253 research outputs found
Doubly resonant optical nanoantenna arrays for polarization resolved measurements of surface-enhanced Raman scattering
We report that rhomb-shaped metal nanoantenna arrays support multiple
plasmonic resonances, making them favorable bio-sensing substrates. Besides the
two localized plasmonic dipole modes associated with the two principle axes of
the rhombi, the sample supports an additional grating-induced surface plasmon
polariton resonance. The plasmonic properties of all modes are carefully
studied by far-field measurements together with numerical and analytical
calculations. The sample is then applied to surface-enhanced Raman scattering
measurements. It is shown to be highly efficient since two plasmonic resonances
of the structure were simultaneously tuned to coincide with the excitation and
the emission wave- length in the SERS experiment. The analysis is completed by
measuring the impact of the polarization angle on the SERS signal.Comment: 13 pages, 5 figure
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Magnetization dynamics of magnetic domain wall imprinted magnetic films
The influence of micromagnetic objects on the dynamic magnetic excitation in magnetic thin films is studied by imprinting periodic domain wall patterns through selective ion irradiation in exchange biased Ni81Fe 19/IrMn structures. For high domain wall densities an increased precessional frequency is achieved. The zero field resonance of the domain wall state hereby depends directly on the stripe period, showing a pronounced increase with decrease of domain wall spacing. With the abrupt annihilation of magnetic domain walls with an applied bias field a jump-like decrease in precessional frequency takes place. The experimental data and micromagnetic simulations prove that the characteristic collective dynamic mode for the domain wall configurations is attributed to strongly coupled tilted magnetization structure. This is evidenced by an overlapping NĆ©el wall structure for the narrowly spaced imprinted antiparallel unidirectional anisotropy state. The controlled introduction of high density frozen-in micromagnetic objects is a novel way to control the dynamic magnetic properties of continuous magnetic thin films
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
A Fermi Surface study of BaKBiO
We present all electron computations of the 3D Fermi surfaces (FS's) in
BaKBiO for a number of different compositions based on the
selfconsistent Korringa-Kohn-Rostoker coherent-potential-approximation
(KKR-CPA) approach for incorporating the effects of Ba/K substitution. By
assuming a simple cubic structure throughout the composition range, the
evolution of the nesting and other features of the FS of the underlying
pristine phase is correlated with the onset of various structural transitions
with K doping. A parameterized scheme for obtaining an accurate 3D map of the
FS in BaKBiO for an arbitrary doping level is developed. We
remark on the puzzling differences between the phase diagrams of
BaKBiO and BaPbBiO by comparing aspects
of their electronic structures and those of the end compounds BaBiO,
KBiO and BaPbO. Our theoretically predicted FS's in the cubic phase are
relevant for analyzing high-resolution Compton scattering and
positron-annihilation experiments sensitive to the electron momentum density,
and are thus amenable to substantial experimental verification.Comment: 12 pages, 7 figures, to appear in Phys. Rev.
Heisenberg exchange enhancement by orbital relaxation in cuprate compounds
We calculate the Heisenberg exchange J in the quasi-2D antiferromagnetic
cuprates La2CuO4, YBa2Cu3O6, Nd2CuO4 and Sr2CuO2Cl2. We apply all-electron
(MC)SCF and non-orthogonal CI calculations to [Cu2O11]18-, [Cu2O9]14-,
[Cu2O7]10- and [Cu2O7Cl4]14- clusters in a model charge embedding. The (MC)SCF
triplet and singlet ground states are well characterized by Cu2+ (dx2-y2) and
O2-. The antiferromagnetic exchange is strongly enhanced by admixing relaxed
(MC)SCF triplet and singlet excited states, in which a single electron is
transferred from the central O ion to Cu. We ascribe this effect to orbital
relaxation in the charge transfer component of the wave function. Close
agreement with experiment is obtained.Comment: publishe
Relationship between nonadiabaticity and damping in permalloy studied by current induced spin structure transformations
By direct imaging we determine spin structure changes in Permalloy wires and disks due to spin transfer torque as well as the critical current densities for different domain wall types. Periodic domain wall transformations from transverse to vortex walls and vice versa are observed, and the transformation mechanism occurs by vortex core displacement perpendicular to the wire. The results imply that the nonadiabaticity parameter Ī² does not equal the damping Ī±, in agreement with recent theoretical predictions. The vortex core motion perpendicular to the current is further studied in disks revealing that the displacement in opposite directions can be attributed to different polarities of the vortex core
Functional characterization of a melon alcohol acyl-transferase gene family involved in the biosynthesis of ester volatiles. Identification of the crucial role of a threonine residue for enzyme activity
Volatile esters, a major class of compounds contributing to the aroma of many fruit, are synthesized by
alcohol acyl-transferases (AAT). We demonstrate here that, in Charentais melon (Cucumis melo var.
cantalupensis), AAT are encoded by a gene family of at least four members with amino acid identity ranging
from 84% (Cm-AAT1/Cm-AAT2) and 58% (Cm-AAT1/Cm-AAT3) to only 22% (Cm-AAT1/Cm-AAT4).
All encoded proteins, except Cm-AAT2, were enzymatically active upon expression in yeast and show
differential substrate preferences. Cm-AAT1 protein produces a wide range of short and long-chain acyl
esters but has strong preference for the formation of E-2-hexenyl acetate and hexyl hexanoate. Cm-AAT3
also accepts a wide range of substrates but with very strong preference for producing benzyl acetate.
Cm-AAT4 is almost exclusively devoted to the formation of acetates, with strong preference for cinnamoyl
acetate. Site directed mutagenesis demonstrated that the failure of Cm-AAT2 to produce volatile esters is
related to the presence of a 268-alanine residue instead of threonine as in all active AAT proteins. Mutating
268-A into 268-T of Cm-AAT2 restored enzyme activity, while mutating 268-T into 268-A abolished
activity of Cm-AAT1. Activities of all three proteins measured with the prefered substrates sharply increase
during fruit ripening. The expression of all Cm-AAT genes is up-regulated during ripening and inhibited in
antisense ACC oxidase melons and in fruit treated with the ethylene antagonist 1-methylcyclopropene
(1-MCP), indicating a positive regulation by ethylene. The data presented in this work suggest that the
multiplicity of AAT genes accounts for the great diversity of esters formed in melon
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