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We compare the normal-state resistivities $\rho$ and the critical temperatures $T_c$ for superconducting $d_{x^2-y^2}$ pairing due to antiferromagnetic (AF) spin fluctuation exchange in the context of the two phenomenological dynamical spin susceptibility models, recently proposed by Millis, Monien, and Pines (MMP) and Monthoux and Pines (MP) and, respectively, by Radtke, Ullah, Levin, and Norman (RULN), for the cuprate high-$T_c$ materials. Assuming comparable electronic bandwidths and resistiviies in both models, we show that the RULN model gives a much lower d-wave $T_c$ ($\lsim20$K) than the MMP model (with $T_c\sim100$K). We demonstrate that these profound differences in the $T_c$'s arise from fundamental differences in the spectral weight distributions of the two model susceptibilities and are {\it{not}} primarily caused by differences in the calculational techniques employed by MP and RULN. The MMP model, claimed to fit NMR data in YBCO, exhibits substantial amounts of spin fluctuation spectral weight up to an imposed cut-off of 400meV, whereas, in the RULN model, claimed to fit YBCO neutron scattering data, the weight is narrowly peaked and effectively cut-off by 100meV. Further neutron scattering experiments, to explore the spectral weight distribution at all wavevectors over a sufficiently large excitation energy range, will thus be of crucial importance to resolve the question whether AF spin fluctuation exchange provides a viable mechanism to account for high-$T_c$ superconductivity. The large high-frequency boson spectral weight, needed to generate both a high d-wave $T_c$ and a low normal-state resistivity, also implies large values, of order unity, for the Migdal smallness parameter, thus casting serious doubt on the validity of the very, Comment: 25 pages, revtex, 5 uuencoded postscript figures

Recent angle-resolved photoemission (ARPES) experiments have indicated that the electronic dispersion in some of the cuprates possesses an extended saddle point near the Fermi level which gives rise to a density of states that diverges like a power law instead of the weaker logarithmic divergence usually considered. We investigate whether this strong singularity can give rise to high transition temperatures by computing the critical temperature $T_c$ and isotope effect coefficient $\alpha$ within a strong- coupling Eliashberg theory which accounts for the full energy variation of the density of states. Using band structures extracted from ARPES measurements, we demonstrate that, while the weak-coupling solutions suggest a strong influence of the strength of the Van Hove singularity on $T_c$ and $\alpha$, strong-coupling solutions show less sensitivity to the singularity strength and do not support the hypothesis that band structure effects alone can account for either the large $T_c$'s or the different $T_c$'s within the copper oxide family. This conclusion is supported when our results are plotted as a function of the physically relevant self- consistent coupling constant, which show universal behavior at very strong coupling., Comment: 11 pages + 4 figures available on request, Revtex 3.0, STCS preprint

There is a great deal of interest in attributing the high critical temperatures of the cuprates to either the proximity of the Fermi level to a van Hove singularity or to structure of the superconducting pairing potential in momentum space far from the Fermi surface. We examine these ideas by calculating the critical temperature Tc for model Einstein-phonon- and spin-fluctuation-mediated superconductors within both the standard, Fermi-surface-restricted Eliashberg theory and the exact mean field theory, which accounts for the full momentum structure of the pairing potential and the energy dependence of the density of states. By using two models of spin-fluctuation-mediated pairing in the cuprates, we demonstrate that our results are independent of the details of the dynamical susceptibility, which is taken to be the pairing potential. We also compare these two models against available neutron scattering data, since these data provide the most direct constraints on the susceptibility. We conclude that the van Hove singularity does not drastically alter Tc from its value when the density of states is constant and that the effect of momentum structure is significant but secondary in importance to that of the energy dependence in the density of states., Comment: 23 pages, 6 figures upon request, revtex version 2, vHs-1

We present a series of detailed band calculations on the various structural phases of doped lanthanum cuprate: HTT, LTO, and LTT. The LTO distortion is shown to have little effect on the electronic density of states (DOS). A fit to the pressure dependence of the superconducting transition temperature indicates that only 2.5% of the DOS is affected by the HTT$\to$ LTO transition. The LTT distortion also has little effect on the DOS for the experimental value of the octahedral tilt angle. Larger tilt angles, though, lead to a dramatic change in the DOS., 9 pages, plain TeX plus macros, 4 figures available from the authors

A method is derived for calculating the pairing kernel in heavy-fermion superconductors including matrix-element effects. Various models for the interaction vertex are considered, including pseudospin exchange, orbital exchange, and quadrupolar exchange. As an example, this formalism is applied to ${\mathrm{UPt}}_{3}$ with the use of relativistic wave functions from a local-density-band calculation.

We evaluate the sum of particle-particle ladder diagrams for a two-dimensional (2D) electron gas in the presence of an external magnetic field. The evaluation depends on the observation that the quantized center-of-mass kinetic energy is conserved in electron-electron scattering events. For a delta-function interaction scattering channels with different center-of-mass kinetic energy separate and the integral equation reduces to an algebraic equation which can be solved explicitly. Divergences associated with superconducting instabilities are identified. In the weak-field limit a connection is established between the different scattering channels and different solutions of the linearized Ginzburg-Landau equations.

Recently, there has been renewed interest in the effect of orbital quantization on the upper critical field of superconductors, given the possibility of reentrant behavior at high fields. In this paper, the Gor'kov equations are solved numerically in the limit where only a few Landau levels are involved, including the effects of impurities and a nonzero g factor, for a set of parameters appropriate to a low-carrier-density superconductor. It is shown that these effects conspire to make the experimental observation of this effect possible, but unlikely.

Photoemission spectra, as calculated using a transition-state analysis within a supercell approach, are found to be in good agreement with experimental data for both PrSb and NdSb. The model assumes localized f electrons in the ground state and a localized f photohole in the final state. By comparing these results to previous ones for CeSb, we determine how the photoemission peak due to f emission moves further away from the Fermi energy for higher Z. Finally, we predict where a second f feature might be seen if resonant photoemission were done on these two compounds.

The Fermi surface of ferromagnetic CeSb is analyzed with use of a model in which the f electrons in the system are treated as localized (unhybridized) j=(5/2), \ensuremath{\mu}=(5/2) states interacting only weakly with the conduction electrons through electrostatic and exchange interactions. The ordering of the f orbital moments along the [001] easy-magnetization axis breaks the cubic symmetry introducing noncubic contributions to the potential seen by the conduction electrons. Because spin-orbit coupling in the conduction bands proves quite significant, the requisite model includes both the spin-orbit coupling and the polarization effects. The application of the model compares quite favorably with the measured de Haas--van Alphen frequencies except that the calculated polarization splittings, while quite small, are still overly large. It is found that the electron masses have little or no enhancement beyond the electron-phonon enhancement.

The local-density supercell impurity approach is used to calculate the excitation spectra due to the emission and absorption of a $3d$ electron in the Mott insulator NiO. Good agreement is found with both photoemission (PES) and inverse photoemission [bremsstrahlung isochromat spectroscopy (BIS)] data. In particular, a BIS excitation gap of 4.1 eV is found compared to 4.3 eV experimentally. The predicted values for the screened Coulomb interaction $U$ is 7.9 eV. These results indicate that there is no conflict between the band picture of Terakura et al. for the ground-state properties of NiO and the excited-state properties derived from PES and BIS by Sawatzky and Allen and by McKay and Henrich.

Cerium and many of its compounds exhibit a double-peaked photoemission spectrum arising from the 4f states. Considerable conjecture as to the origin of this phenomenon has given rise to numerous models for the electronic structure of these materials. In this paper we show that this phenomenon arises from the nature of the excitation process. The initial state including the f states are assumed to be adequately described by a band calculation. The possible final states are then simulated. Two primary channels occur for the f states which are characterized as the fully-screened--poorly-screened peaks: In the fully screened channel, the f hole is screened by f states; in the poorly screened channel, the f hole is screened by d states. As a first approximation, the fully screened final state is described with use of the ground-state density of states. The poorly screened final state is simulated by creating an f hole at a central atom in a large unit cell---a supercell technique which periodically repeats the screened hole at an adequate separation to minimize interaction of the holes. The separation of the two peaks in the photoemission spectra is then determined with use of Slater transition-state arguments. Peak separations are found to be 2.71 eV for CeSb and 2.47 eV for CeP which compare favorably with the experimental values of 2.5 and 2.4 eV, respectively. No stable d-screened f hole is found in CeN, in agreement with the fact that it is not observed.