3,596 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
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
Conventional Superconductivity in Fe-Based Pnictides: the Relevance of Intra-Band Electron-Boson Scattering
Various recent experimental data and especially the large Fe-isotope effect
point against unconventional pairings, since the large intra-band impurity
scattering is strongly pair-breaking for them. The strength of the inter-band
impurity scattering in some single crystals may be strong and probably beyond
the Born scattering limit. In that case the proposed s(+-) pairing (hole(h)-
and electron(el)-gaps are of opposite signs) is suppressed but possibly not
completely destroyed. The data imply that the intra-band pairing in the h- and
in the el-band, which are inevitably due to some nonmagnetic el-boson
interaction (EBI), must be taken into account. EBI is either due to phonons
(EPI) or possibly due to excitons (EEI), or both are simultaneously operative.
We discuss their interplay briefly. The large Fe-isotope effect favors the EPI
and the s(+) pairing (the h- and el-gaps are in-phase).Comment: 7 pages, no figures, explanations and argumentations improved,
references adde
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.
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