The infrared spectra of adsorbed hydrogen and deuterium on c(2×8) and (2×6) GaAs(100) contain a series of bands from 2200 to 1200 (1600 to 1000) ${\mathrm{cm}}^{\mathrm{-}1}$ that are due to arsenic hydrides, terminal gallium hydrides, and bridging gallium hydrides (and deuterides). The latter is the first known example of bridge-bonded hydrogen on a semiconductor surface. Polarized spectra reveal that the AsH and GaH bonds orient along the [1¯10] and [110] axis, respectively. These results are consistent with a GaAs surface structure composed of As and Ga dimers with dimer bonds in the [1¯1 0] and [110] directions.

We studied the self-diffusion at clean, flat as well as stepped Al(111) surfaces using density-functional theory. There are two important steps on fcc(111) surfaces, typically labeled according to their {111} and {100} microfacets. We calculated the formation energies of these steps, and analyzed the diffusion perpendicular and parallel to them. We discuss the general profile of the diffusing-atom potential-energy surface and identify the role of ‘‘normal’’ hopping and exchange mechanisms of diffusion at steps. From these results the equilibrium shape of islands and the temperature dependence of the island shapes under growth conditions are predicted.

The oxygen-induced breakup of vicinal Ag(110) surfaces into (110) facets and step bunches is measured with scanning tunneling microscopy. Faceting transitions are recorded in real time at room temperature for two surface orientations. A vicinal Ag(110) surface misoriented by 2° towards [001] facets via the growth of isolated nuclei. Step-position fluctuations of increasing magnitude are observed prior to faceting, revealing a critical terrace width of ∼100 Å. In contrast, oxygen exposure causes Ag(110) misoriented by 2° towards [33¯1] to facet spontaneously with no evident nucleation barrier.

Using an ab initio potential we perform classical and path-integral Monte Carlo simulations together with finite-size scaling to investigate the head-tail ordering of CO molecules in a full monolayer on graphite. In accordance with experiments, the transition is found to belong to the 2D-Ising universality class and occurs in the correct temperature range; the quantum shift of the transition amounts to 10%. Beyond what is known from experiments we predict a particular ferrielectric ground state, the critical amplitude of the order parameter, and that the ordering is caused by molecular shape asymmetry rather than electrostatic dipole forces.

We show that in epitaxial growth, there is a critical island size ${\mathit{R}}_{\mathit{c}}$ at which a second layer nucleates on top of the island. This provides a simple perspective on the conditions for smooth versus rough growth: For island spacing ${\mathit{L}}_{\mathit{n}}$, if ${\mathit{R}}_{\mathit{c}}$<${\mathit{L}}_{\mathit{n}}$ the islands will nucleate a second layer before coalescence, giving multilayer growth. This occurs when there is a sufficiently large diffusion barrier (the ‘‘Schwoebel barrier’’) at the island edge. We demonstrate that surfactants can enforce layer-by-layer growth by reducing the island spacing. The dependence upon temperature is also explained.

In a high magnetic field, such that the distance between the Landau levels exceeds the exciton Rydberg, the triplet interaction term becomes the lowest state of the system. Under these circumstances, a weak pair interaction in the triplet ground state opens a new opportunity to forming the Bose-Einstein condensate and a superfluid state of excitons at a relatively high temperature. The existence of the Bose condensate due to an essential decrease of the interaction between excitons and an increase of their binding energy in a high magnetic field are established. We show that the excitation spectrum satisfies the Landau criterion for superfluidity, and discuss the observable effects of the Bose condensate of excitons.

We report here our discovery of bulk superconductivity at rather high ${\mathit{T}}_{\mathit{c}}$ in samples having nominal compositions ${\mathrm{YNi}}_4$${\mathrm{BO}}_{0.2}$ (${\mathit{T}}_{\mathit{c}}$≊12.5 K) and ${\mathrm{YNi}}_2$${\mathrm{B}}_3$${\mathrm{O}}_{0.2}$ (${\mathit{T}}_{\mathit{c}}$≊13.5 K). While ${\mathrm{YNi}}_4$${\mathrm{BC}}_{0.2}$ seems to be a single phase material, ${\mathrm{YNi}}_2$${\mathrm{B}}_3$${\mathrm{C}}_{0.2}$ is a multiphase system. Our experimental results show that the two materials are distinct superconductors. Discovery of superconductivity in these materials is of significance since not only is their ${\mathit{T}}_{\mathit{c}}$ high (>10 K) but they also have nickel in large proportions. No nickel-based ternary superconductor was previously known.

We report on a small angle neutron scattering study of the flux-line lattice in single crystal 2H-${\mathrm{NbSe}}_2$. As the magnetic field is tilted away from the crystalline c axis, we find distortions in the flux lattice as would be expected for a mass anisotropy Γ=10.1±0.9. However, we find that the lattice orientation is in disagreement with the predictions of both anisotropic London and Ginzburg-Landau theories. The observed flux lattice orientation remains pinned to that of the crystal lattice for the field orientations studied. The form factors can be quantitatively understood within the framework of Ginzburg-Landau corrections to the London equations.

Nanoparticles of metallic iron on carbon supports have been studied insitu by use of Mössbauer spectroscopy. The magnetic anisotropy energy constant increases with decreasing particle size, presumably because of the influence of surface anisotropy. Chemisorption of oxygen results in formation of a surface layer with magnetic hyperfine fields similar to those of thicker passivation layers, and with a ferromagnetic coupling to the spins in the core of the particles. In contrast, thicker passivation layers have a noncollinear spin structure.

Electron nuclear double resonance spectra in stretch-oriented undoped poly(paraphenylene vinylene) films show clear anisotropy of an unpaired π electron on the polymer chain. The maximum value of the spin density reaches 0.09, which corresponds to the half-width of the spatial extension of the spin over several phenyl rings. Comparison between the dark and photoinduced ESR signals strongly suggests that the observed spins are trapped polarons.

The theory described here shows that the muon spin depolarization rates measured on gadolinium in the critical paramagnetic region give direct information on the longitudinal (along the wave vector q) spin fluctuations. In the Ising regime it is possible to distinguish the parallel (along the c axis) from the perpendicular fluctuations.

The local Zn/Se relative concentration at the interface in ZnSe-GaAs(001) heterostructures synthesized by molecular beam epitaxy was found to be controlled by the Zn/Se flux ratio employed during the early growth stage of ZnSe on GaAs. Correspondingly, the valence band discontinuity varies from 1.20 eV (Zn-rich interface) to 0.58 eV (Se-rich interface). Comparison with the results of first-principles calculations suggests that the observed trend in band offsets is related to the establishment of neutral interfaces with different atomic configurations.

We report the first observation, by computer simulation, of a purely entropic demixing transition in a three-dimensional binary hard-core mixture. This transition is observed in a mixture of large and small cubes. We also find evidence for demixing in other hard-core fluids and, in the case of an athermal polymer solution, we observe a purely entropy-driven polymer collapse. For the study of both the hard-core demixing and polymer collapse, it was essential to use novel collective Monte Carlo moves.

Femtosecond laser pulses, resonant with the Soret bands of myoglobin (Mb) and cytochrome c, are used to probe coherent low frequency nuclear motion of the heme group. The time domain analysis is in good agreement with frequencies obtained independently using spontaneous resonance Raman spectroscopy. The deoxyMb data reveal a strong oscillation near 300 fs (∼ 100 ${\mathrm{cm}}^{\mathrm{-}1}$) and a persistent feature also appears near 50 ${\mathrm{cm}}^{\mathrm{-}1}$. This is near the frequency expected for heme doming motion, which has been associated with the ligand binding reaction coordinate of Mb.

In atomic Stern-Gerlach (polarization) interferometry with time-dependent magnetic fields both the spatial and temporal parts of the atomic phase are generally affected. This leads to a total energy shift and to an inelastic momentum transfer. Some of the related effects are studied using a beam of metastable hydrogen atoms. In particular the scalar Aharonov-Bohm effect and its nondispersivity are considered, in addition to other phase shift effects occurring where the field gradient differs from zero.

Neutron reflectometry is used to obtain a direct measurement of the monomer distribution of a weakly bound end-grafted polystyrene layer in equilibrium with bulk solution, as a function of the polymer concentration and temperature. We report for the first time two new regions in the density profile of the polymers, an excess polymer near the surface and a wetting layer on top of the brush. The physical properties of the polymer layer are strongly affected by the equilibrium between the bound and free polymer molecules.

We describe a nonequilibrium phase winding of the liquid-crystal director at a flat cholesteric-isotropic interface. The physical origin of the driving force is a symmetry-allowed coupling between an equilibrium helix structure and gradients in impurity concentration set up at an interface forced to move by displacing the sample in a temperature gradient. We interpret these observations to evaluate the coupling constant: ‖${\nu }_{\mathit{c}}$‖=3×${10}^{\mathrm{-}4}$ dyn/cm. A second transition is observed at higher displacement speeds to an orientationally disordered state.

Damage spreading (DS) introduces a new dynamic critical phenomenon in both an irreversible monomer-monomer (MM) reaction process and a MM process with one species desorption (MMD). For the MMD model the critical point of the frozen-chaotic transition and the order parameter critical exponent are determined. Also, at criticality the damage is healed according to a power-law decay with a characteristic exponent. For the MM (MMD) model the moments of the damage distribution probability exhibit multifractal (single gap) scaling behavior. DS on a dimer-monomer process (the Ziff-Gulari-Barshad model) is also studied.

The growth of a Cu(100) crystal has been investigated with helium atom beam scattering in real time and examined in the light of the dynamical scaling hypothesis. The associated exponents have been determined. The analysis of both terrace and step correlations during growth provides a detailed scenario of how a singular surface can transform into an arrangement of vicinal surfaces upon unstable growth, resulting in a pyramidlike surface profile. The sides of these pyramids are composed of the (113) and (115) Cu surface for deposition at 160 and 200 K, respectively.

We consider the growth of films by molecular beam epitaxy in the presence of step-edge (Schwoebel) barriers using numerical simulation and experiments. We show that the growth of a singular surface is unstable, but that a miscut above a certain critical slope (which depends on growth conditions) leads to stable growth in a step-flow mode. For singular surfaces the instability gives rise to the formation of large mounded structures on the surface for which the slope is in the stable regime. We identify these in GaAs epitaxy using atomic force and scanning tunneling microscopy. We propose a continuum equation which exhibits these features.