255 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
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
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 Ã…^(-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 CientÃficas y Técnicas. Centro CientÃfico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de EnergÃa Atómica. Centro Atómico Bariloche; ArgentinaFil: FÃ¥k, 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
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
Evidence for magnetic quasiparticle phase separation in a quasi-one-dimensional quantum magnet
Magnetic systems composed of weakly coupled spin-1/2 chains are fertile
ground for hosting the fractional magnetic excitations that are intrinsic to
interacting fermions in one-dimension (1D). However, the exotic physics arising
from the quantum many-body interactions beyond 1D are poorly understood in
materials of this class. Spinons and psinons are two mutually exclusive
low-energy magnetic quasiparticles; the excitation seen depends on the ground
state of the spin chain. Here, we present inelastic neutron scattering and
neutron diffraction evidence for their coexistence in SrCoVO
at milli-Kelvin temperatures in part of the N\'eel phase (2.4 T
H 3.9 T) and possibly also the field-induced spin density
wave phase up to the highest field probed (H 3.9 T,
H = 5.5 T). These results unveil a
novel spatial phase inhomogeneity for the weakly coupled spin chains in this
compound. This quantum dynamical phase separation is a new phenomenon in
quasi-1D quantum magnets, highlighting the non-trivial consequences of
inter-chain coupling.Comment: 8 pages, 6 figure
The effectiveness of instructional software in reading comprehension skills and reading aloud of Omani fourth basic schools’ students
Field-temperature phase diagram of the enigmatic Nd2(Zr1−xTix)2O7 pyrochlore magnets
By combining neutron scattering and magnetization measurements down to 80 mK, we determine the (H, T ) phase diagram of the Nd2(Zr1−xTix )2O7 pyrochlore magnet compounds. In those samples, Zr is partially substituted by Ti, hence tuning the exchange parameters and testing the robustness of the various phases. In all samples, the ground state remains all in/all out, while the field induces phase transitions toward new states characterized by two in–two out or one out–three in/one in–three out configurations. These transitions manifest as metamagnetic singularities in the magnetization versus field measurements. Strikingly, it is found that moderate substitution reinforces the stability of the all in/all out phase: the Néel temperature, the metamagnetic fields along with the ordered magnetic moment, are higher in substituted samples with x < 10%
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