Kraus representation in the presence of initial correlations

We examine the validity of the Kraus representation in the presence of initial correlations and show that it is assured only when a joint dynamics is locally unitary.Comment: REVTeX4, 12 page

Reconstructing the density operator by using generalized field quadratures

The Wigner function for one and two-mode quantum systems is explicitely expressed in terms of the marginal distribution for the generic linearly transformed quadratures. Then, also the density operator of those systems is written in terms of the marginal distribution of these quadratures. Some examples to apply this formalism, and a reduction to the usual optical homodyne tomography are considered.Comment: 17 pages, Latex,accepted by Quantum and Semiclassical Optic

XMM-Newton Optical Monitor observations of LMC X-3

We study the optical counterpart of the black-hole X-ray binary LMC X-3, by using XMM-Newton/OM observations carried out during a low-hard X-ray state. We derive a better constraint for the temperature, mass and radius of the companion star, and we show that the star is likely to be a ~ B5 subgiant filling its Roche lobe. Taking into account the effect of X-ray irradiation, we suggest a value f_M = (1.5 +/- 0.3) M_sun for the mass function in this system, lower than previously thought; we provide a more accurate lower limit to the mass of the compact object.Comment: accepted for publication in the special XMM-Newton issue of A&A

The anisotropy of granular materials

The effect of the anisotropy on the elastoplastic response of two dimensional packed samples of polygons is investigated here, using molecular dynamics simulation. We show a correlation between fabric coefficients, characterizing the anisotropy of the granular skeleton, and the anisotropy of the elastic response. We also study the anisotropy induced by shearing on the subnetwork of the sliding contacts. This anisotropy provides an explanation to some features of the plastic deformation of granular media.Comment: Submitted to PR

Quasi-molecular lines in Lyman wings of cool DA white dwarfs; Application to FUSE observations of G231-40

We present new theoretical calculations of the total line profiles of Lyman alpha and Lyman beta which include perturbations by both neutral hydrogen AND protons and all possible quasi-molecular states of H_2 and H_2^+. They are used to improve theoretical modeling of synthetic spectra for cool DA white dwarfs. We compare them with FUSE observation of G231-40. The appearance of the line wings between Lyman alpha and Lyman beta is shown to be sensitive to the relative abundance of hydrogen ions and neutral atoms, and thereby to provide a temperature diagnostic for stellar atmospheres and laboratory plasmas.Comment: 6 pages, 4 figures, accepted for publication in Astronomy and Astrophysic

Unified force law for granular impact cratering

Experiments on the low-speed impact of solid objects into granular media have been used both to mimic geophysical events and to probe the unusual nature of the granular state of matter. Observations have been interpreted in terms of conflicting stopping forces: product of powers of projectile depth and speed; linear in speed; constant, proportional to the initial impact speed; and proportional to depth. This is reminiscent of high-speed ballistics impact in the 19th and 20th centuries, when a plethora of empirical rules were proposed. To make progress, we developed a means to measure projectile dynamics with 100 nm and 20 us precision. For a 1-inch diameter steel sphere dropped from a wide range of heights into non-cohesive glass beads, we reproduce prior observations either as reasonable approximations or as limiting behaviours. Furthermore, we demonstrate that the interaction between projectile and medium can be decomposed into the sum of velocity-dependent inertial drag plus depth-dependent friction. Thus we achieve a unified description of low-speed impact phenomena and show that the complex response of granular materials to impact, while fundamentally different from that of liquids and solids, can be simply understood

Characterization of the material response in the granular ratcheting

The existence of a very special ratcheting regime has recently been reported in a granular packing subjected to cyclic loading \cite{alonso04}. In this state, the system accumulates a small permanent deformation after each cycle. After a short transient regime, the value of this permanent strain accumulation becomes independent on the number of cycles. We show that a characterization of the material response in this peculiar state is possible in terms of three simple macroscopic variables. They are defined that, they can be easily measured both in the experiments and in the simulations. We have carried out a thorough investigation of the micro- and macro-mechanical factors affecting these variables, by means of Molecular Dynamics simulations of a polydisperse disk packing, as a simple model system for granular material. Biaxial test boundary conditions with a periodically cycling load were implemented. The effect on the plastic response of the confining pressure, the deviatoric stress and the number of cycles has been investigated. The stiffness of the contacts and friction has been shown to play an important role in the overall response of the system. Specially elucidating is the influence of the particular hysteretical behavior in the stress-strain space on the accumulation of permanent strain and the energy dissipation.Comment: 13 pages, 20 figures. Submitted to PR

Binary reaction decays from 24Mg+12C

Charged particle and gamma decays in 24Mg* are investigated for excitation energies where quasimolecular resonances appear in 12C+12C collisions. Various theoretical predictions for the occurence of superdeformed and hyperdeformed bands associated with resonance structures with low spin are discussed within the measured 24Mg* excitation energy region. The inverse kinematics reaction 24Mg+12C is studied at E_lab(24Mg) = 130 MeV, an energy which enables the population of 24Mg states decaying into 12C+12C resonant break-up states. Exclusive data were collected with the Binary Reaction Spectrometer in coincidence with EUROBALL IV installed at the VIVITRON Tandem facility at Strasbourg. Specific structures with large deformation were selectively populated in binary reactions and their associated gamma decays studied. Coincident events associated with inelastic and alpha-transfer channels have been selected by choosing the excitation energy or the entry point via the two-body Q-values. The analysis of the binary reaction channels is presented with a particular emphasis on 24Mg-gamma, 20Ne-gamma and 16O-gamma coincidences. New information (spin and branching ratios) is deduced on high-energy states in 24Mg and 16O, respectively.Comment: 27 pages, 8 figures, 1 tabl

The role of interstitial gas in determining the impact response of granular beds

We examine the impact of a solid sphere into a fine-grained granular bed. Using high-speed X-ray radiography we track both the motion of the sphere and local changes in the bed packing fraction. Varying the initial packing density as well as the ambient gas pressure, we find a complete reversal in the effect of interstitial gas on the impact response of the bed: The dynamic coupling between gas and grains allows for easier penetration in initially loose beds but impedes penetration in more densely packed beds. High-speed imaging of the local packing density shows that these seemingly incongruous effects have a common origin in the resistance to bed packing changes caused by interstitial air.Comment: 5 pages, 4 figures, submitted to EP