Electronic structure studies on neutral, singly and doubly charged ${\mathrm{Be}}_n$ clusters ($n\le 5$) have been carried out within the local spin-density formalism. We present results on their equilibrium geometries and atomization energies and discuss the stability of charged clusters. An extensive study of the Born-Oppenheimer surface of Be trimers is given and it is shown that small doubly charged clusters may exist in metastable states protected from dissociation by large energy barriers. The possibility of creating them by ocne- or two-step ionization processes criticalldepends on their geometries. We also address the issue of charge localization in doubly charged clusters.

We use an averaging technique to estimate the periodic points for attractors near the transition to chaos via period doubling and quasiperiodicity in two experimental systems. Using the averaged experimental data we then evaluate the trajectory scaling functions for both routes to chaos.

It is confirmed experimentally that the self-interaction of electron Bernstein waves occurs by means of nonlinear cyclotron (Landau) damping. This wave-particle interaction occurs in a relatively broad frequency range: $1.46<\mathrm{}\frac{\omega }{{\omega }_c}<1.54\mathrm{}$, $1.60<\mathrm{}\frac{\omega }{{\omega }_c}<1.75\mathrm{}$. The virtual wave is observed most strongly at the half-harmonic electron cyclotron frequency near $\frac{\omega }{{\omega }_c}\cong 1.5$. Although this self-interaction always occurs for $\frac{k_{\parallel }{v}_t}{{\omega }_c}\lesssim 0.1$, the virtual wave cannot be detected for $\frac{k_{\parallel }{v}_t}{{\omega }_c}\gtrsim 0.15$.

Precise x-ray measurements of the tilt angle near the $\mathrm{Smectic}-A-\mathrm{Smectic}-C^{*}$ tricritical point show clear evidence of mean-field to tricritical crossover behavior; the mean-field region shrinks to zero at the tricritical point.

Bond-orientational ordering in the hexatic-$B$ phase of free-standing liquid-crystal films of two to six molecular layers has been studied quantitatively with electron diffraction. By examination of the scaling relation among the $6n$-fold bond-orientational order parameters, it is demonstrated that the two-layer film exhibits truly two-dimensional hexatic behavior. A departure from two-dimensional behavior is observed in the thicker films.

We have determined for the first time the phonon density of states of a nonequilibrium fcc ${\mathrm{Al}}_{0.94}$${\mathrm{Si}}_{0.06}$ solid solution quenched under high pressure by means of inelastic neutron scattering and low-temperature specific heat. When compared with aluminum, a large softening of the transverse acoustic modes is observed similar to that found in amorphous metals. This shows that a phonon softening can be found in nonequilibrium crystalline alloys. The nonequilibrium state is due to the metallic state of silicon atoms trapped in the fcc lattice during quenching.

Magnetic fluctuations seen by inelastic neutron scattering in the paramagnet ${\mathrm{Cr}}_{0.95}$${\mathrm{V}}_{0.05}$ have a structure factor similar to that of the static incommensurate spin-density wave (SDW) in pure Cr, and are thus termed SDW paramagnons. Commensurate-diffuse scattering dominant in the paramagnetic phase of Cr is absent in ${\mathrm{Cr}}_{0.95}$${\mathrm{V}}_{0.05}$. The amplitude of the SDW paramagnons in this alloy increases with temperature and their energy scale extends beyond 40 meV.

Simulations of amorphous silicon formed by quenching of the liquid indicate that $a-\mathrm{Si}$ has a Boltzmann-like distribution of local geometries, corresponding to a "glass temperature" of roughly 700 K. The resulting tetrahedral network exhibits native defects, threefold- and fivefold-coordinated atoms, which have mean formation energies of 0.6 and 0.3 eV, respectively, and which are apparently mobile even at fairly low temperatures. These results are obtained by a novel approach to the analysis, and are relatively insensitive to the empirical interatomic potential used in the simulations.

We present the results of an extensive molecular-dynamics simulation of a dense polymer system. We show for the first time that simulations are able to cover the whole regime from pure Rouse dynamics to reptation dynamics and give strong evidence of the latter. The mean square displacements clearly exhibit a $t^{\frac{1}{4}}$ power law. A mode analysis shows that the high-frequency modes follow the Rouse relaxation while those at lower frequency display reptation relaxation. Both quantities give the same entanglement length.

The zero-temperature entropy of glasses is usually calculated from the specific heat measured during heating. We show that this only gives an upper bound for the entropy; measuring the specific heat during cooling would provide a lower bound. Furthermore, in a computer simulation, measuring the distribution of calorimetrically measured entropies on repeated fast coolings provides information about the internal relaxation mechanisms of the glass. Three different Monte Carlo systems illustrate our results: a model orientational glass, a single two-level system, and a small Ising spin-glass.

X-ray-diffraction studies have been carried out to 255 GPa on various sample and gasket configurations in the diamond-anvil cell. Intense fluorescence emission bands were observed at 793 and 830 nm in some cases which correspond to 360 and 550 GPa on the extrapolated ruby pressure scale. The x-ray measured pressures were only 185 and 201 GPa, respectively. Reasons for this discrepancy between x-ray methods and optical methods at ultrahigh pressures are proposed.

A transition from chain- to island-type structure is observed in Ir adatom cluster nuclei on Ir(100). For five or fewer adatoms the stable configuration is a nucleus in the form of a linear chain, whereas for six or more adatoms a two-dimensional island configuration is stable. This unusual transformation indicates the presence of relatively long-range interactions between the Ir adatoms.

Positrons annihilating at a physisorbed monolayer of methane on graphite have been studied. Photon spectra reveal that positronium is formed and pressure measurements indicate that it rises to a sharp maximum at 50% coverage. The data are in accord with the hypothesis that the Ps formation at the graphite surface is assisted by molecular recoil.

We have discovered a novel anisotropy in a quasi-one-dimensional electronic system in the presence of a magnetic field. The density of states changes dramatically when a magnetic field is applied along different axes of the sample. Further, we do not observe spin splitting of one-dimensional levels in magnetic fields up to 20 T, indicating that enhancement of the Landé $g$ factor is also anisotropic. These results provide new insight into the underlying properties of quantum-confined electrons.

The Hall resistance in a quantum wire is quantitatively and qualitatively different from its two-dimensional counterpart. First there are a finite number of steps, second the step resistances for small to intermediate magnetic fields are not well defined, and most important the small-magnetic-field Hall resistance is altered by a geometrical form factor.

The behavior of the two-particle spectral function, $S(q,\omega )$, is examined in the hydrodynamic regime, at the mobility edge in a model for the integer quantized Hall effect. Results are presented from numerical diagonalization of the Hamiltonian for finite systems. For $\frac{q^2}{\omega }$ small, $S(q,\omega )$ has a conventional, diffusive form. For $\frac{q^2}{\omega }$ large, the novel dependence $S(q,\omega )\sim {\omega }^{-\frac{\eta }{2}}{q}^{-2+\eta }$ is obtained, with $\eta =0.38\mathrm{}\pm 0.04$.

Measurements of the low-temperature specific heat of phosphorus-doped silicon for densities near the metal-insulator transition show an enhancement over the conduction-band itinerant-electron value. The enhancement increases toward lower temperatures but is less than that found for the spin susceptibility. The data are compared with various theoretical models; the large ratio of the spin susceptibility to specific heat indicates the presence of localized spin excitations in the metallic phase as the metalinsulator transition is approached.

Changes with onset of metallic behavior in the position of an impurity-stabilized Fermi level at the GaAs-Ag interface are interpreted in a model based on delocalization of the impurity levels. The resulting energy broadening of interface states appears to be an important mechanism underlying the Schottky behavior; in particular the model obviates the need for separate donor or acceptor properties of the impurity levels.

Band-gap optical recombination from the two-dimensional electron gas in GaAs-(Al,Ga)As quantum wells was measured in the regime of the fractional quantum Hall effect. New intrinsic emission lines emerge at the Landau-level filling factor $\nu \lesssim 1$ and at $\nu <0.8\mathrm{}$. The anomaly near $\nu =1\mathrm{}$ is associated with changes in population of the lowest spin-split Landau level. The spectral doublet observed near $\nu =\frac{2}{3}$ has a striking temperature dependence below $T=2\mathrm{}$ K similar to the magnetoresistance ${\rho }_{\mathrm{xx}}$ associated with the many-body quantum fluid.

We demonstrate, by photoluminescence measurements, the excitation-dependent opposite charging of coupled ${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As/InP single quantum wells. The formation of the dipole layer is associated with band bending lining up the electron levels in the wells. This novel effect is due to the very different tunneling rates of electrons and holes through the barrier and is observed for barrier widths $L_B=40\mathrm{}-100$ Å. On the basis of these findings we discuss a potential new optical bistability.