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Antiferromagnetism is relevant to high-temperature (high-Tc) superconductivity because copper oxide and iron arsenide superconductors arise from electron- or hole-doping of their antiferromagnetic parent compounds. There are two broad classes of explanation for antiferromagnetism: in the ‘local moment’ picture, appropriate for the insulating copper oxides, antiferromagnetic interactions are well described by a Heisenberg Hamiltonian; whereas in the ‘itinerant model’, suitable for metallic chromium, antiferromagnetic order arises from quasiparticle excitations of a nested Fermi surface. There has been contradictory evidence regarding the microscopic origin of the antiferromagnetic order in iron arsenide materials, with some favouring a localized picture and others supporting an itinerant point of view. More importantly, there has not even been agreement about the simplest effective ground-state Hamiltonian necessary to describe the antiferromagnetic order. Here, we use inelastic neutron scattering to map spin-wave excitations in CaFe2As2 (refs 26, 27), a parent compound of the iron arsenide family of superconductors. We find that the spin waves in the entire Brillouin zone can be described by an effective three-dimensional local-moment Heisenberg Hamiltonian, but the large in-plane anisotropy cannot. Therefore, magnetism in the parent compounds of iron arsenide superconductors is neither purely local nor purely itinerant, rather it is a complicated mix of the two. [ABSTRACT FROM AUTHOR]

The density of phonon states for the YbB12 Kondo insulator is calculated from the inelastic neutron scattering spectra of this compound. It is established that thermal vibrations of rare-earth atoms predominantly occur in the low-energy range. These atoms are most weakly bound in the crystal structure of the YbB12 Kondo insulator. The high-energy part of the vibrational spectrum is determined by thermal vibrations of the boron atoms forming a rigidly connected structure of the compound. It is revealed that the temperature dependence of the intensity of the phonon peak attributed to thermal vibrations of the ytterbium atoms exhibits an anomalous behavior. This circumstance suggests that the magnetoelastic coupling occurring in the structure of the YbB12 Kondo insulator is relatively strong and can contribute to the magnetic excitation spectrum of this compound. [ABSTRACT FROM AUTHOR]

The dynamic magnetic response of the intermediate-valence compound EuCu2Si2 has been studied using inelastic neutron scattering. At low temperatures, strong renormalization of the 7 F 0 → 7 F 1 spin-orbit transition energy is detected; it is likely to be related to partial delocalization of the f electrons of Eu. An increase in the temperature increases the valence instability of europium and results in further changes in the magnetic excitation spectrum parameters and the appearance of an intense quasi-elastic component. [ABSTRACT FROM AUTHOR]

The phonon dispersion in RB12 ( R = Yb, Lu) systems is studied in detail using inelastic neutron scattering over a wide range of energies (up to 55 meV). It is demonstrated that the main features of the lattice dynamics in the RB12 crystals can be described in terms of a simple model of force constants with a strong hierarchy of interatomic interactions in the systems: B-B ≫ B- R ≫ R- R. An interesting feature of the low-energy portion of the phonon spectrum is the weakly dispersive optical mode. According to the model calculations, this mode is associated with the relative displacements and “breathing” vibrations of the boron nanoclusters B12. [ABSTRACT FROM AUTHOR]

The multipliers for linear dynamic models have been incorrectly expressed in certain textbooks. This note points out the source of the error and demonstrates the correct formulation of these multipliers. It will be shown that the error implicitly arises from a confusion in the definition of a multiplier. This article therefore, defines a multiplier to be the effect of a unit sustained change from equilibrium in the exogenous variables on the endogenous variables. Therefore, the k-period dynamic multiplier of an sth order linear model is given by the recursion relations. In order to compare the correct formulation of a multiplier with the incorrect version, it can easily be shown that a more general expression for a k-period dynamic multiplier is given by substitution of into. The implication of using the incorrect expression is that a serious misrepresentation of the dynamics of the system will be made if any variable appears in first difference form since those coefficients can bear no influence on the multipliers.

A number of explanations have been offered for the cause of the shift in the relationship between unemployment and unfilled vacancies, but recently Holden and Peel (1975, 1977), Parikh (1977) and Warren (1977) have shown the basic equation of that analysis to be invalid. Warren claims that a disequilibrium approach to the problem reveals that there is no equilibrium relationship, making the discussion of shifts irrelevant. This paper demonstrates that a disequilibrium model with time dependent parameters reveals that there has been both a shift in the flow of quits and lay-offs from employment and a decline in the efficiency of searching for work.
A variety of reasons have been offered for the cause of the shift in the relationship between registered unemployment and unfilled vacancies (the UV relationship). Gujarati (1972), Taylor (1972) and Foster (1974) have agreed that there has been an artificial increase in unemployment since the latter half of 1966 although their explanations for this shift have differed. Gujarati (1972) proposed that the Redundancy Payments Act (1965) and the National Insurance Act (1966) were fundamental in reducing the costs of search for employment, thereby artificially increasing the level of unemployment. Taylor (1972) suggested that the shift in the UV relationship was caused by a deliberate change in the employment policy of firms. [ABSTRACT FROM AUTHOR]

A new family of superconductors containing layers of iron arsenide has attracted considerable interest because of their high transition temperatures (Tc), some of which are >50 K, and because of similarities with the high-Tc copper oxide superconductors. In both the iron arsenides and the copper oxides, superconductivity arises when an antiferromagnetically ordered phase has been suppressed by chemical doping. A universal feature of the copper oxide superconductors is the existence of a resonant magnetic excitation, localized in both energy and wavevector, within the superconducting phase. This resonance, which has also been observed in several heavy-fermion superconductors, is predicted to occur when the sign of the superconducting energy gap takes opposite values on different parts of the Fermi surface, an unusual gap symmetry which implies that the electron pairing interaction is repulsive at short range. Angle-resolved photoelectron spectroscopy shows no evidence of gap anisotropy in the iron arsenides, but such measurements are insensitive to the phase of the gap on separate parts of the Fermi surface. Here we report inelastic neutron scattering observations of a magnetic resonance below Tc in Ba0.6K0.4Fe2As2, a phase-sensitive measurement demonstrating that the superconducting energy gap has unconventional symmetry in the iron arsenide superconductors. [ABSTRACT FROM AUTHOR]