. Hasselmann, and P. Kopietz, cond-mat/0311043.
TT 4.6 Mo 11:30 H19
Reflectionless dynamics of 1D charge and spin modes in atomic
traps — •Lars Kecke, Wolfgang Häusler, and Hermann
Grabert — Physikalisches Institut, Albert- Ludwigs Univerität,
Freiburg
Gases of Fermionic atoms in cigar shaped, one-dimensional traps provide
an ideal means to study the dynamics of correlated Fermi systems
in the Tonks gas limit of repulsive contact interactions. The interaction
strength can be tuned experimentally [1].
Based on the Tomonaga-Luttinger (TL) model we present calculations
on the dynamics of charge and spin density waves. Particularly at the
edge both modes show features that are strikingly different from the
non-interacting case as well as from conventional TL dynamics. For example
we find a suppression of reflections that should be experimentally
observable.
[1] T. Loftus et al PRL 88, 173201 (2002)
TT 4.7 Mo 11:45 H19
Audio Demonstration of Quantum Oscillations: Phase Locking
of Josephson Oscillations in superfluid 3He-B — •S.V.
Pereverzev and G. Eska — Physikalisches Institut, Universität
Bayreuth
We report on a family of dynamic effects which were observed in superfluid
3He flow through a weak link (array of 65x65 holes of 150 nm dia.
in a 50 nm SiN membrane). The flow was driven by a time dependent
pressure difference (∆p(t)) to which an ac-modulation (paccos(ωt + θ))
was added. A self-trapping phase-locked state could be observed when the
Josephson frequency (ωJ(t) = ∆p(t)
) and the ac-drive frequency ω merged
ρκ0
(reverse AC-Josephson effect). This trapped state is a zero net current
state with a nonzero pressure difference across the weak link. Close to
this resonance the observed beat frequency ωJ − ω clearly indicates the
presence of Josephson oscillations which can be heared. Low frequency oscillations
around this quasi-equilibrium state were also observed, as well
as Helmholtz oscillations around the zero pressure-difference equilibrium
state. The frequencies of both of these oscillatory modes depended on
amplitude and frequency of the ac-drive. For the pressure driven flow we
measured unexpected high losses. The origin of these losses is unclear.
TT 4.8 Mo 12:00 H19
Resonant Transport of Bose-Einstein Condensates — •Tobias
Paul, Peter Schlagheck, and Klaus Richter — Institut für theoretische
Physik, Universität Regensburg
Due to the rapid progress of “atomic chip” technology [1], the implementation
of experiments probing the propagation of Bose-Einstein
condensates in mesoscopic waveguides becomes feasible. A particular interesting
waveguide geometry is the double barrier structure created by
a sequence of two constrictions, which can serve as a Fabry-Perot-like
interferometer for the condensate [2]. In this context the question arises
how the presence of repulsive interactions between the condensed atoms
influences the transmission through this double barrier structure. Our numerical
approach is based on the Gross–Pitaevskii equation and employs
absorbing boundary conditions as well as an inhomogeneous source term
in order to obtain a stationary flow of condensate through the guide. We
show that the nonlinear interaction between the atoms leads to a significant
modification of the positions and shapes of the resonance peaks as
compared to the noninteracting case.
[1] H. Ott et al., Phys. Rev. Lett. 87, 230401 (2001); J. Schmiedmayer et
al., J. Mod. Optics 47, 2789 (2000); W. Hänsel et al., Nature 413, 498
(1999).
[2] J. Fortagh and C. Zimmermann, Physik Journal, Juni 2003