3,122 research outputs found
Magnetic confinement of the solar tachocline
We study the physics of the solar tachocline and related MHD instabilities.
We have performed 3-D MHD simulations of the solar radiative interior to check
whether a fossil magnetic field is able to prevent the spread of the
tachocline. Starting with a purely poloidal magnetic field and a latitudinal
shear meant to be imposed by the convection zone at the top of the radiation
zone, we have investigated the interactions between magnetic fields, rotation
and shear, using the spectral code ASH on massive parallel supercomputers. In
all cases we have explored, the fossil field diffuses outward and ends up
connecting with the convection zone, whose differential rotation is then
imprinted at latitudes above 40 deg throughout the radiative interior,
according to Ferraro's law of isorotation. Rotation remains uniform in the
lower latitude region which is contained within closed field lines. We find
that the meridional flow cannot stop the inward progression of the differential
rotation. Further, we observe the development of non-axisymmetric
magnetohydrodynamic instabilities, first due to the initial poloidal
configuration of the fossil field, and later to the toroidal field produced by
shearing the poloidal field through the differential rotation. We do not find
dynamo action as such in the radiative interior, since the mean poloidal field
is not regenerated. But the instability persists during the whole evolution,
while slowly decaying with the mean poloidal field. According to our numerical
simulations, a fossil magnetic field cannot prevent the radiative spread of the
tachocline, and thus it is unable to enforce uniform rotation in the radiation
zone. Neither can the observed thinness of that layer be invoked as a proof for
such an internal fossil magnetic field.Comment: 12 pages, 8 color figures (low res), published in A&A, october 200
Maternal self-reported prenatal depressive symptoms predict infant NR3C1 1F and BDNF IV DNA methylation.
Prenatal maternal psychological distress increases risk for adverse infant outcomes.
However, the biological mechanisms underlying this association remain unclear.
Prenatal stress can impact fetal epigenetic regulation that could underlie changes in
infant stress responses. It has been suggested that maternal glucocorticoids may
mediate this epigenetic effect. We examined this hypothesis by determining the
impact of maternal cortisol and depressive symptoms during pregnancy on infant
NR3C1 and BDNF DNA methylation. Fifty-seven pregnant women were recruited
during the second or third trimester.
Participants self-reported depressive symptoms
and salivary cortisol samples were collected diurnally and in response to a stressor.
Buccal swabs for DNA extraction and DNA methylation analysis were collected from
each infant at two months of age, and mothers were assessed for postnatal depressive
symptoms. Prenatal depressive symptoms significantly predicted increased NR3C1 1F
DNA methylation in male infants ( 2.147 = س , P = 0.044). Prenatal depressive
symptoms also significantly predicted decreased BDNF IV DNA methylation in both
male and female infants ( -3.244 = س , P = 0.013). No measure of maternal cortisol
during pregnancy predicted infant NR3C1 1F or BDNF promoter IV DNA
methylation. Our findings highlight the susceptibility of males to changes in NR3C1
DNA methylation and present novel evidence for altered BDNF IV DNA methylation
in response to maternal depression during pregnancy. The lack of association between
maternal cortisol and infant DNA methylation suggests that effects of maternal
depression may not be mediated directly by glucocorticoids. Future studies should
consider other potential mediating mechanisms in the link between maternal mood
and infant outcome
Double superconducting transition in the filled skutterudite PrOs4Sb12 and sample characterizations
A thorough characterization of many samples of the filled skutterudite
compound PrOs4Sb12 is provided. We find that the double superconducting
transition in the specific heat Tc1~1.89K and Tc2~1.72K tends to appear in
samples with a large residual resistivity ratio, large specific heat jump at
the superconducting transition and with the highest absolute value of the
specific heat above Tc1. However, we present evidence which casts doubt on the
intrinsic nature of the double superconducting transition. The ratio of the two
specific heat jumps \Delta C(Tc1)/\Delta C(Tc2) shows a wide range of values on
crystals from different batches but also within the same batch. This ratio was
strongly reduced by polishing a sample down to 120um. Remarkably, three samples
exhibit a single sharp transition of ~15mK in width at Tc~1.7K. The normalized
specific heat jump (C-Cnormal)/Cnormal at Tc of two of them is higher than ~32%
so larger than the sum of the two specific heat jumps when a double transition
exists. As an evidence of better quality, the slope in the transition is at
least two time steeper.
We discuss the origins of the double transition; in particular we consider,
based on X-ray diffraction results, a scenario involving Pr-vacancies. The
superconducting phase diagram under magnetic field of a sample with a single
transition is fitted with a two-band model taking into account the good values
for the gap as deduced from thermal conductivity measurements.Comment: 10 pages, 9 figures, 2 tables, submitted to Physical review
Plasma engineering of graphene
Recently, there have been enormous efforts to tailor the properties of graphene. These improved properties extend the prospect of graphene for a broad range of applications. Plasmas find applications in various fields including materials science and have been emerging in the field of nanotechnology. This review focuses on different plasma functionalization processes of graphene and its oxide counterpart. The review aims at the advantages of plasma functionalization over the conventional doping techniques. Selectivity and controllability of the plasma techniques opens up future pathways for large scale, rapid functionalization of graphene for advanced applications. We also emphasize on atmospheric pressure plasma jet as the future prospect of plasma based functionalization processes
How do Fermi liquids get heavy and die?
We discuss non-Fermi liquid and quantum critical behavior in heavy fermion
materials, focussing on the mechanism by which the electron mass appears to
diverge at the quantum critical point. We ask whether the basic mechanism for
the transformation involves electron diffraction off a quantum critical spin
density wave, or whether a break-down in the composite nature of the heavy
electron takes place at the quantum critical point. We show that the Hall
constant changes continously in the first scenario, but may ``jump''
discontinuously at a quantum critical point where the composite character of
the electron quasiparticles changes.Comment: Revised version with many new references added. To appear as a
topical review in Journal of Physics: Condensed Matter Physics. Two column
version http://www.physics.rutgers.edu/~coleman/online/questions.ps.g
Divergence of the Grueneisen Ratio at Quantum Critical Points in Heavy Fermion Metals
We present low-temperature volume thermal expansion, , and specific
heat, , measurements on high-quality single crystals of CeNi2Ge2 and
YbRh2(SiGe) which are located very near to quantum
critical points. For both systems, shows a more singular temperature
dependence than , and thus the Grueneisen ratio
diverges as T --> 0. For CeNi2Ge2, our results are in accordance with the
spin-density wave (SDW) scenario for three-dimensional critical
spin-fluctuations. By contrast, the observed singularity in
YbRh2_{0.95}_{0.05}_2$ cannot be explained by the itinerant SDW
theory but is qualitatively consistent with a locally quantum critical picture.Comment: 11 pages, 4 figure
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