8,127 research outputs found
An exact representation of the fermion dynamics in terms of Poisson processes and its connection with Monte Carlo algorithms
We present a simple derivation of a Feynman-Kac type formula to study
fermionic systems. In this approach the real time or the imaginary time
dynamics is expressed in terms of the evolution of a collection of Poisson
processes. A computer implementation of this formula leads to a family of
algorithms parametrized by the values of the jump rates of the Poisson
processes. From these an optimal algorithm can be chosen which coincides with
the Green Function Monte Carlo method in the limit when the latter becomes
exact.Comment: 4 pages, 1 PostScript figure, REVTe
Vibration analysis of the civic tower in Rieti
In the last decades the definition of a suitable monitoring system for identifying the dynamic behavior of structures has had a central position in the civil engineering research area. The vibration analysis leads to the recognition of the reference state of structures which is essential to determine the integrity level when extreme events occur, such as earthquakes. The latest seismic events occurred in the world have shown the essential role of the new passive seismic techniques which aim to protect structures and the importance of supervising the building construction operations and the adopted improvement measures.
In this work the structural monitoring of the civic tower located in Rieti is presented. In the tower a non-conventional TMD has been installed via an inter-story isolation system at the top floor by means of High Damping Rubber Bearings (HDRB).
The general goal is to define a monitoring system suitable with this experimental case through the vibration analysis. Several aspects will be taken into account: the choice of sensors setup, the measured quantities and the extraction of structural information. Firstly this will allow to define the structure’s reference state featured by frequencies, damping ratios and mode shapes. Moreover the effective design of the monitoring system would lead to the characterization of the dynamic behavior of the structure equipped with a passive vibration control system. Different tests have been carried forward: ambient vibration test (AVT), forced vibration test (FVT) with vibrodyne and seismic test (ST). The AVT and the FVT enable to define the monitoring system and check the reliability of the adopted identification tools, among which an Output Only algorithm stands out: the Observer Kalman Filter System Id. On the other hand the ST will point out some preliminary information about the dynamic behaviour of the structure equipped with a non conventional Tuned Mass Damper referring it to higher levels of vibrations
Improved limits on photon velocity oscillations
The mixing of the photon with a hypothetical sterile paraphotonic state would
have consequences on the cosmological propagation of photons. The absence of
distortions in the optical spectrum of distant Type Ia supernov\ae allows to
extend by two orders of magnitude the previous limit on the Lorentz-violating
parameter associated to the photon-paraphoton transition, extracted
from the abscence of distortions in the spectrum of the cosmic microwave
background. The new limit is consistent with the interpretation of the dimming
of distant Type Ia supernov\ae as a consequence of a nonzero cosmological
constant. Observations of gamma-rays from active galactic nuclei allow to
further extend the limit on by ten orders of magnitude.Comment: 10 pages, 4 Postscript figures, use epsfig, amssym
Exact Monte Carlo time dynamics in many-body lattice quantum systems
On the base of a Feynman-Kac--type formula involving Poisson stochastic
processes, recently a Monte Carlo algorithm has been introduced, which
describes exactly the real- or imaginary-time evolution of many-body lattice
quantum systems. We extend this algorithm to the exact simulation of
time-dependent correlation functions. The techniques generally employed in
Monte Carlo simulations to control fluctuations, namely reconfigurations and
importance sampling, are adapted to the present algorithm and their validity is
rigorously proved. We complete the analysis by several examples for the
hard-core boson Hubbard model and for the Heisenberg model
"All on short" prosthetic-implant supported rehabilitations
Objectives. Short implants are increasing their popularity among clinicians who want to fulfill the constant demanding of fixed prosthetic solutions in edentulous jaws. The aim of this report was to propose a new possibility to project and realize an occlusal guided implant cross-arch prosthesis supported by ultra-short implants, describing it presented an edentulous mandible case report. Methods. A 61-year-old, Caucasian, female patient who attended the dental clinic of the University of L’Aquila presented with edentulous posterior inferior jaw and periodontitis and periimplantitis processes in the anterior mandible. The remaining tooth and the affected implant were removed. Six 4-mm-long implants were placed to support a cross-arch metal-resin prosthesis. Results. At 1-year follow-up clinical and radiological assessment showed a good osseointegration of the fixtures and the patient was satisfied with the prosthesis solution. Conclusion. The method, even if it requires further validation, seems to be a valid aid in solving lower edentulous clinical cases, and appears less complex and with more indications of other proposals presented in the current clinical literature. Our case report differs from the current technique All-on-Four, which uses four implants in the mandible to support overdenture prosthesis, assuring a very promising clinical resul
Results from RHIC with Implications for LHC
Results from the PHENIX experiment at RHIC in p-p and Au+Au collisions are
reviewed from the perspective of measurements in p-p collisions at the CERN-ISR
which serve as a basis for many of the techniques used. Issues such as J/Psi
suppression and hydrodynamical flow in A+A collisions require data from
LHC-Ions for an improved understanding. Suppression of high pT particles in
Au+Au collisions, first observed at RHIC, also has unresolved mysteries such as
the equality of the suppression of inclusive pi0 (from light quarks and gluons)
and direct-single electrons (from the decay of heavy quarks) in the transverse
momentum range 4< pT < 9 GeV/c. This disfavors a radiative explanation of
suppression and leads to a fundamental question of whether the Higgs boson
gives mass to fermions. Observation of an exponential distribution of direct
photons in central Au+Au collisions for 1< pT <2 GeV/c where hard-processes are
negligible and with no similar exponential distribution in p-p collisions
indicates thermal photon emission from the medium at RHIC, making PHENIX at the
moment ``the hottest experiment in Physics''.Comment: Invited lectures at the International School of Subnuclear Physics,
47th Course, "The most unexpected at LHC and the status of High Energy
Frontier'', Erice, Sicily, Italy, August 29-September 7. 2009. 32 pages, 22
figure
Resonant, broadband and highly efficient optical frequency conversion in semiconductor nanowire gratings at visible and UV wavelengths
Using a hydrodynamic approach we examine bulk- and surface-induced second and
third harmonic generation from semiconductor nanowire gratings having a
resonant nonlinearity in the absorption region. We demonstrate resonant,
broadband and highly efficient optical frequency conversion: contrary to
conventional wisdom, we show that harmonic generation can take full advantage
of resonant nonlinearities in a spectral range where nonlinear optical
coefficients are boosted well beyond what is achievable in the transparent,
long-wavelength, non-resonant regime. Using femtosecond pulses with
approximately 500 MW/cm2 peak power density, we predict third harmonic
conversion efficiencies of approximately 1% in a silicon nanowire array, at
nearly any desired UV or visible wavelength, including the range of negative
dielectric constant. We also predict surface second harmonic conversion
efficiencies of order 0.01%, depending on the electronic effective mass,
bistable behavior of the signals as a result of a reshaped resonance, and the
onset fifth order nonlinear effects. These remarkable findings, arising from
the combined effects of nonlinear resonance dispersion, field localization, and
phase-locking, could significantly extend the operational spectral bandwidth of
silicon photonics, and strongly suggest that neither linear absorption nor skin
depth should be motivating factors to exclude either semiconductors or metals
from the list of useful or practical nonlinear materials in any spectral range.Comment: 12 pages, 4 figure
Polarization Independent Unidirectional Scattering with Turnstile Nanoantennas
We study the scattering behavior of a dielectric cross-dipole nanoantenna in the near-infrared spectral range when it is excited by a circular polarized plane wave. We theoretically demonstrate, for optimized geometrical parameters of the proposed turnstile structure, the possibility to simultaneously obtain a unidirectional scattering and a specific circular polarization of the scattered field. Our results open new functionalities for metamaterials and optical nanoantennas
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