2,180 research outputs found
Monopoles in the Higgs Phase
We describe new solutions of Yang-Mills-Higgs theories consisting of magnetic
monopoles in a phase with fully broken gauge symmetry. Rather than spreading
out radially, the magnetic field lines form flux tubes. The solution is
topologically stable and, when embedded in N=2 SQCD, preserves 1/4 of the
supercharges. From the perspective of the flux-tube the monopole appears as a
kink. Many monopoles may be threaded onto a single flux tube and placed at
arbitrary separation to create a stable, BPS necklace of solitons.Comment: 8 Pages, 1 Figure. v2: Added references and comments on 3He. v3:
Another reference and corrected term in Lagrangia
Superconducting p-branes and Extremal Black Holes
In Einstein-Maxwell theory, magnetic flux lines are `expelled' from a black
hole as extremality is approached, in the sense that the component of the field
strength normal to the horizon goes to zero. Thus, extremal black holes are
found to exhibit the sort of `Meissner effect' which is characteristic of
superconducting media. We review some of the evidence for this effect, and do
present new evidence for it using recently found black hole solutions in string
theory and Kaluza-Klein theory. We also present some new solutions, which arise
naturally in string theory, which are non-superconducting extremal black holes.
We present a nice geometrical interpretation of these effects derived by
looking carefully at the higher dimensional configurations from which the lower
dimensional black hole solutions are obtained. We show that other extremal
solitonic objects in string theory (such as p-branes) can also display
superconducting properties. In particular, we argue that the relativistic
London equation will hold on the worldvolume of `light' superconducting
p-branes (which are embedded in flat space), and that minimally coupled zero
modes will propagate in the adS factor of the near-horizon geometries of
`heavy', or gravitating, superconducting p-branes.Comment: 22 pages, 2 figure
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
A quantum gate array can be programmed to evaluate the expectation value of any operator
A programmable gate array is a circuit whose action is controlled by input
data. In this letter we describe a special--purpose quantum circuit that can be
programmed to evaluate the expectation value of any operator acting on a
space of states of dimensions. The circuit has a program register whose
state encodes the operator whose expectation value is to be
evaluated. The method requires knowledge of the expansion of in a basis of
the space of operators. We discuss some applications of this circuit and its
relation to known instances of quantum state tomography.Comment: 4 pages, 3 figures include
First-principles study of lattice instabilities in the ferromagnetic martensite NiMnGa
The phonon dispersion relations and elastic constants for ferromagnetic
NiMnGa in the cubic and tetragonally distorted Heusler structures are
computed using density-functional and density-functional perturbation theory
within the spin-polarized generalized-gradient approximation. For
, the TA tranverse acoustic branch along and
symmetry-related directions displays a dynamical instability at a wavevector
that depends on . Through examination of the Fermi-surface nesting and
electron-phonon coupling, this is identified as a Kohn anomaly. In the parent
cubic phase the computed tetragonal shear elastic constant,
C=(CC)/2, is close to zero, indicating a marginal
elastic instability towards a uniform tetragonal distortion. We conclude that
the cubic Heusler structure is unstable against a family of energy-lowering
distortions produced by the coupling between a uniform tetragonal distortion
and the corresponding modulation. The computed relation between the
ratio and the modulation wavevector is in excellent agreement with
structural data on the premartensitic ( = 1) and martensitic ( =
0.94) phases of NiMnGa.Comment: submitted to Phys. Rev.
The Appearance and Disappearance of Ship Tracks on Large Spatial Scales
The 1-km advanced very high resolution radiometer observations from the morning, NOAA-12, and afternoon,
NOAA-11, satellite passes over the coast of California during June 1994 are used to determine the altitudes,
visible optical depths, and cloud droplet effective radii for low-level clouds. Comparisons are made between
the properties of clouds within 50 km of ship tracks and those farther than 200 km from the tracks in order to
deduce the conditions that are conducive to the appearance of ship tracks in satellite images. The results indicate
that the low-level clouds must be sufficiently close to the surface for ship tracks to form. Ship tracks rarely
appear in low-level clouds having altitudes greater than 1 km. The distributions of visible optical depths and
cloud droplet effective radii for ambient clouds in which ship tracks are embedded are the same as those for
clouds without ship tracks. Cloud droplet sizes and liquid water paths for low-level clouds do not constrain the
appearance of ship tracks in the imagery. The sensitivity of ship tracks to cloud altitude appears to explain why
the majority of ship tracks observed from satellites off the coast of California are found south of 358N. A small
rise in the height of low-level clouds appears to explain why numerous ship tracks appeared on one day in a
particular region but disappeared on the next, even though the altitudes of the low-level clouds were generally
less than 1 km and the cloud cover was the same for both days. In addition, ship tracks are frequent when lowlevel
clouds at altitudes below 1 km are extensive and completely cover large areas. The frequency of imagery
pixels overcast by clouds with altitudes below 1 km is greater in the morning than in the afternoon and explains
why more ship tracks are observed in the morning than in the afternoon. If the occurrence of ship tracks in
satellite imagery data depends on the coupling of the clouds to the underlying boundary layer, then cloud-top
altitude and the area of complete cloud cover by low-level clouds may be useful indices for this coupling.This work was supported in part by the Office of Naval Research and by the National Science Foundation through the Center for Clouds, Chemistry and Climate at the Scripps Institution of Oceanography, an NSF Science and Technology Center
First Principles Calculation of Elastic Properties of Solid Argon at High Pressures
The density and the elastic stiffness coefficients of fcc solid argon at high
pressures from 1 GPa up to 80 GPa are computed by first-principles
pseudopotential method with plane-wave basis set and the generalized gradient
approximation (GGA). The result is in good agreement with the experimental
result recently obtained with the Brillouin spectroscopy by Shimizu et al.
[Phys. Rev. Lett. 86, 4568 (2001)]. The Cauchy condition was found to be
strongly violated as in the experimental result, indicating large contribution
from non-central many-body force. The present result has made it clear that the
standard density functional method with periodic boundary conditions can be
successfully applied for calculating elastic properties of rare gas solids at
high pressures in contrast to those at low pressures where dispersion forces
are important.Comment: 4 pages, 5 figures, submitted to PR
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