430 research outputs found
Low-temperature specific heat for ferromagnetic and antiferromagnetic orders in CaRu1-xMnxO3
Low-temperature specific heat of CaRu1-xMnxO3 was measured to clarify the
role of d electrons in ferromagnetic and antiferromagnetic orders observed
above x=0.2. Specific heat divided by temperature C_p/T is found to roughly
follow a T^2 function, and relatively large magnitudes of electronic specific
heat coefficient gamma were obtained in wide x range. In particular, gamma is
unchanged from the value at x=0 (84 mJ/K^2 mol) in the paramagnetic state for
x<=0.1, but linearly reduced with increasing x above x= 0.2. These features of
gamma strongly suggest that itinerant d electrons are tightly coupled with the
evolution of magnetic orders in small and intermediate Mn concentrations.Comment: 4 pages, 2 figures, to be published in J. Phys.: Conf. Ser. (SCES
2011, Cambridge, UK
Temperature Dependent Polarized XANES Spectra for Zn-doped LSCO system
The cuprates seem to exhibit statistics, dimensionality and phase transitions
in novel ways. The nature of excitations [i.e. quasiparticle or collective],
spin-charge separation, stripes [static and dynamics], inhomogeneities,
psuedogap, effect of impurity dopings [e.g. Zn, Ni] and any other phenomenon in
these materials must be consistently understood. Zn-doped LSCO single crystal
were grown by TSFZ technique. Temperature dependent Polarized XANES [near edge
local structure] spectra were measured at the BL13-B1 [Photon Factory] in the
Flourescence mode from 10 K to 300 K. Since both stripes and nonmagnetic Zn
impurities substituted for Cu give rise to inhomogeneous charge and spin
distribution it is interesting to understand the interplay of Zn impurities and
stripes. To understand these points we have used Zn-doping and some of the
results obtained are as follows: The spectra show a strong dependence with
respect to the polarization angle, , as is evident at any temperature
by comparing the spectra where the electric field vector is parallel with
ab-plane to the one where it is parallel to the c-axis. By using the XANES
[temperature] difference spectra we have determined T* [experimentally we find,
T* 160-170 K] for this sample. The XANES difference spectra shows
that the changes in XANES features are larger in the ab-plane than the c-axis,
this trend is expected since zinc is doped in the ab-plane at the copper site.
Our study also complements the results in literature namely that zinc doping
does not affect the c-axis transport.Comment: To appear in Physica C [ISS2001 Special Issue], related talk
presented at ISS2001 as PC-16, 10 pages revtex and 7 pages of figures (pdf
Clear Experimental Signature of Charge-Orbital density wave in NdCaMnO
Single Crystals of NdCaMnO have been prepared by the
travelling floating-zone method, and possible evidence of a charge -orbital
density wave in this material presented earlier [PRB68,092405 (2003)] using
High Resolution Electron Microscopy [HRTEM] and Electron Diffraction [ED]. In
the current note we present direct evidence of charge-orbital ordering in this
material using heat capacity measurements. Our heat capacity measurements
indicate a clear transition consistent with prior observation. We find two main
transitions, one at temperature K, and other at
K. In addition, we may also conclude that there is a strong electron-phonon
coupling in this material.Comment: 7 pages, 8 figure
Revisiting vertical structure of neutrino-dominated accretion disks: Bernoulli parameter, neutrino trapping and other distributions
We revisit the vertical structure of neutrino dominated accretion flows
(NDAFs) in spherical coordinates with a new boundary condition based on the
mechanical equilibrium. The solutions show that NDAF is significantly thick.
The Bernoulli parameter and neutrino trapping are determined by the mass
accretion rate and the viscosity parameter. According to the distribution of
the Bernoulli parameter, the possible outflow may appear in the outer region of
the disk. The neutrino trapping can essentially affect the neutrino radiation
luminosity. The vertical structure of NDAF is like a "sandwich", and the
multilayer accretion may account for the flares in gamma-ray bursts.Comment: 7 pages, 2 figures, Accepted for publication in Astrophysics & Space
Scienc
Ultrastable CO2 Laser Trapping of Lithium Fermions
We demonstrate an ultrastable CO2 laser trap that provides tight confinement
of neutral atoms with negligible optical scattering and minimal laser-noise-
induced heating. Using this method, fermionic 6Li atoms are stored in a 0.4 mK
deep well with a 1/e trap lifetime of 300 sec, consistent with a background
pressure of 10^(-11) Torr. To our knowledge, this is the longest storage time
ever achieved with an all-optical trap, comparable to the best reported
magnetic traps.Comment: 4 pages using REVTeX, 1 eps figur
Dead Zone Formation and Nonsteady Hyperaccretion in Collapsar Disks : A Possible Origin of Short-Term Variability in the Prompt Emission of Gamma-Ray Bursts
The central engine of gamma-ray bursts (GRBs) is believed to be a hot and
dense disk with hyperaccretion onto a few solar-mass black hole. We investigate
where the magnetorotational instability (MRI) actively operates in the
hyperaccretion disk, which can cause angular momentum transport in the disk.
The inner region of hyperaccretion disks can be neutrino opaque, and the
energy- and momentum-transport by neutrinos could affect the growth of the MRI
significantly. Assuming reasonable disk models and a weak magnetic field , it is found that the MRI is strongly suppressed by
the neutrino viscosity in the inner region of hyperaccretion disks. On the
other hand, the MRI can drive active MHD turbulence in the outer
neutrino-transparent region regardless of the field strength. This suggests
that the baryonic matter is accumulated into the inner dead zone where the MRI
grows inactively and the angular momentum transport is inefficient. When the
dead zone gains a large amount of mass and becomes gravitationally unstable,
intense mass accretion onto the central black hole would occur episodically
through the gravitational torque. This process can be a physical mechanism of
the short-term variability in the prompt emission of GRBs. Finally, the origin
of flaring activities in the X-ray afterglow is predicted in the context of our
episodic accretion scenario.Comment: 11pages, 4figures. Accepted for publication in the Astrophysical
Journa
A New Approach to Searching for Dark Matter Signals in Fermi-LAT Gamma Rays
Several cosmic ray experiments have measured excesses in electrons and
positrons, relative to standard backgrounds, for energies from ~ 10 GeV - 1
TeV. These excesses could be due to new astrophysical sources, but an
explanation in which the electrons and positrons are dark matter annihilation
or decay products is also consistent. Fortunately, the Fermi-LAT diffuse gamma
ray measurements can further test these models, since the electrons and
positrons produce gamma rays in their interactions in the interstellar medium.
Although the dark matter gamma ray signal consistent with the local electron
and positron measurements should be quite large, as we review, there are
substantial uncertainties in the modeling of diffuse backgrounds and,
additionally, experimental uncertainties that make it difficult to claim a dark
matter discovery. In this paper, we introduce an alternative method for
understanding the diffuse gamma ray spectrum in which we take the intensity
ratio in each energy bin of two different regions of the sky, thereby canceling
common systematic uncertainties. For many spectra, this ratio fits well to a
power law with a single break in energy. The two measured exponent indices are
a robust discriminant between candidate models, and we demonstrate that dark
matter annihilation scenarios can predict index values that require "extreme"
parameters for background-only explanations.Comment: v1: 11 pages, 7 figures, 1 table, revtex4; v2: 13 pages, 8 figures, 1
table, revtex4, Figure 4 added, minor additions made to text, references
added, conclusions unchanged, published versio
Electronic structure of the strongly hybridized ferromagnet CeFe2
We report on results from high-energy spectroscopic measurements on CeFe2, a
system of particular interest due to its anomalous ferromagnetism with an
unusually low Curie temperature and small magnetization compared to the other
rare earth-iron Laves phase compounds. Our experimental results indicate very
strong hybridization of the Ce 4f states with the delocalized band states,
mainly the Fe 3d states. In the interpretation and analysis of our measured
spectra, we have made use of two different theoretical approaches: The first
one is based on the Anderson impurity model, with surface contributions
explicitly taken into account. The second method consists of band-structure
calculations for bulk CeFe2. The analysis based on the Anderson impurity model
gives calculated spectra in good agreement with the whole range of measured
spectra, and reveals that the Ce 4f -- Fe 3d hybridization is considerably
reduced at the surface, resulting in even stronger hybridization in the bulk
than previously thought. The band-structure calculations are ab initio
full-potential linear muffin-tin orbital calculations within the
local-spin-density approximation of the density functional. The Ce 4f electrons
were treated as itinerant band electrons. Interestingly, the Ce 4f partial
density of states obtained from the band-structure calculations also agree well
with the experimental spectra concerning both the 4f peak position and the 4f
bandwidth, if the surface effects are properly taken into account. In addition,
results, notably the partial spin magnetic moments, from the band-structure
calculations are discussed in some detail and compared to experimental findings
and earlier calculations.Comment: 10 pages, 8 figures, to appear in Phys. Rev. B in December 200
10-kJ PW Laser for the FIREX-I Program
A 10-kJ PW laser (LFEX) is under construction for the
FIREX-I program. This paper reports a design overview of LFEX, the
technological development of a large-aperture arrayed amplifier with
modified four-pass architecture, wavefront correction, a large-aperture
Faraday rotator with a superconducting magnet, a new pulse compressor
arrangement, and focus control
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