619 research outputs found
Separation of the first- and second-order contributions in magneto-optic Kerr effect magnetometry of epitaxial FeMn/NiFe bilayers
The influence of second-order magneto-optic effects on Kerr effect
magnetometry of epitaxial exchange coupled FeMn/NiFe-bilayers is investigated.
A procedure for separation of the first- and second-order contributions is
presented. The full angular dependence of both contributions during the
magnetization reversal is extracted from the experimental data and presented
using gray scaled magnetization reversal diagrams. The theoretical description
of the investigated system is based on an extended Stoner-Wohlfarth model,
which includes an induced unidirectional and fourfold anisotropy in the
ferromagnet, caused by the coupling to the antiferromagnet. The agreement
between the experimental data and the theoretical model for both the first- and
second-order contributions are good, although a coherent reversal of the
magnetization is assumed in the model.Comment: 6 pages, 7 figures, submitted to J. Appl. Phy
Bounds on higher-order Lorentz-violating photon sector coefficients from an asymmetric optical ring resonator experiment
Optical resonators provide a powerful tool for testing aspects of Lorentz
invariance. Here, we present a reanalysis of an experiment where a path
asymmetry was created in an optical ring resonator by introducing a dielectric
prism in one arm. The frequency difference of the two fundamental
counter-propagating modes was then recorded as the apparatus was
orientation-modulated in the laboratory. By assuming that the minimal
Standard-Model Extension coefficients vanish we are able to place bounds on
higher-order parity-odd Lorentz-violating coefficients of the Standard-Model
Extension. The results presented in this work set the first constraints on two
previously unbounded linear combinations of d=8 parity-odd nonbirefringent
nondispersive coefficients of the photon sector.Comment: 6 pages, 4 figures, 3 tables, accepted for publication in Physics
Letters
A theoretical study on the damping of collective excitations in a Bose-Einstein condensate
We study the damping of low-lying collective excitations of condensates in a
weakly interacting Bose gas model within the framework of imaginary time path
integral. A general expression of the damping rate has been obtained in the low
momentum limit for both the very low temperature regime and the higher
temperature regime. For the latter, the result is new and applicable to recent
experiments. Theoretical predictions for the damping rate are compared with the
experimental values.Comment: 15 pages, LaTeX, revised for minor corrections on LaTeX file forma
Origin of four-fold anisotropy in square lattices of circular ferromagnetic dots
We discuss the four-fold anisotropy of in-plane ferromagnetic resonance (FMR)
field , found in a square lattice of circular Permalloy dots when the
interdot distance gets comparable to the dot diameter . The minimum
, along the lattice axes,
differ by 50 Oe at = 1.1. This anisotropy, not expected in
uniformly magnetized dots, is explained by a non-uniform magnetization
\bm(\br) in a dot in response to dipolar forces in the patterned magnetic
structure. It is well described by an iterative solution of a continuous
variational procedure.Comment: 4 pages, 3 figures, revtex, details of analytic calculation and new
references are adde
Two-photon linewidth of light "stopping" via electromagnetically induced transparency
We analyze the two-photon linewidth of the recently proposed adiabatic
transfer technique for ``stopping'' of light using electromagnetically induced
transparency (EIT). We shown that a successful and reliable transfer of
excitation from light to atoms and back can be achieved if the spectrum of the
input probe pulse lies within the initial transparency window of EIT, and if
the two-photon detuning is less than the collective coupling strength
(collective vacuum Rabi-frequency) divided by ,
with being the radiative decay rate, the effective number of atoms
in the sample, and the pulse duration. Hence in an optically thick medium
light ``storage'' and retrieval is possible with high fidelity even for systems
with rather large two-photon detuning or inhomogeneous broadening.Comment: 2 figure
Collective excitations of Bose-Einstein condensed gases at finite temperatures
We have applied the Popov version of the Hartree-Fock-Bogoliubov (HFB)
approximation to calculate the finite-temperature excitation spectrum of a
Bose-Einstein condensate (BEC) of Rb atoms. For lower values of the
temperature, we find excellent agreement with recently-published experimental
data for the JILA TOP trap. In contrast to recent comparison of the results of
HFB--Popov theory with experimental condensate fractions and specific heats,
there is disagreement of the theoretical and recent experimental results near
the BEC phase transition temperature.Comment: 4 pages, Latex, with 4 figures. More info at
http://amo.phy.gasou.edu/bec.htm
Excited states of a dilute Bose-Einstein condensate in a harmonic trap
The low-lying hydrodynamic normal modes of a dilute Bose-Einstein gas in an
isotropic harmonic trap determine the corresponding Bogoliubov amplitudes. In
the Thomas-Fermi limit, these modes have large low-temperature occupation
numbers, and they permit an explicit construction of the dynamic structure
function . The total noncondensate number at zero
temperature increases like , where is the condensate radius measured
in units of the oscillator length. The lowest dipole modes are constructed
explicitly in the Bogoliubov approximation.Comment: 15 pages, REVTE
Status Report on the Hydrodynamic Simulations of a Tapered Plasma Lens for Optical Matching at the ILC Source
The International Linear Collider is a planned electron-positron linear
collider with its positron source producing positrons by aiming undulator
radiation onto a rotating target. The resulting, highly divergent positron beam
requires immediate optical matching to improve the luminosity and therefore the
success of the intended collision experiments. Here, optical matching refers to
the process of capturing particles and making them available for downstream
beamline elements like accelerators. In the past, this has been done with
sophisticated coils, but more recently the usage of a current-carrying plasma,
a so-called plasma lens, has been proposed as an alternative. For the
International Linear Collider idealised particle tracking simulations have
already been done with the purpose of finding the optimal plasma lens design
with respect to the captured positron yield. The proposed design is
characterised by a linearly widened radius in beam direction. Now further
research and development of this design is required, including both experiments
with a prototype set-up as well as corresponding simulations modelling the
hydrodynamics of the current-carrying plasma and the resulting magnetic field.
The accuracy of the latter will benefit greatly from the former. In this work,
first preliminary hydrodynamic simulations instil confidence into further
endeavours.Comment: Talk presented at the International Workshop on Future Linear
Colliders (LCWS 2023), 15-19 May 2023. C23-05-15.
Finite temperature excitations of a trapped Bose gas
We present a detailed study of the temperature dependence of the condensate
and noncondensate density profiles of a Bose-condensed gas in a parabolic trap.
These quantitites are calculated self-consistently using the
Hartree-Fock-Bogoliubov equations within the Popov approximation. Below the
Bose-Einstein transition the excitation frequencies have a realtively weak
temperature dependence even though the condensate is strongly depleted. As the
condensate density goes to zero through the transition, the excitation
frequencies are strongly affected and approach the frequencies of a
noninteracting gas in the high temperature limit.Comment: 4 pages, Latex, 4 postscript figures. Submitted to Physical Review
Letter
Collective excitations of a trapped Bose-condensed gas
By taking the hydrodynamic limit we derive, at , an explicit solution of
the linearized time dependent Gross-Pitaevskii equation for the order parameter
of a Bose gas confined in a harmonic trap and interacting with repulsive
forces. The dispersion law for the
elementary excitations is obtained, to be compared with the prediction
of the noninteracting harmonic oscillator model.
Here is the number of radial nodes and is the orbital angular
momentum. The effects of the kinetic energy pressure, neglected in the
hydrodynamic approximation, are estimated using a sum rule approach. Results
are also presented for deformed traps and attractive forces.Comment: uuencoded file including 12 pages REVTEX and 1 figur
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