The structural properties of silicon clusters are examined to determine at what size clusters with covalentlike open structures become energetically more stable than those with metalliclike close-packed structures. By calculation of the total energy of diamond and face-centered cubic fragments as a function of the number of atoms, an estimate of the "critical" cluster size is made. The estimate is that for clusters of less than about 50 atoms, metallic structures are favored over covalent structures, with the inequality reversed for more than 50 atoms. This result agrees well with recent observations on the relative stability of $\mathrm{Si}_n^{}{}_{}{}^{+}$.

Three sizes of CdSe molecular clusters were studied by ${}_{}{}^{77}{}_{}{}^{}\mathrm{Se}$ NMR spectroscopy. The clusters are synthesized via arrested precipitation in a structured medium and are isolated as a molecular solid after modification of their surfaces with covalently attached organic ligands. While one component of the broad ${}_{}{}^{77}{}_{}{}^{}\mathrm{Se}$ NMR line shape corresponds to the bulk material, increasing proportions appear at shifts upfield as a function of decreasing particle size. The line positions and distribution of chemical shifts can be related to a size dependence of the electronic structure and the local chemical environments.

In a previous paper, Kalmeyer and Laughlin argued that the elementary excitations of the original Anderson resonating-valence-bond model might obey fractional statistics. In this paper, it is shown that an ideal gas of such particles is a new kind of superconductor.

Band-structure results for ordered ${\mathrm{Ba}}_{0.5}$${\mathrm{K}}_{0.5}$Bi${\mathrm{O}}_3$ alloys confirm that the antibonding $\mathrm{Bi}\mathrm{}\left(6s\right)-\mathrm{O}\mathrm{}\left(2p\right)\mathrm{}$ conduction band near $E_F$ in cubic BaBi${\mathrm{O}}_3$ is minimally affected by substitutional K doping at the $A\left(\mathrm{Ba}\right)$ site. This supports the prediction that $A$-site doping would extend the metallic range of ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{BiO}}_3$ closer to half filling (BaBi${\mathrm{O}}_3$) than $B$-site doping (${\mathrm{BaPb}}_{1-y}{\mathrm{Bi}}_y{\mathrm{O}}_3$), thereby increasing both the coupling of the conduction electrons to bond-stretching O phonons and $T_c$. ${\mathrm{Ba}}_{1-x}{\mathrm{T}}_x{\mathrm{PbO}}_3$, where $T$ is trivalent, might also exhibit favorable superconducting properties.

We show, using a proposed oxygen-defect structure for ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ and a tight-binding model based on density-functional calculations, that the distribution of holes between planes and (fragmented) chains causes jumps and plateaus for the hole counts in the planes as a function of the nonstoichiometry, $x$. These features can be correlated with the occurrence of the antiferromagnetic as well as the 60- and 90-K superconducting phases. Our findings are supportive of the view that the hole count is the controlling parameter for the superconductivity.

We show that there exists an infinite near degeneracy of localized eigenstates for the almost-Mathieu equation when a dimensionless coupling constant $\Delta$ exceeds ${\Delta }_c=1\mathrm{}$, the Aubry-André critical value. This is an immediate consequence of the quasiperiodic character of the potential. Concomitant with the transition from localized to extended eigenstates, the near degeneracy gives way to an incipient infinite degeneracy, with the energy differences of pairs of states generally vanishing as ${(\Delta -{\Delta }_c)}^{\gamma }$ for $\Delta \to {\Delta }_c+$.

The electronic structure of substitutional tin impurities in 38 elemental metals and semiconductors is calculated with the linear muffin-tin-orbital Green's-function method. The trends of ${}_{}{}^{119}{}_{}{}^{}\mathrm{Sn}$ isomer shifts in metals are discussed in terms hereof. The isomer shift shows a minimum in the middle of the transition series due to increased hybridization with the metal $d$ bands, while it is largest in the $\mathrm{sp}$ metals to the right of the transition metals. Compared with the $\mathrm{sp}$ metals, the strong covalency of the group-IV semiconductors significantly lowers the isomer shift of substitutional tin atoms.

We use total-yield photoelectron spectroscopy in combination with the Kelvin probe to obtain a simple and direct determination of the distribution of occupied states over nearly the entire band gap in P-doped $a-\mathrm{S}\mathrm{i}:\mathrm{H}$. The large dynamic range (${10}^7$:1) and high energy resolution $\Delta E\le 0.1$ eV) of this electron-emission spectroscopy also allow the distribution of unoccupied states above the Fermi level to be determined by division of the measured spectra by the Fermi occupation function. We exploit this novel capability to prove that the conduction-band tail in P-doped $a-\mathrm{S}\mathrm{i}:\mathrm{H}$ is exactly exponential.

A theoretical result for correlated quantum systems is presented which leads to a noise commutation relation and a generalized Heisenberg uncertainty relation. These relations imply an inherent and unavoidable extra noise in quantum measurements beyond that already included in the Heisenberg lower bound. These relations lead directly to model-independent lower bounds on inherent noise, useful in a variety of applications, including balanced homodyne detection and quantum optical linear amplifiers.

We develop a scaling theory for linear fragmentation processes, for general breakup kernels characterized by a homogeneity index $\lambda >0\mathrm{}$. We discuss the existence of scaling, and show that the scaled cluster-size distribution $\phi \mathrm{}\left(x\right)\mathrm{}$ generally decays with the scaled mass $x$ as $x^{-2\mathrm{}}\exp (-a{x}^{\lambda })$ as $x\to \infty$. For small $x$, $\phi \mathrm{}\left(x\right)\mathrm{}$ approaches the log-normal form, $\exp (-a{\ln }^{2}x)$, if the kernel has a small-size cutoff, and a power-law form in the absence of a cutoff. We also show that $\lambda <0\mathrm{}$ leads to a shattering transition. Finally, we outline the essential features of a nonlinear fragmentation process.

From Monte Carlo calculations of plaquette-Wilson-loop correlations for three-dimensional SU(2) lattice gauge theory, we give unambiguous graphic demonstrations of the formation of non-Abelian chromoelectric flux tubes with constant energy density in the region between a pair of static $q\overline{q}$ sources.

Above-threshold atomic photoemission by intense elliptically polarized light is not symmetric with respect to reflection about either principal polarization axis, contrary to predictions. Simultaneous reflection about both axes is symmetric. These asymmetries occur in helium, krypton, and xenon photoelectron spectra, and depend on the laser intensity and polarization, and the electron energy. Possible mechanisms are discussed.

We report the first observation of the dynamic optical Stark shift on a femtosecond time scale. Pump pulses of 50-fs duration with peak intensities of ${10}^{11}$-${10}^{12}$ W/${\mathrm{cm}}^2$ were used to cause a Stark shift of the $S_0$ to $S_1$ transition in the organic dye molecule rhodamine-B. The perturbed spectra were observed with the use of probe pulses of 6-fs duration. The inferred effective transition dipole moment of 7 D is in good agreement with the value derived from the absorption line strength. At the higher pump powers, line-shape distortions and a reduction in the apparent line strength are also noted and discussed.

A fast beam of ${\mathrm{H}}_2$ molecules in the metastable $c{}_{}{}^3{}_{}{}^{}\Pi _{\mathrm{u}}^{}{}_{}{}^{-}$ state is photoionized with visible laser light in the region 540-670 nm. Very strong photoionization from high vibrational levels is observed with a conversion of up to ten vibrational quanta into electronic energy. This extreme non-Franck-Condon behavior is attributed to avoided crossings between the doubly excited $2p{\sigma }_{\mathrm{u}}2p{\pi }_{\mathrm{u}}{Q}_1{}_{}{}^3{}_{}{}^{}\Pi _{\mathrm{g}}^{}{}_{}{}^{}$ state and the singly excited $1snd{}_{}{}^3{}_{}{}^{}\Pi _{\mathrm{g}}^{}{}_{}{}^{}$ Rydberg states with $n\ge 3$.

The beam-foil technique has been used to record the intensity decay of the $3s-3p$ and $3p-3d$ transitions in Na-like Ti, Fe, Ni, and Cu. Subsequent analysis using the ANDC-CANDY cascade correction method results in lifetime data for the $3p{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$ level which show a divergence from relativistic calculations as the nuclear charge increases. For the $3p{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ level, theory and experiment are found to be in better agreement.

The $L_2\to 2{\pi }^{-2\mathrm{}}{}_{}{}^3{}_{}{}^{}\Sigma _{}^{-}{}_{}{}^{}$ Auger-electron lines from HCl and DCl are recorded in very high resolution. The vibrational structure is resolved which reveals an isotope effect. The $2{\pi }^{-2\mathrm{}}{}_{}{}^3{}_{}{}^{}\Sigma _{}^{-}{}_{}{}^{}$ state is shown to be predissociative with a potential barrier of 0.8(2) eV. The lifetime of the $\mathrm{Cl} 2p_{\frac{1}{2}}$ core-hole state in HCl and DCl was determined to 0.12(2) eV.

In condensed matter there are many boson fields which, like light, can be driven into a squeezed state through anharmonic interactions with an intense pump. Here a single-particle scattering technique is described by which such states can be detected. In particular it is shown that a coherence can be established between a scattering process which involves the emission of a boson and that which involves the absorption of a boson. When this coherence gives rise to destructive interference the scattering rate can be greatly inhibited below that when the boson field is in its vacuum state.

Fully developed convective rolls have been observed in microscopic simulations of a hard-disk fluid driven by opposed temperature and gravitational fields. Under certain conditions a relatively long-lived mode with higher wave number is found to occur instead of the familiar square-cross-section rolls; this mode eventually undergoes a transition to the preferred square-roll state.

The structure of a classical one-component nonneutral plasma at low temperature confined by a magnetic field is analyzed on the basis of a simple electrostatic model assuming a shell structure. When the Madelung energy of charges is properly taken into account, the results of numerical experiments by Rahman and Schiffer are reproduced to good accuracy and the change of the structure with the confining field is predicted.

A smecticlike structure of a simple liquid adjacent to a solid surface gives rise to an attenuating oscillating potential in which the free surface of the film fluctuates. These capillary fluctuations lead to critical wetting phenomena of an earlier unknown origin. The corresponding wetting, layering, and surface melting criteria are obtained by the renormalization-group technique.