Synchrotron x-ray diffraction has been used to study the surface reconstruction of W(001) through its phase transition at around 230 K. We find that, although the superlattice peak intensity decreases a thousand fold over the range studied, the integrated intensity, and hence the structure factor, is nearly constant. This indicates that the surface undergoes an order-disorder transition with little change in the magnitude of the atomic displacements. The critical behavior is compared with predictions of the 2D XY and Ising models.

A high-resolution neutron scattering study of ${\mathrm{Ni}}_{62.5}$${\mathrm{Al}}_{37.5}$ shows that the martensitic transformation is associated with a homogeneous distortion and a nearly seven-fold modulation of the cubic lattice which is predicted by an observed phonon anomaly above $T_M$. In the low-temperature 7R phase a nonuniform spacing of superlattice reflections and unusual line shapes are observed which are believed to arise from stacking faults.

We study the variations of the electrical conductivity σ with thickness d of ultrathin metallic films. In the limit ξ${\mathrm{k}}_{\mathrm{F}}$≪1, where ξ is the correlation length describing the film surface roughness and ${\mathrm{k}}_{\mathrm{F}}$ is the electron Fermi wave vector, we show that σ follows a universal law, σ∼${\mathrm{d}}^{2.3}$, independent of any adjustable parameter. This law accounts for recent experimental data on ${\mathrm{CoSi}}_2$ down to d=10 Å. Moreover, the measurements of σ are well fitted when we introduce in its theoretical expression the values of the surface roughness parameters recently estimated from electron microscopy.

The behavior of band bending in nonreactive interfaces between thin ${\mathrm{CsO}}_{\mathrm{x}}$ films and GaAs(110) is studied by photoemission. Successive cesium and oxygen exposures change the ${\mathrm{CsO}}_{\mathrm{x}}$ stoichiometry in a way that leads to a back-and-forth switching of overlayer metallicity and band bending. For both n- and p-GaAs(110), oxygen-rich nonmetallic overlayers create almost flat-band conditions, while cesium-rich metallic overlayers result in Fermi-level positions close to midgap. These observations demonstrate the dominant influence of metal-induced gap states in nonreactive metal-semiconductor interfaces.

We study the continuum limit of the nonlinear σ model in 2+1 dimensions and at finite temperature T, using Monte Carlo simulation on large lattices. Even though the lattice spacing vanishes, dimensional transmutation occurs which makes the correlation lengths finite. With the assumption that the σ model and the spin-(1/2 antiferromagnetic Heisenberg model are equivalent at low T, we make contact between the two models and find that the latter model must order at T=0. Our results are consistent with neutron-scattering experiments done on ${\mathrm{La}}_2$${\mathrm{CuO}}_4$. .AE

The operation of a high harmonic gyro-traveling-wave-tube amplifier which is based on the synchronous interaction of a rotating beam of large orbit, axis-encircling electrons with a ${\mathrm{TE}}_{\mathrm{n}1}$ cylindrical waveguide mode is described. Principal results include amplification of the ${\mathrm{TE}}_{81}$ mode at the eighth harmonic of the cyclotron frequency mode with a small signal gain of 10 dB, an instantaneous interaction bandwidth of 4.3%, and a saturated power transfer of 0.5 kW.

In this Letter we study the evolution of a two-dimensional soap froth. We present experimental evidence for the existence of a limiting scale-invariant distribution of the number of sides in a bulk froth at long times. We also discuss the apparent failure of the hypothesis that all internal angles in a froth are 120° and the absence of many-sided bubbles at long times. We also suggest a mechanism to explain the observed exponent describing the rate of area growth.

Using formal arguments based on conformal invariance and on the connection between correlated-site percolation and the q-state Potts model with vacancies, we show that the exponents describing Ising clusters at Onsager’s critical point are those of the tricritical q=1 Potts model. This implies, in particular, a fractal dimension d¯=(187/96 and a percolative susceptibility exponent γ=(91/48, in good agreement with existing numerical estimates. This d¯ is also clearly supported by a new very accurate Monte Carlo finite-size scaling determination. We also conjecture an exponent ${\mathrm{y}}_{\mathrm{J}}$=(13/24 controlling the crossover between clusters and droplets.

It is shown that generically cosmic strings will interact with matter via their pure gauge potential. The cross section is purely geometrical, and does not vanish in the limit of zero string size. The mechanism is the Aharonov-Bohm effect, which is already known to cause scattering of charged particles off an infinitesimally thin solenoid. A related mechanism for particle production is also discussed.

The possibility is investigated that the baryon asymmetry of the Universe might have been created at the electroweak phase transition. It is shown that there exists a process which may in certain models with multiple Higgs doublets generate a numerically acceptable asymmetry. This process involves sphalerons and requires strong CP violation at temperatures larger than 1 TeV.

CP phenomenology in ‘‘seesaw’’ models of quark masses is shown to parallel that of the usual left-right-symmetric models with the additional advantage that it provides a natural solution to the strong CP problem. For the case where the third-generation mixing parameter $V_{\mathrm{ub}}$ is extremely small, the neutral-Higgs-boson interactions lead to ε’/ε≃${10}^{\mathrm{-}3}$ and the electric dipole moment of the neutron $d_n$≃${10}^{\mathrm{-}25}$ e cm. Smallness of the neutrino masses is understood as a two-loop effect.

Noncompact quantum electrodynamics in three Euclidean dimensions with N species of four-component Dirac fermions is simulated by lattice-gauge-theory techniques. On an $8^3$ lattice we find chiral-symmetry breaking in the continuum limit for N≤$N_c$ but no symmetry breaking for N≥$N_c$, with $N_c$=3.5±0.5. A physical picture of our results is presented.

A new Monte Carlo dynamics is proposed for ${\mathrm{\phi }}^4$ field theory that reduces critical slowing down. Discrete Ising variables are embedded into the field theory, so that large-scale tunneling events can be induced via a modified Swendsen-Wang algorithm with fluctuating site-bond percolation probabilities. The measured dynamical critical exponents ${\mathrm{z}}_{\mathrm{EL}=0.07\mathrm{}\pm }$0.07, 0.29±0.09, and 0.87±0.20 in one, two, and three dimensions, respectively, are consistent with our conjecture that this embedded dynamics lies in the same universality class as the Swendsen-Wang Ising dynamics.

We have searched for narrow resonance peaks in the process $e^{+}$$e^{\mathrm{-}}$→γγ, utilizing a ${}_{}{}^{22}{}_{}{}^{}\mathrm{Na}$ ${\beta }^{+}$ source and a pair of Ge detectors to detect the final two γ’s. No statistically significant peak is observed at the center-of-mass energy of 1062 keV, where the heavy-ion-collision experiment at LBL has seen a peak. The upper limit to the quantity ${\Gamma }_{\mathrm{ee}}$${\Gamma }_{\gamma \gamma }$/Γ is 1.1×${10}^{\mathrm{-}4}$ eV at the 95% confidence level for a scalar or pseudoscalar resonance at 1062 keV/$c^2$. Similar upper limits are obtained in the mass region between 1045 and 1085 keV/$c^2$

We describe a method for determining the four-momentum distribution of nucleons in nuclei which is particularly sensitive to large momenta. It may also prove useful in probing the differences between bound and free nucleons.

The time scale of fission at initial nuclear temperatures of about 5 MeV is deduced from the number of neutrons evaporated prior to and after scission in the reactions ${}_{}{}^{144}{}_{}{}^{}\mathrm{Sm}$ and ${}_{}{}^{154}{}_{}{}^{}+(838\mathrm{}$ MeV) ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$. The prescission lifetime for fission with symmetric mass splits is longer than for asymmetric mass splits. For symmetric mass splits the excitation energy at scission is only about 60 MeV and independent of the initial excitation energy which is consistent with a prescission lifetime in the order of ${10}^{\mathrm{-}20}$ s.

Partial-wave cross sections for fusion of ${}_{}{}^{64}{}_{}{}^{}\mathrm{Zr}$ and ${}_{}{}^{64}{}_{}{}^{}\mathrm{Zr}$ at energies of 5% and 7% above the Bass-model fusion barrier have been measured with the Darmstadt-Heidelberg Crystal Ball. The distribution for fusion with the more neutron-deficient target ${}_{}{}^{92}{}_{}{}^{}\mathrm{Zr}$ exhausts the unitarity limit for lower angular momenta. In contrast, fusion with the slightly more neutron-rich target ${}_{}{}^{96}{}_{}{}^{}\mathrm{Zr}$ yields a broad angular momentum distribution with cross sections well below unitarity even for small partial waves.

We have measured the emitted energy and the spectral and temporal distributions of the first seven harmonics which are self-generated by the Mark III free-electron laser oscillator at Stanford University. The bandwidth scaling shows that current theory will have to be revised. We present our results and discuss the implications for using free-electron lasers as a source of tunable coherent extreme ultraviolet photon beams.

An unexpected two-beam coupling energy exchange has been observed in ${\mathrm{Bi}}_{12}$${\mathrm{SiO}}_{20}$ without normal (electro-optic) photorefractive effects, using circularly polarized light and a frequency shift of a few hertz or using nonparallel linearly polarized light and no frequency shift. Two-beam coupling based on a photoinduced optical-activity phase grating explains this behavior.

We have investigated electron capture and loss into the continuum of projectiles resulting from single ion-atom collisions. It is revealed that characteristic shape parameters of the well-known cusp in the velocity spectrum observed in the forward direction depend markedly on the experimental resolution. This hitherto unobserved behavior can be explained by higher-order effects in the transition amplitudes of the double-differential production cross section.