The vacuum decay rate, ${\lambda }_{\mathrm{T}}$, of orthopositronium (o-Ps), formed in a gas, has been measured to be ${\lambda }_{\mathrm{T}}$=7.0516±0.0013 μ${\mathrm{s}}^{\mathrm{-}1}$. This result, which is 4 times more accurate than any previous measurement, is ten standard deviations above the theoretical value. Extensive systematic tests included measurement of ${\lambda }_{\mathrm{T}}$ in 3 different gases, each to an accuracy of at least ±0.0025 μ${\mathrm{s}}^{\mathrm{-}1}$ (350 ppm), and the use of two entirely different digital timing systems.

We present experimental measurements of the velocity of a convective vortex front propagating into an unstable conductive state in a Rayleigh-Bénard system for the wide range of ε (ε≡ΔT/Δ${\mathrm{T}}_{\mathrm{c}}$-1) between 4×${10}^{\mathrm{-}4}$ and 2.5×${10}^{\mathrm{-}1}$. The results are found to be in excellent quantitatitve agreement with the theoretical predictions following from an amplitude equation, exhibiting a sharp selection mechanism in the propagation velocity. Wave-number selection by this dynamical mechanism is also studied.

Using a Stark-effect diagnostic, we have explored a new regime of relativistic beam-plasma turbulence, 〈${\mathrm{E}}^2$〉>${\mathrm{nT}}_{\mathrm{e}}$. Measurements of E and of the plasma frequency used nonperturbing optical techniques during the electron-beam pulse. Fields up to 150 kV/cm display an exponential spectrum, exp(-${\mathrm{E}}^2$/${\mathrm{E}}_0^2$) with ${\mathrm{E}}_0$=85 kV/cm. This spectrum does not resemble that given by strong-turbulence theory, and involves stronger fields than are dealt with in current theory. ’

We report measurements of the thermal conductivity of helium in the region from 6×${10}^{\mathrm{-}8}$ to 6×${10}^{\mathrm{-}4}$ K above the lambda point at the vapor pressure. Primarily because of uncertainties in the Kapitza boundary resistance, reliable results were obtained only in the region from 6×${10}^{\mathrm{-}7}$ to 6×${10}^{\mathrm{-}4}$ K above the transition. Within 5×${10}^{\mathrm{-}6}$ K of the transition we find conductivity values significantly lower than expected on the basis of theoretical extrapolations from previous experiments. When the parameter ${\mathrm{R}}_{\mathrm{K}}$ is calculated, we find a maximum about 2×${10}^{\mathrm{-}6}$ K above the transition, in disagreement with the predictions of the two-loop dynamic renormalization-group theory. The uncertainties in our results in this region are discussed in detail.

Concepts based on Si—H bond breakup (normal strength ∼3.5 eV) or weakly bonded H have not accounted satisfactorily for many low-temperature phenomena: rapid decrease in the spin signal in the range 25–300?deC while the Si—H bond density is conserved or decreases; early H evolution stage at 200–450?deC and H diffuision, both with an activation energy of only 1.5 eV, etc. An elegant microscopic explanation of these and other phenomena is given in terms of a novel mechanism that is unique to the amorphous state: defect-mediated H diffusion during which the defects are annihilated.

Results of the first completely ab initio investigation of the microscopic structure of a grain boundary in a semiconductor are presented. By use of the molecular-dynamics–simulated annealing method for performing total-energy calculations within the local-density–functional and pseudopotential approximation, the Σ=5 (001) twist boundary in germanium is studied. A number of rotation-and-translation states are investigated leading to a prediction for the structure of this geometry. Evidence for the possible presence of novel defects and glasslike tunneling mode states at grain boundaries is presented.

We have studied the temperature dependence and the sensitivity to oxygen adsorption of the spin polarization of low-energy secondary electrons from Ni(110). Both types of measurements show a strong surface sensitivity, suggesting a mean magnetic probing depth of only three to four atomic layers, contrary to estimates based on the universal escape-depth curve.

The interaction and the formation of bound states of magnetoexcitons in two-dimensional structures with a fully filled Landau level is investigated. A rich spectrum of bound states with a substantial binding energy -$e^2$/εl with respect to the cyclotron energy 2ħ${\omega }_c$ is found. The dispersion of these states as a function of the total momentum is studied.

The static magnetic susceptibility of ultrafine Mg particles with various diameters of less than 10 nm has been measured in the temperature range from 2 to 240 K. We show for the first time that the temperature-independent paramagnetism is enhanced for smaller particles relative to bulk materials. This finding is compatible with the recent study on Os multinuclear cluster compounds that revealed a size-dependent paramagnetic enchancement. The coexistence of the quantum size effect and the enhancement of paramagnetism has also been observed at low temperatures.

We report time-resolved induced-absorption measurements near the ${\mathrm{Z}}_3$-exciton resonance in CuCl platelets which were excited at the biexciton resonance by laser pulses of 30-ps duration. We find that the excitons created through biexciton decay induce exponential tails on their absorption line. This effect can be explained in terms of exciton phase-space filling if quantum fluctuations in the local exciton density are taken into account.

It was recently argued that, contrary to what has been universally believed, single-particle theory is not adequate to describe the optical spectra of metals and that electron-hole (excitonic) effects play a dominant role. Here, we present theoretical calculations using two different conceptual frameworks which indicate that the opposite is in fact true. We also identify the inappropriate approximations that exaggerated the role of excitonic effects and conclude that the latter play only a minimal role in the optical spectra of metals.

A realistic tight-binding theory, based on the first-principles energy-band results of Mattheiss, is used to study the electron-phonon interaction in the new high-${\mathrm{T}}_{\mathrm{c}}$ superconductors ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$(Ba,Sr${)}_{\mathrm{x}}$${\mathrm{CuO}}_4$. The calculated results include the phonon dispersion curves, phonon linewidths, the Eliashberg function ${\mathrm{\alpha }}^2$(ω)F(ω), and the interaction constant λ. A very strong couplin of specific O phonons to the conduction electrons is found (λ≥2), leading—because of the light oxygen mass—to ${\mathrm{T}}_{\mathrm{c}}$ values of ∼30-40 K.

The observation of the change in the ionospheric density profile and the creation of a density cavity at the critical height by high-power radio waves is made by the time-of-flight radar scattering technique. Small-scale cavitons imbedded inside a large caviton are inferred from the suppression of the photoelectron-enhanced plasma lines near the critical height.

We describe the selection mechanism for novel pattern formation in a driven chain of nonlinear oscillators. Phase organization arises as a consequence of dynamical selection of minimally stable states.

Experimental results are presented which provide the first measurement of the effects of a gravitational field on charged particles, equivalent to the red shift for photons. Two Josephson-effect batteries (V≃300 μV) having a vertical separation of 7.2 cm are connected in opposition by superconducting wires. A voltage difference of 2.35×${10}^{\mathrm{-}21}$ V is maintained between these batteries by means of the gravitational red shift. The emf around this loop is, however, measured to be less than 1×${10}^{\mathrm{-}22}$ V, consistent with the predicted invariance of the gravito-electrochemical potential along the wires.

Nonanalyticities in the generalized dimensions of multifractal sets of physical interest are interpreted as phase transitions. The problem is mapped onto thermodynamics of one-dimensional spin models. The spin Hamiltonians are explicitly constructed and their phase transitions discussed. This mapping can provide insight in both directions.

We present a general sum rule in the Born approximation governing neutrino scattering from local potentials in a crystal. This relation facilitates the placing of stringent bounds on the force exerted on a crystal by a flux of neutrinos. The force is found to depend linearly on the number of scattering centers and no exotic coherent effects are predicted.

The total intensity averaged over all angles of short-wavelength radiation either scattered or spontaneously emitted nearly isotropically from each of a system of N radiators or scatterers is shown to vary linearly with N for large N. Although coherent effects can give intensities proportional to ${\mathrm{N}}^2$ in certain directions (e.g., Bragg angles), this enhancement is mainly at the expense of radiation otherwise emitted in other directions and does not appear in the overall angular distribution.

Under the assumption of the Maiani-Parisi-Petronzio scheme of nonperturbative grand unification, the low-energy couplings of the standard model with n≥8 generations are close to their infrared-stable fixed-point values, and we determine them using analytical expansion methods. For n=9, we find ${\sin }^2$${\mathrm{theta}}_{\mathrm{W}}$=0.193, ${\mathrm{m}}_{\mathrm{H}}$=100 GeV, ${\mathrm{m}}_{\mathrm{top}}$=134 GeV, and ${\mathrm{m}}_{\mathrm{quarks}}$=74 GeV.

The astonishing smoothness of the superdeformed rotational spectrum in ${}_{}{}^{152}{}_{}{}^{}\mathrm{Dy}$, from I=22 to I=60, is used as a basis for inferring a liquid-to-solid transition as the centrifugal force drives the nuclear shape into a closed-shell configuration.