A hydrodynamic model for turbulence in dilute polymer solution is used to describe drag reduction. It is shown that flexible polymers lead to an enhancement of molecular viscosity at small length scales while at intermediate length scales the effective viscosity is decreased. The onset of drag reduction is predicted to occur only above a minimum value of the polymer concentration, ${\mathit{c}}_{\mathit{m}}$. A direct calculation of the dependence of ${\mathit{c}}_{\mathit{m}}$ on the energy input per unit mass, ɛ, shows that ${\mathit{c}}_{\mathit{m}}$∼${\mathrm{\varepsilon }}^{\mathrm{-}1}$. Comparing this result with the scaling theory of de Gennes allows us to conclude that the exponent describing the power-law dependence of polymer deformation on spatial scale is 4/5.

Long-time precursors of order 5 times the global energy confinement time are observed for disrupting plasmas at the high-density limit on the Texas Experimental Tokamak. These precursors, occurring well in advance of any change in the plasma MHD activity, are reflected in the electron particle and heat transport along with the density fluctuation level. Enhanced microturbulence is proposed as the physical mechanism for the confinement degradation and subsequent disruption.

A novel remote suppressor consisting of an injected ion beam has been used for the stabilization of a plasma instability. Plasma instabilities can, for the first time, be remotely suppressed by modulating the ion beam with an appropriately phased and amplified feedback signal derived from a sensor probe. A collisionless curvature-driven trapped-particle instability has been successfully suppressed down to noise levels using this scheme. The ion beam causes only a small perturbation to the plasma, on the order of 1%, so that the plasma equilibrium parameters are not significantly affected.

Multiscaling is shown to be a consequence of multifractality when a lower cutoff ɛ is introduced in calculations of correlation functions. After a suitable rescaling, the correlation function data for different values of ɛ seem to fall onto a single curve. In the multiscaling regime, however, we show that there is not a unique functional form at varying ɛ, but a spread very close to a single curve. For each ɛ, this curve can be computed analytically in terms of the f(α) spectrum which characterizes the multifractal. Part of this spectrum can thus be obtained by computing only one moment of the weights at ɛ≠0.

Many substitutional impurities in crystalline solids diffuse by a hybrid mechanism involving a fast-migrating intermediate species. Nonequilibrium measurements of the migration frequency g and migration length λ for such an impurity can provide definitive data on atomic-scale diffusion mechanisms. We report the first experimental study to exploit this approach. The system investigated is B in crystalline Si. B is shown to diffuse predominantly via a migrating interstitial species ${\mathrm{B}}_{\mathit{i}}$, generated by a kick-out reaction. The observed mechanism and energetics agree with recent total-energy calculations.

Field-ion-microscope observations provide direct evidence that Pt adatoms replace Ni(110) surface atoms at temperatures above 105 K. The directions of the individual displacements in this replacement process are consistent with a model involving concerted motion of the adatom and substrate atom. The activation barrier for the exchange is estimated to be 0.28 eV.

An outstanding goal in the study of crystal growth in the theoretical determination of optimal growth conditions for a specific surface morphology. We have developed a (1+1)-dimensional crystal-growth model which includes both a tunable substrate temperature and growth rate. A kinetic phase diagram, which reveals the dependence of the interface width dynamical exponent on temperature and growth rate, is constructed. The phase diagram contains two limiting phases described by different exponents. It is also demonstrated that the interface width can be minimized through a suitable choice of substrate temperature and deposition rate.

Density-functional calculations with simulated annealing have been performed for aluminum clusters ${\mathrm{Al}}_{\mathit{n}}$ up to n=10. There are many local minima in the energy surfaces, with a rich variety of structures and spin multiplicities. With increasing cluster size we find transitions from planar to nonplanar structures at n=5, and to states with minimum spin degeneracy at n=6. There are stable isomers of ${\mathrm{Al}}_{5–}$${\mathrm{Al}}_{10}$ with buckled planar structures reminiscent of the layers in crystalline α-gallium. All structures show regular patterns of bond and dihedral angles. Trends in binding and ionization energies are compared with experiment and with the predictions of other calculations.

Unlike ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$, ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ba}}_{\mathit{x}}$${\mathrm{CuO}}_4$, x∼${\mathit{x}}_{\mathrm{cr}}$=0.12, undergoes a structural change to a low-temperature tetragonal (LTT) phase below 60 K, and this transformation is known to suppress ${\mathit{T}}_{\mathit{c}}$ drastically. We present self-consistent local-density-functional calculations for this system that indicate the ${\mathrm{CuO}}_6$ octahedrons are unstable to an X-point tilt in any direction, thereby leading to strongly anharmonic X-point vibrations. The LTT phase is most favored energetically, consistent with experiment. The effect of the LTT distortion on the electronic structure accounts for the suppression of ${\mathit{T}}_{\mathit{c}}$ in the LTT phase, and for the enhanced isotope effect at x=${\mathit{x}}_{\mathrm{cr}}$.

We have observed the desorption of ${\mathrm{O}}^{\mathrm{-}}$ and ${\mathrm{Si}}^{\mathrm{-}}$ ions during the electron bombardment of ${\mathrm{SiO}}_2$, and have determined ion yields and kinetic-energy distributions at electron energies >100 eV. The threshold energy for the ${\mathrm{O}}^{\mathrm{-}}$ yield corresponds to the excitation of Si-L-shell core levels. We propose that multielectron excitations cause the ejection of positive ions or neutral atoms from the surface, and that these species can capture electrons in the surface region to form negative ions. Alternatively, the creation of superexcited electronic states of an ${\mathrm{SiO}}_2$ surface complex may lead to the ejection of ${\mathrm{O}}^{\mathrm{-}}$.

In this Letter we stress the importance of final-state effects in photoelectron spectroscopy. In particular, we address the problem of Schottky-barrier formation, as studied via core-level shifts in photoemission. We have calculated the shift of the core-level distribution when a semiconductor surface is covered with a metal, using a wave-vector-dependent image-screening model. We conclude that final-state effects, which are generally neglected in this context, are in fact quite important. This conclusion is supported by experimental observations reported in the literature.

We show that the density for Wigner crystallization to occur for the two-dimensional electron liquid in zero magnetic field can be increased in suitable multiple-quantum-well structures. Data from Monte Carlo calculations are used to determine the properties of each layer in isolation, and the layers interact through Coulomb forces. With this mechanism the maximum solidification density can be raised by as much as a factor of 3. At higher densities, charge-density-wave ground states can occur.

The structure and magnetic properties of yttrium diphthalocyanine [${\mathrm{YPc}}_2$]⋅${\mathrm{CH}}_2$${\mathrm{Cl}}_2$ are reported. Phthalocyanine macrocycles are stacked above each other in one direction, with a staggered angle of 45°. One-dimensional organic ferromagnetism is emphasized for the first time, and discussed on the basis of an orbitally degenerate model. A comparison with the behavior of the nonsolvated derivative α-${\mathrm{YPc}}_2$ is reported.

The spin-canting anomaly, in which fine ferrimagnetic iron-oxide particles are not completely saturated by large magnetic fields, is usually attributed to noncollinear surface moments. Using a simple mean-field model we show that the anomaly is in fact a routine manifestation of the response of a uniaxial ferrimagnetic powder to an applied field. This view is supported by the modeling of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ Mössbauer spectra of Co-adsorbed γ-${\mathrm{Fe}}_2$${\mathrm{O}}_3$ recorded at 4.2 K in applied fields of 6.0 and 7.6 T.

We prove that Berry’s phase arises when the (phenomenological) Hamiltonian for the fast variable in the system violates time-reversal symmetry. Our derivation demonstrates the existence of a current-type Berry’s phase based on the time-reversal-odd part of the Hamiltonian, in contrast to the usual monopole-type Berry’s phase where the degeneracy of states acts as a topological singularity in the parameter space. Effects of this phase on the transverse phonon modes of solids are studied and the results are applied to the anyon model of the high-${\mathit{T}}_{\mathit{c}}$ superconductors.

A novel quantum effect, dynamical localization, is predicted for periodically current-driven Josephson junctions: the quantum-mechanical decrease, as opposed to the classical increase, of the intensity of voltage fluctuations with increasing driving amplitude.

The U=∞ limit of the three-band Hubbard model with nearest-neighbor repulsion V is studied using the slave-boson approach and the large-N expansion technique to order 1/N. A charge-transfer instability is found as in weak-coupling theory. The charge-transfer instability is always associated with a diverging compressibility leading to a phase separation. Near the phase-separation, charge-transfer-instability region we find superconducting instabilities in the s- and d-wave channel. The requirement for superconductivity is that V be on the scale of the Cu-O hopping as suggested by Varma, Schmitt-Rink, and Abrahams.

We report measurements of the de Haas–van Alphen (dHvA) effect in the organic superconductors α-(BEDT-TTF${)}_2$(${\mathrm{NH}}_4$)Hg(SCN${)}_4$ (${\mathit{T}}_{\mathit{c}}$≊1 K) and κ-(BEDT-TTF${)}_2$Cu(NCS${)}_2$ (${\mathit{T}}_{\mathit{c}}$≊10 K) in fields up to 15 T and temperatures down to 0.45 K. For the first time, we determine the angular dependence of the cyclotron effective mass ${\mathit{m}}_{\mathit{c}}$, and the orientation of up to four spin-splitting zeros of the dHvA signal. The angular dependence of the amplitude of the dHvA oscillations is quantitatively explained, and the mass enhancement due to electron-phonon coupling is estimated.

The critical-current anisotropy ${\mathit{j}}_{\mathit{c}}$(B,T,Θ) of epitaxial ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathit{x}}$ films has been measured in detail. Below 70 K, the measured ${\mathit{j}}_{\mathit{c}}$(B,T,Θ) can be fully accounted for by the assumption that only the field component ${\mathit{B}}_{\mathrm{\parallel }}$=B cosΘ influences ${\mathit{j}}_{\mathit{c}}$, as has been proposed by Kes et al.. Above this temperature, a 2D-to-3D dimensional crossover results in additional dissipation. By comparison with results on polycrystalline samples, evidence is given for the absence of any influence of the grain boundaries on ${\mathit{j}}_{\mathit{c}}$(Θ).

We have measured the temperature dependence of the lower and upper critical fields in superconducting ${\mathrm{K}}_3$${\mathrm{C}}_{60}$. From the measurements, we have evaluated the penetration depth (λ=2400 Å) and superconducting coherence length (ξ=26 Å). The parameters are in agreement with a superconducting state formed by a narrow band.