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 $H_r$, found in a square lattice of circular Permalloy dots when the interdot distance $a$ gets comparable to the dot diameter $d$. The minimum $H_r$, along the lattice $$axes, and the maximum, along the$$ axes, differ by $\sim$ 50 Oe at $a/d$ = 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 $\delta$ is less than the collective coupling strength (collective vacuum Rabi-frequency) $g\sqrt{N}$ divided by $\sqrt{\gamma T}$, with $\gamma$ being the radiative decay rate, $N$ the effective number of atoms in the sample, and $T$ 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 $^{87}$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 $S(q,\omega)$. The total noncondensate number $N'(0)$ at zero temperature increases like $R^6$, where $R$ 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 e+e^+ 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 $T=0$, 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 $\omega=\omega_0(2n^2+2n\ell+3n+\ell)^{1/2}$ for the elementary excitations is obtained, to be compared with the prediction $\omega=\omega_0(2n+\ell)$ of the noninteracting harmonic oscillator model. Here $n$ is the number of radial nodes and $\ell$ 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