The transition to spatiotemporal chaos in the neighborhood of a codimension-two point observed in the Taylor-Dean system is investigated. The initial instability is to stationary Dean vortex flow, which becomes unstable to a drift instability, followed by a wavelength-halving instability. This sequence of transitions may be explained in terms of the interaction between the first and second spatial harmonics of the basic pattern.

It is shown that lasing in free-electron devices can be attained without the standard population inversion between the two portions of the electron momentum distribution that contribute to stimulated emission and absorption, respectively. Coherent superpositions of two electronic states in appropriately designed wigglers can strongly suppress stimulated absorption without hampering stimulated emission. The resulting gain curve is symmetric about the emission resonance, and yields a much larger gain than the antisymmetric gain curve of a standard free-electron laser with the same parameters.

Scanning-tunneling-microscope tip-induced migration of divacancies is observed on n-doped GaP(110). The motion occurs only at negative polarity of the tunneling voltage and along the [11¯0] direction, independent of the scanning direction, and is thus purely nonthermal. It is suggested that a field-induced reduction of the barrier for the migration initiates the motion. A quantitative analysis of the migration indicates long-lived excited states, consistent with charged trap states in doped semiconductors.

We present the first scanning tunneling microscopy (STM) study which allows clear discrimination of two chemical species in a metal alloy. Special tunneling conditions, which we attribute to an adsorbate at the STM tip, cause a difference in corrugation between Pt and Ni atoms of 0.3 Å. The STM data reveal chemical short-range order at the surface, which is in agreement with embedded atom simulations and can be understood as small domains of an L$1_0$ ordered phase.

Smectic-A textures limited by special boundaries which do not fix the layer positions but only the direction of the layer normals should obey a simplified elasticity. This elasticity involves no layer dilation but only curvature energy. We present it for smectics limited to planar cells, when all the singularities due to the absence of layer dilation are virtual. We show the possibility of anchoring transitions at low threshold and the existence of a universal cycloid shape in the absence of external constraints. The link with the nucleation of smectic focal conics is discussed.

Reciprocating shear is found to increase the isotropic-to-lamellar transition temperature in a symmetric diblock copolymer melt. The temperature at which the isotropic state becomes unstable rapidly approaches the ordering transition as the shear rate increases. Shear-induced ordering results in lamellae orientation with wave vectors directed normal to the shear and velocity gradient directions. These results are anticipated by a recent theory that accounts for the suppression of fluctuations by a symmetry breaking field in this class of weakly first-order phase transitions.

Polyethylene propylene films of various molecular weights, which would normally wet native-oxide-covered Si surfaces, were observed to dewet the surface when the film thickness became less than the polymer radius of gyration. These films could be made to wet either by increasing the annealing temperature or by chemically modifying the surface. The results are shown to be consistent with an expression for the spreading parameter that incorporates a stretching free energy term for the polymer chains. Measurements of the diffusion constant of the polymer on the silica surface indicated that an activation energy was required to desorb the polymer segments from the surface before diffusion could occur.

Inelastic neutron scattering yielded the first information on superconductivity-induced frequency shifts for q≠0 phonons in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$. For branches starting from the Raman-active mode at 340 ${\mathrm{cm}}^{\mathrm{-}1}$ (10.2 THz), the shifts were found to decrease only slowly with increasing momentum transfer (x=7.0) or even exhibit a maximum at finite (x=6.92) when going along the [100] and [010] directions. They were independent of q in the [001] direction. The shifts were much smaller for the ${\mathit{A}}_{\mathit{u}}$ mode at 307 ${\mathrm{cm}}^{+1\mathrm{}}$ and the ${\mathit{E}}_{\mathit{u}}$ mode at 343 ${\mathrm{cm}}^{\mathrm{-}1}$ and for q≠0 phonons related to these modes.

We study a system in which diffusing particles (species A) are injected into a reactive d-dimensional substrate (species B) at rate λ, with the rule that A+B→C (inert). The amount of species C, C(t), and the number of surviving A particles, A(t), are calculated for substrate dimensions d=1, 2, and 3. We find the surprising results A(t)∼${\mathit{t}}^{2/3}$ for d=3 and C(t)∼ √t lnt for d=1. We confirm our predictions by performing Monte Carlo simulations for d=1, 2, and 3 and experiments for the reaction ${\mathrm{I}}_{2\left(\mathrm{gas}\right)\mathrm{}}$+${2\mathrm{A}\mathrm{g}}_{\mathrm{}\left(\mathrm{solid}\right)\mathrm{}}$→${2\mathrm{A}\mathrm{g}\mathrm{I}}_{\mathrm{}\left(\mathrm{solid}\right)\mathrm{}}$ for d=2.

I study a nonequilibrium lattice model, the pair contact process, in which pairs of particles annihilate with probability p or else create a particle at a vacant nearest neighbor. The model exhibits a continuous phase transition from an active state, with an ongoing production of particles, to an absorbing state without pairs. The model has infinitely many absorbing states. Computer simulations in 1D yielded critical exponents consistent with directed percolation, for the first time placing a model with infinitely many absorbing states firmly in the directed-percolation universality class.

Brightness traces of the solidification of highly undercooled Ta and Re droplets processed in an ultrahigh vacuum drop tube give evidence of two successive phase transformations. First-principles calculations of the structural stability of these metals are developed in order to appreciate the possibilities of obtaining alternative phases. Some prototype transition metal crystal structures were examined and the fcc phase for the Re and the A15 phase for Ta are possible candidates for the observed metastable phases. Good agreement is obtained between theoretical calculations and experimental measurements of their respective melting temperatures.

We report results from an experimental study of coarsening in thin layers of succinonitrile in the presence of impurities. We quench the latter from the liquid phase to different temperatures within the liquid-solid coexistence region thus controling the solid area fraction φ. As φ is increased from 0.13 to 0.40, liquidlike order develops among the coarsening crystals due to diffusional interactions. The latter give rise to local correlations between the size and rate of growth of crystals within a neighborhood of size ξ(φ). We study these and compare our findings with recent theoretical models of coarsening.

The dynamics of simultaneous ordering and phase separation in the presence of a long-range Coulomb interaction is investigated by a computer simulation. It is shown that an intermediate nonstoichiometric ordered single phase appears as a transient phase during decomposition of a binary homogeneous disordered solid solution into an equilibrium two-phase mixture of ordered and disordered phases. Introduction of the long-range Coulomb interactions results in a mesoscale phase which consists of periodical arrays of ordered phase particles embedded in a disordered matrix.

Magnetic ordering in solid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ has been studied by measuring the melting pressure through the various phase transitions. We find a discontinuity in entropy at the high-field–paramagnetic phase transition at low fields, indicating that it is first order. The entropy discontinuity decreases as the field increases and the transition broadens, becoming second order above a field of about 0.65 T. The first precise determination of the first-order phase boundaries has been made, from which thermodynamic data not previously available are obtained.

Three parameters determine the geometry of monolayers in oil-brine-surfactant mixtures dilute in surfactant: the mean bending elastic modulus K, the saddle-splay modulus K¯, and the spontaneous curvature ${\mathit{C}}_0$ of the monolayer. It is found that K¯ and ${\mathit{C}}_0$ of an AOT monolayer vary linearly with ${\mathit{S}}^{\mathrm{-}0.5}$ and ${\mathit{S}}^{0.5}$, respectively (S is the salt concentration) as predicted for monolayers with high charge densities. K¯ is positive for a long-chain oil (12 carbon atoms) and negative for a shorter-chain oil in agreement with the geometry of the phases obtained at optimal salinity where ${\mathit{C}}_0$=0.

Large terraces of the Pt(111) surface are shown to reconstruct in the presence of a Pt gas- phase environment down to 400 K. The lifting of the reconstruction in the absence of the Pt gas-phase is kinetically hindered up to 700 K. The main features of the reconstruction network as seen by the scanning tunneling microscope correspond to those inferred from x-ray scattering for the high-temperature (>1330 K) spontaneous Pt(111) reconstruction. The parallel double corrugation lines forming the network have basically the same structure as those seen on the reconstructed Au(111) surface.

We have studied the growth of Mg overlayers on Pd(111), a system with 16.6% misfit, by means of x-ray photoelectron diffraction. No long-range order is observed by low-energy electron diffraction. Pronounced forward-focusing maxima in Mg 1s emission indicate considerable short-range order. At coverages below 1 monolayer Mg is found to occypy fcc sites distributed over two layers. At higher coverages oriented clusters are formed, accommodating the misfit by alternatively occupying fcc and hcp sites. Comparison to single-scattering calculations suggests an average cluster size of less than 15 Å.

Using a tight-binding Hamiltonian we study the Peierls distorted form of polyaniline, the pernigraniline base polymer. Both bond order and ring torsional degrees of freedom are considered in the conformational relaxation. The geometrical and electronic structures of the localized excitations are calculated. Comparison with available optical and magnetic measurements is made. Particular emphasis is placed on the spectroscopic and magnetic features of neutral solitons.

Transitions between two quantum Hall states or between a quantum Hall state and a Mott insulator induced by periodic potentials are studied in the 1/N expansion. The transitions are found to be continuous in the large-N limit and are described by a critical point that depends on a real parameter θ, which is determined by the topological orders in the quantum Hall states involved in the transition. Some critical exponents and universal quantities are calculated in the large-N limit and shown to be θ dependent.

Theory predicts that an electron or hole added to a conjugated polymer chain causes the chain to distort, forming a large polaron. Recent calculations of the three-dimensional band structure of trans-polyacetylene, t-PA, and poly(phenylene vinylene), PPV, have obtained values for the interchain coupling that are large enough to destabilize the polaron. Nevertheless, there is evidence that polarons exist in these materials. We show that the existence of chain endings and other conjugation breaks can stabilize the polaron in actual samples.