The clean Mo(100) surface reconstructs at low temperature forming a commensurate c(7 √2 × √2 )R45° displacement wave. LEED studies over the range 10 to 300 K reveal a change in the harmonic content of this wave at intermediate temperatures corresponding to a sharpening of antiphase domain walls as T is lowered. Simultaneously, a selection of one of two possible domain orientations occurs if steps are present. The process shows no hysteresis and permits the equilibrium distribution to be observed.

The surface stoichiometry transition from As-covered 2×4 to In-covered 4×2 on InAs(001) misoriented surface is studied by reflection high-energy electron diffraction. The width of the hysteresis due to the first-order phase transition between these two structures is smaller with a surface misoriented toward the [11¯0] direction than that with an exactly oriented surface. The results from Monte Carlo simulation indicate that this phenomenon can be explained by the finite size effect on the first-order surface stoichiometry transition caused by the existence of monomolecular steps.

The occupancy of the K=0 ground state of the biexciton in CuCl is measured by nonlinear optical methods. An increase of the optical phase conjugation signal is observed when a random high density of biexcitons is injected into the crystal. The corresponding increase of coherency is attributed to a quantum attraction towards a Bose-Einstein condensate of biexcitons.

We report the observation of quasibound states in single semiconductor barriers, in heterostructures with both type-I and type-II band alignments. We disucss the localization and the shape of wave functions of such states, as well as their density of states. In structures with type-II band alignment (CdSeZnTe and CdMnSe/ZnTe) we observe transitions involving electrons confined in the conduction band well and quasibound holes localized in the valence band barrier (both in the CdSe layer). In the system with type-I band alignment (ZnMnSe/ZnSe) transitions between states quasibound in the single ZnMnSe barrier also show clear absorption peaks.

We have investigated the behavior of a laterally confined quantum dot in close proximity to a one-dimensional channel in a separate electrical circuit. When this channel is biased in the tunneling regime the resistance is very sensitive to electric fields, and therefore is sensitive to the potential variations on the dot when it is showing Coulomb blockade oscillations. This effect can be calibrated directly, allowing the Coulomb charging energy to be measured. We also found the activation energy of transport through the dot is much lower than expected.

For the first time the distribution function of local densities of states describing an NMR line is calculated for a system of small metallic particles. It is shown that the Knight shift decreases and the line becomes very broad and asymmetric when decreasing the temperature and particle size.

Quasiparticles in inhomogeneous superconductors are subject to two off-diagonal pair potential forces. One of them, the recently discovered supercurrent force, involves the gauge invariant Cooper pair velocity and causes the exchange of Cooper pair momentum between Andreev scattered quasiparticles and the moving condensate. It is shown in the local model of a moving vortex line that the phenomenologically derived interface force of the Noziéres-Vinen-Warren theory can be explained microscopically by the supercurrent force on quasiparticles inside the vortex core.

We report clear evidence for a vortex ‘‘lock-in’’ state in the organic superconductor (BEDTTTF${)}_2$Cu(SCN${)}_2$, from ac susceptibility measurements with crossed ac and dc fields. A dc field applied parallel to the conducting planes causes a rapid decrease in the screening of the ac field. Identical behavior is initially observed for dc fields applied at an angle, but the screening recovers to nearly the zero-field value when the perpendicular field component exceeds a threshold. This indicates that vortices are locked between the layers below the threshold, and move freely along the planes. Above threshold, the vortices pierce the planes, and their motion is drastically inhibited.

A new critical point is established through Monte Carlo simulations for a class of two dimensional XY models with a modified nearest-neighbor spin interaction. The critical exponents found are ν≊1.4, β≊0.6, and an α<0, consistent with hyperscaling. The phase transition is interpreted as a vortex-unbinding transition. Comparisons are made with the phase diagram of the two dimensional Coulomb gas and with the spin-1/2 XY model.

Exposure of Cs surfaces to ${\mathrm{O}}_2$ causes ejection of ${\mathrm{O}}^{\mathrm{-}}$ ions with low yields (∼${10}^{\mathrm{-}8}$ per incident ${\mathrm{O}}_2$ molecule) during the first stages of dissociative chemisorption (followed by exoelectron emission at higher exposures), although the work function of the surface exceeds the electron affinity of O and the energetics of the overall reaction is almost zero. A mechanism is proposed whereafter the release of ${\mathrm{O}}^{\mathrm{-}}$ is a consequence of strong repulsion in ${\mathrm{O}}_2^{2\mathrm{-}}$ species intermediately formed in front of the surface.

Shape transformations of two-compoent vesicles in which the spontaneous curvature depends on the local compositions in the two monolayers are investigated theoretically. Even if the two components do not exhibit lateral phase separation in spherical vesicles, temperature-induced shape transformations cause lateral phase segregation. Vesiculation of smaller buds is favored in the two-component system. In two-component vesicles with intermonolayer phase separation, thermal budding is dominated by the different thermal expansion coefficients of the two monolayers.

A new and general technique for simulating solid-fluid suspensions, which combines molecular dynamics for the solid particles with a lattice-Boltzmann model for the fluid, is described. The many-body hydrodynamic interactions are fully accounted for, both for small particle velocities and at higher Reynolds numbers. Brownian motion of the solid particles is included by adding a fluctuating component to the fluid stress tensor. Simulations of the dynamics of colloidal particles at short times compare favorably with recent diffusing-wave spectroscopy experiments.

We find that the successive increments in the cardiac beat-to-beat interals of healthy subjects display scale-invariant, long-range anticorrelations (up to ${10}^4$ heart beats). Furthermore, we find that the histogram for the heartbeat intervals increments is well described by a Lévy stable distribution. For a group of subjects with severe heart disease, we find that the distribution is unchanged, but the long-range correlations vanish. Therefore, the different scaling behavior in health and disease must relate to the underlying dynamics of the heartbeat.

The growth of microtubules through the so-called dynamical instability is analyzed within a simple theoretical model in which the polymers are nucleated by a flat surface. For an isolated microtubule the model predicts the existence of a transition between bounded and unbounded growth. It is also shown that this transition alters the assembly of dense structures, e.g., by drastically limiting the number of long microtubules grown from the surface. Coupled to the microscopic biochemical control of the growth, such physical effects seem to play an important role in the regulation of the formation of cellular structures (such as the mitotic spindle).

A class of models of deposition and evaporation of dimers, trimers,..., k-mers is studied analytically and by simulation. Correlation functions decay as power laws in time, related to broken symmetries in associated spin Hamiltonians. For k≥3, the number of jammed and evolving steady states increases exponentially with size. Finite size scaling studies support a phenomenological diffusive picture for dynamics and indicate universality over k in many subspaces.

The 2D ‘‘black hole’’ action is obtained by a nonstandard dimensional reduction of pure 3D gravity with a nonzero cosmological constant. Starting from the Chern-Simons formulation of 2+1 gravity, we obtain the (1+1)-dimensional gauge formulation given by Verlinde. Remarkably, the proposed reduction shares the relevant features of the formulation of Cangemi and Jackiw, without the need for a central charge in the algebra. The Lagrange multipliers in these formulations appear naturally as remnants of the three-dimensional theory. The proposed dimensional reduction involves a shift in the three-dimensional connection whose effect is to make the physical length of the extra dimension infinite.

The Weyl curvature inside a black hole formed in a generic collapse grows, classically without bound, near to the inner horizon, due to partial absorption and blueshifting of the radiative tail of the collapse. Using a spherical model, we examine how this growth is modified by quantum effects of conformally coupled massless fields.

We show that the present limit from the CLEO Collaboration on the inclusive decay b→sγ provides strong constraints on the parameters of the charged Higgs boson sector in two-Higgs-doublet models. Only a slight improvement in the experimental bound will exclude the region in the supersymmetric Higgs boson parameter space which is inaccessible to collider searches.

A search for the decay ${\mathit{K}}_{\mathit{L}}^0$→${\mathrm{\mu }}^{\pm }$${\mathit{e}}^{\mp }$ with significantly increased experimental sensitivity has yielded no events. The 90% confidence level limit on the branching ratio is B(${\mathit{K}}_{\mathit{L}}^0$→${\mathrm{\mu }}^{\pm }$${\mathit{e}}^{\mp }$)<3.9×${10}^{\mathrm{-}11}$. When this data set is combined with earlier data the upper limit is 3.3×${10}^{\mathrm{-}11}$.

A search for unconfined fractional charges in Si based on fractional-charge impurity energy-level predictions of Chaudhuri, Coon, and Derkits [Phys. Rev. Lett. 45, 1374 (1980)] is described. An upper limit (95% C.L.) of 2.3×${10}^{\mathrm{-}20}$ fractional charges per atom is obtained using a combination of an infrared photoionization and a field-ionization technique. This optoelectronic approach has the advantage of giving an estimate for the concentration of fractional charges, being repeatable and without mechanical techniques. By comparison, drop type experiments detect the presence of fractional-charge modulo one unit of fractional charge.