The branching ratio for the Λ weak radiative decay has been measured to be B(Λ→n+γ)/(Λ→anything) =[1.78±0.24(stat)${\pm }_{0.16}^{0.14}$(syst)]×${10}^{\mathrm{-}3}$. A low-energy kaon beam was used to produce the Λ hyperons via the reaction ${\mathit{K}}^{\mathrm{-}}$+p→Λ+${\mathrm{\pi }}^0$ at rest. Photons from the signal channel and ${\mathrm{\pi }}^0$ decay were detected with a NaI(Tl) array. The final spectrum contains 287 events after background subtraction, an order of magnitude more events than from the only previous measurement.

An unusual optimum pair basis exists in the nuclear shell-model wave function for an even system of like particles. This prompts the proposal of a generalized pair mean-field method, in which the desired goal of avoiding the dimensional difficulty in the traditional shell-model approaches may be possible.

We discuss and demonstrate the driving of magnetic resonance transitions between members of a dressed atom doublet. A simple extension of existing dressed atom theory, describes the experimental results reasonably well. We mention low intensity, bare atom interpretations of the experiment and conclude with suggestions for further studies and possible applications.

From measurements of the second virial coefficient by small-angle neutron scattering we have found that the theta temperature of linear polyethylene in dilute solution decreases with increasing molecular mass. This behavior supports polymer theories that incorporate both chain connectivity and three-body interactions. We calculate the three-body interaction parameter within the continuous-curve representation of a flexible polymer, and demonstrate from the same model that our value is consistent with the molecular-mass dependence of the critical precipitation temperature.

A new method for nonlinear polychromatic resonant stimulation of conservative nonlinear oscillators is introduced. As an example we consider a Morse potential that serves as a model for the HF molecule. Numerical results show that a large energy transfer to such a conservative oscillator is possible under optimal stimulation with small driving fields. This makes selective excitation of specific modes possible. The classically determined optimal force was also applied to the corresponding quantum system, with similar results in energy transfer and dissociation.

Laboratory experiments on plane channel flow with a streamwise-periodic array of cylinders reveal a bifurcation from two-dimensional traveling waves to three-dimensional spanwise standing waves. The standing waves select a wave number that is independent of the wave number imposed initially by time-periodic disturbances. The stable tertiary flow stands in contrast with most open flows where instability develops directly to turbulence.

Novel opacity calculations, which treat in detail the spectra of medium-Z ions [Rogers and Iglesias, Astrophys. J. Suppl. Ser. 79, 507 (1992)], produce results that are substantially different from opacity calculations extant in the literature. These new opacities provide solutions to a number of outstanding problems in astrophysics, thus providing an indirect validation of the theory. We report the results of an experiment measuring the photoabsorption in the spectral region from 50 to 120 eV of x-ray-heated iron which corroborate these new opacity calculations.

We have observed strongly coupled stimulated Raman backscatter from the interaction of an intense, subpicosecond laser with an underdense plasma. The strongly coupled backscatter spectra have widths greater than the plasma frequency, extend to the blue side of the laser frequency, and at the highest densities, are shifted by less than ${\mathrm{\omega }}_{\mathit{p}}$. These observations are consistent with the predictions of a strongly coupled Raman theory, appropriately modified for short laser pulses.

Since its introduction in 1985, ab initio molecular dynamics has played an important role in the analysis of ionic properties of condensed-matter systems. Extending this method to a Thomas-Fermi model for plasmas, the structure and diffusion constants from molecular-dynamics simulations of the hydrogen plasma are compared to the results of approximate theories of the same model.

10-nm-thick germanium layers have been grown on Si(100) with and without antimony as a surfactant, and investigated by RHEED, TEM, and XPS. We obtained smooth epitaxial germanium layers with the antimony surfactant by passing through an island formation stage. These islands, formed below 400 °C, are of different structure than the islands obtained without surfactants. A possible mechanism for the ‘‘smoothing out’’ of islands developed in the beginning state of surfactant-controlled solid phase epitaxy is proposed.

The structure of ion-irradiated hydrogenated amorphous carbon and hydrogenated graphitic carbon films has been investigated by measuring the trigonal carbon fraction by optical measurements and the hydrogen concentration by elastic recoil of 2.0-MeV ${\mathrm{He}}^{+}$ ions. After irradiation both graphitic and amorphous carbon samples showed a unique relationship between the trigonal bonded carbon and the hydrogen fraction, regardless of the original film structure.

We present results of a helium atom beam scattering experiment on spinodal decomposition in two dimensions and for deep temperature quenches. A system with conserved order parameter and equal volume fractions of two phases is realized by deposition of 0.5 monolayer Cu on a Cu(100) surface. We find that the morphology evolves in a self-similar fashion. Growth exponents close to 1/4 are in accord with ideas of corrections in the Lifshitz-Slyosov growth law due to excess mass transport along domain boundaries in the temperature and time range covered by our experiment.

The pressure-induced transition of ${\mathrm{H}}_2$O into the ice X phase, characterized by symmetric hydrogen bonding, is studied using ab initio molecular dynamics combined with ultrasoft pseudopotentials. A good description of the hydrogen bond is obtained only after gradient corrections to the local-density approximation are included. The transition into ice X is predicted at 49 GPa, in good agreement with experiment, when proton quantum fluctuations are treated within mean-field theory. Molecular-dynamics simulations show that a mode-softening description of the transition is appropriate.

We have studied solid ${\mathrm{C}}_{60}$, measuring the shift of the optical absorption edge with pressure to 35 GPa. In the range of 0–17 GPa, extrapolation of the absorption edge indicates that metallization should occur by 33 GPa. However, from 17 to 25 GPa an irreversible transition to a ‘‘transparent phase’’ occurs. Raman scattering of the depressurized sample shows no trace of ${\mathrm{C}}_{60}$, diamond, or graphite, indicating that the transition involves the collapse of the ${\mathrm{C}}_{60}$ molcules into a new structure of carbon.

We report a quasielastic light scattering measurement of the isotropic-nematic phase transition of a liquid crystal in silica gel. The normalized intensity autocorrelation function, $G_2\mathrm{}\left(t\right)\mathrm{}$, which samples the order-parameter fluctuations, is consistent with the form $G_2\mathrm{}\left(x\right)\mathrm{}\sim \frac{1}{(1+x^2)}$, where $x=\frac{\ln t}{\ln \tau }$. The relaxation time, $\tau$, diverges near a temperature $T^{*}$ and can be quantitatively described by the Vogel-Fulcher law, $T={\tau }_0$ exp[const/($T-T^{*}$)]. The measurements suggest that the silica gel imposes a weak random anisotropy on the system, with the bulk nematic phase being replaced by a "glassy" state. However, hysteresis, normally associated with random-"field" behavior, is absent.

Surface-balance, ellipsometry, and optical second-harmonic-generation techniques are employed to investigate the phase transitions of liquid-crystal 4’-n-octyl-4-cyanobiphenyl films on an air-water interface. We have found, for the first time, a distinctive first-order phase transition between a stable monolayer phase and a stable 3-layer phase of the liquid-crystal film.

By means of quasielastic neutron scattering we have shown that the dynamics of the α relaxation on three different polymeric glasses shows a clear non-Debye behavior on a mesoscopic time scale. The spectral shape is found to be independent of temperature and momentum transfer (Q). The characteristic relaxation time follows a power-law Q dependence, τ(Q)∝${\mathit{Q}}^{\mathrm{-}\mathit{n}}$ (n>2), n beong dependent on the system. We found that the Q behavior and the non-Debye behavior are directly correlated. These results have important implications concerning the physical mechanisms behind the dynamics of the α relaxation.

High-resolution thermal-expansion experiments on (i) insulating ${\mathrm{La}}_2\mathrm{Cu}{\mathrm{O}}_{4+\delta }$, (ii) its Sr- and Ba-doped metallic counterparts with reduced $T_c$ values, and (iii) ${\mathrm{Nd}}_{1.85}{\mathrm{Ce}}_{0.15}\mathrm{Cu}{\mathrm{O}}_{4-\delta }$ ($T_c\simeq 24$ K) reveal distinct lattice instabilities at $T_1=32\mathrm{}\pm 1$ K and $T_2=36\mathrm{}\pm 1$ K. $T_2$ coincides with $T_c^{max}$ found upon 15 at.% Sr doping. For ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.5}$ ($T_c=49\mathrm{}$ K), a lattice instability occurs at $T=90\mathrm{}\pm 1$ K, i.e., again close to $T_c^{max}=92\mathrm{}$ K.

Atomic structures of Al on GaAs(110) are studied by ab initio molecular dynamics for coverages of 1/8 to 1 monolayer (ML). A single chemisorbed Al atom resides at the center of a triangle of one Ga and two As atoms. Al dimers have very long bond lengths and bind due to substrate-mediated interactions. Epitaxial growth of 1 ML of Al is less stable than the formation of islands. Preformed clusters bond strongly to the substrate, which shows that the absence of Fermi-level pinning in samples grown by cluster deposition is due to suppression of reactivity rather than lack of interactions.

Neutron scattering measurements show that the crossover (at T≤10 K) from metallic heavy-fermion to semiconducting behavior coincides with the formation of a gap in the magnetic excitation spectrum of CeNiSn. In contrast to the simple band picture of an insulator, the gap is well defined only at particular values of the momentum transfer Q. While substantial antiferromagnetic correlations in the a-c plane characterize the low-T state, the corresponding zero-frequency response function is Q independent.