878 research outputs found
"Low-state" Black Hole Accretion in Nearby Galaxies
I summarize the main observational properties of low-luminosity AGNs in
nearby galaxies to argue that they are the high-mass analogs of black hole
X-ray binaries in the "low/hard" state. The principal characteristics of
low-state AGNs can be accommodated with a scenario in which the central engine
is comprised of three components: an optically thick, geometrically accretion
disk with a truncated inner radius, a radiatively inefficient flow, and a
compact jet.Comment: 8 pages. To appear in From X-ray Binaries to Quasars: Black Hole
Accretion on All Mass Scales, ed. T. J. Maccarone, R. P. Fender, and L. C. Ho
(Dordrecht: Kluwer
Nambu-Poisson Bracket and M-Theory Branes Coupled to Antisymmetric Fluxes
By using the recently proposed prescription arXiv:0804.3629 for obtaining the
brane action from multiple branes action in BLG theory, we examine
such transition when 11 Dimensional background antisymmetric fluxes couple to
the brane world volume. Such couplings was suggested in arXiv:0805.3427
where it was used the fact that various fields in BLG theory are valued in a
Lie 3-algebra. We argue that this action and promoting it by Nambu-Poisson
bracket gives the expected coupling of fluxes with brane at least at weak
coupling limit. We also study some other aspects of the action for example, the
gauge invariance of the theory.Comment: 14 page
Aspects of U-duality in BLG models with Lorentzian metric 3-algebras
In our previous paper, it was shown that BLG model based on a Lorentzian
metric 3-algebra gives Dp-brane action whose worldvolume is compactified on
torus T^d (d=p-2). Here the 3-algebra was a generalized one with d+1 pairs of
Lorentzian metric generators and expressed in terms of a loop algebra with
central extensions. In this paper, we derive the precise relation between the
coupling constant of the super Yang-Mills, the moduli of T^d and some R-R flux
with VEV's of ghost fields associated with Lorentzian metric generators. In
particular, for d=1, we derive the Yang-Mills action with theta term and show
that SL(2,Z) Montonen-Olive duality is realized as the rotation of two VEV's.
Furthermore, some moduli parameters such as NS-NS 2-form flux are identified as
the deformation parameters of the 3-algebras. By combining them, we recover
most of the moduli parameters which are required by U-duality symmetry.Comment: 27 pages, v2: minor correction
Structure and dynamics of Rh surfaces
Lattice relaxations, surface phonon spectra, surface energies, and work
functions are calculated for Rh(100) and Rh(110) surfaces using
density-functional theory and the full-potential linearized augmented plane
wave method. Both, the local-density approximation and the generalized gradient
approximation to the exchange-correlation functional are considered. The force
constants are obtained from the directly calculated atomic forces, and the
temperature dependence of the surface relaxation is evaluated by minimizing the
free energy of the system. The anharmonicity of the atomic vibrations is taken
into account within the quasiharmonic approximation. The importance of
contributions from different phonons to the surface relaxation is analyzed.Comment: 9 pages, 7 figures, scheduled to appear in Phys. Rev. B, Feb. 15
(1998). Other related publications can be found at
http://www.rz-berlin.mpg.de/th/paper.htm
Resonant dipole-dipole interaction in the presence of dispersing and absorbing surroundings
Within the framework of quantization of the macroscopic electromagnetic
field, equations of motion and an effective Hamiltonian for treating both the
resonant dipole-dipole interaction between two-level atoms and the resonant
atom-field interaction are derived, which can suitably be used for studying the
influence of arbitrary dispersing and absorbing material surroundings on these
interactions. The theory is applied to the study of the transient behavior of
two atoms that initially share a single excitation, with special emphasis on
the role of the two competing processes of virtual and real photon exchange in
the energy transfer between the atoms. In particular, it is shown that for weak
atom-field interaction there is a time window, where the energy transfer
follows a rate regime of the type obtained by ordinary second-order
perturbation theory. Finally, the resonant dipole-dipole interaction is shown
to give rise to a doublet spectrum of the emitted light for weak atom-field
interaction and a triplet spectrum for strong atom-field interaction.Comment: 15 pages, 1 figure, RevTE
Two-dimensional limit of exchange-correlation energy functional approximations in density functional theory
We investigate the behavior of three-dimensional (3D) exchange-correlation
energy functional approximations of density functional theory in anisotropic
systems with two-dimensional (2D) character. Using two simple models, quasi-2D
electron gas and two-electron quantum dot, we show a {\it fundamental
limitation} of the local density approximation (LDA), and its semi-local
extensions, generalized gradient approximation (GGA) and meta-GGA (MGGA), the
most widely used forms of which are worse than the LDA in the strong 2D limit.
The origin of these shortcomings is in the inability of the local (LDA) and
semi-local (GGA/MGGA) approximations to describe systems with 2D character in
which the nature of the exchange-correlation hole is very nonlocal. Nonlocal
functionals provide an alternative approach, and explicitly the average density
approximation (ADA) is shown to be remarkably accurate for the quasi-2D
electron gas system. Our study is not only relevant for understanding of the
functionals but also practical applications to semiconductor quantum structures
and materials such as graphite and metal surfaces. We also comment on the
implication of our findings to the practical device simulations based on the
(semi-)local density functional method.Comment: 21 pages including 9 figures, to be published in Phys. Rev.
Superfluid properties of one-component Fermi gas with an anisotropic p-wave interaction
We investigate superfluid properties and strong-coupling effects in a
one-component Fermi gas with an anisotropic p-wave interaction. Within the
framework of the Gaussian fluctuation theory, we determine the superfluid
transition temperature , as well as the temperature at which
the phase transition from the -wave pairing state to the -wave
state occurs below . We also show that while the anisotropy of the
p-wave interaction enhances in the strong-coupling regime, it
suppresses .Comment: 7 pages, 3 figures, proceedings of QFS 201
Supersymmetric Boost on Intersecting D-branes
We study the effect of the Born-Infeld electric field on the supersymmetric
configuration of various composite D-branes. We show that the generic values of
the electric field do not affect the supersymmetry but, as it approaches
keeping the magnetic field finite, various combinations of the
magnetic fields allow up to 8 supersymmetries. We also explore the unbroken
supersymmetries for two intersecting D-strings which are in uniform or relative
motion. For a finite uniform Lorentz boost, 16 supersymmetries are guaranteed
only when they are parallel. For an infinite one, 8 supersymmetries are
preserved only when both the D-strings are oriented to the forward or backward
direction of the boost. Under a finite relative boost, 8 supersymmetries are
preserved only when the intersecting angle is less than and the
intersecting point moves at the speed of light. As for an infinite relative
boost, 8 supersymmetries are preserved regardless of the values of the
intersecting angle.Comment: 27 pages using REVTeX4, 7 figure
N=8 superconformal gauge theories and M2 branes
Based on recent developments, in this letter we find 2+1 dimensional gauge
theories with scale invariance and N=8 supersymmetry. The gauge theories are
defined by a Lagrangian and are based on an infinite set of 3-algebras,
constructed as an extension of ordinary Lie algebras. Recent no-go theorems on
the existence of 3-algebras are circumvented by relaxing the assumption that
the invariant metric is positive definite. The gauge group is non compact, and
its maximally compact subgroup can be chosen to be any ordinary Lie group,
under which the matter fields are adjoints or singlets. The theories are parity
invariant and do not admit any tunable coupling constant. In the case of SU(N)
the moduli space of vacua contains a branch of the form (R^8)^N/S_N. These
properties are expected for the field theory living on a stack of M2 branes.Comment: 14 pages, no figure
Wigner Crystals Phases in Bilayer Quantum Hall Systems
(This is a substantially shortened version of the original abstract:)
The Wigner crystal phase diagram of the bilayer systems have been studied
using variational methods. Five crystal phases are obtained. As the layer
spacing increases, the system will undergo a sequence of phase transitions. A
common feature of most bilayer Wigner crystals is that they have mixed
(pseudo-spin) ferromagnetic and antiferromagnetic order.Comment: 19 figures. Figures will be provided upon request. Submitted in PRB
in Nov 94
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