52,146 research outputs found
Quantum Levy flights and multifractality of dipolar excitations in a random system
We consider dipolar excitations propagating via dipole-induced exchange among
immobile molecules randomly spaced in a lattice. The character of the
propagation is determined by long-range hops (Levy flights). We analyze the
eigen-energy spectra and the multifractal structure of the wavefunctions. In 1D
and 2D all states are localized, although in 2D the localization length can be
extremely large leading to an effective localization-delocalization crossover
in realistic systems. In 3D all eigenstates are extended but not always
ergodic, and we identify the energy intervals of ergodic and non-ergodic
states. The reduction of the lattice filling induces an ergodic to non-ergodic
transition, and the excitations are mostly non-ergodic at low filling.Comment: 5 pages, 6 figure
Manipulation of the dynamics of many-body systems via quantum control methods
We investigate how dynamical decoupling methods may be used to manipulate the
time evolution of quantum many-body systems. These methods consist of sequences
of external control operations designed to induce a desired dynamics. The
systems considered for the analysis are one-dimensional spin-1/2 models, which,
according to the parameters of the Hamiltonian, may be in the integrable or
non-integrable limits, and in the gapped or gapless phases. We show that an
appropriate control sequence may lead a chaotic chain to evolve as an
integrable chain and a system in the gapless phase to behave as a system in the
gapped phase. A key ingredient for the control schemes developed here is the
possibility to use, in the same sequence, different time intervals between
control operations.Comment: 10 pages, 3 figure
High resolution imaging of the early-type galaxy NGC 1380: an insight into the nature of extended extragalactic star clusters
NGC 1380 is a lenticular galaxy located near the centre of the Fornax Cluster
northeast of NGC 1399. The globular cluster system of this galaxy was
previously studied only from the ground. Recent studies of similar early-type
galaxies, specially lenticular ones, reveal the existence of star clusters that
apparently break up the traditional open/globular cluster dichotomy. With
higher quality photometry from HST/WFPC2 we study the star clusters in NGC
1380, measuring their magnitudes, colours, sizes and projected distances from
the centre of the galaxy. We used deep archival HST/WFPC2 in the B and V bands.
We built colour magnitude diagrams from which we selected a sample of cluster
candidates. We also analysed their colour distribution and measured their
sizes. Based on their location in the luminosity-size diagram we estimated
probabilities of them being typical globular clusters as those found in the
Galaxy. A total of about 570 cluster candidates were found down to V=26.5. We
measured sizes for approximately 200 of them. The observed colour distribution
has three apparent peaks. Likewise for the size distribution. We identified the
smaller population as being mainly typical globular clusters, while the more
extended objects have small probabilities of being such objects. Different
correlations between absolute magnitudes, sizes, colours and location were
inferred for these cluster sub-populations. Most extended clusters (Reff > 4
pc) share similar properties to the diffuse star clusters reported to inhabit
luminous early-type galaxies in the Virgo galaxy cluster such as being of low
surface brightness and fainter than MV ~ -8. We also report on a small group of
(Reff ~ 10 pc), -8< MV < -6, red clusters located near the centre of NGC 1380,
which may be interpreted as faint fuzzies.Comment: accepted for publication in A&
Three-dimensional quasi-Tonks gas in a harmonic trap
We analyze the macroscopic dynamics of a Bose gas in a harmonic trap with a
superimposed two-dimensional optical lattice, assuming a weak coupling between
different lattice sites. We consider the situation in which the local chemical
potential at each lattice site can be considered as that provided by the
Lieb-Liniger solution. Due to the weak coupling between sites and the form of
the chemical potential, the three-dimensional ground-state density profile and
the excitation spectrum acquire remarkable properties different from both 1D
and 3D gases. We call this system a quasi-Tonks gas. We discuss the range of
applicability of this regime, as well as realistic experimental situations
where it can be observed.Comment: 4 pages, 3 figures, misprints correcte
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