The energetics and structures of metal clusters are studied by the Car-Parrinello method with Al chosen as a paradigm. For the 13- and 55-atom clusters which can assume perfect icosahedral and cubic structures, the distortions from these ideal structures are substantial. For the 55-atom cluster, several inequivalent but energetically nearly degenerate structures are found. The origin of this near-degeneracy is traced to the short range of the effective interatomic interactions, which provides an ab initio justification for their parametrization in terms of short-range potentials.

We calculate the critical exponents at the liquid-vapor critical point by using the classical ingredients of the liquid-state theory. Two coupling constants are defined at a microscopic level. The closure of the Ornstein-Zernike equation is given by the Callan-Symanzik equation from which we determine the position of the fixed point. The role of the three-body direct-correlation function is emphasized. A comparison between this work and the standard theory of critical phenomena based on the Landau-Ginzburg-Wilson Hamiltonian is presented.

We have derived the expression of the Ruderman-Kittel-Kasuya-Yosida coupling between two ferromagnetic layers separated by a nonmagnetic spacer, for arbitrary crystal structure and Fermi surface. For the first time, quantitative predictions of the interlayer coupling for Cu, Ag, and Au spacers are given. The results are in good agreement with recent experimental data for fcc Co/Cu and Fe/Cu systems.

We have measured the remanent magnetization of single-crystal EuS from ${10}^{\mathrm{-}4}$ to ${10}^4$ sec after removing an applied field. Using a recent model for magnon relaxation on finite domains, the data yield size-scaling exponents of θ=-0.10±0.02 and ζ=0.669±0.004, in excellent agreement with exact theoretical predictions. As a function of temperature, the remanent magnetization is a maximum above the Curie transition, and for some thermal histories, becomes negative in the ferromagnetic regime. The behavior is consistent with a percolation backbone that is negatively coupled to the finite domains.

First-principles electronic-structure calculations for crystalline ${\mathrm{K}}_6$${\mathrm{C}}_{60}$ are reported, based on the geometry determined by x-ray-diffraction measurements. The ground state is found to be insulating, with an indirect gap of 0.48 eV. The character of both occupied and unoccupied states near the valence-band maximum is completely carbonlike. The K atoms are almost fully ionized, and the ${\mathrm{C}}_{60}$ molecule is stabilized in an unusually high charge state with six additional electrons. Implications of the distribution of excess charge on the ${\mathrm{C}}_{60}$ molecule for orientational ordering are discussed.

Approximately 0.5% of nonmagnetic divalent ions Zn, Cd, and Hg have been alternatively substituted for Ni in the linear chain of antiferromagnetically coupled ${\mathrm{Ni}}^{2+}$ (S=1) spins in [Ni(${\mathrm{C}}_2$${\mathrm{H}}_8$${\mathrm{N}}_2$${)}_2$(${\mathrm{NO}}_2$)]${\mathrm{CIO}}_4$ (NENP). While at low temperature the pure NENP shows no EPR signal, the doped NENP shows identical EPR spectra corresponding to the fractional spin S=1/2 states at the ‘‘open’’ ends of the Ni chains adjacent to the nonmagnetic substitutions.

The boundary between electric-field domains in semiconductor superlattices represents a tunneling barrier. While most of the superlattice is coupled resonantly the current through the superlattice is limited by nonresonant tunneling at the domain boundary. The emitter and collector are purely two dimensional and the system therefore acts as a model system for tunneling between 2D systems. For magnetic fields applied parallel to the layers the average current through the single barrier increases, in contrast to 3D and quasi-2D emitters.

As Ce is adsorbed on a W(110) substrate at 300 K an epitaxial layer develops at a coverage of 0.5 monolayer. LEED patterns indicate a hexagonal pattern consistent with Ce(111). As Ce is added, the lattice parameter shrinks continuously from 9% larger than that for γ-Ce to 3% smaller than that for α-Ce. 4f photoelectron spectra reveal two peaks similar to those of γ-Ce, changing relative intensity with increasing coverage to resemble those of α-Ce.

We have observed a quantized current in a lateral quantum dot, defined by metal gates in the two-dimensional electron gas (2DEG) of a GaAs/AlGaAs heterostructure. By modulating the tunnel barriers in the 2DEG with two phase-shifted rf signals, and employing the Coulomb blockade of electron tunneling, we produced quantized current plateaus in the current-voltage characteristics at integer multiples of ef, where f is the rf frequency. This demonstrates that an integer number of electrons pass through the quantum dot each rf cycle.

We present data on the low-frequency noise of the high-field conducting state in the insulating phase around the 1/5 fractional quantum Hall liquid at B=11.9 T and how it correlates with the strongly nonlinear I-V characteristic. The noise is characteristic of a weakly pinned two-dimensional Wigner crystal reentrant around the 1/5 liquid. For v<1/5, it reaches a peak and then vanishes in the more insulating regime (B≳12.7 T in our sample) where the system is better described as a 2D Wigner glass.

The response of the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ lattice to superconducting order is studied using an ultrahigh-resolution capacitance dilatometer. The onset of superconductivity is accompanied by highly anisotropic jumps of the expansivities in the a-b plane. This leads to a reduction of the orthorhombic splitting below ${\mathit{T}}_{\mathit{c}}$, which suggests that superconductivity favors a symmetric (b=a) ${\mathrm{CuO}}_2$ plane. Little effect is seen along the c axis. Superconducting fluctuations are for the first time clearly observed in the lattice expansion.

The electronic structure of the formally ${\mathrm{Cu}}^{3+}$ (${\mathit{d}}^8$) compound ${\mathrm{NaCuO}}_2$ has been studied by photoemission spectroscopy and subsequent cluster configuration-interaction calculations. We find that the ${\mathit{d}}^8$→${\mathit{d}}^9$L (L: ligand hole) charge-transfer energy is negative and the ground state is dominated by the ${\mathit{d}}^9$L configuration. The band gap corresponds to charge fluctuations of the type ${\mathit{d}}^9$L+${\mathit{d}}^9$L→${\mathit{d}}^9$+${\mathit{d}}^9$${\mathit{L}}^2$ (p-p type), being neither of the Mott-Hubbard (d-d) type nor of the charge-transfer (p-d) type. The gap is stabilized and destablized by the intracluster and intercluster hybridizations, respectively.

The energy spectra of isolated one-dimensional (1D) electron channels have been studied by photoluminescence in zero and finite magnetic fields B. Arrays of periodic quantum wires (period =290–400 nm, width =150–170 nm) were etched from modulation-doped ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs heterojunctions, with special low-concentration Be δ doping in the buffer layer to increase the photoluminescence intensity. We were able to attain the 1D quantum limit, i.e., occupation of only the lowest 1D subband at B=0, and to measure, at small B, a characteristic quadratic B dispersion of the 1D subband edge.

High-quality, single-crystalline films of Fe(110)/Ag(111) were grown by molecular-beam epitaxy in the range of ∼1–3 monolayers of Fe, and investigated in situ by means of the surface magneto-optic Kerr effect. The magnetization exhibits a second-order phase transition at the thickness-dependent Curie temperature. The value of the critical exponent β of 0.137±0.008 is in good agreement with that of the two-dimensional Ising model.

The local density of states of a superconducting vortex core has been measured as a function of disorder in the alloy system ${\mathrm{Nb}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ta}}_{\mathit{x}}$${\mathrm{Se}}_2$ using a low-temperature scanning tunneling microscope. The peak observed in the zero-bias conductance at a vortex center is found to be very sensitive to disorder. As the mean free path is decreased by substitutional alloying the peak gradually disappears and for x=0.2 the density of states in the vortex center is found to be equal to that in the normal state. The vortex-core spectra hence may provide a sensitive measure of the quasiparticle scattering time.

Experimental and ab initio theoretical near-edge x-ray-absorption fine-structure (NEXAFS) spectra of ${\mathrm{C}}_2$/Ag(110) are presented. The results show that chemisorption can strongly affect the NEXAFS of a molecular species. From examination of the molecular orbitals associated with the resonance structures of gas phase and adsorbed ${\mathrm{C}}_2$ it is further demonstrated that near-edge spectroscopy can be used to gain information about the mechanism of molecule-substrate interaction.

Light coupling to energy fluctuations in a nonparabolic conduction band is found to yield a smooth Raman spectrum extending to an energy of 1 eV in the case of intermediate electron-electron scattering between nearly parallel ‘‘nested’’ sections of the Fermi surface. Nesting creates a frequency-dependent damping that produces a Raman line shape which exhibits a surprising proportionality to the optical conductivity. Evidence for the anomalous scattering is found in data on the high-temperature superconductor ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ and in some rare-earth metals.

We study a model of Fichthorn, Gulari, and Ziff for the reaction A+B→0. Their numerical studies showed a transition to a bistable state. We solve the model exactly. Our solution is similar in form to that in diffusion-limited reactions and segregation occurs for low-dimensional substrates. The transition to bistability and saturation for finite-size systems at low desorption probability depend on dimension. The power spectrum of the saturation and the reaction rate are different: The first is universal and the other is sensitive to the geometry of the substrate.

Results of Monte Carlo simulations of folding of a model ‘‘protein,’’ which is a freely joined 27-monomer chain on a simple cubic lattice with nearest-neighbor interactions, are reported. All compact self-avoiding conformations on this chain have been enumerated, and the conformation (‘‘native’’) corresponding to the global minimum of energy is known for each sequence. Only one out of thirty sequences folds and finds the global minimum. For this sequence, the folding process has a two-stage character, with a rapid noncooperative compactization followed by a slower transition over a free-energy barrier to the global minimum. The evolutionary implications of the results are discussed.

We describe a class of nonlinear birational representations of groups generated by a finite number of involutions. These groups are symmetries of the Yang-Baxter equations and their higher-dimensional generalizations. They provide discrete dynamical systems with a variety of behaviors, from chaotic to integrable, according to the number of invariants of the representation.