1,839 research outputs found
Free evolution of an incoherent mixture of states: a quantum mechanical approach to the van Cittert-Zernike theorem
We study the time evolution of an incoherent mixture of quantum states and demonstrate, in very simple terms, a quantum mechanical equivalent of van Cittert–Zernike theorem, which can be easily explained to Quantum Physics students with a basic knowledge of the density matrix theory. Finally, we exemplify this result by applying it to the quantitative analysis of the coherence of a beam of particles in atomic collisions.Fil: Fabre, Ignacio. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Navarrete, Francisco Oscar. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Sarkadi, L.. Hungarian Academy of Sciences; HungríaFil: Barrachina Tejada, Raul Oscar. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin
Rotational Doppler Effect in Magnetic Resonance
We compute the shift in the frequency of the spin resonance in a solid that
rotates in the field of a circularly polarized electromagnetic wave. Electron
spin resonance, nuclear magnetic resonance, and ferromagnetic resonance are
considered. We show that contrary to the case of the rotating LC circuit, the
shift in the frequency of the spin resonance has strong dependence on the
symmetry of the receiver. The shift due to rotation occurs only when rotational
symmetry is broken by the anisotropy of the gyromagnetic tensor, by the shape
of the body, or by magnetocrystalline anisotropy. General expressions for the
resonance frequency and power absorption are derived and implications for
experiment are discussed.Comment: 8 pages, 4 figure
Non-monotonic field-dependence of the ZFC magnetization peak in some systems of magnetic nanoparticles
We have performed magnetic measurements on a diluted system of gamma-Fe2O3
nanoparticles (~7nm), and on a ferritin sample. In both cases, the ZFC-peak
presents a non-monotonic field dependence, as has already been reported in some
experiments,and discussed as a possible evidence of resonant tunneling. Within
simple assumptions, we derive expressions for the magnetization obtained in the
usual ZFC, FC, TRM procedures. We point out that the ZFC-peak position is
extremely sensitive to the width of the particle size distribution, and give
some numerical estimates of this effect. We propose to combine the FC
magnetization with a modified TRM measurement, a procedure which allows a more
direct access to the barrier distribution in a field. The typical barrier
values which are obtained with this method show a monotonic decrease for
increasing fields, as expected from the simple effect of anisotropy barrier
lowering, in contrast with the ZFC results. From our measurements on
gamma-Fe2O3 particles, we show that the width of the effective barrier
distribution is slightly increasing with the field, an effect which is
sufficient for causing the observed initial increase of the ZFC-peak
temperatures.Comment: LaTeX file 19 pages, 9 postscript figures. To appear in Phys. Rev. B
(tentative schedule: Dec.97
Superparamagnetic relaxation in Cu_{x}Fe_{3-x}O_{4} (x=0.5 and x=1) nanoparticles
The scope of this article is to report very detailed results of the
measurements of magnetic relaxation phenomena in the new
CuFeO nanoparticles and known CuFeO
nanoparticles. The size of synthesized particles is (6.51.5)nm. Both
samples show the superparamagnetic behaviour, with the well-defined phenomena
of blocking of magnetic moment. This includes the splitting of
zero-field-cooled and field-cooled magnetic moment curves, dynamical
hysteresis, slow quasi-logarithmic relaxation of magnetic moment below blocking
temperature. The scaling of the magnetic moment relaxation data at different
temperatures confirms the applicability of the simple thermal relaxation model.
The two copper-ferrites with similar structures show significantly different
magnetic anisotropy density and other magnetic properties. Investigated systems
exhibit the consistency of all obtained results.Comment: 18 pages, 8 figure
Relaxation and Landau-Zener experiments down to 100 mK in ferritin
Temperature-independent magnetic viscosity in ferritin has been observed from
2 K down to 100 mK, proving that quantum tunneling plays the main role in these
particles at low temperature. Magnetic relaxation has also been studied using
the Landau-Zener method making the system crossing zero resonant field at
different rates, alpha=dH/dt, ranging from 10^{-5} to 10^{-3} T/s, and at
different temperatures, from 150 mK up to the blocking temperature. We propose
a new Tln(Delta H_{eff}/tau_0 alpha) scaling law for the Landau-Zener
probability in a system distributed in volumes, where Delta H_{eff} is the
effective width of the zero field resonance.Comment: 13 pages, 4 postscript figure
Plasma transition at the flanks of the Venus ionosheath: Evidence from the Venus Express data
Intermittent turbulence, noisy fluctuations and wavy structures in the Venusian magnetosheath and wake
Recent research has shown that distinct physical regions in the Venusian
induced magnetosphere are recognizable from the variations of strength of the
magnetic field and its wave/fluctuation activity. In this paper the statistical
properties of magnetic fluctuations are investigated in the Venusian
magnetosheath and wake regions. The main goal is to identify the characteristic
scaling features of fluctuations along Venus Express (VEX) trajectory and to
understand the specific circumstances of the occurrence of different types of
scalings. For the latter task we also use the results of measurements from the
previous missions to Venus. Our main result is that the changing character of
physical interactions between the solar wind and the planetary obstacle is
leading to different types of spectral scaling in the near-Venusian space.
Noisy fluctuations are observed in the magnetosheath, wavy structures near the
terminator and in the nightside near-planet wake. Multi-scale turbulence is
observed at the magnetosheath boundary layer and near the quasi-parallel bow
shock. Magnetosheath boundary layer turbulence is associated with an average
magnetic field which is nearly aligned with the Sun-Venus line. Noisy magnetic
fluctuations are well described with the Gaussian statistics. Both
magnetosheath boundary layer and near shock turbulence statistics exhibit
non-Gaussian features and intermittency over small spatio-temporal scales. The
occurrence of turbulence near magnetosheath boundaries can be responsible for
the local heating of plasma observed by previous missions
Normalization factors for magnetic relaxation of small particle systems in non-zero magnetic field
We critically discuss relaxation experiments in magnetic systems that can be
characterized in terms of an energy barrier distribution, showing that proper
normalization of the relaxation data is needed whenever curves corresponding to
different temperatures are to be compared. We show how these normalization
factors can be obtained from experimental data by using the
scaling method without making any assumptions about the nature of the energy
barrier distribution. The validity of the procedure is tested using a
ferrofluid of Fe_3O_4 particles.Comment: 5 pages, 6 eps figures added in April 22, to be published in Phys.
Rev. B 55 (1 April 1997
El concepto de nulidad en el derecho procesal civil
Tesis inédita, presentada en la Universidad de Madrid, Facultad de Derecho, 1962.Fac. de DerechoTRUEProQuestpu
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