144 research outputs found
New Magnetic Excitations in the Spin-Density-Wave of Chromium
Low-energy magnetic excitations of chromium have been reinvestigated with a
single-Q crystal using neutron scattering technique. In the transverse
spin-density-wave phase a new type of well-defined magnetic excitation is found
around (0,0,1) with a weak dispersion perpendicular to the wavevector of the
incommensurate structure. The magnetic excitation has an energy gap of E ~ 4
meV and at (0,0,1) exactly corresponds to the Fincher mode previously studied
only along the incommensurate wavevector.Comment: 4 pages, 4 figure
Charging Effects and Quantum Crossover in Granular Superconductors
The effects of the charging energy in the superconducting transition of
granular materials or Josephson junction arrays is investigated using a
pseudospin one model. Within a mean-field renormalization-group approach, we
obtain the phase diagram as a function of temperature and charging energy. In
contrast to early treatments, we find no sign of a reentrant transition in
agreement with more recent studies. A crossover line is identified in the
non-superconducting side of the phase diagram and along which we expect to
observe anomalies in the transport and thermodynamic properties. We also study
a charge ordering phase, which can appear for large nearest neighbor Coulomb
interaction, and show that it leads to first-order transitions at low
temperatures. We argue that, in the presence of charge ordering, a non
monotonic behavior with decreasing temperature is possible with a maximum in
the resistance just before entering the superconducting phase.Comment: 15 pages plus 4 fig. appended, Revtex, INPE/LAS-00
Graph Transformation for Domain-Specific Discrete Event Time Simulation
Proceedings of: Fifth International Conference on Graph Transformation (ICGT 2010). Enschede, The Netherlands, September 27–October 2, 2010.Graph transformation is being increasingly used to express the semantics of domain specific visual languages since its graphical nature makes rules intuitive. However, many application domains require an explicit handling of time in order to represent accurately the behaviour of the real system and to obtain useful simulation metrics. Inspired by the vast knowledge and experience accumulated by the discrete event simulation community, we propose a novel way of adding explicit time to graph transformation rules. In particular, we take the event scheduling discrete simulation world view and incorporate to the rules the ability of scheduling the occurrence of other rules in the future. Hence, our work combines standard, efficient techniques for discrete event simulation (based on the handling of a future event set) and the intuitive, visual nature of graph transformation. Moreover, we show how our formalism can be used to give semantics to other timed approaches.Work partially sponsored by the Spanish Ministry of Science and Innovation, under project “METEORIC” (TIN2008-02081) and mobility grants JC2009-00015 and PR2009-0019, as well as by the R&D programme of the Community of Madrid, project “e-Madrid” (S2009/TIC-1650).Publicad
Sensitivity to measurement perturbation of single atom dynamics in cavity QED
We consider continuous observation of the nonlinear dynamics of single atom
trapped in an optical cavity by a standing wave with intensity modulation. The
motion of the atom changes the phase of the field which is then monitored by
homodyne detection of the output field. We show that the conditional Hilbert
space dynamics of this system, subject to measurement induced perturbations,
depends strongly on whether the corresponding classical dynamics is regular or
chaotic. If the classical dynamics is chaotic the distribution of conditional
Hilbert space vectors corresponding to different observation records tends to
be orthogonal. This is a characteristic feature of hypersensitivity to
perturbation for quantum chaotic systems.Comment: 11 pages, 6 figure
Gamma Ray Bursts as Probes of Quantum Gravity
Gamma ray bursts (GRBs) are short and intense pulses of -rays
arriving from random directions in the sky. Several years ago Amelino-Camelia
et al. pointed out that a comparison of time of arrival of photons at different
energies from a GRB could be used to measure (or obtain a limit on) possible
deviations from a constant speed of light at high photons energies. I review
here our current understanding of GRBs and reconsider the possibility of
performing these observations.Comment: Lectures given at the 40th winter school of theretical physics:
Quantum Gravity and Phenomenology, Feb. 2004 Polan
Experimental study of the quantum driven pendulum and its classical analogue in atoms optics
We present experimental results for the dynamics of cold atoms in a far detuned amplitude-modulated optical standing wave. Phase-space resonances constitute distinct peaks in the atomic momentum distribution containing up to 65% of all atoms resulting from a mixed quantum chaotic phase space. We characterize the atomic behavior in classical and quantum regimes and we present the applicable quantum and classical theory, which we have developed and refined. We show experimental proof that the size and the position of the resonances in phase space can be controlled by varying several parameters, such as the modulation frequency, the scaled well depth, the modulation amplitude, and the scaled Planck's constant of the system. We have found a surprising stability against amplitude noise. We present methods to accurately control the momentum of an ensemble of atoms using these phase-space resonances which could be used for efficient phase-space state preparation
Ultra-Fast Flash Observatory: Fast Response Space Missions for Early Time Phase of Gamma Ray Bursts
One of the unexplored domains in the study of gamma-ray bursts (GRBs) is the early time
phase of the optical light curve. We have proposed Ultra-Fast Flash Observatory (UFFO) to
address this question through extraordinary opportunities presented by a series of small
space missions. The UFFO is equipped with a fast-response Slewing Mirror Telescope that
uses a rapidly moving mirror or mirror array to redirect the optical beam rather than
slewing the entire spacecraft or telescope to aim the optical instrument at the GRB
position. The UFFO will probe the early optical rise of GRBs with sub-second response, for
the first time, opening a completely new frontier in GRB and transient studies. Its fast
response measurements of the optical emission of dozens of GRB each year will provide
unique probes of the burst mechanism and test the prospect of GRB as a new standard
candle, potentially opening up the z > 10 universe. We describe the current limit in
early photon measurements, the aspects of early photon physics, our soon-to-be-launched
UFFO-pathfinder mission, and our next planned mission, the UFFO-100
Enhancing studies of the connectome in autism using the autism brain imaging data exchange II
The second iteration of the Autism Brain Imaging Data Exchange (ABIDE II) aims to enhance the scope of brain connectomics research in Autism Spectrum Disorder (ASD). Consistent with the initial ABIDE effort (ABIDE I), that released 1112 datasets in 2012, this new multisite open-data resource is an aggregate of resting state functional magnetic resonance imaging (MRI) and corresponding structural MRI and phenotypic datasets. ABIDE II includes datasets from an additional 487 individuals with ASD and 557 controls previously collected across 16 international institutions. The combination of ABIDE I and ABIDE II provides investigators with 2156 unique cross-sectional datasets allowing selection of samples for discovery and/or replication. This sample size can also facilitate the identification of neurobiological subgroups, as well as preliminary examinations of sex differences in ASD. Additionally, ABIDE II includes a range of psychiatric variables to inform our understanding of the neural correlates of co-occurring psychopathology; 284 diffusion imaging datasets are also included. It is anticipated that these enhancements will contribute to unraveling key sources of ASD heterogeneity
Management of patients with increased risk for familial pancreatic cancer: updated recommendations from the International Cancer of the Pancreas Screening (CAPS) Consortium
Surgical oncolog
On the Problem of Dynamical Localization in the Nonlinear Schrödinger Equation with a Random Potential
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