9,132 research outputs found

    Cosmological Inflation and the Quantum Measurement Problem

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    According to cosmological inflation, the inhomogeneities in our universe are of quantum mechanical origin. This scenario is phenomenologically very appealing as it solves the puzzles of the standard hot big bang model and naturally explains why the spectrum of cosmological perturbations is almost scale invariant. It is also an ideal playground to discuss deep questions among which is the quantum measurement problem in a cosmological context. Although the large squeezing of the quantum state of the perturbations and the phenomenon of decoherence explain many aspects of the quantum to classical transition, it remains to understand how a specific outcome can be produced in the early universe, in the absence of any observer. The Continuous Spontaneous Localization (CSL) approach to quantum mechanics attempts to solve the quantum measurement question in a general context. In this framework, the wavefunction collapse is caused by adding new non linear and stochastic terms to the Schroedinger equation. In this paper, we apply this theory to inflation, which amounts to solving the CSL parametric oscillator case. We choose the wavefunction collapse to occur on an eigenstate of the Mukhanov-Sasaki variable and discuss the corresponding modified Schroedinger equation. Then, we compute the power spectrum of the perturbations and show that it acquires a universal shape with two branches, one which remains scale invariant and one with nS=4, a spectral index in obvious contradiction with the Cosmic Microwave Background (CMB) anisotropy observations. The requirement that the non-scale invariant part be outside the observational window puts stringent constraints on the parameter controlling the deviations from ordinary quantum mechanics... (Abridged).Comment: References added, minor corrections, conclusions unchange

    Ultrafast (but Many-Body) Relaxation in a Low-Density Electron Glass

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    We present a study of the relaxation dynamics of the photoexcited conductivity of the impurity states in the low-density electronic glass, phosphorous-doped silicon Si:P. Using optical pump-terahertz probe spectroscopy we find strongly temperature and fluence dependent glassy power-law relaxation occurring over sub-ns time scales. Such behavior is in contrast to the much longer time scales found in higher electron density glassy systems. We also find evidence for both multi-particle relaxation mechanisms and/or coupling to electronic collective modes and a low temperature quantum relaxational regime.Comment: 4 pages, 4 figures, Appeared in Phys. Rev. Let

    Doping effects on the electronic and structural properties of CoO2: An LSDA+U study

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    A systematic LSDA+U study of doping effects on the electronic and structural properties of single layer CoO2 is presented. Undoped CoO2 is a charge transfer insulator within LSDA+U and a metal with a high density of states (DOS) at the Fermi level within LSDA. (CoO2)1.0^{1.0-}, on the other hand, is a band insulator with a gap of 2.2 eV. Systems with fractional doping are metals if no charge orderings are present. Due to the strong interaction between the doped electron and other correlated Co d electrons, the calculated electronic structure of (CoO2)x^{x-} depends sensitively on the doping level x. Zone center optical phonon energies are calculated under the frozen phonon approximation and are in good agreement with measured values. Softening of the EgE_g phonon at doping x ~0.25 seems to indicate a strong electron-phonon coupling in this system. Possible intemediate spin states of Co ions, Na ordering, as well as magnetic and charge orderings in this system are also discussed.Comment: 11 pages, 12 figure

    Gauging kinematical and internal symmetry groups for extended systems: the Galilean one-time and two-times harmonic oscillators

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    The possible external couplings of an extended non-relativistic classical system are characterized by gauging its maximal dynamical symmetry group at the center-of-mass. The Galilean one-time and two-times harmonic oscillators are exploited as models. The following remarkable results are then obtained: 1) a peculiar form of interaction of the system as a whole with the external gauge fields; 2) a modification of the dynamical part of the symmetry transformations, which is needed to take into account the alteration of the dynamics itself, induced by the {\it gauge} fields. In particular, the Yang-Mills fields associated to the internal rotations have the effect of modifying the time derivative of the internal variables in a scheme of minimal coupling (introduction of an internal covariant derivative); 3) given their dynamical effect, the Yang-Mills fields associated to the internal rotations apparently define a sort of Galilean spin connection, while the Yang-Mills fields associated to the quadrupole momentum and to the internal energy have the effect of introducing a sort of dynamically induced internal metric in the relative space.Comment: 32 pages, LaTex using the IOP preprint macro package (ioplppt.sty available at: http://www.iop.org/). The file is available at: http://www.fis.unipr.it/papers/1995.html The file is a uuencoded tar gzip file with the IOP preprint style include

    Spoligotype database of Mycobacterium tuberculosis: biogeographic distribution of shared types and epidemiologic and phylogenetic perspectives.

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    We give an update on the worldwide spoligotype database, which now contains 3,319 spoligotype patterns of Mycobacterium tuberculosis in 47 countries, with 259 shared types, i.e., identical spoligotypes shared by two or more patient isolates. The 259 shared types contained a total of 2,779 (84%) of all the isolates. Seven major genetic groups represented 37% of all clustered isolates. Two types (119 and 137) were found almost exclusively in the USA and accounted for 9% of clustered isolates. The remaining 1,517 isolates were scattered into 252 different spoligotypes. This database constitutes a tool for pattern comparison of M. tuberculosis clinical isolates for global epidemiologic studies and phylogenetic purposes

    Proposing a roadmap for HealthGrids

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    Présentation commune des éditeur

    The relative influences of disorder and of frustration on the glassy dynamics in magnetic systems

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    The magnetisation relaxations of three different types of geometrically frustrated magnetic systems have been studied with the same experimental procedures as previously used in spin glasses. The materials investigated are Y2_2Mo2_2O7_7 (pyrochlore system), SrCr8.6_{8.6}Ga3.4_{3.4}O19_{19} (piled pairs of Kagom\'e layers) and (H3_3O)Fe3_3(SO4_4)2_2(OH)6_6 (jarosite compound). Despite a very small amount of disorder, all the samples exhibit many characteristic features of spin glass dynamics below a freezing temperature TgT_g, much smaller than their Curie-Weiss temperature θ\theta. The ageing properties of their thermoremanent magnetization can be well accounted for by the same scaling law as in spin glasses, and the values of the scaling exponents are very close. The effects of temperature variations during ageing have been specifically investigated. In the pyrochlore and the bi-Kagom\'e compounds, a decrease of temperature after some waiting period at a certain temperature TpT_p re-initializes ageing and the evolution at the new temperature is the same as if the system were just quenched from above TgT_g. However, as the temperature is raised back to TpT_p, the sample recovers the state it had previously reached at that temperature. These features are known in spin glasses as rejuvenation and memory effects. They are clear signatures of the spin glass dynamics. In the Kagom\'e compound, there is also some rejuvenation and memory, but much larger temperature changes are needed to observe the effects. In that sense, the behaviour of this compound is quantitatively different from that of spin glasses.Comment: latex VersionCorrigee4.tex, 4 files, 3 figures, 5 pages (Proceedings of the International Conference on Highly Frustrated Magnetism (HFM2003), August 26-30, 2003, Institut Laue Langevin (ILL), Grenoble, France

    Renormalization of Non-Commutative Phi^4_4 Field Theory in x Space

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    In this paper we provide a new proof that the Grosse-Wulkenhaar non-commutative scalar Phi^4_4 theory is renormalizable to all orders in perturbation theory, and extend it to more general models with covariant derivatives. Our proof relies solely on a multiscale analysis in x space. We think this proof is simpler and could be more adapted to the future study of these theories (in particular at the non-perturbative or constructive level).Comment: 32 pages, v2: correction of lemmas 3.1 and 3.2 with no consequence on the main resul

    Non-thermal Processes in Black-Hole-Jet Magnetospheres

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    The environs of supermassive black holes are among the universe's most extreme phenomena. Understanding the physical processes occurring in the vicinity of black holes may provide the key to answer a number of fundamental astrophysical questions including the detectability of strong gravity effects, the formation and propagation of relativistic jets, the origin of the highest energy gamma-rays and cosmic-rays, and the nature and evolution of the central engine in Active Galactic Nuclei (AGN). As a step towards this direction, this paper reviews some of the progress achieved in the field based on observations in the very high energy domain. It particularly focuses on non-thermal particle acceleration and emission processes that may occur in the rotating magnetospheres originating from accreting, supermassive black hole systems. Topics covered include direct electric field acceleration in the black hole's magnetosphere, ultra-high energy cosmic ray production, Blandford-Znajek mechanism, centrifugal acceleration and magnetic reconnection, along with the relevant efficiency constraints imposed by interactions with matter, radiation and fields. By way of application, a detailed discussion of well-known sources (Sgr A*; Cen A; M87; NGC1399) is presented.Comment: invited review for International Journal of Modern Physics D, 49 pages, 15 figures; minor typos corrected to match published versio

    Quantum vibrational resonance in a dual-frequency driven Tietz-Hua quantum well

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    We investigate the response of a quantum particle in the Tietz-Hua quantum potential driven by biharmonic fields – a low and a very high frequency force. The response is characterized by the occurrence of a maximum in the first-order transition probability amplitude, |s|2 , under the influence of the applied fields. It is shown that in the absence of the high-frequency component of the applied fields, |s|2 shows a distinct sequence of resonances; whereas an increase in the amplitude of the high-frequency field induces minima in |s|2. However, the |s|2 maximum occurs in the low-frequency regime where it may be considered otherwise weak in the presence of a single harmonic force
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