The neutron lifetime has been measured by counting the neutrons remaining in a fluid-walled bottle as a function of the duration of storage. Losses of neutrons caused by the wall reflections are eliminated by varying the bottle volume-to-surface ratio. The result obtained is ${\tau }_{\beta }$=887.6±3 s.

Motivated by the recently reported discrepancy between measured and theoretical values for the orthopositronium lifetime, we have searched for exotic decay of orthopositronium into γ plus a long-lived neutral particle X, in the hitherto unexplored mass region of $m_x$ below 100 keV. Our negative result provides new upper limits of 6.4×${10}^{\mathrm{-}5}$ to 7.6×${10}^{\mathrm{-}6}$ on the branching ratio of such decay. Consequently we exclude the hypothesis of Samuel, i.e., that a new pseudoscalar particle with mass less than 5.7 keV is responsible for the discrepancy of the orthopositronium lifetime.

The triple-differential cross sections for elastic breakup of 156-MeV ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ measured at larger relative momenta of the outgoing α-particle and deuteron fragments exhibit conspicuous double-peaked structures in the momentum-transfer dependence. The origin of these features is explored, sensitively revealing interference and off-shell scattering effects in the fragment-target interaction. Elastic breakup data of 56-MeV deuterons and 90-MeV ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ are reanalyzed in view of these effects on the basis of the prior-form distorted-wave Born-approximation theory.

Nucleon-nucleus optical potentials are calculated using full off-shell nucleon-nucleon free t matrices based on the Paris potential. Applications to proton scattering from ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ at 200 and 300 MeV are presented. Significant differences are found between observables calculated using full-folding potentials and tρ approximations to them, demonstrating the importance of an accurate treatment of off-shell effects. The agreement between calculated and measured observables improves substantially using the full-folding model.

Compton polarimeters have been used to measure the linear polarization of resonantly scattered photons. In the deformed nucleus ${}_{}{}^{150}{}_{}{}^{}\mathrm{Nd}$ the first direct and model-independent evidence has been obtained that in a cluster of enhanced dipole excitations near 3 MeV the three strongest transitions are of the same, positive parity.

Laser-cooled sodium atoms pushed up on a vertical trajectory by radiation pressure are observed to turn around due to gravity. The reatively long time the atoms spent freely falling in this ‘‘atomic fountain’’ allowed the ground-state hyperfine splitting to be measured with a linewidth of 2 Hz. After a 1000-s integration time, the center of the line was resolved to ±10 mHz. The absolute splitting was measured to be 1 771 626 129(2) Hz.

The interfacial composition profile bwtween two polymer phases at temperatures below the upper critical solution point has been measured directly, using an ion-beam method based on nuclear reaction analysis. The interface width grows with time to a finite limiting value. The variation of the limiting interfacial width as the critical temperature is approached from below is in quantitative accord with mean-field theories.

Rotation of superfluid ${}_{}{}^3{}_{}{}^{}\mathit{B}$ in an magnetic field enhances the coupling of the nonzero ${\mathrm{m}}_{\mathrm{J}}$ substrates of the real squashing collective mode to the zero sound, and the fivefold line splitting becomes observable even when H is parallel to Ω and to the direction of sound propagation. Equilibrium vortex lattices and vortex-free states can be distinguished by their characteristic absorption spectra. The dependence of the sound attenuation on the angular velocity in magnetic fields up to 32 mT is reported; the data are qualitatively compared with theory.

We present two simple, but accurate, techniques for calculating the finite-temperature atomic structure and free energy of any solid defect (point or extended). The finite-temperature equilibrium atomic structure and thermodynamic properties of defects in solids are obtained self-consistently by minimizing the rree energy of the solid with respect to the coordinates of the atoms. Application of the two methods to a perfect crystal and one with a vacancy show that both methods yield excellent agreement with Monte Carlo calculations for temperatures up to at least 75% of the melting point.

The mobility of vacancies and interstitials trapped on a dissociated edge dislocation in copper is studied by molecular dynamics. Although fast diffusion is thought to occur exclusively in a pipe surrounding the dislocation core, in the present study a quasi-two-dimensional diffusion is observed for defects not only in the cores but also in the stacking fault ribbon. Contrary to current assumptions, the activation energy for diffusion is found to be identical for both defects, which may therefore comparably contribute to mass transport along the dislocations.

We have investigated the role of surface-active species (surfactants) in heteroepitaxial growth. In general, the growth mode is determined by the balance between surface, interface, and film free energies. Thus, if A wets B, B will not wet A. Any attempt at growing an A/B/A heterostructure must overcome this fundamental obstacle. We propose the use of a segregating surfactant to reduce the surface free energies of A and B and suppress island formation, as demonstrated in the growth of Si/Ge/Si(001) with a monolayer of As. Control of growth by amnipulation of surface energetics provides a new avenue to achieve high-quality man-made microstructures against thermodynamic odds.

Using quantitative chemical mapping, we study the low-temperature interdiffusion of HgCdTe/CdTe multilayers, sampling volumes 14 orders of magnitude smaller than previously needed for such measurements. Our results show interdiffusion to be strongly depth dependent and nonlinear, and allow the direct determination of thermodynamic parameters of interest at individual interfaces. The high spatial resolution of our technique establishes that the stability of a layer can depend sensitively on its depth from the surface.

We describe a structural determination of pseudomorphic Si on Ge(001) by means of x-ray-photoelectron diffraction. By comparing the measured angular coordinates of the forward-scattering-induced [011] peak with those calculated by means of single-scattering theory, we find that the lattice constant perpendicular to the interface is 5.34±0.04 Å. This value exceeds that predicted by elastic theory by 0.08 Å.

Resonant optical second-harmonic and sum-frequency generation are applied to probe electronic transitions at the Ca-terminated epi- taxial ${\mathrm{CaF}}_2$/Si(111) interface. A band gap of 2.4 eV is established for the interface states, a value twice as large as that in bulk Si, but only (1/5 of the band gap in ${\mathrm{CaF}}_2$. The experimental three-wave-mixing spectra can be modeled by a two-dimensional band gap and a narrow resonance 150 meV below the band edge, the latter being tentatively assigned to a transition to a bound two-dimensional exciton.

The specific heat C of uncompensated phosphorus-doped silicon with P concentration N between 0.34 and 7.3×${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$, i.e., in the vicinity of the metal-insulator transition, has been measured over a large range of temperature (0.04 K≤T≤3 K), allowing the unambiguous detection of an anomalous contribution ΔC∼$T^{\alpha }$, with α becoming negative for small N. This is attributed to exchange-coupled clusters. The magnetic-field dependence of C (up to 5.7 T) allows us to deduce the relative contributions of localized and delocalized electrons and gives evidence for interactions and correlations between these.

We demonstrate that optical dephasing in disordered semiconductors does not require quasiparticle interaction but can be solely caused by disorder. We show that localization effects strongly influence the optical-dephasing signal by calculating the nonlinear polarization, taking into account disorder nonperturbatively. Decay times of the order of fs to ps can be expected depending on the ratio of diagonal disorder and intersite coupling. It follows that dephasing experiments performed on disordered semiconductors cannot be analyzed simply in terms of quasiparticle interactions.

Initiating Auger transitions using tunable monochromatic x rays from synchrotron, we show that the satellite intensities in $L_2$$M_{45}$$M_{45}$ and $L_3$$M_{45}$$M_{45}$ spectra of Cu and Zn increase with increasing photon energies up to about 200 eV above the thresholds. This indicates a pronounced influence of the history of the core-hole generation on Auger spectra, in sharp contrast to the universally accepted belief, and suggests a breakdown of the sudden approximation. Details of the photon energy dependence of the satellite intensity provide a new interpretation for the origin of the satellite.

The spin-1 Ising model with bilinear (J) and biquadratic (K) nearest-neighbor exchange interactions and a single-ion potential (Δ), a model introduced by Blume, Emery, and Griffiths (BEG), is studied on a honeycomb lattice. The exact solution of the BEG model is obtained for a certain relation K=K(J). Also an approximate treatment of the BEG model outside the surface, in the space of coupling constants, given by the relation above is discussed. Finally a digression is made on the critical temperature of the spin-1 Ising model.

We present a simple theoretical description of recently measured giant magnetoresistance effects in Fe/Cr layered structures. The resistivity is calculated by solving the Boltzmann transport equation with spin-dependent scattering at the interfaces. The magnitude of the effect depends on the ratio of the layer thickness to the mean free path and on the asymmetry in scattering for spin-up and spin-down electrons. Good agreement with experiment is found for both sandwich structures and superlattices.

The magnetization orientations in domain walls at the surfaces of an Fe crystal, a ferromagnetic glass, and a Permalloy film, measured by scanning electron microscopy with polarization analysis, exhibit asymmetric surface Néel wall profiles which (1) are at least twice as wide as interior Bloch walls in bulk and (2) are described quantitatively by our micromagnetic calculations without assuming any special surface parameters. Misinterpretation of domain-wall widths, Bitter patterns, and magnetic-force-microscopy images can result from overlooking the extreme effect of the surface on magnetic microstructure.