886,230 research outputs found
A Short Range Force
Gravitomagnetic and gravitoelectric forces have been studied for sometime and
tests for detecting such forces arising from the earth, are under way. We apply
similar considerations at the level of elementary particles in a formulation
using General Relativity, and deduce the presence of short range forces. A
possible candidate could be the somewhat recently detected but otherwise
mysterious short range force, mediated by massive "photons".Comment: 4 pages, TeX, Based on the paper in the Fifth International
Symposium, Frontiers of Fundamental Physic
Comment on "Scaling of the quasiparticle spectrum for d-wave superconductors"
In a recent Letter Simon and Lee suggested a scaling law for thermodynamic
and kinetic properties of superconductors with lines of gap nodes. However
their crossover parameter between the bulk dominated regime and the vortex
dominated regime is different from that found in our paper (N.B. Kopnin and
G.E. Volovik, JETP Lett., {\bf 64}, 690 (1996); see also cond-mat/9702093). We
discuss the origin of the disagreement.Comment: submitted to Physical Review Letters as "Comment" to the paper by
S.H. Simon and P.A. Lee, Phys. Rev. Lett., 78 (1997) 1548 (cond-mat/9611133
Curvature and Acoustic Instabilities in Rotating Fluid Disks
The stability of a rotating fluid disk to the formation of spiral arms is
studied in the tightwinding approximation in the linear regime. The dispersion
relation for spirals that was derived by Bertin et al. is shown to contain a
new, acoustic instability beyond the Lindblad resonances that depends only on
pressure and rotation. In this regime, pressure and gravity exchange roles as
drivers and inhibitors of spiral wave structures. Other instabilities that are
enhanced by pressure are also found in the general dispersion relation by
including higher order terms in the small parameter 1/kr for wavenumber k and
radius r. These instabilities are present even for large values of Toomre's
parameter Q. Unstable growth rates are determined in four cases: a
self-gravitating disk with a flat rotation curve, a self-gravitating disk with
solid body rotation, a non-self-gravitating disk with solid body rotation, and
a non-self-gravitating disk with Keplerian rotation. The most important
application appears to be as a source of spiral structure, possibly leading to
accretion in non-self-gravitating disks, such as some galactic nuclear disks,
disks around black holes, and proto-planetary disks. All of these examples have
short orbital times so the unstable growth time can be small.Comment: 30 pages, 5 figures, scheduled for ApJ 520, August 1, 199
The Casimir Effect for Generalized Piston Geometries
In this paper we study the Casimir energy and force for generalized pistons
constructed from warped product manifolds of the type where
is an interval of the real line and is a smooth compact
Riemannian manifold either with or without boundary. The piston geometry is
obtained by dividing the warped product manifold into two regions separated by
the cross section positioned at . By exploiting zeta function
regularization techniques we provide formulas for the Casimir energy and force
involving the arbitrary warping function and base manifold .Comment: 16 pages, LaTeX. To appear in the proceedings of the Conference on
Quantum Field Theory Under the Influence of External Conditions (QFEXT11).
Benasque, Spain, September 18-24, 201
Tachyon Effects on the 2-Dim Black Hole Geometry
We study solutions of the tree level string effective action in the presence
of the tachyon mode.We find that the 2-dim. static black hole is stable against
tachyonic perturbations.For a particular ansatz for the tachyon field we find
an exact solution of the equations of motion which exhibits a naked
singularity.In the case of static fields we find numerically that the full
system has a black hole solution,with the tachyon regular at the horizon.Comment: NTUA 45/94, Pages 11, Latex, 3 Figures on reques
Ground Band and a Generalized GP-equation for Spinor Bose-Einstein Condensates
For the spinor Bose-Einstein condensates both the total spin and its
Z-component should be conserved. However, in existing theories, only
the conservation of has been taken into account. To remedy, this paper
is the first attempt to take the conservation of both and into
account. For this purpose, a total spin-state with the good quantum numbers
and is introduced in the trial wave function, thereby a generalized
Gross-Pitaevskii equation has been derived. With this new equation, the ground
bands of the Na and Rb condensates have been studied, where the
levels distinct in split. It was found that the level density is extremely
dense in the bottom of the ground band of Na, i.e., in the vicinity of
the ground state. On the contrary, for Rb, the levels are extremely
dense in the top of the ground band,Comment: 7 page, 5 figure
Flux tubes and the type-I/type-II transition in a superconductor coupled to a superfluid
We analyze magnetic flux tubes at zero temperature in a superconductor that
is coupled to a superfluid via both density and gradient (``entrainment'')
interactions. The example we have in mind is high-density nuclear matter, which
is a proton superconductor and a neutron superfluid, but our treatment is
general and simple, modeling the interactions as a Ginzburg-Landau effective
theory with four-fermion couplings, including only s-wave pairing. We
numerically solve the field equations for flux tubes with an arbitrary number
of flux quanta, and compare their energies. This allows us to map the
type-I/type-II transition in the superconductor, which occurs at the
conventional kappa = 1/sqrt(2) if the condensates are uncoupled.
We find that a density coupling between the condensates raises the critical
kappa and, for a sufficiently high neutron density, resolves the type-I/type-II
transition line into an infinite number of bands corresponding to
``type-II(n)'' phases, in which n, the number of quanta in the favored flux
tube, steps from 1 to infinity. For lower neutron density, the coupling creates
spinodal regions around the type-I/type-II boundary, in which metastable flux
configurations are possible. We find that a gradient coupling between the
condensates lowers the critical kappa and creates spinodal regions. These
exotic phenomena may not occur in nuclear matter, which is thought to be deep
in the type-II region, but might be observed in condensed matter systems.Comment: 14 pages, improved discussion of the effects of varying the
neutron/proton condensate ratio; added reference
Vortex Mass in BCS systems: Kopnin and Baym-Chandler contributions
The Kopnin mass and the Baym-Chandler mass of the vortex have the same
origin. Both represent the mass of the normal component trapped by the vortex.
The Kopnin mass of the vortex is formed by quasiparticles localized in the
vicinity of the vortex. In the superclean limit it is calculated as linear
response exactly in the same way as the density of the normal component is
calculated in homogeneous superfluid. The Baym-Chandler mass is the
hydrodynamical (associated) mass trapped by vortex. It is analogous to the
normal component formed by inhomogeneities, such as pores and impurities. Both
contributions are calculated for the generic model of the continuous vortex
core.Comment: revtex file, 3 pages, 1 figure. Initially appeared as Comment to the
paper by E.B. Sonin et al "Vortex motion in charged and neutral superfluids:
A hydrodynamic approach" (Phys. Rev. B 57, 575 (1998)). The calculation of
the backflow mass is adde
- …