3,750 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
Thermodynamics of the one-dimensional frustrated Heisenberg ferromagnet with arbitrary spin
The thermodynamic quantities (spin-spin correlation functions <{\bf S}_0{\bf
S}_n>, correlation length {\xi}, spin susceptibility {\chi}, and specific heat
C_V) of the frustrated one-dimensional J1-J2 Heisenberg ferromagnet with
arbitrary spin quantum number S below the quantum critical point, i.e. for J2<
|J1|/4, are calculated using a rotation-invariant Green-function formalism and
full diagonalization as well as a finite-temperature Lanczos technique for
finite chains of up to N=18 sites. The low-temperature behavior of the
susceptibility {\chi} and the correlation length {\xi} is well described by
\chi = (2/3)S^4 (|J1|-4J2) T^{-2} + A S^{5/2} (|J1|-4J2)^{1/2} T^{-3/2} and \xi
= S^2 (|J1|-4J2) T^{-1} + B S^{1/2} (|J1|-4J2)^{1/2} T^{-1/2} with A \approx
1.1 ... 1.2 and B \approx 0.84 ... 0.89. The vanishing of the factors in front
of the temperature at J2=|J1|/4 indicates a change of the critical behavior of
{\chi} and {\xi} at T \to 0. The specific heat may exhibit an additional
frustration-induced low-temperature maximum when approaching the quantum
critical point. This maximum appears for S=1/2 and S=1, but was not found for
S>1.Comment: 8 pages, 7 figure
Constraints on the total coupling strength to bosons in iron based superconductors
At present, there is still no consistent interpretation of the normal and
superconducting properties of Fe-based superconductors (FeSCs). The strength of
the el-el interaction and the role of correlation effects are under debate.
Here, we examine several common materials and illustrate various problems and
concepts that are generic for all FeSCs. Based on empirical observations and
qualitative insight from density functional theory, we show that the
superconducting and low-energy thermodynamic properties of the FeSCs can be
described semi-quantitively within multiband Eliashberg theory. We account for
an important high-energy mass renormalization phenomenologically,and in
agreement with constraints provided by thermodynamic, optical, and
angle-resolved photoemission data. When seen in this way, all FeSCs with
40~K studied so far are found to belong to an {\it
intermediate} coupling regime. This finding is in contrast to the strong
coupling scenarios proposed in the early period of the FeSC history.We also
discuss several related issues, including the role of band shifts as measured
by the positions of van Hove singularities, and the nature of a recently
suggested quantum critical point in the strongly hole-doped systems
AFeAs (A = K, Rb, Cs). Using high-precision full relativistic GGA-band
structure calculations, we arrive at a somewhat milder mass renormalization in
comparison with previous studies. From the calculated mass anisotropies of all
Fermi surface sheets, only the -pocket near the corner of the BZ
is compatible with the experimentally observed anisotropy of the upper critical
field. pointing to its dominant role in the superconductivity of these three
compounds.Comment: 19 pages, 9 figure
- …