431 research outputs found
Interaction Quench in the Hubbard model
Motivated by recent experiments in ultracold atomic gases that explore the
nonequilibrium dynamics of interacting quantum many-body systems, we
investigate the opposite limit of Landau's Fermi liquid paradigm: We study a
Hubbard model with a sudden interaction quench, that is the interaction is
switched on at time t=0. Using the flow equation method, we are able to study
the real time dynamics for weak interaction U in a systematic expansion and
find three clearly separated time regimes: i) An initial buildup of
correlations where the quasiparticles are formed. ii) An intermediate
quasi-steady regime resembling a zero temperature Fermi liquid with a
nonequilibrium quasiparticle distribution function. iii) The long time limit
described by a quantum Boltzmann equation leading to thermalization with a
temperature T proportional to U.Comment: Final version as publishe
On a problem of A. Weil
A topological invariant of the geodesic laminations on a modular surface is
constructed. The invariant has a continuous part (the tail of a continued
fraction) and a combinatorial part (the singularity data). It is shown, that
the invariant is complete, i.e. the geodesic lamination can be recovered from
the invariant. The continuous part of the invariant has geometric meaning of a
slope of lamination on the surface.Comment: to appear Beitr\"age zur Algebra und Geometri
Exact results for nonlinear ac-transport through a resonant level model
We obtain exact results for the transport through a resonant level model
(noninteracting Anderson impurity model) for rectangular voltage bias as a
function of time. We study both the transient behavior after switching on the
tunneling at time t = 0 and the ensuing steady state behavior. Explicit
expressions are obtained for the ac-current in the linear response regime and
beyond for large voltage bias. Among other effects, we observe current ringing
and PAT (photon assisted tunneling) oscillations.Comment: 7 page
Symmetry, bifurcation and stacking of the central configurations of the planar 1+4 body problem
In this work we are interested in the central configurations of the planar
1+4 body problem where the satellites have different infinitesimal masses and
two of them are diametrically opposite in a circle. We can think this problem
as a stacked central configuration too. We show that the configuration are
necessarily symmetric and the other sattelites has the same mass. Moreover we
proved that the number of central configuration in this case is in general one,
two or three and in the special case where the satellites diametrically
opposite have the same mass we proved that the number of central configuration
is one or two saying the exact value of the ratio of the masses that provides
this bifurcation.Comment: 9 pages, 2 figures. arXiv admin note: text overlap with
arXiv:1103.627
New theoretical approaches for correlated systems in nonequilibrium
Abstract.: We review recent developments in the theory of interacting quantum many-particle systems that are not in equilibrium. We focus mainly on the nonequilibrium generalizations of the flow equation approach and of dynamical mean-field theory (DMFT). In the nonequilibrium flow equation approach one first diagonalizes the Hamiltonian iteratively, performs the time evolution in this diagonal basis, and then transforms back to the original basis, thereby avoiding a direct perturbation expansion with errors that grow linearly in time. In nonequilibrium DMFT, on the other hand, the Hubbard model can be mapped onto a time-dependent self-consistent single-site problem. We discuss results from the flow equation approach for nonlinear transport in the Kondo model, and further applications of this method to the relaxation behavior in the ferromagnetic Kondo model and the Hubbard model after an interaction quench. For the interaction quench in the Hubbard model, we have also obtained numerical DMFT results using quantum Monte Carlo simulations. In agreement with the flow equation approach they show that for weak coupling the system relaxes to a "prethermalized” intermediate state instead of rapid thermalization. We discuss the description of nonthermal steady states with generalized Gibbs ensemble
Multi-wavelength observations of the young binary system Haro 6-10: The case of misaligned discs
Context. We present a multi-wavelength, high-resolution observational survey
of the young binary system Haro 6-10 (GV Tau, IRAS 04263+2426), which is
harbouring one of the few known infrared companions. Aims. The primary goal of
this project is to determine the physical and geometrical properties of the
circumstellar and circumbinary material in the Haro 6-10 system. Methods.
High-resolution optical (HST/WFPC2) and near-infrared (VLT/NACO) images in
different bands were analysed to investigate the large-scale structures of the
material around the binary.Mid-infrared interferometry (VLTI/MIDI) and
spectroscopy (TIMMI2 at the 3.6m ESO telescope) were carried out to determine
the structure and optical depth of the circumstellar material around the
individual components. Results. The multi-wavelength observations suggest that
both components of the binary system Haro 6-10 are embedded in a common
envelope. The measured extinction indicates a dust composition of the envelope
similar to that of the interstellar medium. Each component of the system has a
circumstellar disc-like structure typical of young stars. The discs are highly
misaligned: the northern component is seen almost edge-on and the southern
component is an almost face-on disc. Conclusions. The two main formation
scenarios of binary systems with misaligned discs are the gravitational capture
of a passing object in a dense environment, and the fragmentation of the
collapsing molecular cloud. Given the low-density environment of the
Taurus-Aurigae star-forming region, the first scenario is unlikely for Haro
6-10. The binary system most probably formed via fragmentation of two different
parts of the collapsing molecular cloud combined with other dynamical processes
related to the cloud and/or the protostars. This can be the explanation also
for other binary systems with an infrared companion.Comment: accepted for publication in A&A on July 15, 201
Chaos around a H\'enon-Heiles-inspired exact perturbation of a black hole
A solution of the Einstein's equations that represents the superposition of a
Schwarszchild black hole with both quadrupolar and octopolar terms describing a
halo is exhibited. We show that this solution, in the Newtonian limit, is an
analog to the well known H\'enon-Heiles potential. The integrability of orbits
of test particles moving around a black hole representing the galactic center
is studied and bounded zones of chaotic behavior are found.Comment: 7 pages Revte
Stellar Encounters with Massive Star-Disk Systems
The dense, clustered environment in which massive stars form can lead to
interactions with neighboring stars. It has been hypothesized that collisions
and mergers may contribute to the growth of the most massive stars. In this
paper we extend the study of star-disk interactions to explore encounters
between a massive protostar and a less massive cluster sibling using the
publicly available SPH code GADGET-2. Collisions do not occur in the parameter
space studied, but the end state of many encounters is an eccentric binary with
a semi-major axis ~ 100 AU. Disk material is sometimes captured by the
impactor. Most encounters result in disruption and destruction of the initial
disk, and periodic torquing of the remnant disk. We consider the effect of the
changing orientation of the disk on an accretion driven jet, and the evolution
of the systems in the presence of on-going accretion from the parent core.Comment: 11 pages, 10 figures, accepted to Ap
Chaos in black holes surrounded by gravitational waves
The occurrence of chaos for test particles moving around Schwarzschild black
holes perturbed by a special class of gravitational waves is studied in the
context of the Melnikov method. The explicit integration of the equations of
motion for the homoclinic orbit is used to reduce the application of this
method to the study of simple graphics.Comment: 15 pages, LaTex
Action minimizing orbits in the n-body problem with simple choreography constraint
In 1999 Chenciner and Montgomery found a remarkably simple choreographic
motion for the planar 3-body problem (see \cite{CM}). In this solution 3 equal
masses travel on a eight shaped planar curve; this orbit is obtained minimizing
the action integral on the set of simple planar choreographies with some
special symmetry constraints. In this work our aim is to study the problem of
masses moving in \RR^d under an attractive force generated by a potential
of the kind , , with the only constraint to be a simple
choreography: if are the orbits then we impose the
existence of x \in H^1_{2 \pi}(\RR,\RR^d) such that q_i(t)=x(t+(i-1) \tau),
i=1,...,n, t \in \RR, where . In this setting, we first
prove that for every d,n \in \NN and , the lagrangian action
attains its absolute minimum on the planar circle. Next we deal with the
problem in a rotating frame and we show a reacher phenomenology: indeed while
for some values of the angular velocity minimizers are still circles, for
others the minima of the action are not anymore rigid motions.Comment: 24 pages; 4 figures; submitted to Nonlinearit
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