652 research outputs found
Doping dependence of the vortex-core energy in bilayer films of cuprates
The energy needed to create a vortex core is the basic ingredient to address
the physics of thermal vortex fluctuations in underdoped cuprates. Here we
theoretically investigate its role on the occurrence of the
Beresinskii-Kosterlitz-Thouless transition in a bilayer film with
inhomogeneity. From the comparison with recent measurements of the penetration
depth in two-unit cell thin films of
YCaBaCuO_{7-\d} (YBCO) by Hetel et al. [Nat. Phys.
3, 700 (2007)] we can extract the value of the vortex-core energy , and
show that scales linearly with at low doping.Comment: 4pages, 3 figures. References added, final versio
Decay of a superfluid current of ultra-cold atoms in a toroidal trap
Using a numerical implementation of the truncated Wigner approximation, we
simulate the experiment reported by Ramanathan et al. in Phys. Rev. Lett. 106,
130401 (2011), in which a Bose-Einstein condensate is created in a toroidal
trap and set into rotation via a phase imprinting technique. A potential
barrier is then placed in the trap to study the decay of the superflow. We find
that the current decays via thermally activated phase slips, which can also be
visualized as vortices crossing the barrier region in the radial direction.
Adopting the notion of critical velocity used in the experiment, we determine
it to be lower than the local speed of sound at the barrier, in contradiction
to the predictions of the zero-temperature Gross-Pitaevskii equation. We map
out the superfluid decay rate and critical velocity as a function of
temperature and observe a strong dependence. Thermal fluctuations offer a
partial explanation of the experimentally observed reduction of the critical
velocity from the phonon velocity.Comment: 15 pages. 11 figure
Experimental and numerical investigation of the earthquake response of crane bridges
© 2014 Elsevier Ltd. The experimental and numerical response of crane bridges is studied in this work. To this end, an experimental campaign on a scale model of an overhead crane bridge was carried out on the shaking table of CEA/Saclay in France. A special similarity law has been used which preserves the ratios of seismic forces to friction forces and of seismic forces to gravity forces, without added masses. A numerical model, composed of beam elements, which takes into account non-linear effects, especially impact and friction, and simulates the earthquake response of the crane bridge, is presented. The comparison of experimental and analytical results gives an overall satisfactory agreement. Finally, a simplified model of the crane bridge, with only a few degrees of freedom is proposed
Critical velocity for a toroidal Bose-Einstein condensate flowing through a barrier
We consider the setup employed in a recent experiment (Ramanathan et al 2011
Phys. Rev. Lett. 106 130401) devoted to the study of the instability of the
superfluid flow of a toroidal Bose-Einstein condensate in presence of a
repulsive optical barrier. Using the Gross-Pitaevskii mean-field equation, we
observe, consistently with what we found in Piazza et al (2009 Phys. Rev. A 80
021601), that the superflow with one unit of angular momentum becomes unstable
at a critical strength of the barrier, and decays through the mechanism of
phase slippage performed by pairs of vortex-antivortex lines annihilating.
While this picture qualitatively agrees with the experimental findings, the
measured critical barrier height is not very well reproduced by the
Gross-Pitaevskii equation, indicating that thermal fluctuations can play an
important role (Mathey et al 2012 arXiv:1207.0501). As an alternative
explanation of the discrepancy, we consider the effect of the finite resolution
of the imaging system. At the critical point, the superfluid velocity in the
vicinity of the obstacle is always of the order of the sound speed in that
region, . In particular, in the hydrodynamic regime
(not reached in the above experiment), the critical point is determined by
applying the Landau criterion inside the barrier region. On the other hand, the
Feynman critical velocity is much lower than the observed critical
velocity. We argue that this is a general feature of the Gross-Pitaevskii
equation, where we have with being a
small parameter of the model. Given these observations, the question still
remains open about the nature of the superfluid instability.Comment: Extended versio
Mathematical formulation of a dynamical system with dry friction subjected to external forces
We consider the response of a one-dimensional system with friction. S.W. Shaw
(Journal of Sound and Vibration, 1986) introduced the set up of different
coefficients for the static and dynamic phases (also called stick and slip
phases). He constructs a step by step solution, corresponding to an harmonic
forcing. In this paper, we show that the theory of variational inequalities
provides an elegant and synthetic approach to obtain the existence and
uniqueness of the solution, avoiding the step by step construction. We then
apply the theory to a real structure with real data and show that the model is
quite accurate. In our case, the forcing motion comes from dilatation, due to
temperature
Phase fluctuations in anisotropic Bose condensates: from cigars to rings
We study the phase-fluctuating condensate regime of ultra-cold atoms trapped
in a ring-shaped trap geometry, which has been realized in recent experiments.
We first consider a simplified box geometry, in which we identify the
conditions to create a state that is dominated by thermal phase-fluctuations,
and then explore the experimental ring geometry. In both cases we demonstrate
that the requirement for strong phase fluctuations can be expressed in terms of
the total number of atoms and the geometric length scales of the trap only. For
the ring-shaped trap we discuss the zero temperature limit in which a
condensate is realized where the phase is fluctuating due to interactions and
quantum fluctuations. We also address possible ways of detecting the phase
fluctuating regime in ring condensates.Comment: 10 pages, 5 figures, minor edit
CONTROL ENCRYPTION OF TECHNIQUE USING THE CLOUD COMPUTING SYD
Information storage and computing information issues can be overcome by mobile applications by using cloud computing. The new model can also make various data based on the cloud to chat, complete the location services and the operating system in real time well and at the same time. By combining cloud computing, security issues may arise, for example, data secrets and user authorization within cloud computing systems, which concern the first restrictions on the development of a mobile computer cloud. In order to provide a safe and powerful process, the hierarchical access control system is proposed using the encryption based on a fixed schedule and a structure under a modified format in this document. In this study, the independent control system is proposed through the encryption of the file according to the structural design of a three-story structural design. The ABE-based access control system uses several tags to distinguish the attributes the authorized user has to have. Within a certain cloud computing environment, large data for all types of mobile devices, such as mobile phones, calls, PDAs, etc., can be controlled and tested by the system, and the data can respond to an unauthorized third party and restricted to legal users as well
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