1,468 research outputs found
Towards the 3D-Imaging of Sources
Geometric details of a nuclear reaction zone, at the time of particle
emission, can be restored from low relative-velocity particle-correlations,
following imaging. Some of the source details get erased and are a potential
cause of problems in the imaging, in the form of instabilities. These can be
coped with by following the method of discretized optimization for the restored
sources. So far it has been possible to produce 1-dimensional emission source
images, corresponding to the reactions averaged over all possible spatial
directions. Currently, efforts are in progress to restore angular details.Comment: Talk given at the Int. Workshop on Hot and Dense Matter in
Relativistic Heavy Ion Collisions, March 24-27, 2004, Budapest; 10 pages, 6
figure
Pressure and linear heat capacity in the superconducting state of thoriated UBe13
Even well below Tc, the heavy-fermion superconductor (U,Th)Be13 has a large
linear term in its specific heat. We show that under uniaxial pressure, the
linear heat capacity increases in magnitude by more than a factor of two. The
change is reversible and suggests that the linear term is an intrinsic property
of the material. In addition, we find no evidence of hysteresis or of latent
heat in the low-temperature and low-pressure portion of the phase diagram,
showing that all transitions in this region are second order.Comment: 5 pages, 4 figure
Microscopic theories for cubic and tetrahedral superconductors: application to PrOs_4Sb_{12}
We examine weak-coupling theory for unconventional superconducting states of
cubic or tetrahedral symmetry for arbitrary order parameters and Fermi surfaces
and identify the stable states in zero applied field. We further examine the
possibility of having multiple superconducting transitions arising from the
weak breaking of a higher symmetry group to cubic or tetrahedral symmetry.
Specifically, we consider two higher symmetry groups. The first is a weak
crystal field theory in which the spin-singlet Cooper pairs have an approximate
spherical symmetry. The second is a weak spin orbit coupling theory for which
spin-triplet Cooper pairs have a cubic orbital symmetry and an approximate
spherical spin rotational symmetry. In hexagonal UPt_3, these theories easily
give rise to multiple transitions. However, we find that for cubic materials,
there is only one case in which two superconducting transitions occur within
weak coupling theory. This sequence of transitions does not agree with the
observed properties of PrOs_4Sb_{12}. Consequently, we find that to explain two
transitions in PrOs_4Sb_{12} using approximate higher symmetry groups requires
a strong coupling theory. In view of this, we finally consider a weak coupling
theory for which two singlet representations have accidentally nearly
degenerate transition temperatures (not due to any approximate symmetries). We
provide an example of such a theory that agrees with the observed properties of
PrOs_4Sb_{12}.Comment: 11 pages,1 figur
Novel dynamic scaling regime in hole-doped La2CuO4
Only 3% hole doping by Li is sufficient to suppress the long-range
3-dimensional (3D) antiferromagnetic order in La2CuO4.
The spin dynamics of such a 2D spin liquid state at T << J was investigated
with measurements of the dynamic magnetic structure factor S(omega,q), using
cold neutron spectroscopy, for single crystalline La2Cu0.94Li0.06O4.
S(omega,q) peaks sharply at (pi,pi) and crosses over around 50K from omega/T
scaling to a novel low temperature regime characterized by a constant energy
scale. The possible connection to a crossover from the quantum critical to the
quantum disordered regime of the 2D antiferromagnetic spin liquid is discussed.Comment: 4 pages, 4 figure
Grain Physics and Rosseland Mean Opacities
Tables of mean opacities are often used to compute the transfer of radiation
in a variety of astrophysical simulations from stellar evolution models to
proto-planetary disks. Often tables, such as Ferguson et al. (2005), are
computed with a predetermined set of physical assumptions that may or may not
be valid for a specific application. This paper explores the effects of several
assumptions of grain physics on the Rosseland mean opacity in an oxygen rich
environment. We find that changing the distribution of grain sizes, either the
power-law exponent or the shape of the distribution, has a marginal effect on
the total mean opacity. We also explore the difference in the mean opacity
between solid homogenous grains and grains that are porous or conglomorations
of several species. Changing the amount of grain opacity included in the mean
by assuming a grain-to-gas ratio significantly affects the mean opacity, but in
a predictable way.Comment: 19 pages, 6 figures, accepted for publication in Ap
Low Temperature Magnetic Properties of the Double Exchange Model
We study the {\it ferromagnetic} (FM) Kondo lattice model in the strong
coupling limit (double exchange (DE) model). The DE mechanism proposed by Zener
to explain ferromagnetism has unexpected properties when there is more than one
itinerant electron. We find that, in general, the many-body ground state of the
DE model is {\it not} globally FM ordered (except for special filled-shell
cases). Also, the low energy excitations of this model are distinct from spin
wave excitations in usual Heisenberg ferromagnets, which will result in unusual
dynamic magnetic properties.Comment: 5 pages, RevTeX, 5 Postscript figures include
Progressive myoclonus epilepsy KCNC1 variant causes a developmental dendritopathy
OBJECTIVE: Mutations in KCNC1 can cause severe neurological dysfunction, including intellectual disability, epilepsy, and ataxia. The Arg320His variant, which occurs in the voltage-sensing domain of the channel, causes a highly penetrant and specific form of progressive myoclonus epilepsy with severe ataxia, designated myoclonus epilepsy and ataxia due to potassium channel mutation (MEAK). KCNC1 encodes the voltage-gated potassium channel KV 3.1, a channel that is important for enabling high-frequency firing in interneurons, raising the possibility that MEAK is associated with reduced interneuronal function. METHODS: To determine how this variant triggers MEAK, we expressed KV 3.1bR320H in cortical interneurons in vitro and investigated the effects on neuronal function and morphology. We also performed electrophysiological recordings of oocytes expressing KV 3.1b to determine whether the mutation introduces gating pore currents. RESULTS: Expression of the KV 3.1bR320H variant profoundly reduced excitability of mature cortical interneurons, and cells expressing these channels were unable to support high-frequency firing. The mutant channel also had an unexpected effect on morphology, severely impairing neurite development and interneuron viability, an effect that could not be rescued by blocking KV 3 channels. Oocyte recordings confirmed that in the adult KV 3.1b isoform, R320H confers a dominant negative loss-of-function effect by slowing channel activation, but does not introduce potentially toxic gating pore currents. SIGNIFICANCE: Overall, our data suggest that, in addition to the regulation of high-frequency firing, KV 3.1 channels play a hitherto unrecognized role in neuronal development. MEAK may be described as a developmental dendritopathy
Colossal Magnetoresistance is a Griffiths Singularity
It is now widely accepted that the magnetic transition in doped manganites
that show large magnetoresistance is a type of percolation effect. This paper
demonstrates that the transition should be viewed in the context of the
Griffiths phase that arises when disorder suppresses a magnetic transition.
This approach explains unusual aspects of susceptibility and heat capacity data
from a single crystal of LaCaMnOComment: 4 page
Chiral Correction to the Spin Fluctuation Feedback in two-dimensional p-wave Superconductors
We consider the stability of the superconducting phase for spin-triplet
p-wave pairing in a quasi-two-dimensional system. We show that in the absence
of spin-orbit coupling there is a chiral contribution to spin fluctuation
feedback which is related to spin quantum Hall effect in a chiral
superconducting phase. We show that this mechanism supports the stability of a
chiral p-wave state.Comment: 8 pages. The final version is accepted for publication in Europhys
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