As a system is diluted, the critical temperature $T_c$ may fall to zero at a concentration $x_c$ greater than the percolation concentration $p_c$, because mere connectivity does not guarantee the transmission of order even at T=0. Detailed results, including bounds on $x_c$, are presented for the three-state Potts antiferromagnet on a triangular lattice and for quadrupolar models of (o-${\mathrm{H}}_2$${)}_{\mathrm{x}}$(p-${\mathrm{H}}_2$${)}_{1\mathrm{-}\mathrm{x}}$ hydrogen mixtures on fcc and triangular lattices.

Inelastic neutron scattering has been used to obtain the density of states, G(E), for both crystalline and amorphous samples of diglycidyl ether of bisphenol A, the monomer basis of most epoxy resins. For the crystalline material G(E) is proportional to ${\mathrm{E}}^2$ at low E while in the amorphous state the results confirm our earlier work in which G(E) while initially proportional to ${\mathrm{E}}^2$, varies as a higher power of E above 1.2 meV. Calculations of the specific heat, C, show that both the amorphous and crystalline samples have a peak in C/${\mathrm{T}}^3$ in the liquid-helium range. Hence the use of C to explain the differences between crystalline and amorphous materials can be misleading.

A simple and general method is presented for the determination of bond lengths (coordination distances) from extended x-ray absorption fine-structure data. A parametrization of the phase shift has been described for the evaluation of bond distances wherein neither any model compound nor the theoretically calculated phase shifts are needed. The method has been applied to platinum and some systems of copper, cobalt, manganese, and nickel.

For particles undergoing fragmentation with a rate a(y)∼${\mathrm{y}}^{\mathrm{\beta }+1\mathrm{}}$, where y is the particle size, a cascading process occurs when β<1 in which smaller particles break up at increasingly rapid rates, resulting in mass being lost to a phase of ‘‘zero’’-size particles. The cascading of the breakup rate, or ‘‘shattering,’’ produces a fractal dust, similar to a Cantor dust, with dimension 0<${\mathrm{D}}_{\mathrm{f}}$<1, and is analogous to, but opposite from, the process of gelation found in aggregating systems.

We report on the first x-ray scattering measurements of pretransitional smectic-C fluctuations in the nematic phase of a side-chain polymeric liquid crystal, C-6 polysiloxane. The data are well described by the mean-field theory of Chen and Lubensky suggesting that the mesogenic side groups are only weakly coupled to the polymeric backbone. Because of disorder in the polymer backbone, the correlation lengths, in the vicinity of the nematic-smectic-C phase transition, are weak functions of temperature and their magnitudes are smaller than those of conventional monomeric liquid crystals.

Amorphization has been studied in electron- ($e^{\mathrm{-}}$) and ion-irradiated Si. Si irradiated at <10 K with 1.0- or 1.5-MeV ${\mathrm{Kr}}^{+}$ became amorphous at <0.4 displacement per atom (dpa), whereas Si irradiated at 10 K to a fluence of ≊14 dpa of 1-MeV $e^{\mathrm{-}}$ in an electron microscope failed to amorphize. However, Si subjected to a simultaneous $e^{\mathrm{-}}$ and ${\mathrm{Kr}}^{+}$ in situ irradiation, at <10 K, to a ${\mathrm{Kr}}^{+}$ fluence of 1.5 dpa retained crystallinity. The critical ratio, at <10 K, of the $e^{\mathrm{-}}$ to ${\mathrm{Kr}}^{+}$ ion displacement rates to maintain a degree of crystallinity is ≊0.5. Atomistic models for these phenomena are presented.

First experiments suggesting the presence of singular vortices in rotating ${}_{}{}^3{}_{}{}^{}\mathit{A}$ are reported. Our studies on the interaction between negative ions and vortex textures verify the existence of the continuous vortices seen earlier by NMR, but evidence for a new vortex state with high ion mobility along the core is also found. The nucleation of the new vortex depends on the external magnetic field and the rotation history of the sample.

A stable and reproducible superconductivity transition between 80 and 93 K has been unambiguously observed both resistively and magnetically in a new Y-Ba-Cu-O compound system at ambient pressure. An estimated upper critical field ${\mathrm{H}}_{\mathrm{c}2}$(0) between 80 and 180 T was obtained.

The pressure effect on the superconducting state above 77 K in the new Y-Ba-Cu-O compound system has been determined. In strong contrast to what is observed in the La-Ba-Cu-O and La-Sr-Cu-O systems, pressure has only a slight effect on the superconducting transition temperature.

We study the number of stable points for spin-glasses and neural networks of higher orders, i.e., with Hamiltonians given by an algebraic form of degree d. For spin-glasses, we derive a rigorous exact expression, in the thermodynamic limit, assuming long-range independent exchange Gaussian interactions among sets of d spins. For neural networks we introduce several upper bounds on the number of programmable stable states, according to different storage schemes.

Inelastic neutron scattering from the antiferromagnetic (${\mathrm{T}}_{\mathrm{N}}$=9.7 K) heavy-fermion system ${\mathrm{U}}_2$${\mathrm{Zn}}_{17}$ reveals magnetic fluctuations which are highly localized in reciprocal space and broad in frequency. At higher energies, the fluctuations persist almost unchanged to temperatures well above ${\mathrm{T}}_{\mathrm{n}}$. Analysis of the neutron and bulk susceptibility data in terms of Kondo-type single-ion response functions modified by Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions between ions indicates that it is the temperature dependence—anticipated theoretically—of the effective RKKY coupling which drives the magnetic phase transition in ${\mathrm{U}}_2$${\mathrm{Zn}}_{17}$.

The momentum distribution of the positronium (Ps) emitted from an Al(100) surface being bombarded by low-energy positrons shows anisotropic lobes that gradually disappear as the initially clean surface is exposed to small amounts of oxygen. A clean Al(111) surface yields a more isotropic spectrum. The simplest model suggests that the anisotropy stems from the energy gaps in the projected nearly-free-electron band structure of Al. We conclude that inelastic many-body effects are weak and that the Ps spectra reflect the electron density of states near the surface.

We calculate the density of states (DOS) for ${\mathrm{Na}}^{+}$ impurity bands in n-type Si/${\mathrm{Sio}}_2$ metal-oxide- semiconductor structures with a Hubbard-type model and using as a basis Martin and Wallis single-impurity states. We show that the intrasite correlation energy increases with the applied electric field on the junction. The same occurs with the bandwidth and with the energy corresponding to the maximum in the DOS. This effect is enhanced by an increase in the impurity concentration, ${\mathrm{N}}_{0\mathrm{x}}$. The asymmetry of the DOS explains why the measured binding energy for finite ${\mathrm{N}}_{0\mathrm{x}}$ is smaller than in the single-impurity case.

We present a method for calculating zero-frequency, spatially varying response and correlation functions around a magnetic impurity in metals. We illustrate the procedure which involves a combination of perturbative scaling and nonperturbative renormalization-group methods for the spin-(1/2) Kondo impurity. We explain the factorization of the temperature and spatial dependence of the Knight shift observed in NMR experiments.

The p(1×1) Co monolayer epitaxially grown on a Cu(001) substrate is ferromagnetic. The remanence magnetization of 1-monolayer and thicker films is zero, although a uniaxial anisotropy perpendicular to the plane of the films exists. Up to 400 K no variation of the magnetization as function of temperature is observed.

A coupled electron-lattice model of a saturated homopolymer, such as a poly(organosilylene), is introduced. It shows that the states available in the polymer for the addition or excitation of electrons, or holes, are localized intragap states that are spontaneously generated by bond-order polaron formation.

We report picosecond transient optical-absorption decays for electrons localized in F-center analogues in molten KCl and KBr. Absorption decays were recorded for 750 ps after electron-hole generation by a 266-nm ultraviolet pulse. The recombination process is analyzed in terms of reactions involving the anionic hole centers ${\mathrm{X}}_2^{\mathrm{-}}$ and ${\mathrm{X}}_3^{\mathrm{-}}$ where X+Cl or Br. The results confirm by direct real-time measurement that F-center electrons are sufficiently mobile to account for the electronic conductivity observed in dilute metal-molten salt solutions. This demonstrates the necessary presence of a new polaronlike transport mechanism involving diffusive modes of ionic motion.

We report a search for photon localization using a new experimental method which measures the effects of multiple elastic scattering on the shape of a short laser pulse injected into a random optical medium. Several systems have been examined which exhibit diffusive photon transport, but no evidence for strong localization. A theoretical assessment of candidate systems which might exhibit a photon mobility edge is presented.

Two simple storing prescriptions are presented for neural network models of N two-state neurons. These rules are local and allow the embedding of correlated patterns without errors in a network of spin-glass type. Starting from an arbitrary configuration of synaptic bonds, up to N patterns can be stored by successive modification of the synaptic efficacies. Proofs for the convergence are given. Extensions of these rules are possible.

The authors report use of ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ and ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ NMR to study the bonding of CO on Pd particles. By ${}_{}{}^{17}{}_{}{}^{}\mathrm{-}_{}^{13}{}_{}{}^{}$C double resonance, they measure the CO bond length to be 1.20±0.03 Å. The ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ resonance frequency is exceptionally high, 310 ppm above values typical for metal carbonyls. Evidence that the shift arises from electron-spin polarization is given from studies of the magnitude and the dependence on temperature and frequency of the ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ spin-lattice relaxation time. A diffusion enegy of 6±2 kcal/mole, half that of CO on Pt, is deduced from motional narrowing of the ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ NMR line.