29,212 research outputs found
Nonhyperbolic step skew-products: Ergodic approximation
We study transitive step skew-product maps modeled over a complete shift of
, , symbols whose fiber maps are defined on the circle and have
intermingled contracting and expanding regions. These dynamics are genuinely
nonhyperbolic and exhibit simultaneously ergodic measures with positive,
negative, and zero exponents.
We introduce a set of axioms for the fiber maps and study the dynamics of the
resulting skew-product. These axioms turn out to capture the key mechanisms of
the dynamics of nonhyperbolic robustly transitive maps with compact central
leaves.
Focusing on the nonhyperbolic ergodic measures (with zero fiber exponent) of
these systems, we prove that such measures are approximated in the weak
topology and in entropy by hyperbolic ones. We also prove that they are in the
intersection of the convex hulls of the measures with positive fiber exponent
and with negative fiber exponent. Our methods also allow us to perturb
hyperbolic measures. We can perturb a measure with negative exponent directly
to a measure with positive exponent (and vice-versa), however we lose some
amount of entropy in this process. The loss of entropy is determined by the
difference between the Lyapunov exponents of the measures.Comment: 43 pages, 5 figure
Effect of the spin-orbit interaction on the thermodynamic properties of crystals: The specific heat of bismuth
In recent years, there has been increasing interest in the specific heat
of insulators and semiconductors because of the availability of samples with
different isotopic masses and the possibility of performing \textit{ab initio}
calculations of its temperature dependence using as a starting point the
electronic band structure. Most of the crystals investigated are elemental
(e.g., germanium) or binary (e.g., gallium nitride) semiconductors. The initial
electronic calculations were performed in the local density approximation and
did not include spin-orbit interaction. Agreement between experimental and
calculated results was usually found to be good, except for crystals containing
heavy atoms (e.g., PbS) for which discrepancies of the order of 20% existed at
the low temperature maximum found for . It has been conjectured that
this discrepancies result from the neglect of spin-orbit interaction which is
large for heavy atoms (1.3eV for the valence electrons of
atomic lead). Here we discuss measurements and \textit{ab initio} calculations
of for crystalline bismuth (1.7 eV), strictly speaking a
semimetal but in the temperature region accessible to us ( 2K) acting as a
semiconductor. We extend experimental data available in the literature and
notice that the \textit{ab initio} calculations without spin-orbit interaction
exhibit a maximum at 8K, about 20% lower than the measured one. Inclusion
of spin-orbit interaction decreases the discrepancy markedly: The maximum of
is now only 7% larger than the measured one. Exact agreement is obtained
if the spin-orbit hamiltonian is reduced by a factor of 0.8.Comment: 4 pages, 3 figure
Coping with Persistent Pain, Effectiveness Research into Self-management (COPERS): statistical analysis plan for a randomised controlled trial
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated
Magnetic transitions in Pr2NiO4 single crystal
The magnetic properties of a stoichiometric Pr2NiO4 single crystal have been examined by means of the temperature dependence of the complex ac susceptibility and the isothermal magnetization in fields up to 200 kOe at T=4.2 K. Three separate phases have been identified and their anisotropic character has been analyzed. A collinear antiferromagnetic phase appears first between TN = 325 K and Tc1 = 115 K, where the Pr ions are polarized by an internal magnetic field. At Tc1 a first modification of the magnetic structure occurs in parallel with a structural phase transition (Bmab to P42/ncm). This magnetic transition has a firstâorder character and involves both the outâofâplane and the inâplane spin components (magnetic modes gx and gxcyfz, respectively). A second magnetic transition having also a firstâorder character is also clearly identified at Tc2 = 90 K which corresponds to a spin reorientation process (gxcyfz to cxgyaz magnetic modes). It should be noted as well that the outâofâphase component of Ïac shows a peak around 30 K which reflects the coexistence of both magnetic configurations in a wide temperature interval. Finally, two fieldâinduced transitions have been observed at 4.2 K when the field is directed along the c axis. We propose that the highâfield anomaly arises from a metamagnetic transition of the weak ferromagnetic component, similarly to La2CuO4
Ptychographic X-ray computed tomography of extended colloidal networks in food emulsions
As a main structural level in colloidal food materials, extended colloidal
networks are important for texture and rheology. By obtaining the 3D
microstructure of the network, macroscopic mechanical properties of the
material can be inferred. However, this approach is hampered by the lack of
suitable non-destructive 3D imaging techniques with submicron resolution.
We present results of quantitative ptychographic X-ray computed tomography
applied to a palm kernel oil based oil-in-water emulsion. The measurements were
carried out at ambient pressure and temperature. The 3D structure of the
extended colloidal network of fat globules was obtained with a resolution of
around 300 nm. Through image analysis of the network structure, the fat globule
size distribution was computed and compared to previous findings. In further
support, the reconstructed electron density values were within 4% of reference
values.Comment: 19 pages, 4 figures, to be published in Food Structur
The X-ray Properties of the Nearby Star-Forming Galaxy IC 342: The XMM-Newton View
We present the X-ray properties of IC342 using XMM-Newton. Thirty-five
sources are detected coincident with the disk of IC342 (more than tripling the
number known), of which ~31 are likely to be intrinsic to IC342. This
population shows a range of spectral properties and has an X-ray luminosity
function slope and infrared luminosity comparable to that of starburst galaxies
such as M82 and the Antennae, while its relative lack of extended X-ray
emission is similar to the properties of quiescent spirals. We do detect
long-term variability between this observation and the 1991 ROSAT and 1993/2000
ASCA observations for five sources. Notably, the second most luminous source
IC342 X-2 is is found to be in its the lowest luminosity state observed for X-2
to date, although the slope of the spectrum is intermediate between the
previously observed low/hard and high/soft states. IC342 X-1, on the other
hand, is found to be in an identical state to that observed in 2000 with ASCA.
Assuming X-1 is in an anomalous very high (VH) state, then either (1) X-1 has
remained in this state between 2000 and 2002, and is therefore the longest
duration VH-state binary ever observed, or (2) it was simply caught in a VH
state by chance in both the 2000 ASCA and 2002 XMM-Newton observations. We have
also confirmed the ROSAT HRI result that the nucleus of IC342 is made up of
both point-like and extended emission. The relative fluxes of the two spectral
components suggest that the nucleus is complex, with a soft extended component
contributing approximately half of the total luminosity. (Abridged)Comment: AJ in press (December 2003), 9 pages, 7 figures, 2 tables,
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Fusion of light proton-rich exotic nuclei at near-barrier energies
We study theoretically fusion of the light proton-rich exotic nuclei F
and B at near-barrier energies in order to investigate the possible role of
breakup processes on their fusion cross sections. To this end, coupled channel
calculations are performed considering the couplings to the breakup channels of
these projectiles. In case of F, the coupling arising out of the
inelastic excitation from the ground state to the bound excited state and its
couplings to the continuum have also been taken into consideration. It is found
that the inelastic excitation/breakup of F affect the fusion cross
sections very nominally even for a heavy target like Pb. On the other hand,
calculations for fusion of the one-proton halo nucleus B on a Pb target
show a significant suppression of the complete fusion cross section above the
Coulomb barrier. This is due to the larger breakup probability of B as
compared to that of F. However, even for B, there is little change
in the complete fusion cross sections as compared to the no-coupling case at
sub-barrier energies.Comment: 11 pages, 3 figures, Revtex.st
Low State, Phase-Resolved IR Spectroscopy of VV Puppis
We present phase-resolved low resolution and higher resolution -band
spectroscopy of the polar VV Pup. All observations were obtained when VV Pup
was in a low accretion state having a K magnitude near 15. The low resolution
observations reveal cyclotron emission in the band during some phases,
consistent with an origin near the active 30.5 MG pole on the white dwarf. The
secondary in VV Pup appears to be a normal M7V star and we find that the
and band fluxes are entirely due to this star at all orbital phases during
the low accretion state. We use our higher resolution Keck spectroscopy to
produce the first -band radial velocity curve for VV Pup. Our orbital
solution yields =414 km sec and leads to mass estimates of
M=0.730.05 M and M=0.100.02 M. We find
that the mass accretion rates during the normal low states of the polars VV
Pup, EF Eri, and EQ Cet are near 10 M yr. The fact
that \.M is not zero in low state polars indicates active secondary stars in
these binary systems, including the sub-stellar donor star present in EF Eri.Comment: Accepted in Astronomical Journal 5 figure
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