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 $T_\mathrm{c} <$ 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 AFe$_2$As$_2$ (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 $\varepsilon$-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.