Using Hubbard operators, which allow one to separate charge and spin channels, we construct mean-field equations for a fluxlike state that coexists with Néel antiferromagnetism (AF). The formal analysis shows that the equations for the real and imaginary parts of the effective hopping amplitude are inconsistent for any θ≠0 (including θ=π/4 which gives flux 1/2). The hopping amplitude is slightly suppressed by exchange renormalization. The equations for the Néel magnetization give two solutions, a band-type solution at δ≠0 and a Heisenberg-type solution for hole density δ==0. The latter solution is absent at δ→0,δ≠0. We explain this by the observation that at Hubbard U==∞ the spectral weight of the minority-spin subband in the Néel AF cannot be moved to the high-energy region at an electron density ρ<1. This forbids the Heisenberg-type of solutions even at J≫t. Therefore, this conclusion is not restricted by the approximation used. At larger δ, band antiferromagnetism (AFM) arises. In this region the Néel magnetization decreases with increasing hole density. The region where band AFM exists decreases with increasing hopping parameter. For t/J≥(t/J${)}_{\mathit{c}}$, there are no magnetic solutions below a certain hole density. Moreover, at large values of t/J (t/J≥1.2) the region where antiferromagnetic order can exist is found to be very narrow and situated near the critical hole density δ=1/3.

The kinematic interactions give rise to the instability of the paramagnetic state with respect to a transition to the superconducting state at δ<0.3 and t>J. The renormalization of the effective bandwidth ${\mathit{W}}_{\mathrm{eff}}$(δ) in the expression for the temperature of the supeconducting instability ${\mathit{T}}_{\mathit{c}}$=${\mathit{W}}_{\mathrm{eff}}$(δ)exp{-1/λ(δ)} by the exchange interaction reduces ${\mathit{T}}_{\mathit{c}}$ in both spin channels, while the renormalization of scattering amplitudes leads to a decrease of λ(δ) in one spin channel and to an increase in the other spin channel.

The magnetic phase diagrams, and other physical characteristics, of the hole-doped ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ and electron-doped ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_4$ high-temperature superconductors are profoundly different. Given that it is envisaged that the simplest Hamiltonians describing these systems are the same, viz., the t-t’-J model, this is surprising. Here we relate these physical differences to their ground states’ single-hole quasiparticles, the spin distortions they produce, and the spatial distribution of carriers for the multiply doped systems. As is well known, the low doping limit of the hole-doped material corresponds to k=(π/2,π/2) quasiparticles, states that generate so-called Shraiman-Siggia long-ranged dipolar spin distortions via backflow. These quasiparticles have been proposed to lead to an incommensurate spiral phase, an unusual scaling of the magnetic susceptibility, as well as the scaling of the correlation length defined by ${\xi }^{\mathrm{-}1}$(x,T)=${\xi }^{\mathrm{-}1}$(x,0)+${\xi }^{\mathrm{-}1}$(0,T), all consistent with experiment. We suggest that for the electron-doped materials the single-hole ground corresponds to k=(π,0) quasiparticles; we show that the spin distortions generated by such carriers are short ranged. Then, we demonstrate the effect of this single-carrier difference in many-carrier ground states via exact diagonalization results by evaluating S(q) for up to four carriers in small clusters.

Consistent with experiment, for the hole-doped materials short-ranged incommensurate spin orderings are induced, whereas for the electron-doped system only commensurate spin correlations are found. Further, we propose that there is an important difference between the spatial distributions of mobile carriers for these two systems: for the hole-doped material the quasiparticles tend to stay far apart from one another, whereas for the electron-doped material we find tendencies consistent with the clustering of carriers, and possibly of low-energy fluctuations into an electronic phase-separated state. Phase separation in this material is consistent with the midgap states found by recent angle-resolved photoemission spectroscopy studies. Last, we propose the extrapolation of an approach based on the t-t’-J model to the hole-doped 123 system.

We describe measurements on trilayers and multilayers of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (YBCO) with intervening layers of ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Pr}}_{\mathit{x}}$${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$, with thicknesses d of YBCO down to that of a single unit cell. The zero-resistance transition temperature ${\mathit{T}}_{\mathit{c}\mathit{o}}$ decreases as d decreases, and increases as x decreases, and we show that a single unit-cell layer is still superconducting. We describe a transition from a ‘‘bulk regime’’ in the interior of the YBCO to a ‘‘surface regime’’ near the interfaces, and show that the ${\mathit{T}}_{\mathit{c}\mathit{o}}$ depression is correlated with a depressed conductance in the region adjacent to the interfaces. The results indicate that the changes are related to the presence of the interfaces, primarily to charge transfer between the layers, with only minor indications of a change in the intrinsic properties of the YBCO from the bulk down to the thickness of a single unit cell. The values of ${\mathit{T}}_{\mathit{c}\mathit{o}}$ in multilayers are consistent with those on trilayers, and so rule out any contributions from very long-range interactions, but they indicate the presence of interactions with length scales of several hundred angstroms across the barrier layers.

Hole and magnon spectral functions of the two-dimensional t-J model are self-consistently calculated for hole concentrations x≲0.3. The employed system of Dyson’s equations was obtained with the use of the Born and spin-wave approximations. It is shown that near x=0.05 the hole spectrum is changed drastically acquiring features of a weakly correlated spectrum. This change is accompanied by a substantial decrease of the carrier effective mass and by entering the chemical potential μ into a sharp maximum of the density of states. With increasing x the maximum moves together with μ up to x≊0.15 and then lags behind. This concentration is also distinguished by the facts that the Fermi surface becomes a simply connected surface and overdamped magnons, which appear at x≊0.02, occupy the widest region in k space. Besides, here usual magnons have the smallest limiting frequency which is approximately three times smaller than the initial one. Significant deviations from the normal Fermi liquid, which partly resemble marginal Fermi liquid, are observed for x≳0.05.

The spin-wave energy, the damping, and the staggered magnetization are calculated for a lightly doped antiferromagnet in the framework of the t-J model. The Dyson equation for the dynamical spin susceptibility is derived and studied at zero temperature. It is shown that the staggered magnetization as well as the spin-wave velocity vanish for some critical hole concentration. The damping of spin waves is computed and found to be very sensitive to the hole doping. The results are in good agreement with experimental data of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$.

Characteristic velocities of the electromagnetic waves propagating in vertically stacked Josephson transmission are theoretically discussed. An equation for solving n velocities of the waves in an n Josephson-junction stack is derived. The solutions of two- and threefold stacks are especially focused on. Furthermore, under the assumption that all parameters of the layers are equal, analytic solutions for a generic N-fold stack are presented. The velocities of the waves in two- and three-junction stacks by Nb-Al-${\mathrm{AlO}}_{\mathit{x}}$-Nb systems are experimentally obtained by measuring the cavity resonance (Fiske step) modes. Comparison of the theory with the experimental data reveals good quantitative agreement.

q-dependent neutron-scattering measurements of the dynamic susceptibility χ’’(q,ω) in superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.6}$ are presented. The observed magnetic scattering near the antiferromagnetic zone center has a nontrivial, approximately flat-topped q dependence. A separable model of the susceptibility, χ’’(q,ω)=χ’’(ω)F(q), is found to describe this scattering at all energies in the range 2 meV ≤ħω≤40 meV. Measurements at temperatures above and below ${\mathit{T}}_{\mathit{c}}$=46 K indicate that the q-dependent ‘‘shape’’ function, F(q), is the same in both the superconducting and normal states. A phenomenological expression for F(q) is presented and the physical significance of its parametrization is discussed. These results are compared with current theories in the literature.

The analysis of several sets of data for the resistivity in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathit{x}}$ films, measured as a function of the temperature, the magnetic field, and the field orientation, allows us to show that a phase-slip-dissipation model is able to accurately describe the experimental results when the fluxon thermal fluctuations are governed by a glasslike activation law A/[T-${\mathit{T}}_{\mathit{g}}$(H)], where ${\mathit{T}}_{\mathit{g}}$(H) is the irreversibility line. In this framework, we obtain the temperature and magnetic-field dependence of the quasi-activation-energy A. A qualitative explanation is given for the temperature and magnetic behavior in terms of a 3D-2D crossover due to the softening of the flux lines with increasing field.

Using the polaron model as an approximation of the t-J model, we have computed in the two-time Green’s-function formalism the spin-wave spectrum and the staggered magnetization at nonzero temperature near the transition to the disordered phase. The incoherent part of the charge spectrum mainly contributes and leads to an estimate of the doping-dependent Néel temperature which turns out to be in fairly good agreement with experiment for high-${\mathit{T}}_{\mathit{c}}$ copper oxides.

The spin dynamics in the doped antiferromagnetic (AF) state of high-${\mathit{T}}_{\mathit{c}}$ copper oxide superconductors has been investigated within the t-t’-J model. We have shown that a small number of doped holes affects the spin dynamics in the ${\mathrm{CuO}}_2$ planes so strongly that above a very small hole concentration, ${\mathit{n}}_{\mathit{c}}$=0.027, the two-dimensional (2D) AF ground state becomes unstable. We have also predicted a great change in the spin excitation spectrum for the doped AF state. For doping close to ${\mathit{n}}_{\mathit{c}}$, a hydrodynamic regime of well-defined magnons collapses, and almost the whole region of long-wavelength spin waves corresponds to the overdamped regime of softened magnons in agreement with inelastic neutron-scattering experiments in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_6$+x reported by Rossat-Mignod et al. Analyzing the imaginary part of the spin susceptibility χ’’(q,ω) we have discovered, in addition to the spin-wave part, also an incoherent band associated with electron-hole excitations. For the overdamped regime the energy ranges of coherent excitations and the incoherent band are overlapped and so it is impossible to separate them. Even for smallest q, when the magnon peak and the band are separated, the relative contribution of the band increases with increasing n and dominates near the critical point.

The resistive transitions of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$-${\mathrm{Y}}_2$${\mathrm{BaCuO}}_5$ granular composites (denoted as ${\mathrm{Y}}_{123}$/${\mathrm{Y}}_{211}$) have been systematically investigated under applied magnetic fields up to 4 kG at a fixed low transport current density of 1.5 mA ${\mathrm{cm}}^{\mathrm{-}2}$. In the region of the superconducting transition the samples showed two distinct sections, a steep part associated with the onset of superconductivity in the individual grains and a transition tail due to the weak links coupling the grains. For each composite the steep section remained unchanged with applied field while the tails moved considerably to lower temperatures with this dissipation region being very sensitive to both transport current and applied field as the percolation limit was approached. The resistivity tails have been interpreted using the Ambegaokar and Halperin (AH) phase-slip model and a four-variable fitting procedure. The AH parameter γ(H${)}_{\mathit{T}=0\mathrm{}}$ has been used to estimate the critical current density, at zero temperature, in the grain boundaries for the different composites at various magnetic fields. The magnetic field dependence of γ(H) was found to be ∝${\mathit{H}}^{\mathrm{-}\mathit{n}}$ with n=0.5 for pure ${\mathrm{Y}}_{123}$ and 0.3 for the other composites. The resistivity data near ${\mathit{T}}_{\mathit{c}}$ (ρ=0) did not conform with the AH model and have been interpreted in terms of the thermally activated flux-creep model. Pinning energies have been determined and were also found to follow the applied magnetic field according to ${\mathit{H}}^{\mathrm{-}\mathrm{\alpha }}$ with α=0.60 for all composites.

The current-voltage (I-V) characteristics are investigated in epitaxial ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ films in magnetic fields parallel to the c axis. Scaling analysis of the nonlinear I-V data demonstrates the existence of a quasi-two-dimensional vortex-glass–liquid transition in which the correlation length in the a-b plane only scales with ‖T-${\mathit{T}}_{\mathit{g}}$‖, where ${\mathit{T}}_{\mathit{g}}$ is the vortex-glass transition temperature. The critical scaling region is very wide: from 4.2 to 50–80 K in fields of 0.5–4.0 T. In the vortex-liquid regime a finite linear resistivity ${\mathrm{\rho }}_{\mathrm{lin}}$=(E/J${)}_{\mathit{J}\to 0}$ is observed, while it appears to vanish in the vortex-glass regime.

The crystal structures, magnetic susceptibilities, and electrical resistivities of the quaternary intermetallic compounds ${\mathrm{LaRh}}_2$${\mathrm{B}}_2$C and ${\mathrm{LaIr}}_2$${\mathrm{B}}_2$C are reported. Although isostructural with the recently reported superconductors L${\mathrm{Ni}}_2$${\mathrm{B}}_2$C and L${\mathrm{Pt}}_2$${\mathrm{B}}_2$C (L=La and Y), they are not superconducting at temperatures above 1.4 K. The magnetic-susceptibility measurements reveal densities of states at the Fermi level intermediate to the Ni- and Pt-based compounds.

We used electron-energy-loss spectrometry to measure the intensities of the Cu white lines found at the onsets of the ${\mathit{L}}_2$ and ${\mathit{L}}_3$ absorption edges of rapidly quenched CuZr, CuTi, CuPd, CuPt, and CuAu. Enhancements of the white lines in the alloys as compared to elemental Cu were correlated with changes in the 3d occupancy of Cu atoms upon alloying. We found that the Cu atoms lost 0.2±0.06 d electrons when alloyed with Ti or Zr, 0.$0_{\mathrm{-}0.0}^{+0.06\mathrm{}}$ d electrons when alloyed with Au or Pd, and 0.${03}_{\mathrm{-}0.03}^{+0.06\mathrm{}}$d electrons when alloyed with Pt. We found no strong correlation between measured 3d- occupancy changes and heats of formation of the alloys, consistent with expectations that outer s electrons play an important role in the alloying of Cu with Pt, Pd, and Au.

Angular distributions of inelastically scattered electrons with energy losses up to 300 eV are presented for Al(100) and Cu(100) surfaces. At a primary energy of 2 keV, the information depth was changed using different angles of incidence. The results show that single energy-loss processes produce intensities preferentially in the forward-scattering direction, whereas multiple excitation processes destroy the angular anisotropy. Differences in the spectra for Al and Cu are ascribed to the localized versus extended nature of excitations.

The N-N stretching band region of a ${\mathrm{Ar}}_{0.3}$(${\mathrm{N}}_2$${)}_{0.7}$ mixed crystal has been studied by infrared spectroscopy. The spectra are interpreted in terms of (i) a static component representing frozen-in degrees of freedom, (ii) a quasielastic Lorentzian component representing the relaxing modes, and (iii) the density of states of the librational and translational modes.

In situ neutron diffuse scattering experiments were performed on disordered ${\mathrm{Ni}}_3$V. Effective pair interactions (EPI) were deduced from the measured short-range order parameters. The EPI were used to predict successfully the disordering temperature and the temperature dependence of the excess free energies of two different antiphases in the ordered phase. We also computed the energy difference between ${\mathit{DO}}_{22}$ and L$1_2$: our result is eight times lower than found by electronic structure calculations at 0 K. This discrepancy is partly due to electronic excitations.

A Raman-scattering investigation of stishovite, a high-pressure crystalline polymorph of ${\mathrm{SiO}}_2$, shows that at around 650 °C it becomes amorphous. The amorphous material produced during the initial stages of amorphization is not identical to ordinary silica since it exhibits features of the high-pressure amorphous polymorph. Above 800 °C there is little remaining evidence of stishovite but the amorphous material has transformed to a state indistinguishable from that of silica. Annealing effects in densified silica are also presented and compared with the stishovite results.

It is shown that optical free-induction decay (OFID) in fractals is governed by a power-law time dependence. The scale invariance of the OFID in the time domain results from the self-similar distribution of the dipolar eigenmodes in fractals.

A numerical scaling analysis is used to show that Nagaoka’s ferromagnetic state in the two-dimensional Hubbard model with one hole is superseded by an antiferromagnetic (AF) state with a discontinuous jump in the total spin due to the AF coupling as the Hubbard U is made finite. The same applies to the two-hole system, which has a spiral spin structure. We can show, via the scaling, that the crossover to an AF state is a precursor of a pathological coalescence of states having the minimum spin and Nagaoka’s state at U=∞ in the thermodynamic limit.