Total reaction cross sections for inclusive pion double charge exchange have been obtained at incident pion energies of 180 and 240 MeV for ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, ${}_{}{}^{6.7}{}_{}{}^{}\mathrm{Li}$, ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$, ${}_{}{}^{103}{}_{}{}^{}\mathrm{Rh}$, and ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$. An explanation of the dependence of these cross sections upon the mass and charge of the target nucleus and the charge of the incident pion is suggested in terms of a phenomenological model in which successive single-charge-exchange processes compete with more probable processes such as quasifree scattering.

We address the problem of broad-band excitation through an arbitrary flip angle γ in two-level systems via single-phase, amplitude-modulated irradiation. The Floquet formalism and perturbation treatment are used to derive the modulation sequences. Sideband amplitudes are a continuous function of γ. Our solutions approach asymptotically the limit of ideal pulses with infinite power.

A quantitative comparison of the fluctuation-induced energy flux with the total energy flux has been made in the edge region of the TEXT tokamak using fluctuation measurements from Langmuir, heavy-ion-beam, and magnetic probes. At all but the lowest densities the convected energy flux due to electrostatic fluctuations dominates the energy losses caused by plasma transport. Energy loss through magnetic fluctuations is insignificant in the edge region.

The low-frequency electron density spectral function S(k,ω), having an ion acoustic peak (at ω∼${\mathrm{kc}}_s$) and an entropy peak (ω=0), has been measured by light scattering in a thermal collisional argon arc plasma. The entropy peak is for the first time observed in a plasma. A prediction of the spectrum derived from Braginskii’s equations is adopted to estimate the transport coefficients. Ion viscosity, ion thermal conductivity, and electron-ion energy-transfer frequency determined from fitting to the data differ from predictions by -39%, +16%, and -22%, respectively.

Neutron reflectivity from annealed thin films of the poly(styrene-b-deuterated methylmethacrylate), P(S-b-D-MMA), reveals the formation of a multilayered morphology parallel to the film surface. This multilayer forms so that PS locates, preferentially, at the air/copolymer and D-PMMA at the substrate/copolymer interfaces with layer thicknesses at these interfaces one-half that found in the bulk. P(D-S-b-MMA) of lower molecular weight shows the first evidence of surface-induced ordering of copolymers in the phase mixed state characterized by an exponentially damped cosine function.

In the isotropic-to-lamellar transition, nonlinear fluctuation terms lower the transition temperature ${\tau }_c$ and drive the transition first order. Here we show that steady shear, by suppressing the fluctuations, raises ${\tau }_c$; in a certain temperature range the lamellar phase can be induced by applying shear. A study ofo the effective potential indicates that the transition remains first order, though becoming very weak at high shear rate. We argue heuristically that the lamellar ordering first occurs with wave vector normal to both the velocity and the velocity gradient.

Using the evanescent-wave-ellipsometry technique, we have studied the orientational wetting behavior of the homologous series of alkyl cyanobiphenyl liquid crystals (nCB, n=5 to 9) on a glass coated with silane surfactant molecules. The results show complete wetting for 6CB to 9CB and partial wetting for 5CB. They compare well with existing theories.

We present a unified description of the thermal boundary resistance $R_K$ (Kapitza resistance) of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ above and below $T_{\lambda }$. Using Dirichlet boundary conditions for the order parameter we determine the critical temperature dependence of $R_K$ without an adjustment of parameters. Good agreement with experimental data below $T_{\lambda }$ is found. Our local renormalization-group treatment goes beyond the conventional hydrodynamic approach and predicts the nonuniversal pressure dependencen of $R_K$ along the λ line.

The heat capacity of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ adsorbed on a graphite substrate has been measured for temperatures between 2 and 200 mK and for coverages corresponding to the region near third-layer promotion. The second layer is observed to undergo a first-order transition between the fluid phase and presumably a registered solid. As a function of coverage, the transition is complete before atoms are promoted into the next layer. At 2.5 mK, the nuclear-spin contribution to the heat capacity of the higher coverage state exhibits a peak which appears to be in consistent with the usual exchange model. One possible explanation attributes the peak to magnetic polarons which form around zero-point vacancies.

The linear stability of a moving first-order phase boundary in the presence of hypercooling and anisotropy is investigated. Applying the results to the A-B transition leads us to conclude that in the temperature regime already explored experimentally the moving planar interface should be linearly stable; we also find that a new theoretical prediction for the terminal velocity of the planar phase boundary is in good quantitative agreement with experiment.

Low-energy positron diffraction (LEPD) is used in conjunction with low-energy electron diffraction (LEED) to determine the relaxed atomic geometries of the CdSe cleavage surfaces. The LEPD analyses yield optimal fits at smaller top-layer perpendicular relaxations than LEED for both cleavage faces, and significantly better agreement between theoretical and experimental intensity profiles.

Differential scanning calorimetry of amorphous silicon (a-Si) prepared by ion implantation shows a one-time low-temperature heat release, equal to one-third of the heat of crystallization. This heat release is direct evidence for structural relaxation of a-Si. It is in agreement with predictions made on the basis of Raman spectrometry.

A variety of experiments have revealed several puzzling properties of hydrogen-impurity pairs. For example, H atoms passivate the electrical activity of some impurities, whereas they induce electrical activity in others; they appear to tunnel around some impurities but not around others. We report first-principles pseudopotential-density-functional calculations for several hydrogen-impurity complexes and unravel the origins and intricacies of the rich behavior of H bound to different substitutional impurities in Si and Ge.

In this Letter we report the experimental determination of the eigenvalue spectrum, density of states, and individual eigenfunctions in an acoustic system analogous to a two-dimensional Schrödinger equation with a quasiperiodic (Penrose tile) potential. The results show features unique to the quasiperiodic symmetry, such as the appearance of gaps and bands with widths which are in the ratio of the golden mean (√5 +1)/2.

We have directly measured the real-space transfer times associated with Γ→X intervalley scattering in type-II GaAs/AlAs superlattices by femtosecond optical pump and probe spectroscopy. We domonstrate that the scattering rate for this unique process connecting electron states in different slabs of the superlattice is related to the spatial overlap of the electronic envelope wave functions in different satellite minima.

Sharp first-order ferroelectric phase transitions as indicated by sharp peaks of the dielectric permittivity arise in ${\mathrm{K}}_{0.937}$${\mathrm{Li}}^{0.063}$${\mathrm{TaO}}_3$ upon cooling or heating in electric fields 75<E<300 kV/m. They cancel the random dipolar fields due to frozen-in off-center ${\mathrm{Li}}_{+}$ ions, which destroy the phase transition and give rise to low-temperature domain states if E=0. Sharp zero-field transitions with a dispersionless peak of ε’ at ${\mathrm{T}}_{\mathrm{c}}^{+}$=104.5 K, which accurately follows the Landau-Devonshire theory at T<${\mathrm{T}}_{\mathrm{c}}^{+}$, are observed after field cooling with E>75 kV/m.

Very thin FeNi films are coupled over nonmagnetic interfacees of varied thickness to bulk FeNi generating exchange couplings ${\mathrm{J}}_{\mathrm{\perp }}$/J of varied strength on a path perpendicular to the surface. The relative magnetization M(T)/M(0) decreases faster at low T for ${\mathrm{J}}_{\mathrm{\perp }}$/J≠0 than for ${\mathrm{J}}_{\mathrm{\perp }}$/J=0. This implies a scarcity of spin waves in uncoupled films which we show to be very likely a universal property of ultrathin 2D ferromagnets on a nonmagnetic substrate.

The effects of quasi-particle tunneling in the long-range properties of an array of ultrasmall Josephson junctions, with a transverse magnetic field, is studied in two dimensions via extensive quantum Monte Carlo simulations. It is found that quasi-particle tunneling suppresses the zero-point quantum-fluctuation effects in the arrays, as well as triggering a discontinuous transition between the low-temperature metastable phase to the stable high-temperature superconducting phase.

dc magnetization measurements in fields up to 5 T on single crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ reveal a well defined onset of diamagnetism allowing a unique magnetic determination of the upper critical field and its anisotropy. The upper-critical-field slopes are -1.9 and -10.5 T/K corresponding to coherence lengths of 16 and 3 Å for fields perpendicular and parallel to the CuO planes. At the magnetically determined nucleation temperatures no distinct features are found in resistive transition curves.

We present evidence that far-infrared radiation of 50.5 ${\mathrm{cm}}^{\mathrm{-}1}$ enhances the recombination rates of photodissociated sperm whale myoglobin at temperatures below 180 K. The enhancement disappears above the glass transition of the glycerol solvent at 180 K. Control experiments show that the enhancement in recombination kinetics cannot be attributed to simple solvent heating but instead is involved with protein absorption of far-infrared radiation.