The shear modulus of two-dimensional electrons at the surface of liquid helium is deduced from measurements of the finite frequency of the $k\to 0$ limit of the coupled electron-substrate transverse sound mode. Its behavior on the approach to melting is compatible with the Kosterlitz-Thouless model for two-dimensional melting.

Axially channeled electrons are captured into bound states of the "atomic-string" potential. When two rows lie in close proximity as in the $〈110〉$ direction of diamond, the potentials overlap, forming a saddle point between the rows. For 4-MeV electrons in the $〈110〉$ diamond potential, the $2p$ level lies above the saddle point, and molecular states are formed. The observation of radiation arising from transitions of these states is reported. Spectral information is used to deduce electron-density enhancement in the C-C bond.

Large-area, uniform amorphous layers have been formed on Si crystals by means of 2.5-nsec, 347-nm laser pulses. Computations indicate that amorphization at the resolidification interface occurs for isotherm threshold velocities of 18 m/sec on Si(001) and 15 m/sec on Si(111). The amorphous Si is impurity free and structurally similar to conventionally formed material. Below the amorphization velocity thresholds Si(001) reforms good single crystal while Si(111) gives twin defects down to ∼6 m/sec—a new defect transition.

Buckling distortions, widely thought to lower the total energy of semiconductor surfaces, are shown to actually raise the energy of Si(111)-2×1. The $\pi$-bonded-chain reconstruction, in contrast, stabilizes the surface, even relative to recently proposed magnetic reconstructions. Calculations for GaAs(110) reveal that the large charge transfers associated with buckling can stabilize the surface of heteropolar semiconductors, by returning the ions of the bulk to neutral atoms at the surface. These conclusions are based on self-consistent pseudopotential calculations.

The specific heat of the linear-chain magnet ${\mathrm{CsNiF}}_3$ has been measured at temperatures between 4 and 18 K, in magnetic fields up to 8 kG (0.8 T). A rounded peak is observed in both the temperature and field dependence. The peak positions in the field dependence are in qualitative agreement with classical soliton theory and they are also consistent with neutron-scattering estimates of the soliton rest energy. Quantitative comparisons indicate the need for extensions of the theory.

SO(6) is identified as the dynamical group of a mean-field Hamiltonian for the coexistence of superconductivity and charge-density waves. The energy spectrum is given in terms of the Casimir invariants. The explicit rotation of the Hamiltonian to canonical Cartan (diagonal) form is found, and expressions for the superconductivity and charge-density-wave order parameters are given in terms of the rotation angles. The descent from the SO(6) to the decoupled BCS-SO(3) and charge-density-wave-U(2) algebras is exhibited.

Within the framework of standard big-bang cosmology and the standard electroweak model, it is shown that $\nu \overline{\nu }$ annihilation on the $Z$ resonance is the only cosmic-ray process sensitive to relic neutrinos. For massive ($m_{\nu }\gtrsim {10}^{-3\mathrm{}}$ eV) neutrinos originating from a $z\sim 3$ red-shifted source, a 15% to 50% absorption dip is predicted at $E_{\nu }\sim {10}^{11}$ GeV.

It is suggested that monopole-antimonopole pair production initiated by pointlike particles (electrons, quarks) has widely different cross sections in the two cases of "point-like" and composite monopoles, respectively. Production of 't Hooft-Polyakov monopoles is expected to be suppressed by a hugh factor, ≥${10}^{30}$. Furthermore, astrophysical evidence is presented suggesting that monopoles with $m_m\approx {10}^4$ GeV/${\mathit{c}}^2$ cannot be point-like.

From three-prong decays of $\tau$ leptons produced in the MAC detector at PEP, the life-time of the $\tau$ is determined to be (4.9±2.0)×${10}^{-13\mathrm{}}$ sec.

A new high-precision measurement of the asymmetry coefficient, $A_{\beta }$, for the positron decay of polarized ${}_{}{}^{56}{}_{}{}^{}\mathrm{Co}$ is reported: $A_{\beta }=+0.359\mathrm{}\pm 0.009$. If one assumes the $V-A$ form of the weak interaction and time-reversal invariance, the Fermi-Gamow-Teller mixing ratio, $y=\frac{C_V{M}_F}{C_A{M}_{\mathrm{GT}}}$, is then -0.091±0.005. Since $y$ is nonzero and isospin is not conserved, ${}_{}{}^{56}{}_{}{}^{}\mathrm{Co}$ ${\beta }^{+}$ decay is a good candidate for sensitive time-reversal invariance tests of the weak interaction.

The ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}(e, e^{\prime }f)$ cross section was measured for excitations between 5 and 11.7 MeV. The sum of the $E2\mathrm{}$ and $E0\mathrm{}$ strength functions was extracted with the aid of available ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}(\gamma , f)$ data. The present results show that the $\frac{E2\mathrm{}}{E0\mathrm{}}$ strength in the fission channel is spread almost uniformly from 7 to 11.7 MeV. The $\frac{E2\mathrm{}}{E0\mathrm{}}$ strength that is found in the fission channel corresponds to 10% of the isoscalar $E2\mathrm{}$ sum rule; if the fission probability is 0.22, this energy region contains 45% of this sum.

Absorption of blackbody radiation by Rydberg atoms in a resonant cavity is shown to be a collective process in which the atoms behave as a Bose gas. The equilibrium energy of this gas was measured and found equal to twice the photon energy (factor of 2 accounting for atomic transition degeneracy). It is shown that this experiment exhibits the Brownian motion of the atomic-system Bloch vector.

A recent study of high-resolution laser spectroscopy in a fast ${\mathrm{N}}_2$${\mathrm{O}}^{+}$ beam indicates that the hyperfine structure of the $\tilde{A}\mathrm{}\left(100\right)\mathrm{}$ state is mainly due to Fermi contact interaction on the outer nitrogen nucleus. This new spectroscopic result is crucial for the interpretation of the laser predissociation of isotopically substituted ${\mathrm{N}}_2$${\mathrm{O}}^{+}$ with a tandem mass spectrometer. Under such circumstances the fast-ion-beam laser-spectroscopy method yields new information on the bond-breakage mechanism during predissociation.

The hyperfine structure of the $6snd{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$ ($10<~n<~50$) Rydberg states of barium has been measured and analyzed in terms of singlet-triplet mixing. A dispersionlike resonant behavior of the mixing coefficient $\Omega$ was observed for barium and strontium in the vicinity of doubly excited configurations. In barium strong disagreement with multichannel-quantum-defect-theory wave functions was found. The present results are compared with the spin polarization and anisotropy of photoelectrons.

Photoionization spectra of alkali atoms in a dc electric field are calculated semianalytically and agree with experiment. Cross sections depend on two distinct sets of parameters: (a) zero-field dipole matrix elements and quantum defects and (b) eigenfunctions of the external Coulomb-Stark potential. The field's effect on photoionization is condensed into a density-of-states matrix ${〈{\Psi }^{\prime }|\Psi 〉}^{-1\mathrm{}}$, whose use may be appropriate to long-range effects in all spectral phenomena.

An exact renormalization-group transformation is developed which describes how the transition to chaos may occur in a universal manner if the frequency ratio in the quasi-periodic regime is held fixed. The principal low-frequency peaks in an experimental spectrum are universally determined at the transition. Our approach is a natural extension of Kolmogorov-Arnold-Moser theory to strong coupling.

It is pointed out that, although squeezing and sub-Poissonian photon statistics need not go together, in the sense that an electromagnetic field may exhibit one but not the other, the method that is normally used to detect a squeezed state automatically generates sub-Poissonian photon statistics. However, when these considerations are applied to the fluorescence from a coherently driven atom, which exhibits both squeezing and sub-Poisson fluctuations, one finds that the statistics of the emitted photons show even larger departures from classical field theory than the squeezing.

Time-resolved measurements of neutron emission from a theta pinch show that the time of peak emission is delayed relative to the time of the peak driving magnetic field. It is demonstrated that this delay is a manifestation of microturbulent ion heating present in the post-implosion phase. A qualitative model predicts such a delay, while numerical simulations including microturbulent transport predict a delay in quantitative agreement with the measurements.

In contrast to recent theoretical electronic structure calculations and analysis of electron energy-loss (ELS) spectra, surface extended-x-ray-absorption fine-structure measurements establish the local structural equivalence of $p(2\mathrm{}\times 2)$ and $c(2\mathrm{}\times 2)$ oxygen on Ni(100). In both cases the oxygen atoms chemisorb in the fourfold hollow site with an O-Ni bond length of 1.96 ± 0.03 Å (i.e., $d_{\perp }=0.86\mathrm{}\pm 0.07$ Å). In the light of the present results the large ELS frequency shift (14 meV) between the $p(2\mathrm{}\times 2)$ and $c(2\mathrm{}\times 2)$ phases remains a puzzle.

High-precision measurements have been made of the specific heat of pure liquid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ for $8<T<\mathrm{}500\mathrm{}$ mK and for $0<P<\mathrm{}32.5\mathrm{}$ bars. Such measurements have importance because of their strong bearing on our basic understanding of both normal and superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and because they provide a valuable input towards determining an absolute temperature scale at low temperatures. The specific-heat results for $T\lesssim 100$ mK differ significantly from all of the previous measurements on ${}_{}{}^3{}_{}{}^{}\mathrm{He}$. Only the new data, however, satisfy several important thermodynamic checks.