365 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
Iron Fluorescent Line Emission from Black Hole Accretion Disks with Magnetic Reconnection-Heated Corona
We investigate the iron K fluorescent line produced by hard X-ray
photons from magnetic reconnection-heated corona. The hot corona with
temperature being about K can irradiate the underlying disk with a
continuum X-ray spectrum produced via thermal Comptonization. Then the iron
atoms in the disk photoelectrically absorb X-ray photons and radiate K
line photons. Therefore, the activity of corona is responsible to the iron line
emission from the underlying disk. In previous studies, oversimplified X-ray
photon sources are often assumed above the disk in order to compute the iron
line profile or power-law line emissivity profiles are assumed with an index
being a free parameter. We adopt the more realistic corona model constructed by
Liu et al. in which the corona is heated by magnetic energy released through
the reconnection of magnetic flux loops and which has no free parameter. Then
the accretion energy is dominantly dissipated in the corona, in which X-ray
photons are efficiently produced and irradiate the underlying disk. We find the
local emmisivity of iron line on the disk is approximated as . The iron line profiles derived from this model
give excellent fits to the observational data of MCG-6-30-15 with the profiles
derived theoretically for for energy band 4-7keV. Possible
origins of line variability are briefly discussed.Comment: 17 pages, 7 figures, accepted for publication in Ap
Neutrino Cooled Disk and Its Stability
We investigate the structure and stability of hypercritical accretion flows
around stellar-mass black holes, taking into account neutrino cooling, lepton
conservation, and firstly a realistic equation of state in order to properly
treat the dissociation of nuclei. We obtain the radial distributions of
physical properties, such as density, temperature and electron fraction, for
various mass accretion rates . We find that,
depending on mass accretion rates, different physics affect considerably the
structure of the disk; most important physics is (1) the photodissociation of
nuclei around for relatively low mass accretion rates
(), (2) efficient neutrino cooling
around for moderately high mass accretion rate (), and (3) neutrino trapping () for
very high mass accretion rate (). We
also investigate the stability of hypercritical accretion flows by drawing the
thermal equilibrium curves, and find that efficient neutrino cooling makes the
accretion flows rather stable against both thermal and viscous modes.Comment: 26 pages, 28 figures, Accepted for publication in Ap
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
Generalized Involution Models for Wreath Products
We prove that if a finite group has a generalized involution model, as
defined by Bump and Ginzburg, then the wreath product also has a
generalized involution model. This extends the work of Baddeley concerning
involution models for wreath products. As an application, we construct a
Gelfand model for wreath products of the form with abelian, and
give an alternate proof of a recent result due to Adin, Postnikov, and Roichman
describing a particularly elegant Gelfand model for the wreath product \ZZ_r
\wr S_n. We conclude by discussing some notable properties of this
representation and its decomposition into irreducible constituents, proving a
conjecture of Adin, Roichman, and Postnikov's.Comment: 29 page
An effective spin-orbital Hamiltonian for the double perovskite SrFeW O: Derivation of the phase diagram
We formulate a superexchange theory of insulating double-perovskite compounds
such as SrFeWO. An effective spin-orbital Hamiltonian is derived in the
strong coupling limit of Hubbard model for d-electrons on Fe and W ions. The
relevant degrees of freedom are the spins S=2 and the three-fold orbital
degeneracy of Fe-ions. W-sites are integrated out by means of a
fourth-order perturbative expansion. The magnetically and orbitally ordered
ground states of the effective Hamiltonia n are discussed as a function of the
model parameters. We show that for realistic values of such parameters the
ground state is antiferromagnetic, as experimentally observed. The order found
is of type-II, consisting of \{111\} ferromagnetic planes stac ked
antiferromagnetically. The orbital order energy scale found is one order of
magnitude less than the spi n one.Comment: 12 pages, 4 figure
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
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