We have discovered that the ${\mathrm{Ce}}^{4+}$ doping and subsequent annealing in reducing atmosphere give rise to 24-K superconductivity in the ${\mathrm{Nd}}_2$${\mathrm{CuO}}_4$-type structure with sheets of Cu-O squares. In contrast to the previously reported high-${\mathrm{T}}_{\mathrm{c}}$ cuprates, the charge carriers in the new superconductors are doped electrons, not holes; this was confirmed by the measurements of Hall and Seebeck coefficients as well as by chemical analysis of the effective copper valence. An anomalous dependence of ${\mathrm{T}}_{\mathrm{c}}$ on the concentration of doped electrons is shown for these electron-doped superconducting suprates.

In the attractive Hubbard Model (and some extended versions of it), the ground state is proved to have spin angular momentum S=0 for every (even) electron filling. In the repulsive case, and with a bipartite lattice and a half-filled band, the ground state has S=(1/2∥B‖-‖A‖‖, where ‖B‖ (‖A‖) is the number of sites in the B (A) sublattice. In both cases the ground state is unique. The second theorem confirms an old, unproved conjecture in the ‖B‖=‖A‖ case and yields, with ‖B‖≠‖A‖, the first provable example of itinerant-electron ferromagnetism. The theorems hold in all dimensions without even the necessity of a periodic lattice structure.

We speculate that quantum electrodynamics near its conjectured ultraviolet-stable fixed point acts as a microscope of unlimited resolving power for short-distance interactions, in close analogy with monopole catalysis of proton decay. We illustrate this phenomenon analytically in the quenched ladder approximation by showing that a perturbatively nonrenormalizable parity-violating interaction becomes renormalizable at the fixed point.

The cross section for the production and subsequent decay to electron and neutrino of the W intermediate vector boson has been measured in 1.8-TeV p¯p collisions at the Fermilab Tevatron Collider. An analysis of events with missing transverse energy greater than 25 GeV and with an electron of transverse energy greater than 15 GeV from a datum sample of 25.3 ${\mathrm{nb}}^{\mathrm{-}1}$ gives σB=2.6±0.6±0.5 nb.

The spin-rotation parameters A and R for ${\pi }^{+}$p and ${\pi }^{\mathrm{-}}$p elastic scattering have been measured at pion momenta from 471 to 625 MeV/c using a frozen spin target polarized in the scattering plane and a recoil-proton polarimeter. Our results show that the major features of the recent πN partial-wave analyses (PWA’s) are correct. This is the first time such confirmation has been possible. Some deviations outside the errors indicate the need for a new round of PWA’s.

We discuss the behavior of the neutron charge form factor in a nuclear environment using the framework of a chiral πρω Lagrangian. For momentum transfers of 0.4 (GeV/c${)}^2$ and larger, the medium renormalization of fundamental meson parameters leads to a sizable decrease in the neutron charge form factor. This effect might be seen in an upcoming high-precision experiment to determine the electric form factor of the neutron through scattering of polarized electrons from polarized ${}_{}{}^3{}_{}{}^{}\mathrm{He}$. We also find that the effective nucleon mass and axial-vector coupling constant $g_A$ are quenched.

Correlations between projectilelike and targetlike fragments have been measured in the reaction ${}_{}{}^{27}{}_{}{}^{}\mathrm{MeV}$/nucleon ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$. These correlations can be consistently interpreted in the framework of an abrasion model including dissipation. However, they can also be described by a binary process in which, before decaying sequentially by particle emission, projectile and target share an approximately equal amount of excitation energy with no significant mass transfer.

Relative populations of particle-unbound states were measured for ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$ nuclei emitted behind the grazing angle for the ${}_{}{}^{14}{}_{}{}^{}$N${+}^{\mathrm{nat}}$Ag reaction at E/A=35 MeV. The populations of states decaying by proton and α-particle emission are inconsistent with purely statistical distributions, indicating that the intrinsic degrees of freedom of the fragmenting system may not be fully equilibrated at the instant the fragment separates from the rest of the system.

Measurements of conversion rates are reported for the first time in one- and two-electron systems, for the 14.4-keV nuclear state in ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$. The total half-life of this state deduced for the two-electron ion is 100±5 ns, and that deduced for the total spin F=1 state of the one-electron ion is 79±6 ns.

Using the 1/N expansion technique, the nature of the crossband transitions in collective models is studied. As an example, the γ→g E2 transition is calculated to next-to-leading order. The closed expressions derived are general and will be useful in the analysis of experimental data in terms of both the sd and sdg boson models. The formalism establishes a direct contact with the geometrical model, illuminating the nature of the γ→g transitions and clarifying some earlier results obtained using an SU(3) basis.

We show that if two upper levels of a four-level laser system are purely lifetime broadened, and decay to an identical continuum, then there will be an interference in the absorption profile of lower-level atoms, and that this interference is absent from the stimulated emission profile of the upper-level atoms. Laser amplification may then be obtained without inversion. Examples include interfering autoionizing levels, and tunneling systems.

Charge-exchange and fragmentation cross-section measurements on ${\mathrm{Au}}_2^{2+}$, ${\mathrm{Au}}_3^{2+}$, ${\mathrm{Au}}_4^{2+}$ and ${\mathrm{Au}}_4^{3+}$ are presented. Limits on the cluster electron affinities are obtained in terms of the ionization energies of Ar, ${\mathrm{N}}_2$, Kr, CO, Xe, and ${\mathrm{O}}_2$. Fragmentation cross sections, largely independent of the gas species, are comparable to those for charge exchange. The metastability of ${\mathrm{Au}}_2^{2+}$ is shown to be implied by the lower bound on its electron affinity.

We have observed an extremely large second-order susceptibility for second-harmonic generation of 9.6–10.8 μm radiation due to intersubband transitions in electric-field-biased GaAs quantum wells. For 92-Å GaAs wells with 309-Å ${\mathrm{Al}}_{0.48}$${\mathrm{Ga}}_{0.52}$As barriers under a bias of 36 kV/cm, the peak value of the susceptibility was 28 nm/V, 73 times larger than bulk GaAs. The magnitude and sign of the susceptibility depend on the bias field, and are in accord with theoretical predictions.

Optical activity measurements in the pretransitional region of the iostropic phase at wavelengths much longer than the pitch were used to determine the first two coefficients in an expansion of the optical activity in terms of the ratio of pitch to wavelength. The wavelength range investigated was from 0.78 to 1.10 μm, and the pitch of the four CE2/7¯S5 mixtures varied from 0.10 and 0.25 μm. Whereas theory predicts that coupling between the structural modes affects the first coefficient, the data indicate that only the second term in the expansion shows the effects of mode coupling.

We have developed a theory of impurity diffusion in silicon under equilibrium and nonequilibrium concentrations of point defects. The results of first-principles calculations of several key quantities are combined with this theory and compared to experimental data. We find that vacancies and self-interstitials mediate the equilibrium diffusion B, P, and As with comparable activation energies, but interstitials are domoinant. Sb diffusion, on the other hand, is mediated primarily by vacancies. We also find that the direct-exchange mechanism plays only a minor role for all dopants studied.

A novel weak-coupling approach is applied to the Su-Schrieffer-Heeger model including interactions for electrons both on sites and bonds as well as phonon dynamics. The influence of electronic interactions on dimerization is investigated. While the phonon frequency controls the overall interplay between instantaneous and retarded interactions, even the competition of different electron-electron interactions depends sensitively on the coupling of electrons with phonons and their frequency.

We report an observation of a single optical phonon emission by monoenergetic hot electrons traversing thin ${\mathrm{n}}^{+}$-type GaAs and thin undoped AlGaAs layers in times much shorter than the classical phonon period. This was done by injecting ballistic electrons into the thin layers with energy around the threshold for optical phonon emission and monitoring their exit energy. We estimate a scattering time of ≃200 fsec for electrons with energy of about 85 meV in ${\mathrm{n}}^{+}$-type GaAs, and ≃550 fsec for 40-meV electrons in undoped AlGaAs.

In the one-dimensional transverse Ising model, we calculate exactly the time evolution and the admittance of the magnetization and also of the energy near the critical point. The results for the above two quantities are shown to coincide with each other, and are explicitly represented in terms of functions of the inverse static susceptibility. The analyses are made by using the recurrence-relations method developed on the basis of Mori’s continued-fraction representation.

For a dilute gas of fermions interacting via an arbitrary pair potential in d=2 dimensions, we show that the many-body ground state is unstable to s-wave pairing at T=0 if and only if a two-body bound state exists. We further obtain, within a variational pairing Ansatz, a smooth crossover from a Cooper-paired state (${\xi }_0$${\mathrm{k}}_{\mathrm{f}}$≫1) to a Bose condensed state of tightly bound pairs (${\xi }_0$${\mathrm{k}}_{\mathrm{f}}$≪1). We briefly discuss non–s-wave superconductors. Insofar as this model is appli- cable to the high-${\mathrm{T}}_{\mathrm{c}}$ materials, they are in the interesting regime with the coherence length ${\xi }_0$ comparable to the interparticle spacing ${\mathrm{K}}_{\mathrm{f}}^{\mathrm{-}1}$.

An Airy-type geophysical experiment was conducted in a 2-km-deep hole in the Greenland ice cap at depths between 213 and 1673 m to test for possible violations of Newton’s inverse-square law. An anomalous gravity gradient was observed. We cannot unambiguously attribute it to a breakdown of Newtonian gravity because we have shown that it might be due to unexpected geological features in the rock below the ice.