A Relativistic Version of the Two-Level Atom in the Rest-Frame Instant Form of Dynamics

We define a relativistic version of the two-level atom, in which an extended atom is replaced by a point particle carrying suitable Grassmann variables for the description of the two-level structure and of the electric dipole. After studying the isolated system "atom plus the electro-magnetic field" in the electric-dipole representation as a parametrized Minkowski theory, we give its restriction to the inertial rest frame and the explicit form of the Poincar\'e generators. After quantization we get a two-level atom with a spin 1/2 electric dipole and the relativistic generalization of the Hamiltonians of the Rabi and Jaynes-Cummings models.Comment: 23 page

Microscopic measurement of the linear compressibilities of two-dimensional fatty acid mesophases

The linear compressibility of two-dimensional fatty acid mesophases has determined by grazing incidence x-ray diffraction. Surface pressure vs molecular area isotherms were reconstructed from these measurements, and the linear compressibility (relative distortion along a given direction for isotropic applied stress) was determined both in the sample plane and in a plane normal to the aliphatic chain director (transverse plane). The linear compressibilities range over two orders of magnitude from 0.1 to 10 m/N and are distributed depending on their magnitude in 4 different sets which we are able to associate with different molecular mechanisms. The largest compressibilities (10m/N) are observed in the tilted phases. They are apparently independent of the chain length and could be related to the reorganization of the headgroup hydrogen-bounded network, whose role should be revalued. Intermediate compressibilities are observed in phases with quasi long-range order (directions normal to the molecular tilt in L_2 or L_2' phases, S phase), and could be related to the ordering of these phases. The lowest compressibilities are observed in the solid untilted CS phase and for 1 direction of the S and L_2'' phases. They are similar to the compressibility of crystalline polymers and correspond to the interactions between methyl groups in the crystal. Finally, negative compressibilities are observed in the transverse plane for L_2' and L_2'' phases and can be traced to subtle reorganizations upon untilting.Comment: 24 pages, 17 figure

On the correlation between fragility and stretching in glassforming liquids

We study the pressure and temperature dependences of the dielectric relaxation of two molecular glassforming liquids, dibutyl phtalate and m-toluidine. We focus on two characteristics of the slowing down of relaxation, the fragility associated with the temperature dependence and the stretching characterizing the relaxation function. We combine our data with data from the literature to revisit the proposed correlation between these two quantities. We do this in light of constraints that we suggest to put on the search for empirical correlations among properties of glassformers. In particular, argue that a meaningful correlation is to be looked for between stretching and isochoric fragility, as both seem to be constant under isochronic conditions and thereby reflect the intrinsic effect of temperature

Disentangling density and temperature effects in the viscous slowing down of glassforming liquids

We present a consistent picture of the respective role of density and temperature in the viscous slowing down of glassforming liquids and polymers. Specifically, based in part upon a new analysis of simulation and experimental data on liquid ortho-terphenyl, we conclude that a zeroth-order description of the approach to the glass transition should be formulated in terms of a temperature-driven super-Arrhenius activated behavior rather than a density-driven congestion or jamming phenomenon. The density plays a role at a quantitative level, but its effect on the viscosity and the structural relaxation time can be simply described via a single parameter, an effective interaction energy that is characteristic of the high temperature liquid regime; as a result, density does not affect the ``fragility'' of the glassforming system.Comment: RevTeX4, 8 pages, 8 eps figure

The Chrono-geometrical Structure of Special and General Relativity: a Re-Visitation of Canonical Geometrodynamics

A modern re-visitation of the consequences of the lack of an intrinsic notion of instantaneous 3-space in relativistic theories leads to a reformulation of their kinematical basis emphasizing the role of non-inertial frames centered on an arbitrary accelerated observer. In special relativity the exigence of predictability implies the adoption of the 3+1 point of view, which leads to a well posed initial value problem for field equations in a framework where the change of the convention of synchronization of distant clocks is realized by means of a gauge transformation. This point of view is also at the heart of the canonical approach to metric and tetrad gravity in globally hyperbolic asymptotically flat space-times, where the use of Shanmugadhasan canonical transformations allows the separation of the physical degrees of freedom of the gravitational field (the tidal effects) from the arbitrary gauge variables. Since a global vision of the equivalence principle implies that only global non-inertial frames can exist in general relativity, the gauge variables are naturally interpreted as generalized relativistic inertial effects, which have to be fixed to get a deterministic evolution in a given non-inertial frame. As a consequence, in each Einstein's space-time in this class the whole chrono-geometrical structure, including also the clock synchronization convention, is dynamically determined and a new approach to the Hole Argument leads to the conclusion that "gravitational field" and "space-time" are two faces of the same entity. This view allows to get a classical scenario for the unification of the four interactions in a scheme suited to the description of the solar system or our galaxy with a deperametrization to special relativity and the subsequent possibility to take the non-relativistic limit.Comment: 33 pages, Lectures given at the 42nd Karpacz Winter School of Theoretical Physics, "Current Mathematical Topics in Gravitation and Cosmology", Ladek, Poland, 6-11 February 200

Connection between slow and fast dynamics of molecular liquids around the glass transition

The mean-square displacement (MSD) was measured by neutron scattering at various temperatures and pressures for a number of molecular glass-forming liquids. The MSD is invariant along the glass-transition line at the pressure studied, thus establishing an ``intrinsic'' Lindemann criterion for any given liquid. A one-to-one connection between the MSD's temperature dependence and the liquid's fragility is found when the MSD is evaluated on a time scale of approximately 4 nanoseconds, but does not hold when the MSD is evaluated at shorter times. The findings are discussed in terms of the elastic model and the role of relaxations, and the correlations between slow and fast dynamics are addressed.Comment: accepted by Phys Rev E (2010

Measuring gravitational lens time delays using low-resolution radio monitoring observations

Obtaining lensing time delay measurements requires long-term monitoring campaigns with a high enough resolution (< 1 arcsec) to separate the multiple images. In the radio, a limited number of high-resolution interferometer arrays make these observations difficult to schedule. To overcome this problem, we propose a technique for measuring gravitational time delays which relies on monitoring the total flux density with low-resolution but high-sensitivity radio telescopes to follow the variation of the brighter image. This is then used to trigger high-resolution observations in optimal numbers which then reveal the variation in the fainter image. We present simulations to assess the efficiency of this method together with a pilot project observing radio lens systems with the Westerbork Synthesis Radio Telescope (WSRT) to trigger Very Large Array (VLA) observations. This new method is promising for measuring time delays because it uses relatively small amounts of time on high-resolution telescopes. This will be important because instruments that have high sensitivity but limited resolution, together with an optimum usage of followup high-resolution observations from appropriate radio telescopes may in the future be useful for gravitational lensing time delay measurements by means of this new method.Comment: 10 pages, 7 figures, accepted by MNRA

Non-invasive Scanning Raman Spectroscopy and Tomography for Graphene Membrane Characterization

Graphene has extraordinary mechanical and electronic properties, making it a promising material for membrane based nanoelectromechanical systems (NEMS). Here, chemical-vapor-deposited graphene is transferred onto target substrates to suspend it over cavities and trenches for pressure-sensor applications. The development of such devices requires suitable metrology methods, i.e., large-scale characterization techniques, to confirm and analyze successful graphene transfer with intact suspended graphene membranes. We propose fast and noninvasive Raman spectroscopy mapping to distinguish between freestanding and substrate-supported graphene, utilizing the different strain and doping levels. The technique is expanded to combine two-dimensional area scans with cross-sectional Raman spectroscopy, resulting in three-dimensional Raman tomography of membrane-based graphene NEMS. The potential of Raman tomography for in-line monitoring is further demonstrated with a methodology for automated data analysis to spatially resolve the material composition in micrometer-scale integrated devices, including free-standing and substrate-supported graphene. Raman tomography may be applied to devices composed of other two-dimensional materials as well as silicon micro- and nanoelectromechanical systems.Comment: 23 pages, 5 figure

Static Observers in Curved Spaces and Non-inertial Frames in Minkowski Spacetime

Static observers in curved spacetimes may interpret their proper acceleration as the opposite of a local gravitational field (in the Newtonian sense). Based on this interpretation and motivated by the equivalence principle, we are led to investigate congruences of timelike curves in Minkowski spacetime whose acceleration field coincides with the acceleration field of static observers of curved spaces. The congruences give rise to non-inertial frames that are examined. Specifically we find, based on the locality principle, the embedding of simultaneity hypersurfaces adapted to the non-inertial frame in an explicit form for arbitrary acceleration fields. We also determine, from the Einstein equations, a covariant field equation that regulates the behavior of the proper acceleration of static observers in curved spacetimes. It corresponds to an exact relativistic version of the Newtonian gravitational field equation. In the specific case in which the level surfaces of the norm of the acceleration field of the static observers are maximally symmetric two-dimensional spaces, the energy-momentum tensor of the source is analyzed.Comment: 28 pages, 4 figures