Paramagnetic spectra of γ and α-Ce measured at the same temperature using high-energy neutrons (∼2 eV) yield a 4f occupancy of ∼0.8 in α-Ce. This phase of Ce also shows single-ion magnetic response but with a high Kondo energy of ∼1800 K. In γ-Ce we observe the ${}_{}{}^2{}_{}{}^{}$${\mathit{F}}_{5/2}$${\to }^2$${\mathit{F}}_{7/2}$ spin-orbit (SO) excitation at ∼260 meV. Remarkably, α-Ce also reveals a broad SO excitation shifted to a higher energy (∼450 meV). These observations reinforce the idea of localized 4f states in α-Ce and k-f hybridization as the driving mechanism for the γ⇆α transition.

We present specific-heat measurements on CePtSi in 0- and in 13-T magnetic field. There is no significant deviation between the two sets of data. This gives further evidence that CePtSi is a heavy-fermion compound and that magnetic correlations as a reason for the large specific heat γ can be excluded.

High-temperature-series expansions of the spin-spin correlation function for the plane-rotator (or XY) model on the sc lattice are extended by three terms through order ${\mathrm{\beta }}^{17}$. Tables of the expansion coefficients are reported for the correlation function spherical moments of order l=0,1,2. Our analysis of the series leads to fairly accurate estimates of the critical parameters.

The influence of the strength and the sign of the interface coupling between a ferromagnetically and an antiferromagnetically ordered film on the layer magnetizations is investigated by a Green-function method. Quantum-mechanical Heisenberg spins on a bcc and a sc lattice are considered. In both cases interesting quantum effects appear at low temperatures at the surface and at the interface of the films.

As discovered earlier, a trapped flux along the c axis of the grain-oriented sample at 4.2 K inhibits the vortex flux produced by an increasing external field (${\mathit{H}}_{\mathit{e}}$) in the a-b plane. It is now found that when ${\mathit{H}}_{\mathit{e}}$ is subsequently cycled, the hysteresis loop shows a pronounced broadening, the remanent vortex-flux magnetization along a-b having increased to about twice its value in the absence of the cross flux along c. Thus, although the cross flux decreases steadily during the changes of ${\mathit{H}}_{\mathit{e}}$, it acts consistently as a pinning agent for the vortices aligned along a-b. However, for large cycling ranges of ${\mathit{H}}_{\mathit{e}}$, the pinning of the cross flux itself is sufficiently overcome that its effects on the vortex behavior in the a-b plane diminish.

In both crystals at 4.2 K, a trapped flux along the c axis is seen to inhibit the initial production of vortex flux by external fields (${\mathit{H}}_{\mathit{e}}$) in the a-b plane and to increase the retention of this vortex flux during subsequent cycling of ${\mathit{H}}_{\mathit{e}}$. Thus, the cross flux along c acts consistently as a pinning agent for the vortices aligned along a-b. The cross-flux effects in both these crystals are as pronounced as those observed in grain-oriented ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ [S. J. Park and J. S. Kouvel, Phys. Rev. B 48, 13 995 (1993)] and they persist undiminished to much larger cycling ranges of ${\mathit{H}}_{\mathit{e}}$ due to the stronger pinning of the cross flux itself.

The magnetic excitation spectra of the electron superconductor ${\mathrm{Pr}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_4$ and related parent compounds ${\mathrm{Pr}}_2$${\mathrm{CuO}}_4$ and ${\mathrm{Nd}}_2$${\mathrm{CuO}}_4$ have been determined by inelastic neutron scattering. We observe crystal-field transitions up to about 100 meV within the ${\mathrm{Pr}}^{3+}$ ${}_{}{}^3{}_{}{}^{}$${\mathit{H}}_4$ and ${\mathrm{Nd}}^{3+}$ ${}_{}{}^4{}_{}{}^{}$${\mathit{I}}_{9/2}$ Russell-Saunders ground multiplets in these materials. We find that a crystal-field treatment of the ${\mathrm{Pr}}^{3+}$ and ${\mathrm{Nd}}^{3+}$ ions can adequately explain the observed excitation spectra for both ${\mathrm{Pr}}_2$${\mathrm{CuO}}_4$ and ${\mathrm{Nd}}_2$${\mathrm{CuO}}_4$. The obtained crystal-field parameters are close to values estimated from a superposition-model calculation and Mössbauer data. The ${\mathrm{Pr}}^{3+}$ and ${\mathrm{Nd}}^{3+}$ wave functions are found to have significant (≊20%) admixture of some states belonging to higher J multiplets. The calculated contributions to the susceptibility of ${\mathrm{Pr}}_2$${\mathrm{CuO}}_4$ and ${\mathrm{Nd}}_2$${\mathrm{CuO}}_4$ from the crystal-field states agree well with experiments. The observed crystal-field transitions in the superconductor ${\mathrm{Pr}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_4$ shift to slightly lower energies and are broadened significantly relative to the present compound due to chemical disorder from Ce doping.

We have measured the ab-plane resistivity on a number of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CuO}}_{\mathit{y}}$ crystals from 4.2 K to 300 K. The as-grown crystals usually exhibit a minimum in their normal-state resistivity. The low-temperature electronic transport can be described by a hopping conduction. As the crystals were annealed in the ambient flowing oxygen, they could become metallic over the whole temperature range. A nonlinear temperature-dependent resistivity was observed and found to be sample dependent. Some ρ(T) curves can be fitted to a power law with the exponent slightly greater than 1, while others appear with a Bloch-Grüneisen-like shape. A discussion of the experimental results is presented.

The pressure-temperature phase diagram for mercury is determined using energy-dispersive x-ray diffraction and a diamond anvil cell. In addition to the well-known structures of Hg, one new modification was found to be stable in the present experimental region. An orthorhombic lattice with four atoms in the unit cell represents all the experimental data for this new phase. The locations of the phase boundaries between the different phases α-β-γ-δ are determined for the extended p-T region. Furthermore, the parameters of the equations of state of the different phases are evaluated.

A formal description of configurational functions in alloys is presented. The approach introduces an infinity of orthogonal and complete basis sets in the configurational space of finite clusters, and leads naturally to generalized cluster expansions. In the present approach, the orthogonality of the basis functions is defined with respect to the scalar product given in terms of unrestricted sums over all possible configurations of the cluster. In the thermodynamic limit, this definition of the scalar product corresponds to sums in the canonical ensemble and, as such, the basis are well adapted to describe functions at any fixed concentration. A general relation between the expansion coefficients in different basis is derived.

Solutions representing multikinks or pairs of single kinks and strongly elongated bonds—antikinks—are found in a Frenkel-Kontorova model with Morse interactions and a misfit between the chain and the periodic potential. They are formed by splitting of the existing single kinks at negative misfits (expanded chains) which are greater in absolute value than the limit of metastability of the single kinks. The energy of the multikinks is higher than that of the single kinks and they are always in a metastable state.

The coupling of the Larmor line to pairs of nuclear spins being flipped by one photon is studied for the nuclear-spin system in the noble metals. The coupling parameter has been evaluated using the spherical model. The separation of the NMR frequencies of the Larmor line and the double-spin-flip line is calculated. The anticrossing feature in the Larmor-line frequency due to the coupling of the two modes is calculated in order to determine if the strength of the Ruderman-Kittel exchange interaction can be measured in Ag and Au.

NMR spin-echo and magnetization measurements performed on a MBE-grown, (110)-oriented Co/Pt superlattice reveal a strong in-plane, uniaxial magnetic anisotropy. This is due to large tensile strain exerted from the Pt lattice onto the Co layers, which leads to an orthorhombic distortion of the Co lattice. The orthorhombic distortion is also reflected in an anisotropy of the hyperfine field detected with the NMR method. This suggests that the magnetic anisotropy is correlated with an anisotropy of the orbital magnetic moments in these structures.

The ab-plane reflectivity of ${\mathrm{La}}_{1.89}$${\mathrm{Ca}}_{1.11}$${\mathrm{Cu}}_2$${\mathrm{O}}_{6\pm \mathrm{\delta }}$ single crystals, whose properties change from semiconductor to superconductor under high-oxygen-pressure annealing, is measured between 250 and 20 000 ${\mathrm{cm}}^{\mathrm{-}1}$ at room temperature. Although the annealing increases the oxygen content very slightly, the spectra change in the same way as seen in other cuprates with carrier doping: a charge-transfer peak disappears and low-energy excitation increases. We suggest the possibility that holes in the apex oxygen redistribute to the ${\mathrm{CuO}}_2$ planes. Mid-infrared absorption is observed in all samples. Relation between the mid-infrared absorption and the influence of La and Ca disorder on the ${\mathrm{CuO}}_2$ planes is also discussed.

The presence of strong vortex fluctuation effects in two families of layered high-temperature superconductors (${\mathrm{HgBa}}_2$${\mathrm{CuO}}_{4+\mathrm{\delta }}$ and ${\mathrm{Tl}}_2$${\mathrm{Ca}}_2$${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{10+\mathrm{\delta }}$) is established from experimental studies of the equilibrium magnetization in the mixed state using the theory of Bulaevskii et al. Detailed information is obtained within the framework of this theory on the London penetration depth ${\lambda }_{\mathit{a}\mathit{b}}$, upper critical field ${\mathit{H}}_{\mathit{c}2}$, and Ginzburg-Landau parameter ${\mathrm{\kappa }}_{\mathit{a}\mathit{b}}$ (with magnetic field perpendicular to the copper oxide planes) of these materials.

The diagrammatic valence-bond technique is adopted to investigate one-dimensional half-filled Hubbard chains with long-range e-e interactions and alternating charge-transfer integrals. Exact results for finite-size systems with different boundary conditions are carefully extrapolated to the infinite-chain limit. In addition to various ground-state properties, such as bond-charge-density wave and bond-order amplitude, we calculate the optical gap and the corresponding oscillator strength. We introduce adiabatic phonons able to modulate on-site energies and investigate the stability of the system with respect to site-charge-density wave distortion. We also evaluate the response of the electronic system to the perturbation induced by adiabatic phonons which modulate charge-transfer integrals, and calculate the corresponding screening of vibrational frequencies.

We report on optical conductivity measurements performed on ${\mathrm{CeAl}}_3$ spanning the photon energy range between approximately zero and 12 eV. From the measurements at micrometer- and millimeter-wave frequencies we obtained the detailed temperature dependence of both components of the complex conductivity. These results as well as the Kramers-Kronig analysis performed on the absorptivity lead, at low temperatures, to a strongly frequency- and temperature-dependent dynamic conductivity. We analyze the optical conductivity in terms of frequency-dependent scattering rate and dynamical mass. The low-temperature effective mass is found to be ${\mathit{m}}^{\mathrm{*}}$/${\mathit{m}}_{\mathit{b}}$=325±50 in the zero-frequency limit and its temperature dependence is compared with that of the thermodynamic parameters.

Both magnetic and transport properties of ${\mathrm{Yb}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Gd}}_{\mathit{x}}$Te solid solutions have been investigated in the whole range 0<x<1, for temperatures 4.2<T<300 K. Antiferromagnetic ordering is observed above the magnetic percolation threshold ${\mathit{x}}_{\mathit{c}}$≊0.45, due to the indirect exchange interaction between the localized magnetic moments carried by ${\mathrm{Gd}}^{3+}$ ions, mediated via the free carriers. The ${\mathrm{Gd}}^{3+}$ ions also act as donors, hence a metal-insulator transition at ${\mathit{x}}_{\mathit{m}}$≊0.2. This concentration is one order of magnitude larger than predicted by the Mott criterion, due to the important role played by magnetic fluctuations in the localization process. Pretransitional effects are investigated in the metallic side, within the framework of existing theories on disordered metals. A model taking into account the electron scattering by phonons, localized spin fluctuations, and Coulomb interactions allows for an overall agreement between the theoretical and experimental resistivity curves, at any x. In particular, a T-linear dependence of the conductivity is observed in the strong diffusion regime ${\mathit{x}}_{\mathit{m}}$<x<0.5, in a range of temperature that extends up to 50 K. We attribute this behavior to the electron-electron scattering that dominates the transport properties in a material which proves to be a unique example of disordered metals with a high electron density (due to the high value of ${\mathit{x}}_{\mathit{c}}$).

An experimental study of the photorefractive effect in a polymeric material containing carbazole and the second-order tricyanovinylcarbazole moieties as a side chain is presented. This polymeric system exhibits intrinsically both photoconductivity and the electro-optic effect. Absorptive and photorefractive gratings have been evidenced by four-wave-mixing experiments and electro-optic measurements. The properties of the photorefractive gratings are studied by investigating the electric-field dependence of the diffraction efficiency. The dynamics of the erase-write behavior of the gratings, as well as permanent photobleaching of the polymer, are described.

The exact solution of the Hubbard-Peierls model is given in a cluster of six atoms and three spinless electrons. Phonons with wave vector k=π are retained and quantum mechanically analyzed. Energy level and several averages are evaluated, thus determining the physical properties of the model in terms of the parameters of the system. The regime of the distorted lattice is obtained. The exact and Born-Oppenheimer (BO) results are compared, concluding that the BO enhances quantum fluctuations; this peculiar behavior is explained.