299 research outputs found
Superfluid 4He dynamics beyond quasiparticle excitations
The dynamics of superfluid 4He at and above the Landau quasiparticle regime
is investigated by high precision inelastic neutron scattering measurements of
the dynamic structure factor. A highly structured response is observed above
the familiar phonon-maxon-roton spectrum, characterized by sharp thresholds for
phonon-phonon, maxon-roton and roton-roton coupling processes. The experimental
dynamic structure factor is compared to the calculation of the same physical
quantity by a Dynamic Many-body theory including three-phonon processes
self-consistently. The theory is found to provide a quantitative description of
the dynamics of the correlated bosons for energies up to about three times that
of the Landau quasiparticles.Comment: 5 pages, 3 figure
Formalizing the transformations of a cognitive universe
International audienceIn an effort to continue the pioneering work of Harary in USA and Flament in France, we have undertaken to develop, on an experimental basis, a formalized theory of systems of beliefs and their modifications. This theory uses the psycho-social concepts of theories of cognitive consistency and of the tools of discrete mathematics, such as rewriting and intervals within graphs. The axioms and rewriting rules are elaborated from experimental data, and we demonstrate that the system we have built has the property of termination. This result is in accordance with experimental observations that show that every subject having an inconsistent system of beliefs (i.e., one containing contradictions) makes this system evolve towards consistency to reach a simple, consistent reference framework
Man-made noise in our living environments
The ITU’s (International Telecommunication Union’s) man-made noise levels are based on measurements performed in the 1970s. Some measurements have been carried out since then, showing that noise caused by automotive ignition systems has been reduced, but manmade noise in business areas and city centers increased, especially due to the widespread use of electronic systems. The interference scenario also changed, from analog communication systems in relatively free-space conditions, to digital systems in living areas, often semi-enclosed such as offi ces, industrial production plants, and even inside cars and trains. Several measurements have therefore been carried out to estimate the level of man-made noise in these semi-enclosed environments
The dispersion relation of Landau elementary excitations and the thermodynamic properties of superfluid He
The dispersion relation of the elementary excitations of
superfluid He has been measured at very low temperatures, from saturated
vapor pressure up to solidification, using a high flux time-of-flight neutron
scattering spectrometer equipped with a high spatial resolution detector
(10 'pixels'). A complete determination of is achieved, from
very low wave-vectors up to the end of Pitaeskii's plateau. The results compare
favorably in the whole the wave-vector range with the predictions of the
dynamic many-body theory (DMBT). At low wave-vectors, bridging the gap between
ultrasonic data and former neutron measurements, the evolution with the
pressure from anomalous to normal dispersion, as well as the peculiar
wave-vector dependence of the phase and group velocities, are accurately
characterized. The thermodynamic properties have been calculated analytically,
developing Landau's model, using the measured dispersion curve. A good
agreement is found below 0.85 K between direct heat capacity measurements and
the calculated specific heat, if thermodynamically consistent power series
expansions are used. The thermodynamic properties have also been calculated
numerically; in this case, the results are applicable with excellent accuracy
up to 1.3 K, a temperature above which the dispersion relation itself becomes
temperature dependent.Comment: 35 pages, 59 figures; Supplemental Material: Text, Tables, data file
Microscopic dynamics of superfluid 4He: A comprehensive study by inelastic neutron scattering
The dynamic structure factor of superfluid 4 He has been investigated at very low temperatures by inelastic neutron scattering. The measurements combine different incoming energies resulting in an unprecedentedly large dynamic range with excellent energy resolution, covering wave vectors Q up to 5 Å^(-1) and energies ω up to 15 meV. A detailed description of the dynamics of superfluid 4He is obtained from saturated vapor pressure up to solidification. The single-excitation spectrum is substantially modified at high pressures, as the maxon energy exceeds the roton-roton decay threshold. A highly structured multiexcitation spectrum is observed at lowenergies, where clear thresholds and branches have been identified. Strong phonon emission branches are observed when the phonon or roton group velocities exceed the sound velocity. The spectrum is found to display strong multiexcitations whenever the single excitations face disintegration following Pitaevskii's type a or b criteria. At intermediate energies, an interesting pattern in the dynamic structure factor is observed in the vicinity of the recoil energy. All these features, which evolve significantly with pressure, are in very good agreement with the dynamic many-body calculations, even at the highest densities, where the correlations are strongest.Fil: Beauvois, K.. Universite Grenoble Alpes. Institut Nanosciences et Cryogenie - Commissariat a L´Energie Atomique et Aux Energies Alternatives. Institut Nanosciences et Cryogenie; Francia. Institut Laue Langevin; FranciaFil: Dawidowski, Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; ArgentinaFil: Fåk, B.. Institut Laue Langevin; FranciaFil: Godfrin, H.. Universite Grenoble Alpes. Institut Nanosciences et Cryogenie - Commissariat a L´Energie Atomique et Aux Energies Alternatives. Institut Nanosciences et Cryogenie; FranciaFil: Krotscheck, E.. State University of New York; Estados UnidosFil: Ollivier, J.. Institut Laue Langevin; FranciaFil: Sultan, A.. Universite Grenoble Alpes. Institut Nanosciences et Cryogenie - Commissariat a L´Energie Atomique et Aux Energies Alternatives. Institut Nanosciences et Cryogenie; Franci
Presentation and Preliminary Results of DROÏD Project: Development of a Distributed Optical Fibre Dosimeter
International audienceABSTRACT DROÏD project is intended to develop a distributed optical fibre dosimeter based on Radiation-Induced Attenua-tion (RIA). The RIA will be measured by a high resolution Optical Time Domain Reflectometry (OTDR) technique that allows to locate the irradiated fibre section. The first part of the project focuses on designing a high radiation sensitive fibre. As a preliminary work, the attenu-ation of several fibres with various compositions has been recorded in situ during and after irradiation. The experimental setup and RIA values are presented and discussed. Several dopants and their combinations have been identified as a good starting point to design a highly radiation sensitive fibre
Magnetic and electronic structure of the topological semimetal YbMnSb
The antiferromagnetic (AFM) semimetal YbMnSb has recently been identified
as a candidate topological material, driven by time-reversal symmetry breaking.
Depending on the ordered arrangement of Mn spins below the N\'{e}el
temperature, = 345 K, the electronic bands near the Fermi energy
can ether have a Dirac node, a Weyl node or a nodal line. We have investigated
the ground state magnetic structure of YbMnSb using unpolarized and
polarized single crystal neutron diffraction. We find that the Mn moments lie
along the axis of the space group and are arranged in a C-type AFM
structure, which implies the existence of gapped Dirac nodes near the Fermi
level. The results highlight how different magnetic structures can critically
affect the topological nature of fermions in semimetals
Do All Lives Have the Same Value? Support for International Military Interventions as a Function of Political System and Public Opinion of the Target States
This research examined the support for international military interventions as a function of the political system and the public opinion of the target country. In two experiments, we informed participants about a possible military intervention by the international community towards a sovereign country whose government planned to use military force against a secessionist region. They were then asked whether they would support this intervention whilst being reminded that it would cause civilian deaths. The democratic or nondemocratic political system of the target country was experimentally manipulated, and the population support for its belligerent government policy was either assessed (Experiment 1) or manipulated (Experiment 2). Results showed greater support for the intervention when the target country was nondemocratic, as compared to the democratic and the control conditions, but only when its population supported the belligerent government policy. Support for the external intervention was low when the target country was democratic, irrespective of national public opinion. These findings provide support for the democracy-as-value hypothesis applied to international military interventions, and suggest that civilian deaths (collateral damage) are more acceptable when nondemocratic populations support their government's belligerent policy
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