The theoretically expected directional anisotropy of surface self-diffusion was experimentally verified for a clean Ni(110) surface. The diffusion coefficients were determined by the sinusoidal profile decay technique at 800°C in [001] and [$1\overline{1}0$] directions as 2.2 × ${10}^{-7\mathrm{}}$ ${\mathrm{cm}}^2$ ${\sec }^{-1\mathrm{}}$ and 3.0 × ${10}^{-6\mathrm{}}$ ${\mathrm{cm}}^2$ ${\sec }^{-1\mathrm{}}$, respectively. The cleanliness and structural integrity of the Ni(110) surface were checked with Auger spectroscopy and low-energy electron diffraction, respectively.

The clustering of atoms around slow positrons in helium gases is experimentally studied and the results are interpreted in terms of liquid drop formation. The critical temperature and pressure of the droplet phase are found to be 6.6 K, 5.4 atm in ${}_{}{}^2{}_{}{}^{}\mathrm{He}$ and 8.4 K, 7.6 atm in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. The critical densities coincide with those of the helium liquids.

The theory of mobility of negative ions in superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ is presented. The calculated $B$-phase mobility agrees well with experiment.

When a smetic-$A$ phase is composed of chiral molecules, it exhibits an electroclinic effect, i.e., a direct coupling of molecular tilt to applied field. The pretransitional behavior of the electroclinic effect in the $A$ phase is used to study the critical behavior near the second order, smectic-$A$—smectic-$C$ phase transition. This behavior is measured by monitoring the change in birefringence of a sample as the electroclinic effect causes a tilt of the molecules. A large pretransitional effect is measured, and constants describing the critical behavior are determined.

An aluminum single crystal has been irradiated at 4.6 K with fast neutrons to a Frenkel defect concentration of 810 ppm. Inelastic neutron scattering experiments revealed a clear shift of the [$00\zeta$] transverse phonon frequencies with respect to the annealed crystal. In contrast to theoretical predictions, no obvious resonancelike behavior has been observed in the wave-vector range $0.05<\mathrm{}\zeta <0.19\mathrm{}$ ($\zeta$ is in reduced units).

In charge- or spin-density-wave systems, interaction between Landau levels and manybody effects produces nonlinear de Haas-van Alphen oscillations. This yields, inter alia, frequencies which are difference frequencies between ordinary oscillations with cyclotron mass equal to the sum of the ordinary masses. The new oscillations may have an amplitude as large as 10% of the ordinary ones, but they are in a new region of the spectrum where they can easily be detected. The effect is larger for weakly stable desntiy waves.

The first experimental evidence for distinct spectral and spatial energy transfer in ruby via nonradiative coupling is presented. Monochromatic laser excitation of the $R_1$ line results in a rise of the inhomogeneous background on a millisecond time scale, consistent with a one-phonon—assisted process and contrary to recent theory. However, time-resolved studies of trap fluorescence suggest a rapid, coherent, spatial transfer within the $R_1$ line which does not shift frequency.

The unbroadened Mössbauer resonance from the 13.3-keV level of ${}_{}{}^{73}{}_{}{}^{}\mathrm{Ge}$ is observed. The resonance shows a marked dispersive asymmetry, $2\xi =-9.5\mathrm{}\left(1.9\right)\%$. This is far larger than has been observed for other $E2\mathrm{}$ transitions, but is in agreement with recent calculations of Goldwire and Hannon. The absence of broadening [$2{\Gamma }_{\mathrm{meas}}=6.9\mathrm{}\left(9\right)\mathrm{}$ μm/sec] of this high-resolution ${\frac{5}{2}}^{+}\to {\frac{9}{2}}^{+}$ resonance in a cubic lattice places an upper limit of 28 kHz on possible hexadecapole energy splittings in either nuclear level.

We report the first experimental observation of resonant Brillouin scattering of excitonic polaritons in a semiconductor. The usual symmetric Brillouin doublet becomes an asymmetric multiplet when the incident light energy is scanned through the $n=1\mathrm{}$ exciton resonance and allows a direct measurement of the polariton dispersion curve. In GaAs we determine a longitudinal-transverse splitting of 0.08 ± 0.02 meV and a translational mass $M_{\mathrm{ex}}=(0.6\mathrm{}\pm 0.1)m_0$ for the [100] heavy exciton. The excitions couple preferentially to LA phonons.

The $n$-component Ginzburg-Landau-Wilson model for a semi-infinite system is solved exactly at $T=T_c$ in the limit $n\to \infty$. In the scaling regime the spin-spin correlation function is $G(\overrightarrow{\rho }, z, z^{\prime }, T_c)=\mathrm{const}{\{{[{\rho }^2+{(z-z^{\prime })}^2]}^{-1\mathrm{}}-{[{\rho }^2+{(z+z^{\prime })}^2]}^{-1\mathrm{}}\}}^{\frac{\mathrm{}(d-2\mathrm{})\mathrm{}}{2}}$, for dimensionalities $d$ in the range $2<d<\mathrm{}4\mathrm{}$, where $z$, $z^{\prime }$ are the distances of the two spins from the surface and $\overrightarrow{\mathrm{p}}$ is their separation parallel to the surface. The critical exponents ${\eta }_{\perp }$ and ${\eta }_{\parallel }$ are $\frac{\mathrm{}(d-2\mathrm{})\mathrm{}}{2}$ and ($d-2\mathrm{}$), respectively.

A unified geometric formulation of gravitation and supergravity is presented. The action for these theories is constructed out of the components of the curvature tensor for bundle spaces with four-dimensional Lorentz base manifold and structure groups Sp(4) for gravity and OSp(1,4) for supergravity. The requirement of invariance under reflections, local Lorentz transformations, and general coordinate transformations uniquely determines the action and ensures the existence of local supersymmetry in supergravity.

Double-differential cross sections for the electroproduction of pions of both charges have been measured. We compare the data obtained for production near threshold from ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ and ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ with theoretical treatments employing both shell-model and sum-rule nuclear descriptions with full inclusion of the final-state interaction.

The branching ratio for the reaction ${\pi }^{-}d\to {\pi }^0\mathrm{nn}$ has been measured for the first time by stopping the ${\pi }^{-}$ in a liquid-deuterium target and detecting the two decay $\gamma \text{'}\mathrm{s}$ in two large NaI detectors. A branching ratio $(\mathrm{B}.\mathrm{R}.\mathrm{})=\frac{\omega ({\pi }^{-}d\to {\pi }^{0}nn)}{\omega ({\pi }^{-}d\to \mathrm{all})}$ of (1.45 ± 0.19) × ${10}^{-4\mathrm{}}$ has been measured; the predicted B.R. lies in the range 1.39 × ${10}^{-4\mathrm{}}$ to 1.59 × ${10}^{-4\mathrm{}}$.

The unique high-resolution and high-energy features of the electron-scattering facility at the MIT-Bates Accelerator were used to locate a dominant narrow resonance at 15.045 ± 0.035 MeV in ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$. A spin-parity assignment of $6^{-}$ and an isospin $T=1\mathrm{}$ assignment were made. The $M6\mathrm{}$ form factor was measured and compared to the prediction of a theoretical calculation which uses the open-shell random-phase approximation on a shell-model ground state for ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$.

The $\pi$-nucleus Low equation, under certain assumptions (implicit in conventional multiple-scattering theory), is equivalent to a linear equation. The propagator of this equation includes a factor of ${\left(\frac{z}{H_0}\right)}^2$ which vastly reduces off-shell scattering. As a result, the Kisslinger singularity is eliminated, and the size of the Lorentz-Lorenz effect and local-field corrections is reduced.

Using two coherent time-delayed short pulses, we have demonstrated that one can obtain, in the profile of Doppler-free two-photon resonances, interference fringes with a splitting $\frac{1}{2T}$ ($T$, delay time between the two pulses) much smaller than the spectral width $\frac{1}{\tau }$ of each pulse ($\tau$ duration of each pulse). This method should lead to a number of important improvements in the presently available techniques of ultrahigh-resolution laser spectroscopy of atoms and molecules.

We have studied Doppler-free two-photon excitation of atoms with a train of phase-coherent standing-wave light pulses, originating from the same laser pulse. Quantum interference effects produce narrow spectral fringes which have a physical origin similar to that of Ramsey fringes. Linewidths much less than the Fourier-transform limit of an individual light pulse have been observed for the sodium $3S-5S$ transition; and a dramatic enhancement of the resonant signal is possible.

Superfluorescence has been observed at 3 μm on the $7P$-to-$7S$ transition in atomic cesium after 2-ns excitation of a $6S$-to-$7P$ transition. Both in a magnetic field and in zero field, quantum beats have been observed. The beat frequencies correspond not only to initial-level splittings but also to combinations of initial- and final-level splittings. Superfluorescence beats are therefore basically different from single-atom quantum beats.

We illustrate the usefulness of the extended x-ray absorption fine-structure effect for determining the chemical state, structure, and location of adsorbed species by a study of 0.2 monolayers of bromine adsorbed on graphite at room temperature. Bromine adsorbs as molecules with one atom fixed above the basal-plane hexagonal site while the molecular axis is free to flop around.

The temperature dependence of the intensity and of the correlation time of laser light scattered by the network density fluctuations in polyacrylamide gels is reported. The density fluctuations show critical behavior at - 17°C. This behavior is quantitatively explained as a phase transition of a binary mixture of the covalently cross-linked polymer network and the fluid medium.