Complete replacement of Cu by Fe in R${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ leads to R${\mathrm{Ba}}_2$${\mathrm{Fe}}_3$${\mathrm{O}}_8$ (R=Y and Eu). We investigated ${\mathrm{YBa}}_2$${\mathrm{Fe}}_3$${\mathrm{O}}_8$ and ${\mathrm{EuBa}}_2$${\mathrm{Fe}}_3$${\mathrm{O}}_8$ by several complementary experimental techniques. These compounds crystallize in a tetragonal structure with the configuration of the atoms in the unit cell very similar to that in the superconductor ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$. Magnetic-susceptibility and Mössbauer-spectroscopy measurements of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ and ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ in R${\mathrm{Ba}}_2$${\mathrm{Fe}}_3$${\mathrm{O}}_8$ (R=Y and Eu) at temperatures from 4.2 to 800 K have been performed. In both compounds the iron moments order antiferromagnetically at the same Néel temperature ${\mathit{T}}_{\mathit{N}}$∼700 K. In ${\mathrm{YBa}}_2$${\mathrm{Fe}}_3$${\mathrm{O}}_8$ the Mössbauer spectra reveal two inequivalent iron sites, probably corresponding to iron in the Fe(2) site (fivefold oxygen coordination) and in the Fe(1) site (distorted octahedral oxygen coordination). The quadrupole splitting of Fe(2) is zero. Specific-heat (${\mathit{C}}_{\mathit{p}}$) measurements at different magnetic fields have been carried out between 1.5 and 30 K. The ${\mathit{C}}_{\mathit{p}}$(T,H) curves can be resolved into a contribution of the form ${\mathit{C}}_{\mathit{e}}$(T)=${\gamma }^{\mathrm{*}}$T with finite ${\gamma }^{\mathrm{*}}$ and a lattice contribution that consists of Debye terms. The pronounced high ${\gamma }^{\mathrm{*}}$=10.5 mJ/mol ${\mathrm{K}}^2$ for ${\mathrm{YBa}}_2$${\mathrm{Fe}}_3$${\mathrm{O}}_8$ is discussed.

The superconducting transition temperatures (${\mathit{T}}_{\mathit{c}}$’s) of ${\mathrm{HgBa}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{6+\mathrm{\delta }}$ with different dopings and different ambient ${\mathit{T}}_{\mathit{c}}$’s were examined magnetically under hydrostatic pressures. We observed that ${\mathit{T}}_{\mathit{c}}$ increases with pressure linearly at a rate of 0.22±0.02 K/kbar up to 18 kbar for as-synthesized samples with an ambient ${\mathit{T}}_{\mathit{c}}$∼111 K, but at a similar initial rate only below ∼4 kbar, and at a reduced rate of 0.11±0.02 K/kbar above, for the oxygenated samples with an ambient ${\mathit{T}}_{\mathit{c}}$∼117 K. The observation suggests the possible existence of a Van Hove singularity in the electronic energy spectrum of ${\mathrm{HgBa}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{6+\mathrm{\delta }}$ and a possible pressure-induced Fermi-surface-topology change in the oxygenated compound.

A square-lattice gauge-theory model for the holon liquid existing in the unconventional metallic phases of strongly interacting two-dimensional electron liquids is introduced. The phase of the holon order parameter couples in a gauge-invariant way to a statistical gauge field, which arises from strong-interaction physics. We find that the superfluid transition that exists in the absence of statistical gauge-field excitations in the holon liquid turns into a crossover phenomenon in the presence of such fluctuations. The corresponding high-temperature phase shows a T-linear resistance, while the low-temperature phase exhibits a ${\mathit{T}}^2$ dependence for the resistance. The crossover temperature scale ${\mathit{T}}_{\mathit{c}0}$ vanishes, however, in the limit of maximum statistical fluctuations. While we find that statistical gauge-field excitations are strictly gapless, the residue of the pole corresponding to such excitations becomes exponentially small at temperatures small in comparison to ${\mathit{T}}_{\mathit{c}0}$.

Measurements of the temperature dependence of the absolute thermopower (S) on samples of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_{3\mathrm{-}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ with 0≤x≤0.3 and δ<0.05 show that, between 100–200 K, S has a maximum that depends on x, according to the relation ${\mathit{S}}_{\mathit{m}}$(x)≊(${\mathit{k}}_{\mathit{B}}$/‖e‖) ln[(1+x)/(1-x)]. This is the expected saturation value of S, for noninteracting carriers in a narrow conduction band with x doped electrons above half-filling. The temperature dependence of the resistivity (ρ) exhibits a crossover from wide-band to narrow-band behavior, in a conductor with a mean free path limited by impurities. The drop of S at higher temperatures, accompanied by a drop in dρ/dT, can be attributed to excitation of electrons from the conduction band to higher-energy levels associated with the dopant, or to the contribution of activated hopping to the transport coefficients.

The electronic structure of the first member of the Aurivillius family of phases ${\mathrm{Bi}}_2$${\mathrm{O}}_2$(${\mathit{M}}_{\mathit{m}\mathrm{-}1}$${\mathrm{NbO}}_{3\mathit{m}+1\mathrm{}}$) with m=1 has been calculated using the linear muffin-tin orbital (LMTO) method. The band structure of ${\mathrm{Bi}}_2$${\mathrm{NbO}}_6$ is similar to that of ${\mathrm{Bi}}_2$${\mathrm{O}}_3$, which is the best oxygen ionic conductor. The Fermi level is located at the strong O 2p peak and reveals the formation of a large number of oxygen holes in the valence band. The substitution of fluorine for oxygen is shown to lead to a stabilization of the structure and the formation of ${\mathrm{Bi}}_2$${\mathrm{NbO}}_5$F. The preferred sites for fluorine substitution have been determined based on substitution energy estimates. The role of oxygen vacancies on the electronic structure and stability of the phases is investigated. Ideal ${\mathrm{Bi}}_2$${\mathrm{NbO}}_6$ is shown to be unstable and tends to contain a large number of oxygen vacancies. Vacancy formation energies have been found for different oxygen sites of the ideal crystals.

The ${\mathrm{Y}}_2$${\mathrm{Ba}}_4$${\mathrm{Cu}}_7$${\mathrm{O}}_{14+\mathrm{\delta }}$ family of superconductors is composed of alternating layers of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathrm{\delta }}$ and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$ materials along the c axis. The distribution of the hole-carrier density in the two-dimensional ${\mathrm{CuO}}_2$ planes in this compound can be quite complex due to the differing nature of these layers. Results of electronic-structure calculations are presented for three oxygen stoichiometries (δ=0.0, 0.5, 1.0) which shed some light on the distribution of the hole-carrier density in different ${\mathrm{CuO}}_2$ planes. We find that the hole density in the ${\mathrm{CuO}}_2$ planes associated with the single chains is, generally speaking, lower than for double-chain layers. The difference is, however, relatively small.

For the compound ${\mathrm{Y}}_2$${\mathrm{Ba}}_4$${\mathrm{Cu}}_7$${\mathrm{O}}_{14}$, we obtain an average value of ∼0.110 hole/${\mathrm{CuO}}_2$ for the hole density which is slightly above the threshold value of ∼0.06 hole/${\mathrm{CuO}}_2$ for the appearance of superconductivity to give it a low value of ${\mathit{T}}_{\mathit{c}}$∼15 K, despite the fact that the single chains in this compound have no oxygen atoms. In ${\mathrm{Y}}_2$${\mathrm{Ba}}_4$${\mathrm{Cu}}_7$${\mathrm{O}}_{15}$ and in ${\mathrm{Y}}_2$${\mathrm{Ba}}_4$${\mathrm{Cu}}_7$${\mathrm{O}}_{14.5}$ in the full, empty, full,. . . sequence for single chains, the hole densities are much larger, in agreement with much higher ${\mathit{T}}_{\mathit{c}}$ values observed in these compounds. The effect of oxygen disorder in the single chains on the hole densities has also been investigated for the latter two compounds, and is found to be much less drastic than in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathrm{\delta }}$. This is due to the presence of double chains which act as a cushion. The effective valence of Cu at different sites, which is an alternative way of presenting the hole densities, is also calculated and is shown to depend intimately not only on the oxygen stoichiometry but also on the degree of oxygen ordering in the single chains.

Electron-emission distribution curves of ordered and disordered graphite surfaces excited by primary electrons of energies in the 30–100 eV range have been measured. The entire inelastic-loss and secondary-electronic-emission spectra were studied in order to examine correlations between bulk plasmon deexcitation and the structure above the ‘‘zero’’ peak in the secondary-electron-emission spectrum. Our measurements show that no correlation exists between the prominent structure at ∼17 eV in the secondary spectrum and the appearance of bulk plasmons in the loss spectrum, as a function of primary energy. These results indicate that plasmon deexcitation in graphite does not result in single-electron excitations that contribute to prominent features in the secondary-electron-emission spectrum with threshold kinetic energies at ${\mathit{E}}_{\mathrm{plasmon}}$-e (work function). It is suggested that structure in the low-energy secondary-electron emission originates from single-electron transitions induced by the primary-electron beam.

Electrical conductivity of undoped and Pb-doped ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{10\mathrm{-}\mathrm{\delta }}$ glasses is reported in the temperature range 150–450 K. The temperature dependence of the conductivity can be interpreted consistently in terms of nonadiabatic polaron hopping theories proposed for amorphous semiconductors.

We propose a cell renormalization group for self-avoiding walks (SAW’s) in the square lattice based on a new center rule. The results for large cells are obtained by simulation. We calculate the effective fractal dimension ${\mathit{d}}_{\mathit{f}}$(b) for SAW’s in cells of size b (3≤b≤22) at criticality. Small deviations of ${\mathit{d}}_{\mathit{f}}$(b) from the asymptotic value indicate a correction-to-scaling exponent Δ=1.

Inelastic neutron scattering was used in a detailed measure of the [ζζζ] transverse-acoustic (TA) and the [ζ00] longitudinal-acoustic (LA) branches in ${\mathrm{Ni}}_{50}$${\mathrm{Al}}_{50}$ alloy. Anomalies are observed in the TA branch near ζ=0.28 and the LA branch near ζ=0.30 which are the precise wave vectors predicted by Zhao and Harmon in their calculations of the electron-phonon coupling in ${\mathrm{Ni}}_{50}$${\mathrm{Al}}_{50}$.

We first consider the effects of the crystal field on both the 4f ground state and the 5d excited state of a ${\mathrm{Ce}}^{3+}$ ion in the paramagnetic medium ${\mathrm{CeF}}_3$, and further take account of the splitting of the crystal-field ground-state levels caused by the effective superexchange field ${\mathit{H}}_{\mathit{v}}$ and the applied field ${\mathit{H}}_{\mathit{e}}$; then, with a model of a double transition that includes ground-state splitting, we calculate quantitatively the specific Faraday rotation ${\mathrm{\theta }}_{\mathit{F}}$, the Verdet constant V, the magnetic susceptibility χ, and their temperature dependences in ${\mathrm{CeF}}_3$. Theoretical calculations show that the magneto-optical effect and its temperature dependence in ${\mathrm{CeF}}_3$ are closely related to the superexchange interaction between ${\mathrm{Ce}}^{3+}$ ions. ${\mathit{V}}^{\mathrm{-}1}$ is linearly dependent on T and V/χ=-18 353(1+63.01/T) in the temperature range 60<T<=300 K. The theory is in good agreement with the experimental results.

The q-states Potts model on the Sierpínski gasket with ferromagnetic and antiferromagnetic interactions is analyzed with use of the transfer-matrix method. It leads to a set of four coupled maps for the free energy and correlation length. Attention is focused on the behavior of the residual entropy, when 1, 2, or 3 bonds within a unit cell is antiferromagnetic. In the isotropic case, with q=4, the value for ${\mathit{s}}_0$ is larger than that of the triangular lattice. The isotropic ferromagnetic model shows an unusual sharp peak in the specific heat in the limit of large q.

Taking into account quadratic corrections to the linearized renormalization transformation, a simple expression for the transient behavior of the local eigenvalues obtained from a Monte Carlo renormalization-group (MCRG) analysis is derived, which constitutes a basis for extrapolation procedures commonly used in MCRG calculations. The method is applied to investigate the effect of irrelevant and relevant scaling fields for the d=2 Ising model. It is shown that the transient effect for the magnetic eigenvalue is very fast because it is dominated not by the largest, but by the second largest irrelevant scaling exponent. Peculiarities observed for the effect of the relevant thermal scaling field τ=${\mathit{a}}_{\pm }$(T-${\mathit{T}}_{\mathit{c}}$)/${\mathit{T}}_{\mathit{c}}$ may be interpreted by assuming ${\mathit{a}}_{+}$≠${\mathit{a}}_{\mathrm{-}}$, i.e., a nonanalytic scaling field.

The alloys ${\mathrm{CeAg}}_{0.75}$${\mathrm{Ga}}_{3.25}$, ${\mathrm{CePd}}_{0.3}$${\mathrm{Ga}}_{3.7}$, and ${\mathrm{CeAg}}_{0.6}$${\mathrm{Al}}_{3.4}$ reported to crystallize in the ${\mathrm{BaAl}}_4$-type tetragonal structure, are investigated by magnetization M, electrical resistivity, and magnetoresistance measurements. All of these alloys appear to order ferromagnetically below about ${\mathit{T}}_{\mathit{c}}$=4 K. M tends to saturate at low fields (<10 kOe). The paramagnetic Curie temperature ${\mathrm{\theta }}_{\mathit{p}}$ is positive and the magnitude is comparable to ${\mathit{T}}_{\mathit{c}}$. This observation indicates a negligible influence of the Kondo effect in these alloys.

The magnetic structures of the ordered transition-metal compounds FePd and FeRh are found in both the CsCl and CuAu atomic structures by first-principles total-energy calculations. The ground state of FePd is shown to be the CuAu structure and ferromagnetic, in agreement with experiment. However, at equilibrium in the CsCl structure, where FePd is metastable, FePd behaves just like FeRh and has an antiferromagnetic ground state. A ferromagnetic phase of FePd lies 0.5 mRy higher in energy at the equilibrium volume and, like FeRh, but more readily, undergoes a first-order phase transition from the antiferromagnetic phase to the ferromagnetic phase at an expanded lattice constant or in a magnetic field.

The linearized magnetic excitations (magnons) in a two-dimensional ±J Heisenberg spin glass are investigated in the limit where the field is assumed to be strong enough to set up complete alignment of the ground state. The density of states, the localization indices, and the dynamic structure factor at zero temperature are computed by numerical simulation techniques. A comparison is made between the numerical results for the density of states and the dynamic structure factor and the predictions of the coherent-exchange approximation. Numerical results at zero field are also presented.

The fully frustrated triangular antiferromagnetic Ising model is found to order for higher spin values at finite temperatures. The ordered sublattices exhibit unsaturated magnetizations down to zero temperature, except for S=∞, which saturates. These results are obtained by the recently developed hard-spin mean-field theory, which is highly successful for frustrated systems. The S=1/2 case, as expected, and the S=1 case show no finite-temperature ordering. Thus, ordering behavior is enhanced with increased spin values, in contrast to ferromagnetic systems.

It is shown that an Ising spin system in which both spins and interactions evolve in time according to dynamical laws suggested by neural processes, but with widely separated timescales, leads to a thermodynamic equilibrium corresponding to a system of averaged replicas, where the replica number can take any real value determined by the ratio of characteristic temperatures. The resultant phase structure has interesting features which are explored.

A large magnetothermopower (MTP) has been found to accompany the magnetoresistance (MR) in a ${\mathrm{Ag}}_{80}$${\mathrm{Co}}_{20}$ granular system. The zero-field thermopower is negative and its magnitude increases as the magnetic field increases. The MTP is inversely proportional to the resistance in the temperature range where the diffusion thermopower dominates. These observations support the spin-split density-of-states (DOS) model which yields both the MTP and the MR for multilayers and granular systems, even with a spin-independent potential at the interfaces.

We show that a special kind of slave-boson mean-field approximation, which allows for the symmetry-broken states appropriate for a bipartite lattice, can give essentially the same results as those by the variational-wave-function approach proposed by Gula´csi, Strack, and Vollhardt [Phys. Rev. B 47, 8594 (1993)]. The advantages of our approach are briefly discussed.