6,535 research outputs found
Localization in momentum space of ultracold atoms in incommensurate lattices
We characterize the disorder induced localization in momentum space for
ultracold atoms in one-dimensional incommensurate lattices, according to the
dual Aubry-Andr\'e model. For low disorder the system is localized in momentum
space, and the momentum distribution exhibits time-periodic oscillations of the
relative intensity of its components. The behavior of these oscillations is
explained by means of a simple three-mode approximation. We predict their
frequency and visibility by using typical parameters of feasible experiments.
Above the transition the system diffuses in momentum space, and the
oscillations vanish when averaged over different realizations, offering a clear
signature of the transition
Numerical study of the localization length critical index in a network model of plateau-plateau transitions in the quantum Hall effect
We calculate numerically the localization length critical index within the
Chalker-Coddington (CC) model for plateau-plateau transitions in the quantum
Hall effect. Lyapunov exponents have been calculated with relative errors on
the order . Such high precision was obtained by considering the
distribution of Lyapunov exponents for large ensembles of relatively short
chains and calculating the ensemble average values. We analyze thoroughly
finite size effects and find the localization length critical index .Comment: 4 pages, 4 figure
Neutral and ionic dopants in helium clusters: interaction forces for the and
The potential energy surface (PES) describing the interactions between
and and an extensive
study of the energies and structures of a set of small clusters,
, have been presented by us in a previous series of
publications [1-3]. In the present work we want to extend the same analysis to
the case of the excited and of the
ionized Li moiety. We thus show here calculated
interaction potentials for the two title systems and the corresponding fitting
of the computed points. For both surfaces the MP4 method with cc-pV5Z basis
sets has been used to generate an extensive range of radial/angular coordinates
of the two dimensional PES's which describe rigid rotor molecular dopants
interacting with one He partner
Actively stressed marginal networks
We study the effects of motor-generated stresses in disordered three
dimensional fiber networks using a combination of a mean-field, effective
medium theory, scaling analysis and a computational model. We find that motor
activity controls the elasticity in an anomalous fashion close to the point of
marginal stability by coupling to critical network fluctuations. We also show
that motor stresses can stabilize initially floppy networks, extending the
range of critical behavior to a broad regime of network connectivities below
the marginal point. Away from this regime, or at high stress, motors give rise
to a linear increase in stiffness with stress. Finally, we demonstrate that our
results are captured by a simple, constitutive scaling relation highlighting
the important role of non-affine strain fluctuations as a susceptibility to
motor stress.Comment: 8 pages, 4 figure
Competition of the connectivity with the local and the global order in polymer melts and crystals
The competition between the connectivity and the local or global order in
model fully-flexible chain molecules is investigated by molecular-dynamics
simulations. States with both missing (melts) and high (crystal) global order
are considered. Local order is characterized within the first coordination
shell (FCS) of a tagged monomer and found to be lower than in atomic systems in
both melt and crystal. The role played by the bonds linking the tagged monomer
to FCS monomers (radial bonds), and the bonds linking two FCS monomers (shell
bonds) is investigated. The detailed analysis in terms of Steinhardt's
orientation order parameters Q_l (l = 2 - 10) reveals that increasing the
number of shell bonds decreases the FCS order in both melt and crystal.
Differently, the FCS arrangements organize the radial bonds. Even if the
molecular chains are fully flexible, the distribution of the angle formed by
adjacent radial bonds exhibits sharp contributions at the characteristic angles
{\theta} = 70{\deg}, 122{\deg}, 180{\deg}. The fractions of adjacent radial
bonds with {\theta} = 122{\deg}, 180{\deg} are enhanced by the global order of
the crystal, whereas the fraction with 70{\deg} < {\theta} < 110{\deg} is
nearly unaffected by the crystallization. Kink defects, i.e. large lateral
displacements of the chains, are evidenced in the crystalline state.Comment: J. Chem. Phys. in pres
Multi--hump soliton--like structures in interactions of lasers and Bose--Einstein condensates
An investigation is made of multi-hump and periodic solutions of the
semi-classical coupled equations describing laser radiation copropagating with
a Bose-Einstein condensate. Solutions reminiscent of optical vector solitons
have been found and have been used to gain understanding of the dynamics
observed in the numerical simulations, in particular to shed light on the
phenomenon of jet emission from a condensate interacting with a laser.Comment: 6 pages, 4 figures; submitted to European Physics Letter
Coherent open-loop optimal control of light-harvesting dynamics
We apply theoretically open-loop quantum optimal control techniques to
provide methods for the verification of various quantum coherent transport
mechanisms in natural and artificial light-harvesting complexes under realistic
experimental constraints. We demonstrate that optimally shaped laser pulses
allow to faithfully prepare the photosystem in specified initial states (such
as localized excitation or coherent superposition, i.e. propagating and
non-propagating states) and to probe efficiently the dynamics. These results
provide a path towards the discrimination of the different transport pathways
and to the characterization of environmental properties, enhancing our
understanding of the role that coherent processes may play in biological
complexes.Comment: 12 pages, 15 figure
Critical behavior of charmonia across the phase transition: A QCD sum rule approach
We investigate medium-induced change of mass and width of J/psi and eta_c
across the phase transition in hot gluonic matter using QCD sum rules. In the
QCD sum rule approach, the medium effect on heavy quarkonia is induced by the
change of both scalar and twist-2 gluon condensates, whose temperature
dependences are extracted from the lattice calculations of energy density and
pressure. Although the stability of the operator product expansion side seems
to break down at T > 1.06Tc for the vector channel and T>1.04Tc for the
pseudoscalar channel, we find a sudden change of the spectral property across
the critical temperature Tc, which originates from an equally rapid change of
the scalar gluon condensate characterized by e-3p. By parameterizing the ground
state of the spectral density by the Breit-Wigner form, we find that for both
J/psi and eta_c, the masses suddenly decrease maximally by a few hundreds of
MeV and the widths broaden to ~100 MeV slightly above Tc. Implications for
recent and future heavy ion experiments are discussed.
We also carry out a similar analysis for charmonia in nuclear matter, which
could serve as a testing ground for observing the precursor phenomena of the
QCD phase transition. We finally discuss the possibility of observing the mass
shift at nuclear matter at the FAIR project at GSI.Comment: 18 pages, 21 figures, 2 figures are added and discussion on effect of
dynamical quarks is extended. version to appear in Phys.Rev.
Effects of hydrogen/deuterium absorption on the magnetic properties of Co/Pd multilayers
The effects of hydrogen (H2) and deuterium (D2) absorption were studied in
two Co/Pd multilayers with perpendicular magnetic anisotropy (PMA) using
polarized neutron reflectivity (PNR). PNR was measured in an external magnetic
field H applied in the plane of the sample with the magnetization M confined in
the plane for {\mu}_o H= 6.0 T and partially out of plane at 0.65 T. Nominal
thicknesses of the Co and Pd layers were 2.5 {\AA} and 21 {\AA}, respectively.
Because of these small values, the actual layer chemical composition,
thickness, and interface roughness parameters were determined from the nuclear
scattering length density profile ({\rho}_n) and its derivative obtained from
both x-ray reflectivity and PNR, and uncertainties were determined using Monte
Carlo analysis. The PNR {\rho}_n showed that although D2 absorption occurred
throughout the samples, absorption in the multilayer stack was modest (0.02 D
per Pd atom) and thus did not expand. Direct magnetometry showed that H2
absorption decreased the total M at saturation and increased the component of M
in the plane of the sample when not at saturation. The PNR magnetic scattering
length density ({\rho}_m) revealed that the Pd layers in the multilayer stack
were magnetized and that their magnetization was preferentially modified upon
D2 absorption. In one sample, a modulation of M with twice the multilayer
period was observed at {\mu}_o H= 0.65 T, which increased upon D2 absorption.
These results indicate that H2 or D2 absorption decreases both the PMA and
total magnetization of the samples. The lack of measurable expansion during
absorption indicates that these changes are primarily governed by modification
of the electronic structure of the material.Comment: to appear in Physics review B, 201
Hysteresis effects in rotating Bose-Einstein condensates
We study the formation of vortices in a dilute Bose-Einstein condensate
confined in a rotating anisotropic trap. We find that the number of vortices
and angular momentum attained by the condensate depends upon the rotation
history of the trap and on the number of vortices present in the condensate
initially. A simplified model based on hydrodynamic equations is developed, and
used to explain this effect in terms of a shift in the resonance frequency of
the quadrupole mode of the condensate in the presence of a vortex lattice.
Differences between the spin-up and spin-down response of the condensate are
found, demonstrating hysteresis phenomena in this system.Comment: 16 pages, 7 figures; revised after referees' report
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