525 research outputs found
Efficient implementation of the nonequilibrium Green function method for electronic transport calculations
An efficient implementation of the nonequilibrium Green function (NEGF)
method combined with the density functional theory (DFT) using localized
pseudo-atomic orbitals (PAOs) is presented for electronic transport
calculations of a system connected with two leads under a finite bias voltage.
In the implementation, accurate and efficient methods are developed especially
for evaluation of the density matrix and treatment of boundaries between the
scattering region and the leads. Equilibrium and nonequilibrium contributions
in the density matrix are evaluated with very high precision by a contour
integration with a continued fraction representation of the Fermi-Dirac
function and by a simple quadratureon the real axis with a small imaginary
part, respectively. The Hartree potential is computed efficiently by a
combination of the two dimensional fast Fourier transform (FFT) and a finite
difference method, and the charge density near the boundaries is constructed
with a careful treatment to avoid the spurious scattering at the boundaries.
The efficiency of the implementation is demonstrated by rapid convergence
properties of the density matrix. In addition, as an illustration, our method
is applied for zigzag graphene nanoribbons, a Fe/MgO/Fe tunneling junction, and
a LaMnOSrMnO superlattice, demonstrating its applicability to a wide
variety of systems.Comment: 20 pages, 11 figure
Dynamic critical phenomena in the AdS/CFT duality
In critical phenomena, singular behaviors arise not only for thermodynamic
quantities but also for transport coefficients. We study this dynamic critical
phenomenon in the AdS/CFT duality. We consider black holes with a single
R-charge in various dimensions and compute the R-charge diffusion in the linear
perturbations. In this case, the black holes belong to model B according to the
classification of Hohenberg and Halperin.Comment: 17 pages, ReVTeX4; v2: added references and discussio
Modeling Hadronic Gamma-ray Emissions from Solar Flares and Prospects for Detecting Non-thermal Signatures from Protostars
We investigate gamma-ray emission in the impulsive phase of solar flares and
the detectability of non-thermal signatures from protostellar flares. Energetic
solar flares emit high-energy gamma rays of GeV energies, but their production
mechanism and emission site are still unknown. Young stellar objects, including
protostars, also exhibit luminous X-ray flares, but the triggering mechanism of
the flaring activity is still unclear due to the strong obscuration.
Non-thermal signatures in mm/sub-mm and gamma-ray bands are useful to probe
protostellar flares owing to their strong penetration power. We develop a
non-thermal emission model of the impulsive phase of solar flares, where
cosmic-ray protons accelerated at the termination shock produce high-energy
gamma rays via hadronuclear interaction with the evaporation plasma. This model
can reproduce gamma-ray data in the impulsive phase of a solar flare. We apply
our model to protostellar flares and show that Cherenkov Telescope Array will
be able to detect gamma rays of TeV energies if particle acceleration in
protostellar flares is efficient. Non-thermal electrons accelerated together
with protons can emit strong mm and sub-mm signals via synchrotron radiation,
whose power is consistent with the energetic mm/sub-mm transients observed from
young stars. Future gamma-ray and mm/sub-mm observations from protostars,
coordinated with a hard X-ray observation, will unravel the triggering
mechanism of non-thermal particle production in protostellar flares.Comment: 19 pages, 5 figures, 1 tabl
Symposium on the biology of cells modified by viruses or antigens. II.: On the Analysis of Antibody Synthesis at the Cellular Level
The title of this symposium implies a similarity which is not obvious between the cellular responses to virus infection and to antigenic stimulation. In fact, no analogy between these two types of cellular response is apparent either from a consideration of the natures of the stimuli,
a specific nucleotide sequence on the one hand and almost any foreign chemical configuration on the other, or from an examination of the products of the response, identical units in the case of the virus and complementary antibody
units in the case of the antigen
Causality violation and singularities
We show that singularities necessarily occur when a boundary of causality
violating set exists in a space-time under the physically suitable assumptions
except the global causality condition in the Hawking-Penrose singularity
theorems. Instead of the global causality condition, we impose some
restrictions on the causality violating sets to show the occurrence of
singularities.Comment: 11 pages, latex, 2 eps figure
Mirror effect induced by the dilaton field on the Hawking radiation
We discuss the string creation in the near-extremal NS1 black string
solution. The string creation is described by an effective field equation
derived from a fundamental string action coupled to the dilaton field in a
conformally invariant manner. In the non-critical string model the dilaton
field causes a timelike mirror surface outside the horizon when the size of the
black string is comparable to the Planck scale. Since the fundamental strings
are reflected by the mirror surface, the negative energy flux does not
propagate across the surface. This means that the evaporation stops just before
the naked singularity of the extremal black string appears even though the
surface gravity is non-zero in the extremal limit.Comment: 15 page
Stability and Thermodynamics of AdS Black Holes with Scalar Hair
Recently a class of static spherical black hole solutions with scalar hair
was found in four and five dimensional gauged supergravity with modified, but
AdS invariant boundary conditions. These black holes are fully specified by a
single conserved charge, namely their mass, which acquires a contribution from
the scalar field. Here we report on a more detailed study of some of the
properties of these solutions. A thermodynamic analysis shows that in the
canonical ensemble the standard Schwarzschild-AdS black hole is stable against
decay into a hairy black hole. We also study the stability of the hairy black
holes and find there always exists an unstable radial fluctuation, in both four
and five dimensions. We argue, however, that Schwarzschild-AdS is probably not
the endstate of evolution under this instability.Comment: 18 pages, 4 figure
- …