3,847 research outputs found
Bunching Transitions on Vicinal Surfaces and Quantum N-mers
We study vicinal crystal surfaces with the terrace-step-kink model on a
discrete lattice. Including both a short-ranged attractive interaction and a
long-ranged repulsive interaction arising from elastic forces, we discover a
series of phases in which steps coalesce into bunches of n steps each. The
value of n varies with temperature and the ratio of short to long range
interaction strengths. We propose that the bunch phases have been observed in
very recent experiments on Si surfaces. Within the context of a mapping of the
model to a system of bosons on a 1D lattice, the bunch phases appear as quantum
n-mers.Comment: 5 pages, RevTex; to appear in Phys. Rev. Let
Exact Dynamical Correlation Functions of Calogero-Sutherland Model and One-Dimensional Fractional Statistics
One-dimensional model of non-relativistic particles with inverse-square
interaction potential known as Calogero-Sutherland Model (CSM) is shown to
possess fractional statistics. Using the theory of Jack symmetric polynomial
the exact dynamical density-density correlation function and the one-particle
Green's function (hole propagator) at any rational interaction coupling
constant are obtained and used to show clear evidences of the
fractional statistics. Motifs representing the eigenstates of the model are
also constructed and used to reveal the fractional {\it exclusion} statistics
(in the sense of Haldane's ``Generalized Pauli Exclusion Principle''). This
model is also endowed with a natural {\it exchange } statistics (1D analog of
2D braiding statistics) compatible with the {\it exclusion} statistics.
(Submitted to PRL on April 18, 1994)Comment: Revtex 11 pages, IASSNS-HEP-94/27 (April 18, 1994
Hydrodynamics of cold atomic gases in the limit of weak nonlinearity, dispersion and dissipation
Dynamics of interacting cold atomic gases have recently become a focus of
both experimental and theoretical studies. Often cold atom systems show
hydrodynamic behavior and support the propagation of nonlinear dispersive
waves. Although this propagation depends on many details of the system, a great
insight can be obtained in the rather universal limit of weak nonlinearity,
dispersion and dissipation (WNDD). In this limit, using a reductive
perturbation method we map some of the hydrodynamic models relevant to cold
atoms to well known chiral one-dimensional equations such as KdV, Burgers,
KdV-Burgers, and Benjamin-Ono equations. These equations have been thoroughly
studied in literature. The mapping gives us a simple way to make estimates for
original hydrodynamic equations and to study the interplay between
nonlinearity, dissipation and dispersion which are the hallmarks of nonlinear
hydrodynamics.Comment: 18 pages, 3 figures, 1 tabl
Elementary excitations of the symmetric spin-orbital model: The XY limit
The elementary excitations of the 1D, symmetric, spin-orbital model are
investigated by studying two anisotropic versions of the model, the pure XY and
the dimerized XXZ case, with analytical and numerical methods. While they
preserve the symmetry between spin and orbital degrees of freedom, these models
allow for a simple and transparent picture of the low--lying excitations: In
the pure XY case, a phase separation takes place between two phases with
free--fermion like, gapless excitations, while in the dimerized case, the
low-energy effective Hamiltonian reduces to the 1D Ising model with gapped
excitations. In both cases, all the elementary excitations involve simultaneous
flips of the spin and orbital degrees of freedom, a clear indication of the
breakdown of the traditional mean-field theory.Comment: Revtex, two figure
New Algebraic Quantum Many-body Problems
We develop a systematic procedure for constructing quantum many-body problems
whose spectrum can be partially or totally computed by purely algebraic means.
The exactly-solvable models include rational and hyperbolic potentials related
to root systems, in some cases with an additional external field. The
quasi-exactly solvable models can be considered as deformations of the previous
ones which share their algebraic character.Comment: LaTeX 2e with amstex package, 36 page
Circumstellar Na I and Ca II lines of type Ia supernovae in symbiotic scenario
Formation of circumstellar lines of Na I and Ca II in type Ia supernovae is
studied for the case, when supernova explodes in a binary system with a red
giant. The model suggests a spherically-symmetric wind and takes into account
ionization and heating of the wind by X-rays from the shock wave and by
gamma-quanta of ^{56}Ni radioactive decay. For the wind density typical of the
red giant the expected optical depth of the wind in Na I lines turnes out too
low (\tau<0.001}) to detect the absorption. For the same wind densities the
predicted optical depth of Ca II 3934 \AA is sufficient for the detection
(\tau>0.1). I conclude that the absorption lines detected in SN 2006X cannot
form in the red giant wind; they are rather related to clouds at distances
larger than the dust evaporation radius (r>10^{17} cm). From the absence in SN
2006X of Ca II absorption lines not related with the similar Na I components I
derive the upper limit of the mass loss rate by the wind with velocity u:
\dot{M}<10^{-8}(u/10 km/s) M_{\odot} yr^{-1}.Comment: 10 pages, 6 figures, Astronomy Letters (accepted
Spectral Mapping Reconstruction of Extended Sources
Three dimensional spectroscopy of extended sources is typically performed
with dedicated integral field spectrographs. We describe a method of
reconstructing full spectral cubes, with two spatial and one spectral
dimension, from rastered spectral mapping observations employing a single slit
in a traditional slit spectrograph. When the background and image
characteristics are stable, as is often achieved in space, the use of
traditional long slits for integral field spectroscopy can substantially reduce
instrument complexity over dedicated integral field designs, without loss of
mapping efficiency -- particularly compelling when a long slit mode for single
unresolved source followup is separately required. We detail a custom
flux-conserving cube reconstruction algorithm, discuss issues of extended
source flux calibration, and describe CUBISM, a tool which implements these
methods for spectral maps obtained with ther Spitzer Space Telescope's Infrared
Spectrograph.Comment: 11 pages, 8 figures, accepted by PAS
Shape Invariance in the Calogero and Calogero-Sutherland Models
We show that the Calogero and Calogero-Sutherland models possess an N-body
generalization of shape invariance. We obtain the operator representation that
gives rise to this result, and discuss the implications of this result,
including the possibility of solving these models using algebraic methods based
on this shape invariance. Our representation gives us a natural way to
construct supersymmetric generalizations of these models, which are interesting
both in their own right and for the insights they offer in connection with the
exact solubility of these models.Comment: Latex file, 23 pages, no picture
Physical Conditions in the Seyfert Galaxy NGC 2992
This paper presents long slit spectral maps of the bi-cone shaped extended
narrow line region (ENLR) in the Seyfert galaxy NGC 2992. We investigate the
physical properties of the ENLR via emission line diagnostics, and compare the
observations to shock and photoionization models for the excitation mechanism
of the gas. The line ratios vary as a function of position in the ENLR, and the
loci of the observed points on line ratio diagrams are shown to be most
consistent with shock+precursor model grids. We consider the energetics of a
nuclear ionizing source for the ENLR, and perform the q-test in which the rate
of ionizing photons from the nucleus is inferred from measurements of the
density and ionization parameter. The q-test is shown to be invalid in the case
of NGC 2992 because of the limitations of the [S II]6717/6731 density
diagnostic. The excitation of the gas is shown to be broadly consistent with
the kinematics, with higher [N II]6583/H-alpha present in the more dynamically
active region. We also show that the pressure associated with the X-ray
emitting plasma may provide a large fraction of the pressure required to power
the ENLR via shocks.Comment: 55 pages, 49 figures, ApJ accepted September 9, 1998. Figures 1a-f
are provided in jpeg forma
Magnetic structure of Yb2Pt2Pb: Ising moments on the Shastry-Sutherland lattice.
Neutron diffraction measurements were carried out on single crystals and powders of Yb2Pt2Pb, where Yb moments form two interpenetrating planar sublattices of orthogonal dimers, a geometry known as Shastry-Sutherland lattice, and are stacked along the c axis in a ladder geometry. Yb2Pt2Pb orders antiferromagnetically at TN=2.07K, and the magnetic structure determined from these measurements features the interleaving of two orthogonal sublattices into a 5×5×1 magnetic supercell that is based on stripes with moments perpendicular to the dimer bonds, which are along (110) and (−110). Magnetic fields applied along (110) or (−110) suppress the antiferromagnetic peaks from an individual sublattice, but leave the orthogonal sublattice unaffected, evidence for the Ising character of the Yb moments in Yb2Pt2Pb that is supported by point charge calculations. Specific heat, magnetic susceptibility, and electrical resistivity measurements concur with neutron elastic scattering results that the longitudinal critical fluctuations are gapped with ΔE≃0.07meV
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