It is shown in a simple Hubbard model that through a mechanism called η pairing one can construct many eigenstates of the Hamiltonian possessing off-diagonal long-range order. The intrapair distance is small. It is shown that these eigenstates are metastable and possess an energy gap.

Using a Lanczos technique we study the frustrated spin-(1/2 Heisenberg model on square lattices of 16 and 20 sites. Frustration is introduced by an interaction along the diagonals of the plaquettes with coupling $J_2$≥0. For large $J_2$ we found that the ground state breaks (spontaneously) the lattice rotational symmetry. For intermediate values of $J_2$, the squares of order parameters associated with spin-Peierls and ‘‘twisted’’ states have a peak while a similar quantity for a chiral state shows no interesting structure.

Keirstead and Huberman proposed dynamical rules for a simple, fault-tolerant computing architecture capable of reliable detection of one-dimensional motion in highly noisy environments. We solve analytically their model and show that the collective nature of the detection process produces a sharp transition in the performance of the system as a function of noise. Moreover, we introduce improved dynamical rules which allow the system to detect arbitrary motion, in principle in any dimension. We present numerical results for these new rules in order to establish their efficiency.

Irregularity of the eigenvalue sequence in a quantum nonintegrable system is investigated from the viewpoint of algorithmic complexity theory. An example of a real symmetric matrix whose eigenvalue sequence has null complexity but whose nearest-neighbor level spacing shows the Wigner-type distribution is presented. Numerical examinations demonstrate that the algorithmic complexity of eigenvalues of a nonintegrable Hamiltonian system also turns out to be null.

We have performed a test of Newton’s universal theory of gravitation by comparing gravity measured on a tower to an upward continuation of the surface gravity field. We measured gravity at 12 heights on a 465-m tower at the Nevada Test Site and, in addition, made measurements at 281 locations on the ground. The surface points fell within 91 optimally chosen sectors that extended out to 2.6 km from the tower. These data were combined with 60 000 additional surface gravity measurements within 300 km of the tower. We used a surface integral derived from Laplace’s equation to upward continue the surface gravity field and our observations are consistent with the Newtonian predictions to within (-60±95)×${10}^{\mathrm{-}8}$ m ${\sec }^{\mathrm{-}2}$ at the top of the tower.

The effect of an electric dipole moment (EDM) of the W boson is studied in the radiative decay $W^{\mathrm{-}}$→ūdγ. It is shown that the radiation amplitude zero which is present in this process provides a sensitive test of the possible compositeness of the W. The two possibilities, a large EDM (λ≥1) and a significant magnetic moment (κ≠1) can in principle be distinguished.

By studying $e^{+}$$e^{\mathrm{-}}$ annihilations in the center-of-mass energy range between 50 and 60.8 GeV, we have established a 95%-confidence-level lower limit on the mass of a fourth-generation charge -(1/3 quark b’ of 27.2 GeV. In contrast with all previous searches, this limit has been obtained through consideration of the decay processes b’→bγ and b’→bg aswell as b’→${\mathrm{cW}}^{\mathrm{-}}$. For the cases where any one of the three decay modes dominates, we obtain higher mass limits.

In terms of perturbative QCD we calculate the basic features of high-mass ($M_X$) diffractive dissociation of hadrons at large momentum transfer t. We find that the distribution over 1-x=$M_X^2$/s should change from 1/(1-x) at small t to &/ ${\ln }^3$[1/(1-x)] at large t [where ${\omega }_0$=${\alpha }_S$(t)12 ln(2)/π], and that the gluon distribution in the Pomeron should flatten with increasing ‖t‖.

I identify a new realization of chiral SU(3)×SU(3) symmetry which, in a sense, encompasses both the Wigner-Weyl mode and the Nambu-Goldstone mode. The chiral symmetry is unbroken, but there are Goldstone bosons. Furthermore, the light vector mesons, as well as the pseudoscalar mesons, are intimately involved because the chiral partners of the pseudoscalars are the longitudinal components of the vectors. I discuss the connection of this realization with vector dominance, the Kawarabayashi-Suzuki-Riazuddin-Fayyazuddin relation, and the large-N approximation.

The implications of precise new Z- and W-mass and weak-neutral-current data for the top-quark and Higgs-boson masses are analyzed. It is found that (a) ${\sin }^2$${\mathrm{\theta }}_{\mathrm{W}}$=0.226±0.005 for arbitrary $m_t$ and for all $M_H$<1 TeV; (b) $m_t$<210 GeV at 90% C.L.; (c) $m_t$>40 GeV at 90% C.L., even if the direct lower limit $m_t$>78 GeV does not apply; and (d) there is no constraint on the Higgs-boson mass.

A canonical-ensemble calculation of the specific heat in ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$ using the eigenstates of a number of realistic interactions and the energies of the states obtained experimentally exhibits a prominent peak at T≃1.7–2.5 MeV which is associated with the deformed-to-spherical phase transition seen in finite-temperature mean-field calculations. This phase transition arises from a change in the relevant degrees of freedom describing the system and will occur for any nucleus which has a low-lying collective spectrum. Including a continuum in addition to the discrete spectrum does not produce an additional peak in the specific heat at low temperatures.

Searching for evidence of ‘‘cold’’ nuclear fusion in deuterium-loaded Ti and Pd foils with plastic track detectors, we detected the emission of α particles from trace-heavy-element decay, but found no evidence of dd fusion. Cycling ${\mathrm{TiD}}_2$ and ${\mathrm{PdD}}_{>0.4\mathrm{}}$, in high-pressure ${\mathrm{D}}_2$ cells between 1 and 15 bars and 77 and 300 K, gave an upper bound of 0.7 ${\mathrm{cm}}^{\mathrm{-}3}$ ${\mathrm{s}}^{\mathrm{-}1}$ for the mean rate of dd${\to }^3$He+n fusion. For electrolytically deuterated ${\mathrm{PdD}}_{0.8}$ our upper bound is 0.0018 ${\mathrm{cm}}^{\mathrm{-}3}$ ${\mathrm{s}}^{\mathrm{-}1}$ for the mean rate of dd→p+t. This is ∼1.5×${10}^6$ and 180 times lower than ‘‘cold’’-fusion rates reported by Fleischmann, Pons, and Hawkins and Jones et al., respectively.

We have measured the emerging charge-state distribution of 27-MeV/u ${\mathrm{Xe}}^{35+}$ beams channeled through a thin Si single crystal, and deduced electron impact ionization cross sections for ${\mathrm{Xe}}^{35+}$ to ${\mathrm{Xe}}^{45+}$ by 14.7-keV electrons. They are ≊2 to 4 times higher than predicted by usually accepted empirical estimations. We have also measured the energy loss versus emerging charge state. For hyperchanneled ${\mathrm{Xe}}^{\mathrm{Q}+}$ ions, the stopping power depends only on the mean (and not on the actually sampled) density of valence electrons and compares well with the prediction of the electron gas model.

The extreme ultraviolet photoabsorption spectrum of a laser-produced lanthanum plasma has been recorded and found to contain a number of discrete features in the 130-eV region. These have been analyzed as 4${\mathrm{d}}^{10}$→4${\mathrm{d}}^9$nf,np transitions in ${\mathrm{La}}^{3+}$. We show that the 4f transition, which is expected to be the strongest, is not in evidence. The reason is that this resonance, after the collapse of the 4f wave function, has a large autoionization width. We conclude that the 4f orbital in ${\mathrm{Ba}}^{2+}$ is only partially collapsed, which settles a long-standing discussion of this point.

Cross sections for projectile K-shell ionization were measured for 0.75–3.5-MeV/N ${\mathrm{C}}^{5+}$ and ${\mathrm{O}}^{7+}$ projectiles in collisions with ${\mathrm{H}}_2$ and He targets. The experimental results agree with plane-wave Born-approximation calculations which take into account the interaction between projectile and target electrons. We demonstrate that for energies where the target electrons have sufficient kinetic energy in the projectile frame to ionize the projectile electron, the electron-electron interaction can lead to a significant increase in the total ionization cross section.

Diffusion of muonic deuterium (μd) atomd in ${\mathrm{D}}_2$ at 300 K was studied at pressures of 0.1–10 bars by measuring the distributions of time intervals between entry of negative muons into the gas and detection of photons emitted upon the arrival of μd atoms at foils spaced along the muon beam axis. Results indicate an approximately Maxwellian velocity distribution with a mean energy of 1.8±0.1 eV for the μd atoms, and μd scattering cross sections in agreement with theory.

We report the first observation of an excited state of ${\mathrm{Li}}_3$. With an origin band at 14 583 ${\mathrm{cm}}^{\mathrm{-}1}$, this state has been identified as $2^2$E’. Analysis based on the dynamic Jahn-Teller model reveals very weak distortion and localization characteristics (${\mathrm{E}}_{\mathrm{stab}=57\mathrm{}}$ ${\mathrm{cm}}^{\mathrm{-}1}$, ${\mathrm{E}}_{\mathrm{b}}$=17 ${\mathrm{cm}}^{\mathrm{-}1}$, ${\mathrm{\omega }}_0$=191 ${\mathrm{cm}}^{\mathrm{-}1}$). The lowest vibronic state is located above the conical intersection, implying a nearly ${\mathrm{D}}_{3\mathrm{h}}$ geometry. The frequency of the completely symmetric mode is 326 ${\mathrm{cm}}^{\mathrm{-}1}$. Analyses of rotational states and Franck-Condon factors are also presented.

We employ the concepts of band theory to describe the behavior of electromagnetic waves in three dimensionally periodic face-centered-cubic (fcc) dielectric structures. This can produce a ‘‘photonic band gap’’ in which optical modes, spontaneous emission, and zero-point fluctuations are all absent. In the course of a broad experimental survey, we have found that most fcc dielectric structures have ‘‘semimetallic’’ band structure. Nevertheless, we have identified one particular dielectric ‘‘crystal’’ which actually has a ‘‘photonic band gap.’’ This dielectric structure, consisting of 86% empty space, requires a refractive index contrast greater than 3 to 1, which happens to be readily obtainable in semiconductor materials.

A model for subcritical parity-breaking bifurcation of a periodic pattern is introduced. It is found that nucleated regions of the new asymmetric state propagate with a well-defined velocity in a direction determined by their parity, and leave in their wake a pattern with an altered wavelength. Successive passage of these ‘‘parity bubbles’’ enables the system to relax to a selected wavelength. The possible relevance of these findings to recent observations of ‘‘solitary modes’’ in directional solidification and hydrodynamic experiments is discussed.

We report a study of a fingering instability which occurs during the spreading of a spinning drop. We measure the time evolution of the drop profile, the critical radius for instability onset, and the fastest growing instability wavelength. The experimental results are compatible with the predictions of lubrication theory when the centrifugal effect is not too large compared to the capillary one.