89,514 research outputs found
Spin tunneling in magnetic molecules: Quantitative estimates for Fe8 clusters
Spin tunneling in the particular case of the magnetic molecular cluster
octanuclear iron(III), Fe8, is treated by an effective Hamiltonian that allows
for an angle-based description of the process. The presence of an external
magnetic field along the easy axis is also taken into account in this
description. Analytic expressions for the energy levels and barriers are
obtained from a harmonic approximation of the potential function which give
results in good agreement with the experimental results. The energy splittings
due to spin tunneling is treated in an adapted WKB approach and it is shown
that the present description can give results to a reliable degree of accuracy.Comment: 17 pages, 2 figures, preprint submitted to Physica
Measurements of surface impedance of superconductors as a function of frequency in microwave range
We report measurements of the complex resistivity in YBCO and MgB thin
films over a continuous frequency spectrum in the microwave range, making use
of a Corbino disk geometry. The paper mainly focuses on the extraction of the
resistivity from raw data, displaying data analysis procedure and its limits of
validity. We obtain and show resistivity curves as a function of frequency and
temperature denoting a frequency dependent widening of the superconducting
transition.Comment: 12 pages, 7 figures. Extended and revised version of cond-mat/0307143
(2003
New aspects of microwave properties of Nb in the mixed state
We present a study of the frequency dependence of the vortex dynamics in a
conventional superconductor. We have employed a swept-frequency, Corbino-disk
technique to investigate the temperature (3.6K-Tc) and high-field (from Hc2/2
to Hc2) microwave complex resistivity in Nb thin (20-40 nm) films as a function
of the frequency (1-20 GHz). We have found several previously unnoticed
features: (i) a field-dependent depinning frequency in the GHz range; (ii)
deviations from the accepted frequency dependence, that can be ascribed to some
kind of vortex creep; (iii) the presence of switching phenomena, reminiscent of
vortex instabilities. We discuss the possible origin of the features here
reported.Comment: 5 pages, 3 figures, presented at VORTEX VI Conference, to appear on
Physica
Microwave properties of Nb/PdNi/Nb trilayers. Observation of flux flow in excess of Bardeen-Stephen theory
We combine wideband (1-20 GHz) Corbino disk and dielectric resonator (8.2
GHz) techniques to study the microwave properties in Nb/PdNi/Nb trilayers,
grown by UHV dc magnetron sputtering, composed by Nb layers of nominal
thickness =15 nm, and a ferromagnetic PdNi layer of thickness = 1, 2,
8 and 9 nm. We focus on the vortex state. Magnetic fields up to were
applied. The microwave resistivity at fixed increases with ,
eventually exceeding the Bardeen Stephen flux flow value.Comment: 6 pages. Submitted to Journal of Superconductivity and Novel
Magnetis
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