[en] Checkerboard patterns have been proposed to explain the real space structure observed in scanning tunneling microscopy experiments on BSCCO and Na-CCOC. However, simple checkerboard patterns have low energy incommsensurate (IC) spin peaks rotated 45 deg. from the direction of the charge IC peaks, contrary to what is seen in neutron scattering. Here, we study modulated checkerboard patterns which can resolve the low frequency inconsistency. Using spin-wave theory, we explore the spin response of these superstructures and find that the high energy response is inconsistent with neutron scattering results. In particular, the modulated checkerboard structures are incapable of supporting the experimentally well-established resonance peak at (π,π)

[en] In the normal state of the high temperature superconductors Bi₂Sr ₂CaCu₂O_8+δ and La_2-xSr _xCuO₄ , and in the related ''stripe ordered'' material, La_1.25 Nd_0.6Sr _0.15CuO ₄ , there is sharp structure in the measured single hole spectral function, A^<(rvec k,ω ) , considered as a function of rvec k at fixed small binding energy ω . At the same time, as a function of ω at fixed rvec k on much of the putative Fermi surface, any structure in A^<(rvec k,ω ) , other than the Fermi cutoff, is very broad. This is characteristic of the situation in which there are no stable excitations with the quantum numbers of the electron, as is the case in the one-dimensional electron gas

[en] The one-dimensional electron gas exhibits spin-charge separation and power-law spectral responses to many experimentally relevant probes. Ordering in a quasi-one-dimensional system is necessarily associated with a dimensional crossover, at which sharp quasiparticle peaks, with small spectral weight, emerge from the incoherent background. Using methods of Abelian bosonization, we derive asymptotically correct expressions for the spectral changes induced by this crossover. Comparison is made with experiments on the high-temperature superconductors, which are electronically quasi-one-dimensional on a local scale. (c) 2000 The American Physical Society

[en] We report inelastic neutron scattering studies of magnetic excitations in antiferromagnetically ordered SrFe₂As₂ (T_N=200-220 K), the parent compound of the FeAs-based superconductors. At low temperatures (T=7 K), the magnetic spectrum S(Q,(ℎ/2π)ω) consists of a Bragg peak at the elastic position ((ℎ/2π)ω=0 meV), a spin gap (Δ≤6.5 meV), and sharp spin-wave excitations at higher energies. Based on the observed dispersion relation, we estimate the effective magnetic exchange coupling using a Heisenberg model. On warming across T_N, the low-temperature spin gap rapidly closes, with weak critical scattering and spin-spin correlations in the paramagnetic state. The antiferromagnetic order in SrFe₂As₂ is therefore consistent with a first order phase transition, similar to the structural lattice distortion