649 research outputs found
Collective motional resonances and instabilities of an electron cloud stored in a Penning trap
We have experimentally investigated the behavior of an electron cloud confined in a Penning trap at weak superimposed magnetic fields. Exciting the motional frequencies of the electrons by an external drive field we found the axial mode split into two components which were identified as center-of-mass and individual electron oscillations. When the trapping potential was varied, rapid electron loss appeared at numerous values of the applied voltage. They are determined by the relation n z ω z + n m ω m =ω c . ω z ,ω m ,ω c are the axial, magnetron, and cyclotron frequency of the trapped electrons, respectively. The reason for this loss is attributed to higher order contributions to the ideal quadrupole trapping potential
Large Deviations in the Superstable Weakly Imperfect Bose Gas
The superstable Weakly Imperfect Bose Gas {(WIBG)} was originally derived to
solve the inconsistency of the Bogoliubov theory of superfluidity. Its
grand-canonical thermodynamics was recently solved but not at {point of} the
{(first order)} phase transition. This paper proposes to close this gap by
using the large deviations formalism and in particular the analysis of the Kac
distribution function. It turns out that, as a function of the chemical
potential, the discontinuity of the Bose condensate density at the phase
transition {point} disappears as a function of the particle density. Indeed,
the Bose condensate continuously starts at the first critical particle density
and progressively grows but the free-energy per particle stays constant until
the second critical density is reached. At higher particle densities, the Bose
condensate density as well as the free-energy per particle both increase
{monotonously}
Low-temperature heat transfer in nanowires
The new regime of low-temperature heat transfer in suspended nanowires is
predicted. It takes place when (i) only ``acoustic'' phonon modes of the wire
are thermally populated and (ii) phonons are subject to the effective elastic
scattering. Qualitatively, the main peculiarities of heat transfer originate
due to appearance of the flexural modes with high density of states in the wire
phonon spectrum. They give rise to the temperature dependence of the
wire thermal conductance. The experimental situations where the new regime is
likely to be detected are discussed.Comment: RevTex file, 1 PS figur
Effect of phonon scattering by surface roughness on the universal thermal conductance
The effect of phonon scattering by surface roughness on the thermal
conductance in mesoscopic systems at low temperatures is calculated using full
elasticity theory. The low frequency behavior of the scattering shows novel
power law dependences arising from the unusual properties of the elastic modes.
This leads to new predictions for the low temperature depression of the thermal
conductance below the ideal universal value. Comparison with the data of Schwab
et al. [Nature 404, 974 (2000)] suggests that surface roughness on a scale of
the width of the thermal pathway is important in the experiment.Comment: 6 pages, 3 figure
Elastic Wave Transmission at an Abrupt Junction in a Thin Plate, with Application to Heat Transport and Vibrations in Mesoscopic Systems
The transmission coefficient for vibrational waves crossing an abrupt
junction between two thin elastic plates of different widths is calculated.
These calculations are relevant to ballistic phonon thermal transport at low
temperatures in mesoscopic systems and the Q for vibrations in mesoscopic
oscillators. Complete results are calculated in a simple scalar model of the
elastic waves, and results for long wavelength modes are calculated using the
full elasticity theory calculation. We suggest that thin plate elasticty theory
provide a useful and tractable approximation to the full three dimensional
geometry.Comment: 35 pages, including 12 figure
Bloch-Wall Phase Transition in the Spherical Model
The temperature-induced second-order phase transition from Bloch to linear
(Ising-like) domain walls in uniaxial ferromagnets is investigated for the
model of D-component classical spin vectors in the limit D \to \infty. This
exactly soluble model is equivalent to the standard spherical model in the
homogeneous case, but deviates from it and is free from unphysical behavior in
a general inhomogeneous situation. It is shown that the thermal fluctuations of
the transverse magnetization in the wall (the Bloch-wall order parameter)
result in the diminishing of the wall transition temperature T_B in comparison
to its mean-field value, thus favouring the existence of linear walls. For
finite values of T_B an additional anisotropy in the basis plane x,y is
required; in purely uniaxial ferromagnets a domain wall behaves like a
2-dimensional system with a continuous spin symmetry and does not order into
the Bloch one.Comment: 16 pages, 2 figure
Measurements of the Separated Longitudinal Structure Function FL From Hydrogen and Deuterium Targets at Low Q2
Structure functions, as measured in lepton-nucleon scattering, have proven to be very useful in studying the partonic dynamics within the nucleon. However, it is experimentally difficult to separately determine the longitudinal and transverse structure functions, and consequently there are substantially less data available in particular for the longitudinal structure function. Here, we present separated structure functions for hydrogen and deuterium at low four-momentum transfer squared, Q2 \u3c 1GeV2, and compare them with parton distribution parametrization and kT factorization approaches. While differences are found, the parametrizations generally agree with the data, even at the very low-Q2 scale of the data. The deuterium data show a smaller longitudinal structure function and a smaller ratio of longitudinal to transverse cross section, R, than the proton. This suggests either an unexpected difference in R for the proton and the neutron or a suppression of the gluonic distribution in nuclei
Bound Magnetic Polaron Interactions in Insulating Doped Diluted Magnetic Semiconductors
The magnetic behavior of insulating doped diluted magnetic semiconductors
(DMS) is characterized by the interaction of large collective spins known as
bound magnetic polarons. Experimental measurements of the susceptibility of
these materials have suggested that the polaron-polaron interaction is
ferromagnetic, in contrast to the antiferromagnetic carrier-carrier
interactions that are characteristic of nonmagnetic semiconductors. To explain
this behavior, a model has been developed in which polarons interact via both
the standard direct carrier-carrier exchange interaction (due to virtual
carrier hopping) and an indirect carrier-ion-carrier exchange interaction (due
to the interactions of polarons with magnetic ions in an interstitial region).
Using a variational procedure, the optimal values of the model parameters were
determined as a function of temperature. At temperatures of interest, the
parameters describing polaron-polaron interactions were found to be nearly
temperature-independent. For reasonable values of these constant parameters, we
find that indirect ferromagnetic interactions can dominate the direct
antiferromagnetic interactions and cause the polarons to align. This result
supports the experimental evidence for ferromagnetism in insulating doped DMS.Comment: 11 pages, 7 figure
Applications of quark-hadron duality in F2 structure function
Inclusive electron-proton and electron-deuteron inelastic cross sections have
been measured at Jefferson Lab (JLab) in the resonance region, at large Bjorken
x, up to 0.92, and four-momentum transfer squared Q2 up to 7.5 GeV2 in the
experiment E00-116. These measurements are used to extend to larger x and Q2
precision, quantitative, studies of the phenomenon of quark-hadron duality. Our
analysis confirms, both globally and locally, the apparent violation of
quark-hadron duality previously observed at a Q2 of 3.5 GeV2 when resonance
data are compared to structure function data created from CTEQ6M and MRST2004
parton distribution functions (PDFs). More importantly, our new data show that
this discrepancy saturates by Q2 ~ 4 Gev2, becoming Q2 independent. This
suggests only small violations of Q2 evolution by contributions from the
higher-twist terms in the resonance region which is confirmed by our
comparisons to ALEKHIN and ALLM97.We conclude that the unconstrained strength
of the CTEQ6M and MRST2004 PDFs at large x is the major source of the
disagreement between data and these parameterizations in the kinematic regime
we study and that, in view of quark-hadron duality, properly averaged resonance
region data could be used in global QCD fits to reduce PDF uncertainties at
large x.Comment: 35 page
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