Band-gap renormalization due to many-particle effects has been investigated at the $\Gamma$ and $X$ points of the Brillouin zone in ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ as a function of alloy composition. The composition dependence of the distribution of the photoexcited electrons among direct and indirect conduction-band valleys has been used to separate experimentally the electron-exchange contribution to band-gap narrowing.

The ideas of supersymmetric quantum mechanics are applied to the tunneling problem for double-well potentials. We evaluate the tunneling by developing a systematic perturbation expansion whose leading term is an improvement over the standard WKB result. We find that the perturbation series converges rapidly.

A phenomenological Landau-type free energy forms the basis of a theory for the low-temperature magnetic properties of CsNi${\mathrm{Cl}}_3$ under the influence of an applied magnetic field. Three magnetically ordered phases, as well as the paramagnetic phase, are found to coexist at a novel type of multicritical point representing the intersection of one line of first-order transitions and three lines of second-order transitions in the $H-T$ phase diagram. The temperature dependences of the critical fields are in good agreement with corresponding experimental data. Similar results are predicted for CsNi${\mathrm{Br}}_3$.

We have studied the excitation and decay of surface plasmons at a silver-air interface using a time-resolved optical technique. Monitoring the surface-plasmon propagation enabled us to measure the dephasing of surface plasmons via their mean free path and a momentum lifetime of 48 fs (13 μm) was found. After a very rapid plasmon decay the energy is transferred into a strain wave and an increase in the temperature of the silver film. We have observed the acoustic phonons, associated with the strain, bouncing up and down the silver film and the heat diffusion on a picosecond time scale.

We identify the physical origin of the excess capacitance at Schottky diodes without an interfacial layer, i.e., intimate Schottky contacts. Measured capacitance in excess of the space-charge capacitance is shown to be caused by the injection of minority carriers into the bulk semiconductor, rather than by the presence of interface states, as previously thought. Minority-carrier injection depends sensitively on the properties of the Ohmic back-contact.

Photodesorption and photodissociation of NO molecularly adsorbed on Si(111)7×7 at 90 K have been observed at wavelengths in the range from the visible to near-uv. An enhancement in the photodesorption intensity with photon energies above the transition energy of the first direct band gap of Si clearly shows that photogenerated carriers in Si initiate the reactions. Detailed data analysis suggests that the reactions are induced by hot carriers.

An extensive experimental and theoretical study of the x-ray Raman spectrum and electronic structure of polycrystalline boron has been performed. Anomalous peaks in the spectrum are directly related to the structure of the electronic density of states at high unoccupied bands. The present technique can probe the density of states very deep in the unoccupied conduction bands. The $d$-like nature of the "anomalous" peaks leads to a dipole approximation reevaluation.

We study the simplest example of reacting polymers, a single chain with reacting end groups. We develop a direct renormalization formalism for the cyclization rates: For long free-draining polymers, the ratio of the longest relaxation rate for the unperturbed system to the reaction rate is found to be a universal constant independent of the chemistry, of the reacting ends. For non-free-draining models we find that the analogous ratio vanishes to order $\epsilon$ ($\epsilon =4-d$) and we predict that mean-field theory holds asymptotically.

A late-decaying particle, e.g., the gravitino, initiates a new phase of nucleosynthesis in the kiloelectronvolt era. The abundances of all light elements, except ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, are given by the fixed points of the rate equations, agree with observations, and are insensitive to the pre-kiloelectronvolt physics if ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ is not underproduced. Constraints from ordinary nucleosynthesis evaporate, allowing a wide range of values for ${\Omega }_B$ and the expansion rate: The Universe can be closed by baryons; the number of light $v$ species can be greater than four; the era of conventional nucleosynthesis can be matter or vacuum-energy dominated.

We present an exact analytic computation of the Lyapunov dimension of the universal attractor of the complex Ginzburg-Landau partial differential equation for a finite range of its parameter values. We obtain upper bounds on the attractor's dimension when the parameters do not permit an exact evaluation by our methods. The exact Lyapunov dimension agrees with an estimate of the number of degrees of freedom based on a simple linear stability analysis and mode-counting argument.

A search for sequential heavy leptons has been performed at the center-of-mass energy of 52 GeV at the KEK colliding-beam accelerator TRISTAN. We have found no evidence for the production of heavy leptons with the integrated luminosity of 2.9 ${\mathrm{pb}}^{-1\mathrm{}}$. A lower mass limit of 25.0 GeV/${\mathit{c}}^2$ at 95% confidence level was obtained.

The $\mathrm{D}(e, e^{\prime }p)$ coincidence cross section was measured at angles out of the plane, at an excitation energy of approximately 18 MeV and a momentum transfer of 0.33 ${\mathrm{fm}}^{-1\mathrm{}}$. This is the first reported measurement of the longitudinal-transverse and transverse-transverse interference terms of the electrodisintegration cross section of the deuteron. The longitudinal plus transverse cross section is also reported. The results are in good agreement with a theoretical calculation carried out with use of the Paris potential.

The magnetic moment of ${}_{}{}^{190m}{}_{}{}^{}\mathrm{Os}$ ($I^{\pi }={10}^{-}$; $T_{\frac{1}{2}}=9.9\mathrm{}$ min; $E=1705\mathrm{}$ keV; configuration ${\left|n_1\frac{9}{2}\mathrm{}\right[505\left]\mathrm{}{n}_2\frac{11}{2}\mathrm{}\right[615]\mathrm{}〉}_{{10}^{-}}$) has been measured with the new technique of reorientation-nuclearorientation. As the spin contributions of the two neutrons to the magnetic moment cancel almost completely, it is possible to determine the anomalous neutron orbital $g$ factor directly. The result, $-0.12\mathrm{}\le \delta g_l\mathrm{}\left(n\right)\mathrm{}\le -0.07\mathrm{}$ (90% confidence level), tends to be larger than $\delta g_l\mathrm{}\left(n\right)\mathrm{}\simeq -0.05\mathrm{}$ adopted up to now.

We present new data from numerical simulations of microwave excitation of a two-dimensional hydrogen atom, which show that in a wide parameter range under the one-dimensional delocalization border, the quantum localization phenomenon persists. We theoretically reconsider the problem of two-dimensional localization, by using an appropriately constructed four-dimensional map over an orbital period of the electron, and thus explain the numerical results.

Clouds of two to about fifty simultaneously stored, laser-cooled ${\mathrm{Mg}}^{+}$ ions in a Paul trap were observed in two phases, which are clearly distinguishable by their excitation spectra. Transitions between these phases can be induced either by a variation of the power of the laser radiation used to cool the ions or by a change of the size of the radio-frequency voltage applied to the trap. Transitions between a "crystalline" phase and a "gaseous" phase can be repeatedly observed by variation of the appropriate parameters. The two phases and the transitions between them have also been recorded by a photon-counting image system.

We observe small numbers of laser-cooled ${\mathrm{Hg}}^{+}$ ions, which are confined in a Paul radio-frequency ion trap, to crystallize into regular arrays or clusters. We investigate the structure of these clusters by direct imaging, optical spectroscopy, and numerical calculations. The spectroscopy of such "pseudomolecules" is unique in that we can probe individual atoms of the molecule separately. The ratio of Coulomb potential energy per ion to $k_{\mathrm{B}}T$ in these clusters is observed to be as high as 120.

We have investigated the associative ionization of Na atoms in two counterpropagating thermal beams independently excited by various combinations of linearly and circularly polarized light. In this way information about coherence contributions to the ion signal has been obtained for the first time. These contributions which correspond to off-diagonal elements of the detection matrix have hitherto been neglected, but are of the same magnitude as the cross sections themselves and depend on the velocity.

Dynamical effects of a surface-induced softening of the H—H bond during ${\mathrm{H}}_2$-surface collisions have been studied by numerical wave-packet simulations. The calculation shows that the zero-point vibrational energy released by the bond softening is fully transferred to the translational motion of the molecule (except for incident energies > 0.2 eV, where vibrational excitation also takes place), and that inelastic processes can trap the molecule in a mobile and long-lived state confined near the surface.

We have observed barium Rydberg states with principal quantum numbers up to $n=520\mathrm{}$. Such highly excited states allow quantitative in situ measurements of small electric fields on the order of tens of microvolts per centimeter. Because of the effective suppression of stray electric fields we have observed quasi-Landau resonances in external magnetic fields on the order of tens of gauss. The energy dependence of the spacing between neighboring quasi-Landau resonances was found to be in excellent agreement with calculations involving classical trajectories.

The degree of orientation of C${\mathrm{H}}_3$I in rotation-state—selected, oriented-molecule beams has been measured via the asymmetry of the angular distribution of the $\mathrm{I}\left({}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}\right)$ photodissociation product from the linearly-polarized-laser—induced photofragmentation of C${\mathrm{H}}_3$I in (parent) rotational states $|JKM〉=|111\mathrm{}〉, |212\mathrm{}〉, |211\mathrm{}〉, \mathrm{and} |222\mathrm{}〉$. For the latter state the up/down asymmetry is nearly $\frac{4}{1}$ (less for the other states). Partial disorientation of the C${\mathrm{H}}_3$I due to hyperfine coupling in a weak electric field has also been demonstrated.