A partial-wave analysis of recent low-energy pion-absorption data on a diproton (${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$) in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ is supplemented by proton-neutron final-state-interaction phase shifts and mixing parameters, yielding a model-independent determination of the absorption amplitude. Several possible amplitudes describe the existing angular distribution, but these can essentially be distinguished by a polarization measurement.

We have searched for 180° (c.m.) correlated equal-energy two-photon decays produced in 6-MeV/nucleon U + Th collisions. In the summed-energy region between 1.5 and 1.8 MeV, we set an upper limit of 3 × ${10}^{-10\mathrm{}}$ decay per projectile for the yield integrated over a target thickness of 3.6 mg/${\mathrm{cm}}^2$, which corresponds to a cross section of 3 × ${10}^{-29\mathrm{}}$ ${\mathrm{cm}}^2$ averaged over the target. This can be compared to a production cross section of (1-2) × ${10}^{-28\mathrm{}}$ ${\mathrm{cm}}^2$, averaged over a target thickness of 0.3-0.6 mg/${\mathrm{cm}}^2$ found by others for correlated electron-positron pairs in the same reaction.

For the reaction ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}(\pi ,{\pi }^{\prime }){}_{}{}^{12}{}_{}{}^{}\mathrm{C}_{}^{*}{}_{}{}^{}(2^{+},4.44\mathrm{} \mathrm{MeV})$, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}_{}^{*}{}_{}{}^{}\to {}_{}{}^{12}{}_{}{}^{}\mathrm{C}(\mathrm{g}.\mathrm{s}.\mathrm{})+\gamma$, pion-$\gamma$ angular correlations have been measured in and out of the reaction plane. The data taken at a pion momentum transfer of $q=0.47\mathrm{}$ ${\mathrm{fm}}^{-1\mathrm{}}$ for incident kinetic energies of 116, 140, 162, 180, and 226 MeV confirm $\Delta$-hole model predictions, in which the $\Delta$ propagation in the nucleus is explicitly taken into account.

The first ion-atom-collision data obtained with antiprotons are presented. We measured the single- and double-ionization cross section for 0.5-5-MeV antiprotons and protons colliding with helium. For ion energies above ∼ 2 MeV, the single-ionization cross section is the same for protons and antiprotons. However, surprisingly, the double-ionization cross section for antiprotons is approximately a factor of 2 larger than that for protons. The present data constitute a challenge for future theoretical models of charged-particle-atom collisions.

It is demonstrated that Doppler-free two-photon polarization spectroscopy makes possible Stark profile measurements without Doppler-broadened background under high-density and high-temperature conditions, e.g., in dense arc plasmas, where other sub-Doppler laser-spectroscopic techniques introduced so far fail. This method, based on the polarization rotation connected with a two-photon transition, is not limited by collisions changing the velocity or the excitation of the atoms or by photoionization of the upper level.

The Stark-broadened line profile observed with Doppler-free two-photon polarization spectroscopy of the hydrogen resonance line ${\mathrm{L}}_{\alpha }$ in a plasma is investigated theoretically. With use of computer simulations for the numerical evaluation of line profiles, strong ion dynamical effects in the line core are found at moderate plasma densities.

It is shown that a broad class of two-dimensional vortices occurring in the flow of an incompressible, inviscid fluid are unstable to three-dimensional perturbations. At short wavelengths along the vortex axis the growth rate becomes independent of wavelength, and the eigenmode becomes concentrated near the center of the vortex.

A theory is presented for Pierrehumbert's three-dimensional short-wave inviscid instability of the simple two-dimensional elliptical flow with velocity field $\mathbf{u}\mathrm{}(x,y,z)=\Omega (-Ey,E^{-1\mathrm{}}x,0)$. The fundamental modes, which are also exact solutions of the nonlinear equations, are plane waves whose wave vector rotates elliptically around the $z$ axis with period $\frac{2\pi }{\Omega }$. The growth rates are the exponents of a matrix Floquet problem, and agree with those calculated by Pierrehumbert.

Tunneling times for absorptive optical bistability with a squeezed vacuum as input to the cavity are calculated. Using a squeezed input field to reduce the fluctuations in the critical quadrature, we show that the tunneling times may be increased. This may substantially improve the intrinsic stability of the device.

The intensity pattern generated by a monochromatic point source in a random medium is studied. The intensity-intensity correlation function is calculated and it is shown that the intensity, as a function of coordinate, exhibits large fluctuations (the speckle pattern). The sensitivity of this speckle pattern to small changes in the source frequency is also studied.

We have substantially increased the output power and extraction efficiency of a free-electron laser operating at 34.6 GHz by tapering the wiggler magnetic field. In the exponential-gain regime, the laser exhibited a measured gain of 34 dB/m. With a 50-kW input signal, the amplifier saturated in 1.3 m with a 180-MW output signal. By using a taper that brought the magnetic field at the end of the wiggler down to 45% of its initial (peak) value, we increased the output signal to 1.0 GW. This corresponds to an extraction efficiency of 34%.

A three-dimensional inhomogeneous magnetohydrodynamic equation is investigated numerically. It is found that a spatially ordered structure is formed at a saturated energy level by the excitation of the convective cell modes due to the three-dimensional nonlinear mode coupling. It is shown that the saturated energy level and radial plasma flux are proportional to the magnitude of the background density gradient. The experimental observation of a self-organized coherent structure in a finite-$\beta$ collisional edge plasma is expected.

We present and analyze a new class of continuous phase transitions to incommensurate structures. This class is characterized by a small order parameter and diverging susceptibility, in common with instability-type transitions. However, the fundamental wave vector vanishes at the transition and the harmonic components of the order parameter are not small compared with the fundamental component, in common with nucleation-type transitions. These properties are shown to result from a gradient-cubic term in the free energy.

The critical behavior of the wetting transition of a two-dimensional solid-on-solid model with a random substrate is calculated exactly. By resumming of the most divergent diagrams of a field-theoretical version of the model, the disorder is found to be marginal and to lead to logarithmic corrections to the critical behavior of the pure system.

A microscopic model of mixed crystals with orientational disorder and translation-rotation coupling is proposed. The substitutional halogens generate random strain fields which couple to the orientational modes. At large fields, the ferroelastic phase transition is suppressed. The anomalous behavior of acoustic phonons is studied. The central peak is obtained as a static phenomenon. Qualitative agreement with a number of experimental results is obtained.

We have measured the transition from partial to complete wetting of Pyrex capillaries in a critical mixture of 2,6-lutidine and water with different amounts of dissolved potassium chloride. With increasing concentration of KCl the difference between the critical and wetting temperatures decreases. At a concentration of 7.3 × ${10}^{-2\mathrm{}}$ mol/l KCl we observe only partial wetting and can thus determine the difference between the critical exponent of the order parameter, ${\beta }_1$, at the (Pyrex) boundary, and $\beta$, in the bulk, from the capillary rise close to $T_c$. The result ${\beta }_1=0.83\mathrm{}\pm 0.05$ is in good agreement with theoretical predictions.

We have performed tunneling measurements on ultrathin, homogeneous, strong-coupled superconducting Pb films in the regime where $T_c$ varies with sheet resistance. The energy gap $\Delta$ and the strong-coupling corrections to the density of states have been measured as functions of the sheet resistance $R_{\square }$. We conclude that the standard picture of enhanced Coulomb repulsion with increasing sheet resistance is too naive and that, in the presence of disorder, an analysis more sophisticated than Eliashberg theory is necessary.

Differential resistance measurements of Nb${\mathrm{Se}}_3$-insulator-lead tunnel junctions as a function of dc bias voltage are reported. The amplitude of the charge-density-wave gap, $2\Delta$, which occurs in Nb${\mathrm{Se}}_3$ at $T_2=59\mathrm{}$ K is found equal to 70 ± 5 meV. This value decreases when pressure is applied but the ratio $\frac{2\Delta }{k{T}_2}\sim 14$ is pressure independent up to 2.5 kbars. This large magnitude of the charge-density-wave gap, at variance with the BCS value, is interpreted as resulting from the strong-coupling nature of electron-phonon interactions.

The residual energy, $\epsilon \left(\tau \right)$, left after cooling to zero temperature in a finite time $\tau$ is analyzed for various disordered systems, including spin-glasses and random-field magnets. We argue that the generic behavior for such frustrated systems is $\epsilon \left(\tau \right)\sim {\left(\ln \tau \right)}^{-\zeta }$ for large $\tau$, with the exponent $\zeta$ depending on the system. This result is dominated in some cases by a distribution of classical two-level systems with low excitation energies, and in other cases by large-scale nonequilibrium effects.

We have directly observed fluctuating dipolar clusters in the mixed crystal K${\mathrm{Ta}}_{0.991}$ ${\mathrm{Nb}}_{0.009}$${\mathrm{O}}_3$ using inelastic-light-scattering techniques for temperatures between 1.8 and 25 K and for electric fields up to 2 kV/cm. Together with earlier dielectric-relaxation data, our data span over nine decades in frequency and obey a Vogel-Fulcher relaxation model over the temperature range 6-15 K with an extrapolated transition at 3.0 K to a glassy dipolar state.