We have measured with 0.5-psec resolution the reshaping of small-area 7-psec laser pulses to $0\pi$ pulses due to their passage through an optically thick sodium cell. The reshaped pulses have a picosecond oscillatory structure characteristic of $0\pi$ pulses that extends over more than 100 psec. These observations are compared in detail with the predictions of theory and good agreement is obtained

Transverse outputs of an optical resonator are predicted to undergo transitions to turbulence via three well-established scenarios. The instabilities are induced by self-focusing nonlinearities. A bifurcation involving period doubling of invariant circles in a parameter region where attractors coexist is identified.

We demonstrate a new all-optical, frequency-multiplexed, and fast technique for gas-phase ultrasonic absorption spectroscopy. A pulsed laser produces a short-duration acoustic pulse in the gas sample, and the acoustic pulse profiles at two distances are monitored by focused continuous probe beams. Fourier decompositions of these probe deflection signals provide the absorption spectrum, and examples for C${\mathrm{O}}_2$ and a C${\mathrm{O}}_2$+${\mathrm{H}}_2$O mixture are given. Ultrasonic propagation speeds in the megahertz regime are also obtained for several gases.

The perpendicular heating in cyclotron waves tends to pile up the resonant particles toward the low magnetic field side with their banana tips localized to the resonant surface. A poloidal electric field with an $\overrightarrow{\mathrm{E}}\times \overrightarrow{\mathrm{B}}$ drift comparable to the ion vertical drift in a toroidal magnetic field may result. With the assumption of anomalous electron and neoclassical ion transport, density variations due to wave heating are discussed.

Bonds between neighboring atoms in a solid are imperfect harmonic oscillators. Through consideration of the nonlinearities, it is shown that vacancy-interstitial defect pairs can be described in terms of phonon dynamical variables. The effective spring constants are determined by self-consistency requirements. An inhomogeneous solution is found to the equations of the self-consistent spring constants; it describes an atom breaking its bonds to its neighbors, resulting in a defect pair.

Inelastic incoherent neutron spectra below 4.1 THz have been obtained for the $\sqrt{3}\times 4$ "herringbone" and $\sqrt{3}\times \sqrt{3}$ "on-end" monolayer phases of ethane adsorbed on graphite basal planes. One-phonon cross sections calculated independently with empirical atom-atom potentials make possible identification of the collective vibrational modes observed and reproduce qualitatively the rather different spectra of the two phases.

Using magic-angle rotation it has been possible to obtain specific high resolution of the narrow component of the classical NMR proton spectra of amorphous Si: H. Two different lines could be observed in samples prepared by reactive sputtering, the most important one being attributed to the protons of mobile hydrogen molecules, and the other to protons of Si-H bonds diluted in the amorphous structure.

The thermal conductivity of high-quality polymer single crystals has been measured for the first time. The expected conductivity maximum observed in good single crystals was absent and replaced by a weak ($\sim T^{\frac{1}{2}}$) temperature dependence between 2 and 40 K. This anomalous behavior is attributed to scattering of acoustic phonons by low-energy optical phonons with a coupling matrix element of 40 ${\mathrm{cm}}^{-1\mathrm{}}$.

The phase diagram of quantized vortices in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-$B$ has been obtained with two techniques. Our NMR measurements show that magnetization displays a pressure-dependent first-order phase change at $T_V\mathrm{}\left(P\right)\mathrm{}$. Our gyroscopic experiments reveal a transition curve $T_G\mathrm{}\left(P\right)\mathrm{}$ which marks an abrupt change in the critical velocity. Both curves show a weak magnetic field dependence, run similarly in the ($P$, $T$) plane, and seem to mark the same vortex-core transition.

Shear waves are shown to propagate in the two-dimensional electron solid on liquid helium. Their velocity and damping are measured and used to deduce the shear components of the viscoelastic tensor up to melting. A linear temperature variation at low temperatures contrasts with the premelting region, where the sharper renormalization of the elastic component is accompanied by a very rapid increase in the viscous component.

We have used small-angle neutron scattering to study surfactant interactions in an inverted micellar and a related microemulsion system. The structure-factor fits imply an attractive interaction between micelles with a short-ranged potential due to the overlapping of the surfactant hydrocarbon tails. The linear increase in this interaction with size accounts for the critical transition observed in the microemulsion. This analysis is confirmed by Monte Carlo simulations.

A method is developed for analyzing the outer part of the small-angle x-ray or neutron scattering curve for porous scatterers in which the pore boundaries can be described by fractals. When the results are applied to the scattering data from a lignite coal, the fractal dimension of the boundary surface of the pores in this coal is found to be 2.56 ±0.03.

We have measured both ultraviolet luminescence and ejection of atomic argon from solid argon films electronically excited by megaelectronvolt light ions. The two phenomena reflect (respectively) the radiative and the nonradiative parts of the energy release during electronic recombination and deexcitation. The measurements can be correlated by a model of diffusion of ionic excitons followed by formation and decay of self-trapped excimers.

We report donor-acceptor-pair line luminescence from Na-doped ZnSe. The luminescence is "type II," and analysis gives an energy of 20 ±2 meV for the donor. From the spectrum type and the energy value we conclude that the donor must be an alkali-metal (Na or Li) interstitial ion. Prior evidence for such species in II-VI compounds has been circumstantial, and no reliable energy values have been available. Moreover, the magnitude of the energy value raises serious questions on the theory of shallow levels in compound semiconductors.

Magnetophonon resonance at high electric fields in thin (1 to 9 μm) $n^{+}$ $n{n}^{+}$ GaAs structures reveals a new mechanism of magnetoconduction, in which an electron in the $n\mathrm{th}$ Landau level can scatter elastically or quasielastically into the $\mathrm{}(n+1\mathrm{})\mathrm{}\mathrm{th}$ level.

We report on the experimental study of the behavior of unoccupied exchange-split bands in ferromagnets on approaching the Curie temperature. Using spin-polarized isochromat spectroscopy, we observe minority and majority states near the $H$ point in bcc Fe to merge together with increasing temperature, providing evidence for a "collapsing" band state. The flat minority band, generating most of the empty density of states, is sampled at two other points in $k$ space by use of different crystal surfaces. At these locations the position of the peak remains unchanged, indicating the existence of "noncollapsing" band states.

We present a simple derivation of the exponential band tails universally observed in three-dimensional disordered materials. The physical picture employed is that of Halperin and Lax in which states are localized by long-wavelength potential fluctuations. When the effect of the small-scale fluctuations of the potential are included as well, via the scaling arguments of Thouless, there results an exponential dependence of the density of states on energy below an energy $E_1$. The magnitudes and dependences of $E_1$ and the width of the tail on disorder agree with the experiment.

We study the angular dependence of the field-cooled susceptibility ${\chi }_{\mathrm{eq}}$ of ${\mathrm{Fe}}_2$Ti${\mathrm{O}}_5$, a spin-glass with a strong uniaxial anisotropy, by rotating the measuring field $\overrightarrow{\mathrm{H}}$ with respect to the cooling field ${\overrightarrow{\mathrm{H}}}_c$. Unlike the behavior found in classical spin-glasses (i) the anisotropy in ${\chi }_{\mathrm{eq}}$ depends on the direction of ${\overrightarrow{\mathrm{H}}}_c$ and (ii) the irreversible magnetization does not rotate with $\overrightarrow{\mathrm{H}}$. The de Almeida-Thouless line is identified via the field and temperature dependence of the anisotropy in ${\chi }_{\mathrm{eq}}$.

Observational tests of the hypothesis that the universe is flat and dominated by dark matter in the form of massive photinos include the production of significant fluxes of cosmic rays and gamma rays in our galactic halo. Specification of the cosmological photino density and the masses of scalar quarks and leptons determines the present annihilation rate. The predicted number of low-energy cosmic-ray antiprotons is comparable to the observed flux.

The vacuum expectation value of the stress-energy tensor for the Hartle-Hawking state in Schwarzschild space-time has been calculated for the conformal scalar field. $〈T_{\mu }^{\nu }〉$ separates naturally into the sum of two terms. The first coincides with an approximate expression suggested by Page. The second term is a "remainder" that may be evaluated numerically. The total expression is in good qualitative agreement with Page's approximation. These results are at variance with earlier results by Fawcett whose error is explained.