The analyzing power $A_N$ in inclusive ${\pi }^0$ production has been measured with use of the new 185-GeV/c Fermilab polarized proton beam. We obtain the value $A_N=0.10\mathrm{}\pm 0.03$ for ${\pi }^0\text{'}s$ in the kinematic region $0.2<\mathrm{}{x}_F<0.8\mathrm{}$ and $0.3<\mathrm{}{p}_T<1.2\mathrm{}$ GeV/c. In certain models of particle production this suggests that the spin of the proton is carried by its valence quarks.

We have measured the differential cross sections for coherent Compton photon scattering in the reaction ${}_{}{}^4{}_{}{}^{}\mathrm{He}(\gamma ,\gamma ){}_{}{}^4{}_{}{}^{}\mathrm{He}$ at laboratory angles of 24°, 30°, 45°, and 60° with an average laboratory photon energy of 320 MeV, at 22° with 358 MeV, and at 30° with 260 MeV. These measurements are the first unambiguous test of the $\Delta$-hole formalism for this reaction near the peak of the cross section for the $\Delta$ resonance. The results are compared to theoretical calculations in the isobar-hole model. Agreement is good for data at the energy corresponding to the $\Delta$ peak.

A macroscopic approach to giant dipole resonances (GDR's) in hot rotating nuclei is presented. It is based on the Landau theory of nuclear shape transitions and provides a unified description of thermal fluctuations in all quadrupole shape degrees of freedom. With all parameters fixed by the zero-temperature nuclear properties the theory shows a very good agreement with existing GDR measurements in hot nuclei. The sensitivity of the GDR peak to the shape of hot nuclei is critically examined. Low-temperature experimental results in Er show clear evidence for changes in the nuclear energy surface, while higher-temperature results are dominated by the fluctuations.

In the microscopic approach the decay constant of cluster radioactivity is determined by the preformation probability for the open channel multiplied with the Gamov penetrability. The preformation probability is found to possess a simple mass dependence on the emitted cluster. This observation leads to a formula for order-of-magnitude estimates of absolute decay constants. The estimates are in excellent agreement with available experimental data. Predictions for as-yet unmeasured decay rates are made.

Modulations of the intensity of two-pulse photon echoes due to ultrasonic waves are described for alexandrite and ruby. Enhancements (up to a factor of 8) of the echo intensity are observed. The main features of the echo intensity changes are well described by a model in which the ultrasonic waves modulate the optical transition frequencies of the echo-producing ${\mathrm{Cr}}^{3+}$ ions effecting a spatially dependent phase accumulation which compensates for the phase mismatch.

Circular atoms, i.e., Rydberg atoms with maximum angular momentum, are semiclassical atoms of great interest for Rydberg constant metrology and QED tests. Following a suggestion by Delande and Gay, we have prepared circular atoms by a new method, involving the adiabatic switching of crossed electric and magnetic fields. The method, which is demonstrated for Rydberg states of lithium (principal quantum number $n=20\mathrm{} \mathrm{to} 25$, is applicable over a wide variety of experiment conditions.

We investigate the effect of temporal modulation on the spatial patterns produced by a spatially extended system undergoing a Hopf bifurcation. It is shown that physically this modulation can stabilize standing waves, which would otherwise be stable to traveling waves. Mathematically a codimension-3 point is naturally introduced which encompasses a Takens-Bogdanov codimension-2 point with vanishing Hopf frequency. Experimentally this setup is attractive since the three relevant parameters are easily accessible and the steady bifurcation can be made forward as well as backward near a codimension-2 point.

The steady-state behavior of two-level atoms, driven by a strong field and relaxing into a frequency-dependent vacuum reservoir, has been studied in an experiment involving Ba atoms in a confocal cavity. Enhancement or suppression of the atomic excitation is observed when the cavity is tuned to either of the strong-field fluorescence sidebands. These effects result from a redistribution of the population among the dressed atom-field states induced by the cavity-modified vacuum field. The enhancement effect may be useful in achieving significant steady-state inversion.

The influence of quenched disorder on the critical behavior of superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ is studied by confinement of the helium to the pores of three different porous media: Vycor, xerogel, and aerogel glasses. In each case a well defined power-law behavior for the superfluid density is observed as the transition temperature is approached. In Vycor the superfluid critical exponent is bulklike, whereas strikingly different critical exponents are seen in the other two media.

We report the first observation of a sharp heat-capacity signature related to the superfluid transition in thin ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films adsorbed on porous Vycor and xerogel glasses. The transition temperatures of these films range from 0.14 to 1.1 K. This new feature is found in addition to the broad peak centered at a higher temperature reported in a number of earlier studies.

A theoretical and computer simulation analysis suggests that two collective modes can propagate in liquid water for wave vectors $k>\mathrm{}0.64\mathrm{}$ ${\mathrm{\AA }}^{-1\mathrm{}}$. The low-frequency mode (ordinary sound) turns out to be associated with the oxygen density fluctuations, whereas the hydrogen density fluctuations are found to be governed by both modes, thus indicating that the high-frequency collective motion is related to fast librational oscillations. This allows a simply physical interpretation of the existing experimental and computer simulation data.

We present neutron and x-ray measurements of the diffuse scattering from single-grain samples of the icosahedral phase of Al-Li-Cu. The x-ray measurements were analyzed to determine the integrated intensity of the diffuse scattering relative to the sharp diffraction peaks, as well as the length scale of the correlations which produce the diffuse scattering. The neutron data show that these correlations are static rather than dynamic in nature. In addition, we present a simple model based on the disordered packing of icosahedral clusters of atoms, which reproduces the unusual ringlike nature of this scattering. Quantitatively, this model overestimates the degree of disorder present in these alloys.

The reconstruction of large and complex unit cells from x-ray or neutron diffraction data is time consuming. If we can represent the unit cell by a set of pixels, or Ising spins, then a rapid reconstruction is possible, with use of the method of simulated annealing. This Letter discusses the requirements on the scattering density for this method to succeed in the case of very large and highly disordered unit cells. It is shown that experimental errors and contributions of delocalized electrons must be less than 1% in order to avoid a "spin-glass" ground state.

An exactly solvable model of three-dimensional wetting is described which has a logarithmically divergent specific-heat singularity.

It is proposed that surfaces of crystals which reconstruct with broken orientational symmetry and exhibit an anisotropic intrinsic stress tensor are unstable to the formation of elastic-stress domains. Thus the ground state of such a surface is not uniform, contrary to previous expectations. Evidence of this for the Si(001) surface is discussed.

We present calculations, based on the embedded-atom method of phonon energies and polarizations on the clean Cu(100) surface. The atomic relaxations and the dynamical matrix are obtained, without fitting to surface properties, in one consistent calculation which is no more computationally demanding then pair potentials. Excellent agreement with experiment is obtained. We find that the force constants are qualitatively different from those obtained by fitting to two-body central-potential models, reflecting the many-body nature of the method and suggesting that fits based on central-potential models may be misleading.

Perturbed $\gamma -\gamma$ angular correlation experiments reveal an unexpected dynamical behavior of ${}_{}{}^{111}{}_{}{}^{}\mathrm{In}$ atoms in a trivacancy complex in irridium. The measured quadrupole interaction is static at 12 K but becomes strongly time dependent around 100 K resulting in a typical damping of the interaction pattern. At 293 K again a static interaction of different magnitude is observed. This behavior can be described by a jump motion of the indium atom between a stable position in the center of the <111> three-vacancy triangle at low temperatures and the surrounding vacant sites at high temperatures.

We present evidence of off-diagonal long-range order in the incompressible fractional quantum Hall states at $\nu =\frac{1}{3} \mathrm{and} \frac{2}{5}$ Landau-level fillings. We construct correlation functions that exhibit this property only for incompressible quantum-Hall-effect states. In a compressible state no off-diagonal long-range order is observed.

An unusual photocurrent multiplication effect at the $n$-silicon/electrolyte interface which gives rise to quantum efficiencies approaching 400% has been investigated by intensity-modulated photocurrent spectroscopy. This technique has shown that the process involves hole capture followed by the successive injection of three electrons by surface intermediates. Kinetic analysis of this effect shows that the rate constants for electron injection are unusually low at between 1 and ${10}^3$ ${\mathrm{s}}^{-1\mathrm{}}$.

The valence-band-offset controversy in HgTe/CdTe superlattices can be simply resolved. It is shown that, while the superlattice becomes semimetallic with increasing valence-band offset, it reverts to semiconducting behavior as the offset is increased yet further. The observed electron-cyclotron mass and the band gap can be better explained for the offset of 350 meV measured by photoemission than for the smaller offset ∼40 meV which coincidentally is also in fair agreement with the magneto-optical data.