70,629 research outputs found

### Comment on "Classical and Quantum Interaction of the Dipole"

In this paper I have presented Comment on Anandan's paper (J. Anandan, Phys.
Rev. Lett. 85, 1354 (2000)) [hep-th/9910018].Comment: 1 page, revtex; small changes, mainly typos, according to the
published version in Phys. Rev. Let

### Comment on "R\"{o}ntgen Quantum Phase Shift: A Semiclassical Local Electrodynamical Effect?''

This paper is Comment on the paper: S.A.R. Horsley and M. Babiker, Phys. Rev.
Lett. 95, 010405 (2005).Comment: minor changes in the text, some references are changed, according to
the version which is accepted for publication in Phys. Rev. Let

### Electric-Magnetic Duality and Topological Insulators

We work out the action of the SL(2,Z) electric-magnetic duality group for an
insulator with a non-trivial permittivity, permeability and theta-angle. This
theory has recently been proposed to be the correct low-energy effective action
for topological insulators. As applications, we give manifestly SL(2,Z)
covariant expressions for the Faraday rotation at orthogonal incidence at the
interface of two such materials, as well as for the induced magnetic and
electric charges, slightly clarifying the meaning of expressions previously
derived in the literature. We also use electric-magnetic duality to find a
gravitational dual for a strongly coupled version of this theory using the
AdS/CFT correspondence.Comment: 4 pages; version accepted by PR

### Biot-Savart-like law in electrostatics

The Biot-Savart law is a well-known and powerful theoretical tool used to
calculate magnetic fields due to currents in magnetostatics. We extend the
range of applicability and the formal structure of the Biot-Savart law to
electrostatics by deriving a Biot-Savart-like law suitable for calculating
electric fields. We show that, under certain circumstances, the traditional
Dirichlet problem can be mapped onto a much simpler Biot-Savart-like problem.
We find an integral expression for the electric field due to an arbitrarily
shaped, planar region kept at a fixed electric potential, in an otherwise
grounded plane. As a by-product we present a very simple formula to compute the
field produced in the plane defined by such a region. We illustrate the
usefulness of our approach by calculating the electric field produced by planar
regions of a few nontrivial shapes.Comment: 14 pages, 6 figures, RevTex, accepted for publication in the European
Journal of Physic

### The total nucleon-nucleon cross section at large N_c

It is shown that at sufficiently large $N_c$ for incident momenta which are
much larger than the QCD, the total nucleon-nucleon cross section is
independent of incident momentum and given by $\sigma^{\rm total}=2 \pi
\log^2(N_c) / (m^2_{\pi})$. This result is valid in the extreme large $N_c$
regime of $\log(N_c) \gg 1$ and has corrections of relative order $\log
(\log(N_c))/\log(N_c)$. A possible connection of this result to the
Froissart-Martin bound is discussed.Comment: 4 page

### A simple variational principle for classical spinning particle with anomalous magnetic momentum

We obtain Bargmann-Michel-Telegdi equations of motion of classical spinning
particle using Lagrangian variational principle with Grassmann variables.Comment: 3 pages, late

### Towards a bulk theory of flexoelectricity

Flexoelectricity is the linear response of polarization to a strain gradient.
Here we address the simplest class of dielectrics, namely elemental cubic
crystals, and we prove that therein there is no extrinsic (i.e. surface)
contribution to flexoelectricity in the thermodynamic limit. The flexoelectric
tensor is expressed as a bulk response of the solid, manifestly independent of
surface configurations. Furthermore, we prove that the flexoelectric responses
induced by a long-wavelength phonon and by a uniform strain gradient are
identical.Comment: 5 pages, 1 figure (2 panels

### The effect of starspots on the radii of low-mass pre-main sequence stars

A polytropic model is used to investigate the effects of dark photospheric
spots on the evolution and radii of magnetically active, low-mass (M<0.5Msun),
pre-main sequence (PMS) stars. Spots slow the contraction along Hayashi tracks
and inflate the radii of PMS stars by a factor of (1-beta)^{-N} compared to
unspotted stars of the same luminosity, where beta is the equivalent covering
fraction of dark starspots and N \simeq 0.45+/-0.05. This is a much stronger
inflation than predicted by the models of Spruit & Weiss (1986) for main
sequence stars with the same beta, where N \sim 0.2 to 0.3. These models have
been compared to radii determined for very magnetically active K- and M-dwarfs
in the young Pleiades and NGC 2516 clusters, and the radii of tidally-locked,
low-mass eclipsing binary components. The binary components and ZAMS K-dwarfs
have radii inflated by \sim 10 per cent compared to an empirical
radius-luminosity relation that is defined by magnetically inactive field
dwarfs with interferometrically measured radii; low-mass M-type PMS stars, that
are still on their Hayashi tracks, are inflated by up to \sim 40 per cent. If
this were attributable to starspots alone, we estimate that an effective spot
coverage of 0.35 < beta < 0.51 is required. Alternatively, global inhibition of
convective flux transport by dynamo-generated fields may play a role. However,
we find greater consistency with the starspot models when comparing the loci of
active young stars and inactive field stars in colour-magnitude diagrams,
particularly for the highly inflated PMS stars, where the large, uniform
temperature reduction required in globally inhibited convection models would
cause the stars to be much redder than observed.Comment: MNRAS in press, 13 page

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