By an exact calculation on the planar Ising model we show that for all subcritical temperatures, the bubble model predicts the correct average distribution of matter contributing to the pair-spin correlation function in the two-particle sector. This considerably substantiates the bubble model which is generalized to higher dimensions with an external field h and then used to show that the free energy has an essential singularity in h of Andreev-Fisher form at all points along the phase boundary. We argue that such a singularity would also occur in the Baxter model.

We consider a compact source of gravitational waves of frequency ω, in or near a massive spherically symmetric distribution of matter or a black hole. Recent calculations have led to apparently contradictory results for the influence of the massive body on the propagation of the waves. We show here that the results are in fact consistent and in agreement with the ‘‘standard’’ viewpoint in which the high-frequency compact source produces the radiation as if in a flat background, and the background curvature affects the propagation of these waves.

Graviational instantons, solutions to the Euclidean Einstein equations, with toplogy ${\mathit{R}}^3$×${\mathit{S}}^1$ arise naturally in finite-temperature quantum gravity. It is shown here that all such instantons must have the same asymptotic structure as the Schwarzschild instanton. From this follows that if the Ricci tensor of such a manifold is non-negative it must be flat. Hence there is no nontrivial vacuum gravitational instanton on ${\mathit{R}}^3$×${\mathit{S}}^1$. This places a significant restriction on the instabilities of hot flat space. Another consequence is that any static vacuum Lorentzian Kaluza-Klein solution is flat.

The smallest known three-dimensional closed manifold of curvature k≡-1 was discovered a few years ago by Weeks. This kind of manifold is constructed from a hyperbolic polyhedron with faces pairwise identified. Here it is used as the comoving spatial section of a Friedmann cosmological model, in the spirit of Ellis and Schreiber’s idea of small universes. Its nontrivial global topology has the effect of producing multiple images of single cosmic sources, and this is the basis of an attempt to solve a famous controversy about the redshifts of quasars.

We study bosonic string theory in a gravitational background. We show that either left movers or right movers are the only background-independent nonperturbative solutions of the field equations for an arbitrary static metric. They are stable and have a conserved topological charge, therefore being topological solitons. The action vanishes for these solutions and hence they provide the dominating contribution in a path integral quantization.

Critical dynamics constrains models of dynamical electroweak symmetry breaking in which the scale of high-energy physics is far above 1 TeV. A big hierarchy requires the high-energy theory to have a second-order chiral phase transition, near which the theory is described by a low-energy effective Lagrangian with composite ‘‘Higgs’’ scalars. As scalar theories with more than one ${\mathrm{\Phi }}^4$ coupling can have a Coleman-Weinberg instability and a first-order transition, such dynamical EWSB models cannot always support a large hierarchy. If the large-${\mathit{N}}_{\mathit{c}}$ Nambu–Jona-Lasinio model is a good approximation to the top-condensate and strong extended technicolor models, they will not produce acceptable EWSB.

We have observed 729±15 ${\mathit{K}}_{\mathit{L}}$→${\mathrm{\pi }}^{\pm }$${\mathrm{\pi }}^0$${\mathit{e}}^{\mp }$ν¯(ν) decays in the data set collected in the 1987–1988 run of Fermilab experiment E731. We found the ${\mathit{K}}_{\mathit{e}4}$ branching ratio to be [5.16±0.20(stat)±0.22(sys)]×${10}^{\mathrm{-}5}$. We also performed the first measurement of the form factors parametrizing the decay. Our branching ratio result agrees with a prediction using a vector-meson-exchange model. An interpretation of the data in terms of chiral perturbation theory implies a value ${\mathit{L}}_3$=(3.4±0.4)×${10}^{\mathrm{-}3}$ in the O(${\mathit{p}}^4$) chiral Lagrangian.

Several resonances of s symmetry in the continuum of the muonic molecule dtμ that cluster below the (tμ${)}_{2\mathit{s}}$+d threshold have been analyzed. It has been found that the fusion rates for some of these states are comparable with their Coulomb decay rates, and that the associated sticking probability is approximately 1 order of magnitude lower than that of the ground state. This feature indicates that muon-catalyzed fusion might proceed more efficiently if it were occurring from the resonant states formed from the excited (n=2) states of muonic atoms.

We present high sensitivity electron energy spectra for xenon in a strong 50 ps, 1.053 μm laser field. The above threshold ionization distribution is smoothly decreasing over the entire kinetic energy range (0–30 eV), with no abrupt changes in the slope. This is in direct contrast to the sharp cutoff observed in xenon optical harmonic generation spectra. Calculations using the single active electron approximation show excellent agreement with the observed electron distributions. These results directly address the unresolved relationship between the electron and photon emission from an atom in an intense field.

We have observed unusual second- and fourth-order interference patterns when entangled photons with center-wavelength differences ≊40 nm are mixed together at single or dual Mach-Zehnder interferometers. The difference frequency of the photons appears directly in the spectrum of the coincidence rate. This phase-sensitive behavior can reveal center-frequency differences from zero to hundreds of THz.

The gain-guided nature of an amplifier can lead to more noise in the output than would be predicted based on the quantum noise limit of 1 photon per mode. We report the results of an experiment which demonstrate that the gain-guided nature of a focused-gain Raman amplifier can lead to excess noise in the amplified field. The results are compared with a theoretical analysis of the output of an amplifier which incorporates the effects of gain guiding as well as the focused nature of the gain profile.

The energy spectrum for a tight binding model with on-site potentials forming a Cantor set is seen to have a globally self-similar structure, and an exact correlation of the wave functions with the energies is pointed out. With periodic driving the quasienergy spectrum is obtained from an equivalent lattice problem and is seen to imply nonrecurrent time evolution of states. A model system with a discrete spectrum shows similar behavior. Implications for the problem of microwave ionization of hydrogen atoms are indicated.

Recently, ultranarrow resonances have been observed in the diamagnetic Rydberg spectrum of the lithium atom. We show that they originate from an accidental destructive interference, which, in turn, is due to the chaotic character of the classical behavior. Our numerical simulations show that their statistical properties are well described by a random-matrix model.

It is shown that a gas composed of inelastically colliding particles is unstable to the formation of high density clusters. A possible physical mechanism underlying this instability is proposed. A theoretical analysis, based on the Jenkins-Richman equations, as well as a numerical simulation of the dynamics of an unforced system of hard disks in a periodic rectangular enclosure, renders support to the proposed mechanism. In particular, a simple formula for the characteristic intercluster distance is derived and found to be in agreement with the numerical results. Applications to granular systems of engineering interest as well as to astrophysics are briefly outlined.

An accelerator concept is outlined which enables ${10}^{15}$ to ${10}^{18}$ ions in the form of a charge neutralized ion ring to be accelerated to GeV energies. A repetition rate of 10 Hz will deliver an average current in the range of 0.1 A.

A theory is given for the nonlinear dynamical evolution of the m=1 instability, governed by a generalized Ohm’s law that includes the Hall term. An island equation is derived for the entire nonlinear evolution which includes an almost explosive growth phase followed by a rapid decay phase. The predictions of the theory are compared with recent numerical results and experiments.

Direct observations of a Pt adatom migrating on the Pt(001) plane in the presence of an externally applied electric field show that a positive field raises, and a negative field lowers, the activation barrier for the adatom–substrate-atom exchange process. At field strengths of 1.5 V/Å and above, inhibition of the exchange mechanism is sufficient to change the diffusion mode from exclusively exchange displacements to a mode that includes hops. The effect of the electric field is explained by an electron-transfer process which changes the bonding of the adatom to neighboring surface atoms.

Neutron scattering experiments on ${\mathrm{Fe}}_{3\left(1\mathrm{-}\mathrm{\delta }\right)}$${\mathrm{O}}_4$ single crystals show that, for δ=0, the well established additional Bragg peaks occur at (h,0,1+1/2), below ${\mathit{T}}_{\mathit{V}}$=122.5 K with abrupt disappearance of the diffuse planar scattering associated with 1D correlations along the cubic axis. For δ=0.006, the superlattice peaks never become resolution limited, long-range order is not attained, and the diffuse scattering increases continuously with cooling. This composition exceeds the percolation limit beyond which the transition is inhibited by insufficient average charge per ordered unit cell.

We have studied theoretically the conditions for the occurrence of spontaneous off-center atomic displacements in pure zinc-blende semiconductors. A pseudo Jahn-Teller coupling of a chemically active valence d band with an s-like conduction band is predicted to lead to such a metastability. All-electron density functional calculations confirm that this is the case in CuCl. The unusual experimental manifestations of this metastability are outlined.

Small voids are formed in Si by MeV He implantation and annealing. We measure the equilibrium shape of these voids and hence extract the surface energy curve γ(θ) for Si. γ(111) is the global minimum, with γ(100)≊1.1γ(111) and all other cusps on the surface being relatively small. The experimental γ(θ) is compared with theoretical predictions and earlier experiments. Step energies obtained from dγ/dθ are ≊28±10 meV/atom on (100) and ≊140±20 meV/atom on (111); these values are compared with scanning tunneling microscope experiments.