2,379 research outputs found
Diagnosing order by disorder in quantum spin systems
In this paper we study the frustrated J1-J2 quantum Heisenberg model on the
square lattice for J2 > 2J1, in a magnetic field. In this regime the classical
system is known to have a degenerate manifold of lowest energy configurations,
where standard thermal order by disorder occurs. In order to study its quantum
version we use a path integral formulation in terms of coherent states. We show
that the classical degeneracy in the plane transverse to the magnetic field is
lifted by quantum fluctuations. Collinear states are then selected, in a
similar pattern to that set by thermal order by disorder, leaving a Z2
degeneracy. A careful analysis reveals a purely quantum mechanical effect given
by the tunneling between the two minima selected by fluctuations. The effective
description contains two planar (XY -like) fields conjugate to the total
magnetization and the difference of the two sublattice magnetizations. Disorder
in either or both of these fields produces the locking of their conjugate
observables. Furthermore, within this scenario we argue that the quantum state
is close to a product state.Comment: 8 pages, 3 figure
Statistical transmutation in doped quantum dimer models
We prove a "statistical transmutation" symmetry of doped quantum dimer models
on the square, triangular and kagome lattices: the energy spectrum is invariant
under a simultaneous change of statistics (i.e. bosonic into fermionic or
vice-versa) of the holes and of the signs of all the dimer resonance loops.
This exact transformation enables to define duality equivalence between doped
quantum dimer Hamiltonians, and provides the analytic framework to analyze
dynamical statistical transmutations. We investigate numerically the doping of
the triangular quantum dimer model, with special focus on the topological Z2
dimer liquid. Doping leads to four (instead of two for the square lattice)
inequivalent families of Hamiltonians. Competition between phase separation,
superfluidity, supersolidity and fermionic phases is investigated in the four
families.Comment: 3 figure
Daylight operation of a free space, entanglement-based quantum key distribution system
Many quantum key distribution (QKD) implementations using a free space
transmission path are restricted to operation at night time in order to
distinguish the signal photons used for a secure key establishment from
background light. Here, we present a lean entanglement-based QKD system
overcoming that imitation. By implementing spectral, spatial and temporal
filtering techniques, we were able to establish a secure key continuously over
several days under varying light and weather conditions.Comment: 13 pages, 6 figure
Temporal characterization of individual harmonics of an attosecond pulse train by THz streaking
We report on the global temporal pulse characteristics of individual
harmonics in an attosecond pulse train by means of photo-electron streaking in
a strong low-frequency transient. The scheme allows direct retrieval of pulse
durations and first order chirp of individual harmonics without the need of
temporal scanning. The measurements were performed using an intense THz field
generated by tilted phase front technique in LiNbO_3 . Pulse properties for
harmonics of order 23, 25 and 27 show that the individual pulse durations and
linear chirp are decreasing by the harmonic order
Experimental quantum key distribution based on a Bell test
We report on a complete free-space field implementation of a modified Ekert91
protocol for quantum key distribution using entangled photon pairs. For each
photon pair we perform a random choice between key generation and a Bell
inequality. The amount of violation is used to determine the possible knowledge
of an eavesdropper to ensure security of the distributed final key.Comment: 5 pages ReVTeX, 3 figures; version v2 with updated references and
minor corrections, author spelling fixe
Construction and evaluation of classifiers for forensic document analysis
In this study we illustrate a statistical approach to questioned document
examination. Specifically, we consider the construction of three classifiers
that predict the writer of a sample document based on categorical data. To
evaluate these classifiers, we use a data set with a large number of writers
and a small number of writing samples per writer. Since the resulting
classifiers were found to have near perfect accuracy using leave-one-out
cross-validation, we propose a novel Bayesian-based cross-validation method for
evaluating the classifiers.Comment: Published in at http://dx.doi.org/10.1214/10-AOAS379 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Coastal bathymetry estimation using an ensemble of synthetic aperture radar images from Sentinel-1
In this study, coastal bathymetry is estimated with a wave ray-tracing algorithm using wave parameters retrieved from Synthetic Aperture Radar (SAR) images acquired by the Sentinel-1 satellites. The method relies on the long swell wave’s detection by SAR imagery and the wave’s properties adjustment to the underwater topography, which can be mathematically related using the linear dispersion relation. The ray-tracing algorithm tracks the shoaling waves until the wave breaking zone, using the wavelength and wave direction retrieved from the 2D directional spectra applied at consecutive sub-images. Then, by inverting the linear wave dispersion relationship, the depth is calculated based on the mean wavelength obtained for each sub-image and maintaining the wave period retrieved at the first offshore position, which is computed using a mean depth from an independent bathymetric source. The output of the algorithm is a bathymetric model that results from the interpolation of the depth computed at each tracking position to a uniform grid and the results are compared with bathymetric information from the General Bathymetric Chart of the Ocean. The use of a monthly ensemble of SAR images, instead of individual ones, to reproduce the bathymetry near Aveiro, Portugal, resulted in a smoother topography with lower relative errors, suggesting that the final bathymetric model retrieved from SAR should result from a combination of SAR images. The methodology presented here to infer the bathymetry using space-borne SAR imagery can be useful to retrieve the mean bottom topography (especially in remote areas where the traditional hydrographic surveying methods are not performed regularly) and to reproduce new underwater structures, such as banks, reefs or bars, which are important to detect for the safety of navigation.Peer Reviewe
SOLAR SOFTWARE APPLIED TO DESIGN A PHOTOVOLTAIC SYSTEM TO SUPPLY THE ENERGY DEMAND OF AN ITALIAN SCHOOL
This paper has as objective to describe the use of SOLAR 1.1 software todesign a system for supplying a part of the electric demand of a schoolsituated in Umbertide, comparing the use o SOLAR 1.1 to real results of asystem implanted yet. This software version helps also to conduce theeconomic analysis for grid connected or standalone photovoltaic systemsfor the choice of convenient values of investment rate and annuity factorand it calculates the payback period of investment in the photovoltaic plant
Current jumps in flat-band ladders with Dzyaloshinskii-Moriya interactions
Localized magnons states, due to flat bands in the spectrum, is an intensely studied phenomenon and can be found in many frustrated magnets of different spatial dimensionality. The presence of Dzyaloshinskii-Moriya (DM) interactions may change radically the behavior in such systems. In this context, we study a paradigmatic example of a one-dimensional frustrated antiferromagnet, the sawtooth chain in the presence of DM interactions. Using both path integrals methods and numerical Density Matrix Renormalization Group, we revisit the physics of localized magnons and determine the consequences of the DM interaction on the ground state. We have studied the spin current behavior, finding three different regimes. First, a Luttinger-liquid regime where the spin current shows a step behavior as a function of parameter D, at a low magnetic field. Increasing the magnetic field, the system is in the Meissner phase at the m = 1/2 plateau, where the spin current is proportional to the DM parameter. Finally, further increasing the magnetic field and for finite D there is a small stiffness regime where the spin current shows, at fixed magnetization, a jump to large values at D = 0, a phenomenon also due to the flat band.Fil: Acevedo, Santiago Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Pujol, P.. Université Paul Sabatier; Francia. Université de Toulouse; FranciaFil: Lamas, Carlos Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentin
SOLAR 1.1: DEVELOPMENT OF SOFTWARE FOR SELECTING COMMERCIALLY AVAILABLE PHOTOVOLTAIC PANELS, INCLUDING THE ECONOMIC ANALYSIS
Photovoltaic energy represents an opportunity to produce electricity in a clean manner. It can be applied in all world places, in particular in the developing countries, where there are places where electricity grids are unreliable or non-existent and is inconvenient to make investments in a grids expansion. In remote locations photovoltaic power supplies often the most economic and cleaner option to produce electric energy. In addition, many developing countries have high radiation levels year round because of their latitude. The software SOLAR 1.1 was developed with purpose of helping the choice of photovoltaic panels available commercially including electric needs calculation for the installation. This new version of program also help to conduce the economic analysis for grid connected or stand alone photovoltaic systems for the choice of convenient values of interest rate and payback period. In this version of the software is possible to select the language among English, Italian and Portuguese. The software choices the panels in its archive that contains more of 250 types of photovoltaic modules made by 35 producers. The selection provides as output three modules, the cheapest for each cell type: monocrystalline, multicrystalline and amorphous. The software archive can be updated adding new item or editing the inserted items. The economic analysis can be operated by SOLAR 1.1 in each of the chosen panels. This analysis gives as output all the values of the costs in the photovoltaic system and the diagrams with the electricity cost and the expected annual saving trend with variation of the amortisation period and for different values of the interest rate and the governmental subsidy rate
- …