${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ NMR relaxation is used to derive information on the low-energy spin excitations induced in antiferromagnetic (AF) CuO by the heterovalent substitution of one percent of ${\mathrm{Cu}}^{2+}$ by ${\mathrm{Li}}^{+}$. The energy gap ${\mathit{E}}_{\mathit{g}}$ between localized and itinerant states, where the extra hole diffuses in the AF matrix, is obtained as ${\mathit{E}}_{\mathit{g}}$≃600 K while the effective correlation time of the hole is ${\mathrm{\tau }}_{\mathit{h}}$≃(1/${\mathrm{\omega }}_{\mathrm{exch}}$)/${\mathit{xe}}_{\mathit{g}}^{\mathrm{-}\mathit{E}}$/T, in agreement with the main conclusions of a previous work in ${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Li}}_{\mathit{x}}$O for larger doping [P. Carretta, M. Corti, and A. Rigamonti, Phys. Rev. B 48, 3433 (1993)]. The experimental findings are discussed in comparison with the ones in lightly doped AF ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$, where a drastic changeover in the x dependence of the spin fluctuation properties has been recently found for x≲0.02 [F. C. Chou et al., Phys. Rev. Lett. 71, 2323 (1993)]. Finally, mostly on the basis of ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ relaxation rates at different measuring frequencies, it is conjectured that in ${\mathrm{Cu}}_{0.99}$${\mathrm{Li}}_{0.01}$O one has a localization of the extra hole, possibly in the ${\mathrm{CuO}}_4$ plaquette [R. J. Gooding, Phys. Rev. Lett. 66, 2266 (1991)], establishing a condition of fast spin fluctuations.

We consider multiband effects on the superconductivity in the alkali-doped fullerenes. It is shown that the multiband superconducting gap is reduced to the well-known Bardeen-Cooper-Schrieffer expression due to a cyclic symmetry in the orientationally ordered phases, thereby justifying many previous works assuming single-band s-wave pairing without considering several conduction bands explicitly.

The reflectance of undoped single-crystal ${\mathrm{C}}_{60}$ has been measured from 100 to 1600 ${\mathrm{cm}}^{\mathrm{-}1}$ at high resolution above and below the orientational-ordering transition. All of the ${\mathit{F}}_{1\mathit{u}}$ modes allowed in the free molecule are observed in the solid at ≊526, 576, 1183, and 1430 ${\mathrm{cm}}^{\mathrm{-}1}$. Below the orientational-ordering transition, all the lines split into quartets, with the exception of the 576 ${\mathrm{cm}}^{\mathrm{-}1}$ line, which narrows and remains unsplit. The appearance of quartets is attributed to the possibility of triplet splitting of the ${\mathit{F}}_{1\mathit{u}}$(->3${\mathit{F}}_{\mathit{u}}$) modes due to the local crystal field in the low-temperature phase, as well as the perturbation of the local crystal field due to the merohedral disorder, that may lead to further line splitting.

We investigate numerically the shape of cracks obtained in hydraulic fracturing at constant pressure using a square-lattice beam model with disorder. We consider the case in which only beams under tension can break, and discuss the conditions under which the resulting cracks may develop fractal patterns. We also determine the opening volume of the crack and the elastic stress field in the bulk, quantities that are accessible experimentally.

We have investigated hydrogenated amorphous carbon at a density (2.6 g/${\mathrm{cm}}^3$) and H content (16%) corresponding to hard films, by means of first-principles molecular dynamics. Our computer-generated sample consists of about 55% fourfold- 40% threefold- and a small fraction of twofold-coordinated carbon sites, in agreement with recent measurements. All H atoms are bonded to C; a great majority of them is bonded to C atoms linked to three other carbons, giving rise to ${\mathrm{C}}_4$-H units, and thus promoting ${\mathit{sp}}^3$ bonding. Only monohydrated C sites are found in our simulation. Our results for total and partial correlation functions compare well with those obtained in neutron-scattering experiments.

Exchange interactions between ferromagnetic layers through paramagnetic spacers have been widely observed and studied in thin multilayers. In this paper we report an experimental analysis of the exchange correlation length in soft ferromagnetic nanocrystals. It has been observed that for an average distance between crystallites below 5 nm they are coupled by exchange interactions even above the Curie temperature of the amorphous matrix. The experiment indicates that exchange can propagate through noncrystalline paramagnetic layers. An estimation of the exchange strength has been made.

Elastic neutron-scattering experiments performed in the low-carrier system CeP show interesting magnetic diffraction patterns under a magnetic field. They can be explained by the stacking of the ferromagnetically coupled ${\mathrm{\Gamma }}_8$ double Ce layers among the antiferromagnetically coupled ${\mathrm{\Gamma }}_7$ layers with a period of eleven Ce layers along the field direction, even though the crystal-field level energy of ${\mathrm{\Gamma }}_8$ is 160 K. The result gives direct evidence for the existence of the magnetic-polaron state in CeP proposed by Kasuya et al.

Neutron-diffraction experiments have been performed in a field of up to 5 T to investigate the magnetic structure of a single crystal of the heavy-fermion material ${\mathrm{UPd}}_2$${\mathrm{Al}}_3$ in the normal state (T>2 K). We find no evidence for the complex phase transitions and structures proposed on the basis of bulk magnetic measurements. Our results are consistent with a domain reorientation of the antiferromagnetic domains when a field is applied. The domains align to have their moments perpendicular to the applied field. Our results suggest that the hexagonal axis 〈100〉 is the easy axis. No evidence for incommensurate modulations at any applied field is found. The field necessary to align the domains decreases with increasing temperature.

A very efficient energy transfer from ${\mathrm{Fe}}^{3+}$ (d tetrahedral sites) to ${\mathrm{Tm}}^{3+}$ (c dodecahedral sites) in yttrium aluminum garnet and gadolinium gallium garnet crystals was observed. The site-selective spectroscopy shows that in case of near-neighbor Fe(d)-Tm pairs the transfer efficiency is 100%. For the other pairs it is less efficient and increases with temperature due to a better superposition of ${\mathrm{Fe}}^{3+}$ emission and ${}_{}{}^3{}_{}{}^{}$${\mathit{H}}_4$ ${\mathrm{Tm}}^{3+}$ absorption bands.

The out-of-plane free hole mass and the long-distance asymptotics of the localized hole wave function in ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ are shown to be substantially affected by the subtle changes in magnetic ordering, in magnetic fields which are both parallel and perpendicular to the copper oxide planes. This explains the unusually large magnetoconductivity and the magnetic field dependence of the dielectric constant at the corresponding phase transitions.

We report linear electrical transport in perpendicular magnetic fields of very thin amorphous MoGe superconducting films and MoGe/Ge multilayers. These measurements avoid the influence of sample edges, which were sometimes dominant in earlier experiments. We demonstrate that the magnetic coupling in this system is not significant. Measurements of Josephson-coupled multilayers reveal the existence of a characteristic mass anisotropy that divides qualitatively different regimes of behavior. This characteristic anisotropy may be related to the strength of the random pinning in the system.

Real-space ordering of doped holes in ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{NiO}}_4$ is revealed in magnetic and electronic-transport anomalies at low temperatures. The ordering is most pronounced for a hole concentration ${\mathit{n}}_{\mathit{h}}$ of 1/3 below 235 K where a superlattice with a modulation wave vector δ of 1/3 has been found. For ${\mathit{n}}_{\mathit{h}}$=1/2, another charge-ordered state with δ=0 is also indicated below ∼340 K. Our results are, furthermore, suggestive of mesoscopic hole segregation for 0.2≤x≤0.4 forming the ${\mathit{n}}_{\mathit{h}}$=1/3 and δ=1/3 state below 235 K.

We have calculated the field dependence of the magnetoresistance and the Hall constant for the N=2 insulating phase of the Anderson lattice Hamiltonian using a slave-boson approach. At low temperatures, the magnetoresistance is negative and its field dependence as well as that of the Hall coefficient are in qualitative agreement with the experimental results for ${\mathrm{Ce}}_3$${\mathrm{Bi}}_4$${\mathrm{Pt}}_3$. At high temperatures, the magnetoresistance has a small positive quadratic field dependence. We predict an almost identical value of the transverse and longitudinal magnetoresistances for this compound at low temperatures.

The electronic structure and properties of the noble-metal monoxides, PdO, PtO, and AgO have been determined by the full-potential linearized augmented-plane-wave (FLAPW) and full-potential linear-muffin-tin-orbital (FLMTO) methods. The calculated band structures show PdO and PtO to be poor metals with very low densities of states at the Fermi level. Thus, as in the 3d oxides, both methods used within the scope of the local-density approximation fail to produce the band gaps observed experimentally for PdO and PtO; they do, however, show that these band gaps are of the type that occur from crystal-field effects rather than being of the Mott-Hubbard or charge-transfer type. For AgO, the monoclinic crystal-field splitting of the d states is strong enough to induce a small direct band gap, which partially separates the electronic states of two nonequivalent silver atoms and results in ${\mathrm{Ag}}^{1+}$ and ${\mathrm{Ag}}^{2+}$ configurations rather than ${\mathrm{Ag}}^{1+}$ and ${\mathrm{Ag}}^{3+}$. Thus, correlation effects appear to be important for the detailed description of electronic states near ${\mathit{E}}_{\mathit{F}}$ not only for 3d metal monoxides, but for noble-metal oxides with much-less-localized metallic d states. Finally, the excellent agreement between the FLAPW and FLMTO results shows the possible advantage of using the much-less time-consuming FLMTO method in quantitative band-structure calculations of complex crystals.

A family of heavy-fermion materials that exhibits semiconducting-like properties has recently been discovered. A typical representative of this family is the system ${\mathrm{Ce}}_3$${\mathrm{Bi}}_4$${\mathrm{Pt}}_3$. Behavior similar to these rare-earth materials has also been observed in the compound FeSi. Using a two-band model and treating the electronic correlations beyond the mean-field level, we calculate the thermodynamic properties of this model for the case of two electrons per unit cell. We find that a hybridization gap appears above a critical value of the mixing term which depends on the ratio between the effective masses of the electrons in the large and narrow bands. The two-band model provides a unified description of the rare-earth and transition-metal-based systems. The results obtained for the temperature-dependent susceptibility are in good qualitative agreement with experimental data.

Seebeck coefficient data have been measured for ${\mathrm{Fe}}_{3\mathrm{-}\mathit{y}}$${\mathrm{Ti}}_{\mathit{y}}$${\mathrm{O}}_4$ and for ${\mathrm{Fe}}_{3\mathrm{-}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$${\mathrm{O}}_4$ single crystals in the range 75–300 K and for 0≤y≤0.9 and 0≤x≤0.29. The data have been rationalized in terms of a small polaron model, on the basis of an adaptation of the standard order-disorder theory. Very good agreement is obtained between the two-parameter theory and experiment. In this model the Ti sites do not participate in electronic conduction.

The pair-excitation probability in the weak-field regime is calculated using a fully quantized interaction Hamiltonian, for both the rotating- and fixed-atom cases. The peak of the absorption occurs at the sum of the final-state energies of the two atoms and the transition probability is shown to be proportional to the inverse square of the laser detuning and to the inverse cube of the nearest-neighbor distance. In addition, retardation effects result in a system size dependence for the transition rate not previously predicted. The significance of retardation effects is shown to depend on the excited-state coherence lifetime, the distance of closest approach, and the wavelength of the transition.

The screening and renormalization of the Coulomb repulsion in the conduction band of a two-band model are considered. It is shown that when the second band lies above the conduction band and it is empty at low temperatures, the transfer of pairs of charge carriers between the two bands is the relevant interband process for reducing the Coulomb repulsion in the conduction band. By considering a two-band situation arising from the hybridization of the conduction band with shallow impurity states having a large intrasite repulsion, it is shown that the pair-transfer process is effective in reducing the Coulomb repulsion even in situations where the second band is very narrow and possesses by itself a very large repulsion. The implications for enhanced pairing in systems with the appropriate band structure or in those near the metal-insulator transition caused by doping, are discussed.

The electronic energy band and ground-state properties for the existing body-centered-cubic (bcc) and body-centered-tetragonal (bct) crystals, and the predicted hexagonal-close-packed (hcp) structure of elemental copper have been calculated by using first-principles density-functional linear muffin-tin orbital methods in a unified scheme. Results are presented in the form of the energy-band structure in k space and the total energy as a function of the lattice constant. A recent proposed generalized gradient approximation scheme gives more accurate values than the standard local-density approximation. The calculated band structure of bct-Cu is in good agreement with that measured in photoemission experiments, on Cu films grown epitaxialy on Pd{001} and on Pt{001}. The equilibrium lattice constants given by us are in good agreement with those obtained from experiments on bct-Cu and bcc-Cu films. The possibility of the existence of an artificial structure of hcp-Cu has been discussed.

The linear-response approach to density-functional calculations in the local-density approximation is more efficient than traditional supercell and dielectric matrix techniques for the calculation of physical properties such as phonon frequencies. We have implemented this approach within the framework of the linearized augmented plane-wave method, which should bring it to bear on systems containing transition-metal and other elements that have localized orbitals. The formulation for this implementation is presented. The method is applied to the calculation of phonon spectra, Born effective charges, and dielectric constants for semiconductors. Results for phonon spectra and Born effective charges are in good agreement with experiment.