5,297 research outputs found
Dynamical charge density fluctuations pervading the phase diagram of a Cu-based high-Tc superconductor
Charge density waves are a common occurrence in all families of high critical
temperature superconducting cuprates. Although consistently observed in the
underdoped region of the phase diagram and at relatively low temperatures, it
is still unclear to what extent they influence the unusual properties of these
systems. Using resonant x-ray scattering we carefully determined the
temperature dependence of charge density modulations in
(Y,Nd)BaCuO for three doping levels. We discovered
short-range dynamical charge density fluctuations besides the previously known
quasi-critical charge density waves. They persist up to well above the
pseudogap temperature T*, are characterized by energies of few meV and pervade
a large area of the phase diagram, so that they can play a key role in shaping
the peculiar normal-state properties of cuprates.Comment: 34 pages, 4 figures, 11 supplementary figure
Thermodynamic Properties and Phase Transitions in a Mean-Field Ising Spin Glass on Lattice Gas: the Random Blume-Emery-Griffiths-Capel Model
The study of the mean-field static solution of the Random
Blume-Emery-Griffiths-Capel model, an Ising-spin lattice gas with quenched
random magnetic interaction, is performed. The model exhibits a paramagnetic
phase, described by a stable Replica Symmetric solution. When the temperature
is decreased or the density increases, the system undergoes a phase transition
to a Full Replica Symmetry Breaking spin-glass phase. The nature of the
transition can be either of the second order (like in the
Sherrington-Kirkpatrick model) or, at temperature below a given critical value,
of the first order in the Ehrenfest sense, with a discontinuous jump of the
order parameter and accompanied by a latent heat. In this last case coexistence
of phases takes place. The thermodynamics is worked out in the Full Replica
Symmetry Breaking scheme, and the relative Parisi equations are solved using a
pseudo-spectral method down to zero temperature.Comment: 24 pages, 12 figure
Running coupling in electroweak interactions of leptons from f(R)-gravity with torsion
The f(R)-gravitational theory with torsion is considered for one family of
leptons; it is found that the torsion tensor gives rise to interactions having
the structure of the weak forces while the intrinsic non-linearity of the f(R)
function provides an energy-dependent coupling: in this way, torsional f(R)
gravity naturally generates both structure and strength of the electroweak
interactions among leptons. This implies that the weak interactions among the
lepton fields could be addressed as a geometric effect due to the interactions
among spinors induced by the presence of torsion in the most general f(R)
gravity. Phenomenological considerations are addressed.Comment: 9 pages. arXiv admin note: text overlap with arXiv:1012.5529 by other
author
Control and optimization of semi-passively actuated multibody systems
The controlled multibody systems are under the consideration. At the lecture special emphasis is put on the study of underactuated and overactuated systems having different type of actuators (external powered drives, unpowered spring-damper like drives, etc.). Several questions are addressed about the role of inherent dynamics, and how much multibody system should be governed by external powered drives and how much by the systems inherent dynamics. The lecture consists of the following parts: introduction to the subject in question; mathematical statement of the optimal control problems that are suitable for modelling of controlled motion and optimization of semi-passively controlled multibody systems with different degrees of actuation; description of the methodology and the numerical algorithms for solution of control and optimization problems for semi-passively actuated multibody systems. The solutions of several optimal control problems for different kind of semi-passively actuated multibody systems are presented. Namely, the energy-optimal control of planar semi-passively controlled three-link manipulator robot, the energy-optimal control of closed-loop chain semi-passively actuated SCARA-like robot; optimization of the hydraulic and pneumatic drives of the multibody system modelled the human locomotor apparatus with above-knee prostheses, and others. Future perspectives in area of control and optimization problems of the semi-passively actuated multibody systems are discussed
XMM-Newton observation of PSR B2224+65 and its jet
We have investigated the pulsar PSR B2224+65 and its X-ray jet with
XMM-Newton. Apart from the long X-ray jet which is almost perpendicular to the
direction of proper motion, a putative extended feature at the pulsar position,
which oriented in the opposite direction of the proper motion, is also
suggested by this deep X-ray imaging. Non-detection of any coherent X-ray
pulsation disfavors the magnetospheric origin of the X-rays observed from the
position of PSR B2224+65 and hence suggest that the interpretation of pulsar
wind nebula is more viable. We have also probed the origin of PSR B2224+65 and
identified a runaway star, which possibly originated from the Cygnus OB9
association, as a candidate for the former binary companion of the neutron
star's progenitor.Comment: 24 pages, 8 figures, 3 tables, accepted for publication in Ap
Conceivable security risks and authentication techniques for smart devices
With the rapidly escalating use of smart devices and fraudulent transaction of users’ data from their devices, efficient and reliable techniques for authentication of the smart devices have become an obligatory issue. This paper reviews the security risks for mobile devices and studies several authentication techniques available for smart devices. The results from field studies enable a comparative evaluation of user-preferred authentication mechanisms and their opinions about reliability, biometric authentication and visual authentication techniques
Algorithms for Game Metrics
Simulation and bisimulation metrics for stochastic systems provide a
quantitative generalization of the classical simulation and bisimulation
relations. These metrics capture the similarity of states with respect to
quantitative specifications written in the quantitative {\mu}-calculus and
related probabilistic logics. We first show that the metrics provide a bound
for the difference in long-run average and discounted average behavior across
states, indicating that the metrics can be used both in system verification,
and in performance evaluation. For turn-based games and MDPs, we provide a
polynomial-time algorithm for the computation of the one-step metric distance
between states. The algorithm is based on linear programming; it improves on
the previous known exponential-time algorithm based on a reduction to the
theory of reals. We then present PSPACE algorithms for both the decision
problem and the problem of approximating the metric distance between two
states, matching the best known algorithms for Markov chains. For the
bisimulation kernel of the metric our algorithm works in time O(n^4) for both
turn-based games and MDPs; improving the previously best known O(n^9\cdot
log(n)) time algorithm for MDPs. For a concurrent game G, we show that
computing the exact distance between states is at least as hard as computing
the value of concurrent reachability games and the square-root-sum problem in
computational geometry. We show that checking whether the metric distance is
bounded by a rational r, can be done via a reduction to the theory of real
closed fields, involving a formula with three quantifier alternations, yielding
O(|G|^O(|G|^5)) time complexity, improving the previously known reduction,
which yielded O(|G|^O(|G|^7)) time complexity. These algorithms can be iterated
to approximate the metrics using binary search.Comment: 27 pages. Full version of the paper accepted at FSTTCS 200
Cold fronts and multi-temperature structures in the core of Abell 2052
The physics of the coolest phases in the hot Intra-Cluster Medium (ICM) of
clusters of galaxies is yet to be fully unveiled. X-ray cavities blown by the
central Active Galactic Nucleus (AGN) contain enough energy to heat the
surrounding gas and stop cooling, but locally blobs or filaments of gas appear
to be able to cool to low temperatures of 10^4 K. In X-rays, however, gas with
temperatures lower than 0.5 keV is not observed. Using a deep XMM-Newton
observation of the cluster of galaxies Abell 2052, we derive 2D maps of the
temperature, entropy, and iron abundance in the core region. About 130 kpc
South-West of the central galaxy, we discover a discontinuity in the surface
brightness of the hot gas which is consistent with a cold front. Interestingly,
the iron abundance jumps from ~0.75 to ~0.5 across the front. In a smaller
region to the North-West of the central galaxy we find a relatively high
contribution of cool 0.5 keV gas, but no X-ray emitting gas is detected below
that temperature. However, the region appears to be associated with much cooler
H-alpha filaments in the optical waveband. The elliptical shape of the cold
front in the SW of the cluster suggests that the front is caused by sloshing of
the hot gas in the clusters gravitational potential. This effect is probably an
important mechanism to transport metals from the core region to the outer parts
of the cluster. The smooth temperature profile across the sharp jump in the
metalicity indicates the presence of heat conduction and the lack of mixing
across the discontinuity. The cool blob of gas NW of the central galaxy was
probably pushed away from the core and squeezed by the adjacent bubble, where
it can cool efficiently and relatively undisturbed by the AGN. Shock induced
mixing between the two phases may cause the 0.5 keV gas to cool non-radiatively
and explain our non-detection of gas below 0.5 keV.Comment: 11 pages, 9 figures, A&A, in pres
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