We found that dense electronic excitation of RbI at 4.2 K by two-photon absorption produces the $F$ band and a new luminescence band, of which the intensities are proportional to the square of the density of excitation. Such creation of $F$ centers and new luminescence centers has been shown to arise from interaction between free excitons and self-trapped excitons.

Results of depolarized Raman spectroscopy on fractal silica aerogels are presented at frequencies from 0.3 to 50 ${\mathrm{cm}}^{-1\mathrm{}}$. Three distinct vibration regimes are observed. In the fracton range, a power-law dependence of the Raman susceptibility on the frequency is found. It allows a determination of the superlocalization exponent $d_{\phi }$. Alternatively, with use of the "resistance" expression for the exponent $d_{\phi }$, a spectral dimension $\bar{\overline{d}}$ in fair agreement with previous measurements results.

In recent years, the behavior of hydrogen in crystalline silicon has been the object of intense experimental interest, but understanding has been limited by the absence of reliable theoretical calculations. Here we use state-of-the-art techniques to investigate systematically the properties of H in Si: stable configurations, migration paths, charge-state effects, cooperative interactions, etc. The calculations are based on local-density-functional theory, using ab initio pseudopotentials, in a supercell geometry. The results are used to scrutinize and supplement existing understanding of the observed phenomena. A novel mechanism for H-induced damage is proposed.

The existence of the magnetic-field-induced liquid-to-solid phase transition in an extreme quantum-limit 2D electron plasma is established for electrons at a high-quality GaAs/GaAlAs heterojunction by detection of a gapless magnetophonon excitation branch with radio-frequency spectroscopy. The phase diagram, determined for Wigner-Seitz to Bohr radius ratio $1.6<\mathrm{}{r}_s<2.5\mathrm{}$, extrapolates to a zero-temperature critical Landau-level filling factor ${\nu }_c=0.23\mathrm{}\pm 0.04$ and a zero-filling-factor melting temperature close to the classical ($r_s\to \infty$) limit.

Co-Au multilayers with Co layer thicknesses between 22 and 5 ÅA, prepared by ion-beam sputtering, have an easy-plane magnetic anisotropy. Their saturation magnetization indicates the presence of diffuse interfaces, caused by ion-beam mixing. Annealing of the multilayers at 250-300°C leads to a perpendicular anisotropy for Co thicknesses below about 14 ÅA. This effect is attributed to a stronlgy enhanced interface anisotropy, which is due to sharpening of the interfaces as revealed by x-ray diffraction.

The dependences of the $\frac{l}{f}$ noise in CCu metal-insulator composites on temperature, frequency, magnetic field, and electric field and the noise symmetry properties were measured in the range $2 \mathrm{K}<T<\mathrm{}300\mathrm{}$ K, $0.1 \mathrm{Hz}<f<\mathrm{}3\mathrm{}$ kHz, $B<\mathrm{}80\mathrm{}$ kG, and $E<\mathrm{}500\mathrm{}$ V/cm. In the regime $T<\mathrm{}50\mathrm{}$ K all properties were consistent with a picture of noise from double-well systems coupled to the conductivity via the universal-conductance-fluctuation term.

Molecular angular-momentum polarization produced in gas-surface collisions is investigated by the magnetic field effect on a Knudsen flow. The effects for ${\mathrm{H}}_2$ isotopes scattered by LiF(001) surfaces are found to be 2 to 3 orders of magnitude larger than previously observed for metal surfaces. This is attributed to the large corrugation of the LiF crystal face, since the polarization type involved is produced by in-plane forces. Both first- and second-rank polarizations ("orientation" and "alignment") are observed, the production matrix elements being of order ${10}^{-1\mathrm{}}$ to ${10}^{-2\mathrm{}}$.

Neutron-diffraction measurements have been taken on single crystals of $\mathrm{Nd}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ to explore the nature of the magnetic order as a function of oxygen concentration $x$. The Cu-O planes are known to order antiferromagnetically at $T_{\mathrm{N}1\mathrm{}}$, with $T_{\mathrm{N}1\mathrm{}}\left(x\approx 0\right)\simeq 450$ K. At lower temperatures we find that the oxygen-deficient Cu planes also order magnetically [$T_{\mathrm{N}2\mathrm{}}(x\approx 0.1)\simeq 80$ K], with a large ordered moment of $0.46{\mu }_{\mathrm{B}}$ and a simple antiferromagnetic sequencing along the $c$ axis. Both ordering temperatures decrease with increasing $x$, with $T_{\mathrm{N}2\mathrm{}}$ being much more sensitive to $x$ than $T_{\mathrm{N}1\mathrm{}}$.

Time-dependent spin-autocorrelation functions at $T=\infty$ for classical Heisenberg magnets in dimensionalities $d=1, 2, \mathrm{and} 3$ are investigated by means of a computer simulation. These functions are shown to exhibit power-law long-time tails of form $t^{-{\alpha }_d}$ with characteristic exponents ${\alpha }_d$ which differ significantly from the values ${\alpha }_d^{\mathrm{}\left(\mathrm{SD}\right)\mathrm{}}=\frac{d}{2}$ predicted by the phenomenological spin-diffusion theory: ${\alpha }_1=0.609\mathrm{}\pm 0.005$, ${\alpha }_2=1.050\mathrm{}\pm 0.025$, ${\alpha }_3\simeq 1.6$. The method to employ computer simulations for this problem differs from methods previously employed. Anomalous spin diffusion is confirmed by existing proton spin-lattice relaxation data for the quasi-1D $s=\frac{5}{2}$ Heisenberg antiferromagnet ${\left(\mathrm{C}{\mathrm{H}}_3\right)}_4$NMn${\mathrm{Cl}}_3$.

We report the first observation of the photoinduced absorption spectrum of the emeraldine base form of polyaniline. The spectrum contains both subgap photoinduced bleaching at 1.8 eV and photoinduced absorptions at 0.9, 1.4, and 3.0 eV, as well as band-edge bleaching above 3.5 eV. The existence of the subgap bleaching peak is consistent with a model of optically excited localized molecular excitons. The energies of the three photoinduced absorption peaks and the dependence of the peak amplitudes on pump power give evidence for the photoexcitation of polaron pairs that recombine bimolecularly.

By use of an effective Hamiltonian which takes into account the constraints on the motion of a hole in a quantum antiferromagnet, the spectral function of the hole is calculated. For small exchange and away from the antiferromagnetic zone boundary, it is found to be dominated by incoherent multiplespin-wave processes. The dispersion of the quasiparticle part and the possible implications for disordering of the quantum antiferromagnet are discussed.

Electron diffraction patterns of the Tl-based superconductor ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$ Ca${\mathrm{Cu}}_2$${\mathrm{O}}_8$ display characteristic rings of diffuse intensity about space-group-forbidden reflections. There exists a close relationship between this diffuse intensity distribution and the incommensurate modulation observed in the structurally related Bi-based superconductor ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$ Ca${\mathrm{Cu}}_2$${\mathrm{O}}_8$.

Diffuse neutron-scattering experiments on the martensitic fcc-hcp phase transition in single crystals of pure cobalt and Co-32% Ni reveal the presence of satellites around (111) fcc reflections within a temperature range where the fcc and hcp phases coexist. The satellites indicate a modulation of the (111) fcc lattice planes with a wavelength $\lambda =6\mathrm{}{d}_{111}$. This result can be explained by the occurrence of an intermediate structure where platelike nuclei of the hcp phase are coherently inserted into the fcc matrix.

The nucleation rate of interacting thermal kinks and antikinks in a sine-Gordon chain is evaluated within the dilute-gas approximation. Our novel result describes both the diffusive and nondiffusive limits. In comparison with the nondiffusive and noninteracting nucleation rate calculated previously by Büttiker and Landauer, interactions modify the temperature dependence of the prefactor and, in the diffusive limit, the Arrhenius factor as well.

We tested Newton's inverse-square law of gravitation by comparing gravity measured on a 600-m tower with gravity calculated from ground measurements. A significant departure from the law was detected, approaching (- 500±35)×${10}^{-8\mathrm{}}$ ${\mathrm{ms}}^{-2\mathrm{}}$ at the top of the tower and suggestive of a rapidly attenuating non-Newtonian attractive force. These results are marginally consistent with a one-term Yukawa-type attractive force, but they are fully consistent with two Yukawa-type forces, attractive and repulsive, and then also with Airy and Cavendish experiments.

We formulate a geometrical theory of self-dual scalar fields in two dimensions with a local Lorentzinvariant action. Path-integral quantization is used to derive chiral bosonization by demonstrating equivalence of correlation functions of currents and energy-momentum tensors in our theory with those for Weyl fermions.

QED in 2+1 dimensions, analyzed in the $\frac{1}{N}$ expansion, is shown to exhibit a critical behavior as the number $N$ of fermions approaches $\frac{32}{{\pi }^2}$. The dynamical mass has a universal scaling behavior at the critical point. The existence of criticality is discovered first by an analytic study of the gap equation and the effective potential. Its existence and universality is then confirmed numerically. Similarities to four-dimensional theories are discussed.

Possible interpretations for the $\zeta \mathrm{}\left(2230\right)\mathrm{}$ are discussed in accordance with new data from Baltrusaitis et al., Aston et al., and Binon et al., and it is argued that the $\zeta \mathrm{}\left(2230\right)\mathrm{}$ is likely to be a ${\mathrm{}\left(\mathrm{qs}\right)\mathrm{}}_{3*}-{\left(\overline{q}\overline{s}\right)}_3\mathrm{}(q=u,d)\mathrm{}$ baryonium state, which couples strongly to $\Lambda \Lambda$ and decays to $K\overline{K}$, $K{\overline{K}}^{*}$, ${\overline{K}}^{*}{K}^{*}$, $\eta \eta ,\eta {\eta }^{\prime }$, ${\eta }^{\prime }{\eta }^{\prime }$, and $\omega \phi$, via quark rearrangement, and also to $\pi \pi$, $\omega \omega$, and $\phi \phi$ via quark annihilation, and therefore has a narrow width. A possible $s\overline{s}g$ hybrid state, which can decay to $\phi \phi$, $\omega \phi$, and other mesons, is also suggested.

We present the complete calculation for the short-distance flavor-changing process $b\to s{l}^{+}{l}^{-}$ and discuss the effect of QCD enhancement, and the resulting $e-\mu$ rate difference. We estimate the exclusive rates for $B\to k{l}^{+}{l}^{-}$ and $B\to k^{*}{l}^{+}{l}^{-}$ using the relativistic constituent-quark model to calculate the different form factors that include recoil effects. The effect of the fourth generation in enhancing the rates is also discussed.

We use data taken by the Mark II detector at the SLAC storage ring PEP to study inclusive lepton production in $e^{+}{e}^{-}$ annihilation at 29 GeV. From fits to the inclusive lepton momentum and transverse-momentum spectra we measure the semileptonic branching ratios for charm and bottom hadrons. We also determine with good precision the mean value of the bottom fragmentation function using both electron and muon samples.