J. F. Cooke

A spin polaron model of high-Tc superconductivity

Abstract Spin-polaron model has been studies as a basis for high- T c superconductivity. Parameterized antiferromagnetic band calculations for a CuO 2 plane show how a Mott-Hubbard (M-H) gap opens as a consequence of an electron repulsion energy U at the Cu sites. The parameters are chosen to give good agreement with measured density of states and the magnetic moment per Cu site. Variation of the hole concentration by Sr doping in La 2− x Sr x CuO 4 and O depletion in YBa 2 Cu 3 O 7− x moves the Fermi level relative to the M-H band edge. The holes induce spin deviations on the neighboring Cu sites to form small spin polarons, with effective masses several times the band masses. Two spin polarons have an attractive interaction because the spin deviations are partially “healed” when the polarons approach one another. The proximity of the Fermi level to the M-H band edge and the interplay of O 2pσ and 2pπ bands can provide fits to the variations of T c with x in La 2− x Sr x CuO 4 and YBa 2 Cu 3 O 7− x . A discussion of various aspects and implications of the model is given.

Canonical transform theory of spin‐waves in strongly anisotropic magnets

We have developed a new infinite‐order perturbation approach for treating strongly anisotropic magnets. This formalism is based on a canonical transformation of a given system into a new system with an effective two‐ion anisotropy which, for example, can be treated by conventional spin‐wave techniques. An expression for the spin‐wave energy for the general conical magnetic moment configuration with arbitrary single‐ion anisotropy has been derived using this formalism. It was found that an analysis of the transverse spin‐wave spectra can at most result in a determination of a renormalized exchange integral and two renormalized anisotropy constants. A comparison of the theoretical predictions of this formalism with experimentally determined spin‐wave spectra indicate that a substantial part of the large two‐ion anisotropy, which had been previously introduced to describe the spin‐wave spectra in the heavy rare earth series, actually resulted from the fitting procedure which was used and not from any physica...

Band theoretic interpretation of neutron scattering experiments in metallic ferromagnets

Abstract Magnetic excitations in the 3d transition metal ferromagnets nickel and iron have been found to possess unusual properties not found in other magnetic systems. For example, inelastic neutron scattering experiments have revealed that spinwaves do not exist at wave-vectors above a particular cut-off value while below this cut-off point they have been observed well above the Curie temperature. This behaviour is not consistent with predictions of localized spin models but can be explained by the band theoretic, or itinerant electron theory. In fact, first principles calculations of the low temperature neutron scattering cross-section based on the itinerant model have been shown to be in excellent agreement with experiment. In addition, the prediction of an optical spin-wave mode has been recently confirmed. The finite temperature extrapolation of the low temperature theory based on the traditional concept of a temperature-dependent spin-splitting of the electronic energy bands appears to be inconsistent with experiment. A more realistic first principles approach to develop a correct finite temperature theory is under investigation.

Theoretical study of magnetic excitations in Ni3Al

Inelastic neutron scattering studies of the transition metal alloy Ni3Al have revealed highly unusual spin‐wave behavior, that spin waves have been observed only in a small region around the Brillouin zone center (q≂0). Results from calculations of the inelastic neutron scattering cross section based on itinerant electron theory for Ni3Al have led to a relatively simple explanation of this phenomenon. To our knowledge, this is the first calculation of this type for an alloy system. The calculations yield the well‐defined Goldstone mode (spin wave) at q=0 but no spin‐wave peaks were found for the smallest calculable nonzero momentum transfer, which was just outside the range of q where spin waves were observed experimentally. The reason is simply that the spin wave runs immediately into a region of high density of Stoner excitations (single‐particle spin‐flip excitations) as q is increased from zero. This system, therefore, represents the extreme limiting case of the itinerant electron theory prediction of...

Paramagnetic form factors from itinerant electron theory

Elastic neutron scattering experiments performed over the past two decades have provided accurate information about the magnetic form factors of paramagnetic transition metals. These measurements have traditionally been analyzed in terms of an atomic‐like theory. There are, however, some cases where this procedure does not work, and there remains the overall conceptual problem of using an atomistic theory for systems where the unpaired‐spin electrons are itinerant. We have recently developed computer codes for efficiently evaluating the induced magnetic form factors of fcc and bcc itinerant electron paramagnets. Results for the orbital and spin contributions have been obtained for Cr, Nb, V, Mo, Pd, and Rh based on local density bands. By using calculated spin enhancement parameters, we find reasonable agreement between theory and neutron form factor data. In addition, these zero parameter calculations yield predictions for the bulk susceptibility on an absolute scale which are in reasonable agreement wit...

Role of electron-electron interactions in the RKKY theory of magnetism

The theory of magnetism in heavy rare earth metals is based on the RKKY theory. In this formalism the indirect exchange interaction between the local 4f spins is mediated by the conduction electrons. When carried to second order in the 4f‐conduction electron interaction, traditional pertubation theory leads to a Heisenberg‐like interaction between the local spins which depends on the electronic energy bands and 4f‐conduction electron exchange matrix elements. This derivation neglects the detailed behavior of electron‐electron interaction within the conduction band, which is known to be important in metallic systems. By using an equation of motion method, an expression for the inelastic neutron scattering cross‐section has been derived which includes, in an approximate way, this electron‐electron interaction. The results of this calculation indicate that spin‐wave peaks can be broadened and shifted if the spin‐wave band lies near the conduction electron Stoner continuum. The origin of this effect is similar to that found in itinerant electron systems where the spin‐wave band actually intersects the Stoner continuum, resulting in the disappearance of the spin‐wave mode.

Dynamic susceptibility calculations in ferromagnetic iron

Calculations of the low temperature dynamic susceptibility of ferromagnetic iron have been carried out within the framework of an itinerant electron model. The formalism incorporates band and wave vector dependent matrix element effects and is essentially the same as that used in previous work on nickel. The spin wave dispersion curves calculated along the three principal symmetry directions are found to be in excellent agreement with neutron scattering experiments. In addition the calculations predict correctly the experimentally observed spin wave disappearance. These results indicate that low temperature spin waves in both nickel and iron can be adequately described in terms of an itinerant electron model without recource to ad hoc assumptions about local moment behavior.

High-energy spin waves in cubic and hcp transition metals

Abstract We report on some new calculations of the magnetic susceptibility of hcp cobalt. Both acoustic and optic spin waves are obtained over a limited range of wave vector transfer. The results are in broad agreement with the available experimental data. Comparison is made with earlier work on the cubic ferromagnets.

A fast and accurate method for the calculation of orbital susceptibility and form factor of paramagnetic transition metals

Abstract We describe a new method for calculating the orbital contribution to the susceptibility and neutron magnetic form factor of paramagnetic transition metals. The method has been developed by us over the past four years, and the computational details and results will be published elsewhere. In this paper we discuss the theoretical issues, which include a justification of the basic approximation and the estimate of errors. It is shown that it gives reliable results for the size of the susceptibility and the overall shape of the form factor, but is probably not sufficiently accurate to account for the anisotropy in the orbital moment distribution.

Paramagnetic form factors of hcp transition metals

Abstract Recently we have developed a numerical technique to compute the total form factor of the itinerant electrons in hcp transition metals. The calculated orbital contributions deviate from atomic orbital form factors. There is reasonable overall agreement between theory and elastic neutron scattering experiments.