9,560 research outputs found
Field dependence of the magnetic spectrum in anisotropic and Dzyaloshinskii-Moriya antiferromagnets: I. Theory
We consider theoretically the effects of an applied uniform magnetic field on
the magnetic spectrum of anisotropic two-dimensional and Dzyaloshinskii-Moriya
layered quantum Heisenberg antiferromagnets. The first case is relevant for
systems such as the two-dimensional square lattice antiferromagnet
Sr(2)CuO(2)Cl(2), while the later is known to be relevant to the physics of the
layered orthorhombic antiferromagnet La(2)CuO(4). We first establish the
correspondence betwenn the low-energy spectrum obtained within the anisotropic
non-linear sigma model and by means of the spin-wave approximation for a
standard easy-axis antiferromagent. Then, we focus on the field-theory approach
to calculate the magnetic field dependence of the magnon gaps and spectral
intensities for magnetic fields applied along the three possible
crystallographic directions. We discuss the various possible ground states and
their evolution with temperature for the different field orientations, and the
occurrence of spin-flop transitions for fields perpendicular to the layers
(transverse fields) as well as for fields along the easy axis (longitudinal
fields). Measurements of the one-magnon Raman spectrum in Sr(2)CuO(2)Cl(2) and
La(2)CuO(4) and a comparison between the experimental results and the
predictions of the present theory will be reported in part II of this research
work [L. Benfatto et al., cond-mat/0602664].Comment: 21 pages, 11 figures, final version. Part II of the present work is
presented in cond-mat/060266
Particle Creation by a Moving Boundary with Robin Boundary Condition
We consider a massless scalar field in 1+1 dimensions satisfying a Robin
boundary condition (BC) at a non-relativistic moving boundary. We derive a
Bogoliubov transformation between input and output bosonic field operators,
which allows us to calculate the spectral distribution of created particles.
The cases of Dirichlet and Neumann BC may be obtained from our result as
limiting cases. These two limits yield the same spectrum, which turns out to be
an upper bound for the spectra derived for Robin BC. We show that the particle
emission effect can be considerably reduced (with respect to the
Dirichlet/Neumann case) by selecting a particular value for the oscillation
frequency of the boundary position
Supergraph Approach in a Higher-order LDE Calculation of the Effective Potential for F-type Broken SUSY
In this work, we adopt the simplest model that spontaneously breaks
supersymmetry, namely, the minimal O'Raifeartaigh model. The effective
potential is computed in the framework of the linear delta expansion (LDE)
approach up to the order , conjugated with superspace and supergraph
techniques. The latter can be duly mastered even if supersymmetry is no longer
exact and the efficacy of the superfield approach in connection with the LDE
procedure is confirmed according to our investigation. That opens up a way for
a semi-nonperturbative superspace computation which allows us to deal with
spontaneously broken supersymmetric models and encourages us to go further and
apply this treatment to the Minimal Supersymmetric Standard Model (MSSM)
precision tests.Comment: 42 pages, 22 figures, text modified, new paragraph added in the
conclusions, revtex
The Schwarzschild-de Sitter solution in five-dimensional general relativity briefly revisited
We briefly revisit the Schwarzschild-de Sitter solution in the context of
five-dimensional general relativity. We obtain a class of five-dimensional
solutions of Einstein vacuum field equations into which the four-dimensional
Schwarzschild-de Sitter space can be locally and isometrically embedded. We
show that this class of solutions is well-behaved in the limit of lambda
approaching zero. Applying the same procedure to the de Sitter cosmological
model in five dimensions we obtain a class of embedding spaces which are
similarly well-behaved in this limit. These examples demonstrate that the
presence of a non-zero cosmological constant does not in general impose a rigid
relation between the (3+1) and (4+1)-dimensional spacetimes, with degenerate
limiting behaviour.Comment: 7 page
Skyrmions in a Doped Antiferromagnet
Magnetization and magnetoresistance have been measured in insulating
antiferromagnetic La_{2}Cu_{0.97}Li_{0.03}O_{4} over a wide range of
temperatures, magnetic fields, and field orientations. The magnetoresistance
step associated with a weak ferromagnetic transition exhibits a striking
nonmonotonic temperature dependence, consistent with the presence of skyrmions.Comment: 4+ pages, 3 figures (some low resolution), supplementary material (3
pages); discussion expanded, references added; as publishe
Casimir forces between arbitrary compact objects: Scalar and electromagnetic field
We develop an exact method for computing the Casimir energy between arbitrary
compact objects, both with boundary conditions for a scalar field and
dielectrics or perfect conductors for the electromagnetic field. The energy is
obtained as an interaction between multipoles, generated by quantum source or
current fluctuations. The objects' shape and composition enter only through
their scattering matrices. The result is exact when all multipoles are
included, and converges rapidly. A low frequency expansion yields the energy as
a series in the ratio of the objects' size to their separation. As examples, we
obtain this series for two spheres with Robin boundary conditions for a scalar
field and dielectric spheres for the electromagnetic field. The full
interaction at all separations is obtained for spheres with Robin boundary
conditions and for perfectly conducting spheres.Comment: 24 pages, 3 figures, contribution to QFEXT07 proceeding
Self-dual vortices in a Maxwell-Chern-Simons model with non-minimal coupling
We find self-dual vortex solutions in a Maxwell-Chern-Simons model with
anomalous magnetic moment. From a recently developed N=2-supersymmetric
extension, we obtain the proper Bogomol'nyi equations together with a Higgs
potential allowing both topological and non-topological phases in the theory.Comment: 12 pages, 9 figures, 2 tables; some typos corrected, one reference
updated. To be published in the Int. J. Mod. Phys. A (1999
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