3,768 research outputs found
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
Spin polarization of the magnetic spiral in NaCu_2O_2, as seen by NMR
The incommensurate (IC) spin ordering in quasi-1D edge-shared cuprate
NaCu_2O_2 has been studied by ^{23}Na nuclear magnetic resonance spectroscopy
in an external magnetic field near 6 Tesla applied along the main
crystallographic axes. The NMR lineshape evolution above and below T_N\approx12
K yields a clear signature of an IC static modulation of the local magnetic
field consistent with a Cu^{2+} spin spiral polarized in the bc-plane rather
than in the ab-plane as reported from earlier neutron diffraction data.Comment: 5 pages, 4 figure
Magnetic ordering of weakly coupled frustrated quantum spin chains
The ordering temperature of a quasi-one-dimensional system, consisting of
weakly interacting quantum spin-1/2 chains with antiferromagnetic
spin-frustrating couplings (or zig-zag ladder) is calculated. The results show
that a quantum critical point between two phases of the one-dimensional
subsystem plays a crucial role. If the one-dimensional subsystem is in the
antiferromagnetic-like phase in the ground state, similar to the phase of a
spin chain without frustration, weak couplings yield magnetic ordering of the
Neel type. For intra-chain spin-frustrating interactions larger than the
critical one (at which the quantum phase transition takes place), the
quasi-one-dimensional spin system manifests a spiral magnetic incommensurate
ordering. The obtained results of our quantum theory are compared with the
quasi-classical approximations. The calculated features of magnetic ordering
are expected to be generic for weakly coupled quantum spin chains with gapless
excitations and spin-frustrating nearest and next-nearest neighbor
interactions.Comment: 6 pages, 2 figure
Comment on "Competition between helimagnetism and commensurate quantum spin correlations in LiCu2O2"
The microscopic origin of the recently observed helical structure in LiCu2O2
[1] is considered. It is shown that the frustrated antiferromagnetic double
chain scenario adopted in Ref.1 is unrealistic. It should be replaced by a
frustrated single-chain (CuO2) scenario proposed in Ref. 2 with ferromagnetic
nearest neighbor and antiferromagnetic next nearest neighbor exchange integrals
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[1] T. Masuda et al. Phys. Rev. Lett. 92 (2004), 177201. [2] A. Gippius et
al. Phys. Rev. B 70 (2004), R01426; cond-mat/0312576.Comment: one page, one figur
Internal structure of structurally stitched NCF preform
The paper addresses the experimental investigation of the unit cell architecture in a structurally stitched multilayer carbon-fibre preform. Each layer is a multiaxial multiply non-crimp fabric (NCF) knit with a non-structural stitching. The term “structural” presumes here that the stitching yarn does not only consolidate the plies (as the non-structural one does) but also forms a 3D reinforcement. One stitching technique — tufting — is studied, with 120 tex aramide yarn. The experimental data reveals a considerable irregularity of the piercing pattern and fibre distribution
Electron-Phonon Coupling Origin of the resistivity in YNi_{2}B_{2}C Single Crystals
Resistivity measurements from 4.2 K up to 300 K were made on YNi_{2}B_{2}C
single crystals with Tc=15.5 K. The resulting rho(T) curve shows a perfect
Bloch-Grueneisen (BG) behavior, with a very small residual resistivity which
indicates the low impurity content and the high cristallographic quality of the
samples. The value lambda_{tr}=0.53 for the transport electron-phonon coupling
constant was obtained by using the high-temperature constant value of d(rho)/dT
and the plasma frequency reported in literature. The BG expression for the
phononic part of the resistivity rho_{ph}(T) was then used to fit the data in
the whole temperature range, by approximating alpha^{2}_{tr}F(Omega) with the
experimental phonon spectral density G(Omega) multiplied by a two-step
weighting function to be determined by the fit. The resulting fitting curve
perfectly agrees with the experimental points. We also solved the real-axis
Eliashberg equations in both s- and d-wave symmetries under the approximation
alpha^{2}F(Omega)= alpha^{2}_{tr}F(Omega). We found that the value of
lambda_{tr} here determined in single-band approximation is quite compatible
with Tc and the gap Delta experimentally observed. Finally, we calculated the
normalized tunneling conductance, whose comparison with break-junction tunnel
data gives indication of the possible s-wave symmetry for the order parameter
in YNi_{2}B_{2}C.Comment: 6 pages, 5 figures. Proceedings of SATT10 Conference, to be published
in Int. J. Mod. Phys.
Influence of the van Hove singularity on the specific heat jump in BCS superconductors
Within the weak-coupling BCS scheme we derive a general form of the
coefficients in the Ginzburg-Landau expansion of the free energy of a
superconductor for the case of a Fermi level close to a van Hove singularity
(VHS). A simple expression for the influence of the VHS on the specific heat
jump is then obtained for the case where gaps for different bands are distinct
but nearly constant at the corresponding sheets of the Fermi surface.Comment: 8 pages, 2 figures, LaTeX2
Soldered Bundle Background for the De Sitter Top
We prove that the mathematical framework for the de Sitter top system is the
de Sitter fiber bundle. In this context, the concept of soldering associated
with a fiber bundle plays a central role. We comment on the possibility that
our formalism may be of particular interest in different contexts including
MacDowell-Mansouri theory, two time physics and oriented matroid theory.Comment: 12 pages, Latex; some improvements introduced, reference added, typos
correcte
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