We observed two distinct peaks, 205-MeV/c ${\pi }^{\mathrm{-}}$ and 235-MeV/c ${\mu }^{\mathrm{-}}$, associated with $K_{\pi 2}^{\mathrm{-}}$ and $K_{\mu 2}^{\mathrm{-}}$ decays at rest, respectively, from negative kaons stopped in liquid helium. These peaks were found to be delayed with respect to the stopping $K^{\mathrm{-}}$, showing that stopped $K^{\mathrm{-}}$ mesons of about 2% fraction are trapped in metastable states with an overall lifetime of about 40 nsec. This observation provides a direct evidence for Condo’s trapping hypothesis for the at-rest decay components of $K^{\mathrm{-}}$ and ${\pi }^{\mathrm{-}}$ in liquid helium.

For the first time, Lamb shifts of the 2 ${}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$-2 ${}_{}{}^3{}_{}{}^{}\mathrm{P}_0^{}{}_{}{}^{}$,1,2 transitions in helium have been measured directly with an LNA laser. The absolute wavelengths have been measured to a precision of 1 part in ${10}^8$, or ${10}^{\mathrm{-}4}$ ${\mathrm{cm}}^{\mathrm{-}1}$. The Lamb shifts have been determined to 6 parts in ${10}^4$. Our results agree well with previous, indirect measurements, but represent a fivefold improvement in precision. The experimental results are compared with recent precision calculations. The obtained experimental Lamb shifts are 1 order of magnitude more precise than current theory.

Intense laser light filamentation in a hydrogen plasma is investigated using a two-dimensional hydrodynamic simulation code. The laser paraxial wave equation is used to study the focusing which results from a Gaussian hot-spot intensity perturbation. It is found that diffraction increases the scale size of small scale off-axis filamentation, and significantly raises the ponderomotive self-focusing intensity threshold. Radially propagating density fluctuations, contributing to small scale focusing, are observed due to the self-consistent treatment of hydrodynamics in the simulations.

Path-integral Monte Carlo calculations have been used to study ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ clusters at low temperatures. We develop a fluctuation formula for the superfluid fraction in terms of a projected area swept out by a path. Manifestations of superfluid behavior are shown to exist in a cluster of 64 atoms and a remnant of the λ transition persists in a cluster of 128 atoms. The temperature dependence of the superfluid fraction is similar to that observed in the liquid.

A first-principles global theory of unstable thermodynamic systems is developed which properly describes the growth kinetics of quenched systems from early through late times and includes sharp interfaces in the late stage description.

Accurate measurements on the nuclear-spin waves observed in the NMR spectrum of spin-polarized atomic hydrogen provide a quantitative test of theory. Results for temperatures between 160 and 537 mK and densities between 7×${10}^{15}$ and 5×${10}^{16}$ ${\mathrm{cm}}^{\mathrm{-}3}$ display the expected magnitude and $T^{\mathrm{-}1/2}$ divergence predicted for the spin-wave quality factor μ. It is shown that spin waves persist at low enough densities (Knudsen regime) that elastic wall collisions are more probable than atom-atom exchange collisions. An anomalous increase in spin-wave widths at low temperatures is explained in terms of the enhanced effects of sticking collisions of the atoms with the walls.

We calculate the temperature and normal fluid velocity of superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ between parallel plates in the presence of a stationary heat flow normal to the plates. The system is modeled by a Landau-Boltzmann equation and a diffuse scattering mechanism at the boundaries. In the hydrodynamic regime the temperature jump at the wall turns out to be small. The three Onsager surface coefficients proposed recently by Grabinski and Liu are determined. In the Knudsen regime the thermal boundary resistance is found to increase exponentially with decreasing temperature.

The effect of external fields on the structural and electronic properties of Ag(110) is investigated using local-density-functional theory. We find that Ag(110) undergoes a (1×2) missing row reconstruction as an excess charge of ≃0.05 electron per surface atom is added onto the surface. Our result supports an electron-donation mechanism for the alkali-metals-induced reconstruction on the (110) surface of the 3d and 4d fcc metals. The surface interlayer spacings are found to be insensitive to the applied field in the absence of field-induced surface reconstruction.

A clear Pb(110) surface roughening phase transition has been observed at ${\mathrm{T}}_{\mathrm{r}}$∼415 K, which is ∼185 K below the bulk melting temperature, using the high-resolution low-energy electron diffraction technique. It is shown that the height-height correlation function diverges logarithmically at ${\mathrm{T}}_{\mathrm{r}}$. The measured value of the roughening transition temperature agrees remarkably well with recent molecular-dynamics calculations. Evidence of surface melting at about 520 K is also seen from the anomalous behavior in the diffraction intensity.

We have measured negative secondary-ion yields from phenylalanine, CsI, and Au bombarded by complex ions of 5–28 keV. The primary ions were organic monomer and dimer ions and clusters of CsI. Large enhancements occur in the measured secondary-ion yields for more complex projectiles. We show that secondary-ion yields of ${\mathrm{Au}}^{\mathrm{-}}$ atomic species are related to the square of the projectile momentum.

We have observed dramatic changes in the time distribution of synchrotron x rays resonantly scattered from ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ nuclei in a crystal of yttrium iron garnet, which depend on the deviation angle of the incident radiation from the Bragg angle. These changes are caused by small shifts in the effective energies of the hyperfine-split nuclear resonances, an effect of dynamical diffraction for the coherently excited nuclei in the crystal. The very high brightness of the synchro- tron x-ray source allows this effect to be observed in a 15-min measurement.

Inelastic-light-scattering measurements show that exchange Coulomb interactions in the two-dimensional electron gas of GaAs microstructures are more important than previously anticipated. Small-wave-vector spectra from modulation-doped quantum wells exhibit unexpected single-particle intersubband transitions in addition to collective spin-density and charge-density modes. From the measured spectral energies the direct and exchange intersubband Coulomb interactions are determined and found to be of comparable strengths.

We have measured the electron intersubband relaxation time due to the emission of longitudinal-optical (LO) phonons in GaAs-AlGaAs quantum wells using time-resolved anti-Stokes Raman scattering. When the energy separation of the subbands is greater than the LO phonon energy we find the lifetime of electrons in the n=2 level to be ≤1 ps which is in excellent agreement with theoretical calculations of two-dimensional electron-phonon interactions.

In optical experiments with laterally patterned GaAs quantum wells we observe confinement of free carriers to motion in one dimension. Optical recombination of band-gap transitions shows that free electrons and holes are confined in spatially indirect type-II multiple quantum wires. Resonant-inelastic-light-scattering spectra show intersubband excitations of the one-dimensional electron gas. From the optical measurements we obtain the Fermi energy as well as subband spacing and determine the linear free-electron density.

Variable frequency ferromagnetic resonance (FMR) has been used to directly observe the coupled resonance modes in single-crystal Fe/Cr/Fe(001) sandwiches grown by molecular-beam epitaxy. Magnetization M and magnetoresistance measurements also were carried out on these samples, which exhibited antiferromagnetic (AF) layer alignment for 12 Å<t(Cr)<25 Å. The FMR data reveal two resonance modes with complex frequency dependences for the AF-aligned samples. The detailed FMR and M vs H behavior can be quantitatively explained by an AF coupling parameter J which has a thickness dependence peaked about t(Cr)=16 Å.

Several Fe(110)/Ag(111) superlattices with the Fe component 2 monolayers thick and the Ag component of varying thickness were produced by molecular-beam epitaxy. The Curie temperature was found to be higher than 500 K and a central feature in the Mössbauer spectra is shown to come from thermal magnetic relaxation of Fe islands. The magnetic hyperfine field has a linear temperature dependence and this is explained by a simple model based on the island structure of the Fe.

Using our earlier thermodynamic force field we have calculated the equilibrium structures of medium-size ${\mathrm{Si}}_{\mathrm{n}}$ clusters (n=1–25). We find the very surprising result that beginning at n=7, near n=13 and near n=19, these clusters follow a pentagonal growth pattern (icosahedra plus associated face capping). Our results are fully consistent with magic numbers recently found in the reaction rates for addition of first ${\mathrm{C}}_2$${\mathrm{H}}_4$ molecules to ${\mathrm{Si}}_{\mathrm{n}}^{+}$ clusters.

It is shown that a very small asymmetric ellipicity of the pulsar discovered in SN 1987A could cause cyclic appearance and disappearance of the signals due to the slow rotation about the body symmetry axis which could carry the magnetic axis out of our line of sight. Taking account of gravitational-radiation damping, eccentricities that are consistent with the measured small rate of change of the millisecond period would yield a period for the disappearances ranging from about an hour to the age of the Universe. We also examine precession as a possible explanation of the observed eight-hour modulation of the millisecond pulses. While a sinusoidal frequency modulation of the correct period can be found, the amplitude of the modulation is much too small compared to observation.

A search was made for heavy stable charged particles produced in 1.8-TeV proton-antiproton collisions. No such particles were found in 26.2 ${\mathrm{nb}}^{\mathrm{-}1}$ of data. Cross-section limits are presented and mass limits of the order of 100 GeV are set for particles containing excited quarks in higher color representations.

CP asymmetries in neutral-B decays to CP eigenstates are studied in the standard model. Whereas usually one assumes a single decay amplitude which induces CP violation via B-B¯ mixing, we investigate additional effects due to two interfering decay amplitudes. We estimate these effects in characteristic cases and suggest ways to experimentally distinguish between these two sources of asymmetries. The effects, which show up as time-integrated asymmetries at symmetric $e^{+}$$e^{\mathrm{-}}$ colliders operating at the Υ(4S), are quite small in Kobayashi-Maskawa- (KM) allowed decays such as as $B_d^0$→$K_S$ψ and become large in KM-suppressed decays.