Ju H. Kim

Microscopic Fermi liquid theory of NMR relaxation and neutron scattering in the metallic copper oxides

Abstract We calculate the NMR relaxation rates for the Cu and O sites, the temperature dependent magnetic susceptibility and the inelastic neutron scattering cross section using the dynamical RPA susceptibility derived previously. This approach is a Fermi liquid based scheme in which the Cu d electrons are (on the basis of considerable experimental evidence) assumed to be quasi-localized. These narrow band effects lead to low energy scales, T coh below which the Cu d electrons are fully coherent and a “one component” picture is applicable. Above this characteristic temperature the d electrons begin to lose their coherence, although they are still strongly coupled to the p orbitals. This coherence energy scale, along with the exchange interactions J H completely determine the characteristic temperatures and frequencies which appear in NMR and neutron data. Our calculations of 1/ T 1 on the Cu sites suggest that the cuprates are not as close to a magnetic instability as has been assumed elsewhere. This is because the coherence energy scale (which appears in the “bare” Lindhard function for the renormalized band structure) already provides an intrinsic “soft” frequency. Consequently, we find that antiferromagnetic interactions are playing a less dominant (but not insignificant) role in magnetic measurements. Analysis of the oxygen relaxation suggests that the sharp antiferromagnetic peaks invoked elsewhere to yield form factor cancellation effects may not be sufficiently robust to explain the data. Indeed, careful numerical calculations yield some degree of non-Korringa behavior on the oxygen sites due to imperfect cancellations of the transfer-hyperfine-coupling induced relaxation. Finally, our calculations of the neutron cross section show incommensurate peaks in the diagonal ( q, q ) and off-diagonal ( q , π) directions, which are found to reflect the detailed Fermi surface geometry of the LaSrCuO system. These peaks become increasingly more incommensurate as the hole concentration increases. These predictions and their comparison with future experiments may help to determine the applicability of Fermi liquid based approaches to the cuprates.

THE KONDO BOSON THEORY OF THE DYNAMIC SUSCEPTIBILITY OF HEAVY FERMIONS

Abstract We apply the Kondo-boson - 1/ N expansion to the microscopic Anderson lattice model, properly generalized to include the effects of spin-orbit coupling. We compute the imaginary susceptibility for a generic cubic band structure including the effects of the leading order Fermi liquid interactions. We show that the antiferromagnetic correlations (at the zone boundary) which occur at frequencies of the order of the Kondo temperature, arise naturally as a consequence of transitions across the hybridization gap or the Fermi surface. Finite frequency structure at the zone center derives from f-spin non conserving interactions mediated by exchange of Kondo boson propagators. We discuss alternate “interacting impurities” approaches in light of the experimental evidence for coherence and a sharp Fermi surface at low temperatures.

Current Status of Fermi Liquid Based Approaches to the Cuprates

A central issue in the field of high temperature superconductivity is the nature of the normal state. Here we review normal state transport, thermodynamic, magnetic and spectroscopic data and discuss their interpretation within a Fermi liquid based framework. It is demonstrated that the cuprate data are not consistent with canonical Fermi liquid behavior. However, if the Fermi liquid is characterized by low energy scales, arising from narrow bandwidths or soft spin fluctuations, then canonical behavior is not expected at the relatively “high” temperatures of the normal state. We discuss two main Fermi liquid based schools, i.e., the “almost localized” and “almost magnetic” descriptions. Both approaches have addressed the data with some success and it is clear that they are, in many respects, closely related. Here it is claimed that the strongest support for a Fermi liquid based approach to the cuprates derives from the comparison with other strongly correlated Fermi liquid systems, which exhibit similar normal state anomalies.

Metallic copper oxide as an almost localized fermi liquid

Abstract In view of recent photoemission data, a Fermi liquid description of the metallic copper oxides must be taken as a serious contender for a theory of the normal state. We summarize evidence to support a picture in which the Fermi liquid consists of quasi-localized d electrons. The associated electronic and magnetic structure is discussed.

Electron-phonon interactions in copper-oxides

Abstract We calculate electron-phonon coupling constants for the copper oxide planes using a renormalized bandstructure as well as a diagrammatic 1/N approach. The effects of infinitely strong Coulomb repulsion lead to a dramatic reduction in the coupling. Phononic, polaronic and some charge transfer descriptions of the superconductivity seem unlikely in view of these results.