3,529 research outputs found
Improved ferrous shielding for flat cables
To improve shielding of flat multicore cables, a thin, seamless ferrous shield around all cores optimizes low frequency magnetic shielding. Such shielding is covered with an ultrathin seamless coat of highly conductive nonferrous material
On quantum and parallel transport in a Hilbert bundle over spacetime
We study the Hilbert bundle description of stochastic quantum mechanics in
curved spacetime developed by Prugove\v{c}ki, which gives a powerful new
framework for exploring the quantum mechanical propagation of states in curved
spacetime. We concentrate on the quantum transport law in the bundle,
specifically on the information which can be obtained from the flat space
limit. We give a detailed proof that quantum transport coincides with parallel
transport in the bundle in this limit, confirming statements of Prugove\v{c}ki.
We furthermore show that the quantum-geometric propagator in curved spacetime
proposed by Prugove\v{c}ki, yielding a Feynman path integral-like formula
involving integrations over intermediate phase space variables, is Poincar\'e
gauge covariant (i.e. is gauge invariant except for transformations at the
endpoints of the path) provided the integration measure is interpreted as a
``contact point measure'' in the soldered stochastic phase space bundle raised
over curved spacetime.Comment: 25 pages, Plain TeX, harvmac/lanlma
Exact one- and two-particle excitation spectra of acute-angle helimagnets above their saturation magnetic field
The two-magnon problem for the frustrated XXZ spin-1/2 Heisenberg Hamiltonian
and external magnetic fields exceeding the saturation field Bs is considered.
We show that the problem can be exactly mapped onto an effective tight-binding
impurity problem. It allows to obtain explicit exact expressions for the
two-magnon Green's functions for arbitrary dimension and number of
interactions. We apply this theory to a quasi-one dimensional helimagnet with
ferromagnetic nearest neighbor J1 < 0 and antiferromagnetic next-nearest
neighbor J2 > 0 interactions. An outstanding feature of the excitation spectrum
is the existence of two-magnon bound states. This leads to deviations of the
saturation field Bs from its classical value Bs(classical) which coincides with
the one-magnon instability. For the refined frustration ratio |J2/J1|> 0.374661
the minimum of the two-magnon spectrum occurs at the boundary of the Brillouin
zone. Based on the two-magnon approach, we propose general analytic expressions
for the saturation field Bs, confirming known previous results for
one-dimensional isotropic systems, but explore also the role of interchain and
long-ranged intrachain interactions as well as of the exchange anisotropy.Comment: 21 pages, 6 Figures. submitted to Phys. Rev.
Theoretical de Haas-van Alphen Data and Plasma Frequencies of MgB2 and TaB2
The de Haas-van Alphen-frequencies as well as the effective masses for a
magnetic field parallel to the crystallographic c-axis are calculated within
the local spin density approximation (LSDA) for MgB2 and TaB2. In addition, we
analyze the plasma frequencies computed for each Fermi surface sheet. We find a
large anisotropy of Fermi velocities in MgB2 in difference to the nearly
isotropic behavior in TaB2. We compare calculations performed within the
relativistic non-full potential augmented-spherical-wave (ASW) scheme and the
scalar-relativistic full potential local orbital (FPLO) scheme. A significant
dependence for small cross sections on the bandstructure method is found. The
comparison with the first available experimental de Haas-van Alphen-data by
Yelland et al. (Ref. 19) shows deviations from the electronic structure
calculated within both L(S)DA approaches although the cross section predicted
by FPLO are closer to the experimental data. The elucidation of the relevant
many-body effects beyond the standard LDA is considered as a possible key
problem to understand the superconductivity in MgB2.Comment: Typos corrected, 3references added. Extended and corrected version of
S. Elgazzar et al., Solid State Comm. v. 121, 99 (2002). 7pages, 4figures,
AIP Conference Proc. "Correlated Electron Systems and High-Tc
Superconductors" (ed. F. Mancini) (October 2001, Salerno, Italy
New insight into the physics of iron pnictides from optical and penetration depth data
We report theoretical values for the unscreened plasma frequencies Omega_p of
several Fe pnictides obtained from DFT based calculations within the LDA and
compare them with experimental plasma frequencies obtained from reflectivity
data. The sizable renormalization observed for all considered compounds points
to the presence of many-body effects beyond the LDA. From the large empirical
background dielectric constant of about 12-15, we estimate a large arsenic
polarizability of about 9.5 +- 1.2 Angstroem^3 where the details depend on the
polarizabilities of the remaining ions taken from the literature. This large
polarizability can significantly reduce the value of the Coulomb repulsion U_d
about 4 eV on iron known from iron oxides to a level of 2 eV or below. In
general, this result points to rather strong polaronic effects as suggested by
G.A. Sawatzky et al., in Refs. arXiv:0808.1390 and arXiv:0811.0214 (Berciu et
al.). Possible consequences for the conditions of a formation of bipolarons are
discussed, too. From the extrapolated muon spin rotation penetration depth data
at T= 0 and the experimental Omega_p we estimate the total coupling constant
lambda_tot for the el-boson interaction within the Eliashberg-theory adopting a
single band approximation. For LaFeAsO_0.9F_0.1 a weak to intermediately strong
coupling regime and a quasi-clean limit behaviour are found. For a pronounced
multiband case we obtain a constraint for various intraband coupling constants
which in principle allows for a sizable strong coupling in bands with either
slow electrons or holes.Comment: 34 pages, 10 figures, submitted to New Journal of Physics
(30.01.2009
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