Cross sections for the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(${\mathrm{\pi }}^{\pm }$,${\mathrm{\pi }}^{\pm }$’p${)}^{11}$B reaction have been measured and compared with the results of distorted-wave impulse-approximation calculations that employ the vactirozation approximation for quasifree scattering. Near the giant dipole resonance of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ the calculations underestimate the cross sections for (${\pi }^{\mathrm{-}}$,${\pi }^{\mathrm{-}}$’p). The angular distributions for (${\pi }^{+}$,${\pi }^{+}$’p) and (${\pi }^{\mathrm{-}}$,${\pi }^{\mathrm{-}}$’p) are different and indicate that in this region there is interference between direct decay and semidirect decay.

We have observed dielectronic and direct excitation of H-like ${\mathrm{S}}^{15+}$ and ${\mathrm{Ca}}^{19+}$ and He-like ${\mathrm{Ti}}^{20+}$ ions in silicon channels caused by collision with weakly bound target electrons which behave as a free-electron gas. As in vacuo, relaxation of the doubly excited states can occur radiatively leading to ions of decreased charge, but in a crystal channel collisional effects can cause double ionization. The effects are seen in both the x-ray yields and charge-state fractions, and, in the case of ${\mathrm{Ti}}^{20+}$, in charge-state x-ray coincidences.

We report observations of inhomogeneous broadening of the vibrational line profiles of a gas in the "impact" density regime. In measurements up to 27 amagat (where the spectra are dominated by collision broadening), non-Lorentzian, asymmetric features are observed in Raman $Q$-branch transitions of ${\mathrm{H}}_2$ dilute in a heavy perturber gas. We compare these measurements with an inhomogeneous line-profile model based on collisional shifts that strongly depend on the (thermal) speed of the ${\mathrm{H}}_2$ radiator. Quantitative agreement is obtained only when speed-changing collisions are considered, which result in spectral line narrowing.

Total ${\mathrm{H}}^0$-${\mathrm{H}}^0$ scattering cross section at 2 K is inferred from intrabeam scattering to be about 100 A${\mathrm{̊}}^2$. This value is somewhat higher than computations published during the past two decades. These results provide the first experimental clue regarding the magnitude of ${\mathrm{H}}^0$-${\mathrm{H}}^0$ scattering at low temperatures.

We apply for the first time a novel coherent transient, the cw photon echo. The propagation direction of the optical coherence prepared in a medium by a broadband cw laser beam is reversed by a pair of counterpropagating laser pulses, resulting in a continuous emission of the signal. The signal decays with a phase relaxation time ${\mathrm{T}}_2$ and can have a quantum-beat-type modulation, giving the same wave form in one single shot as the plot of echo intensity versus pulse separation in a standard two-pulse echo experiment. Experimental demonstration is given in a ${\mathrm{Eu}}^{3+}$:${\mathrm{YAlO}}_3$ crystal.

Electron-impact ionization cross sections for He-like ${\mathrm{Li}}^{+}$ ions have been measured with extreme precision (0.01% statistics). Thus, two new mechanisms in the ionization of positive ions both involving two-electron excitations could be quantitatively investigated. We see (1) direct excitation of both K electrons into the L shell with subsequent autoionization and (2) resonant capture of the incident electron by the ion with correlated KK-LL excitation of both core electrons leading to a triply excited Li atom which then decays by the emission of two correlated electrons. Strong interference of process (2) with direct ionization is observed.

We study multiphoton transitions in helium Rydberg atoms subjected to a microwave electric field of fixed frequency but varying intensity. For each principal quantum number in the range n=25–32, the $n^3$S to $n^3$(L>2), n=25–32, transition probability exhibits very sharp structures as a function of the field amplitude. Their positions could be reproduced precisely using a Floquet Hamiltonian for the interaction between atom and field. Their shapes are determined by the transients of field turn-on and turn-off in a way that makes a close analogy with the theory of slow atomic collisions.

Molecular-dynamics simulations of two immiscible fluids confined between solid walls and sheared in a Couette geometry were performed as a function of shear rate for a range of wall and fluid properties. Changes with capillary number in the interface shape and dynamic contact angle were consistent with previous analytic results. Computed velocity fields showed that the usual no-slip boundary condition broke down within ∼2 atomic spacings from the contact line. The slip appears to be associated with the breakdown of local hydrodynamic theory at atomic scales.

The H mode is observed by the edge heating of a tokamak plasma solely by the electron cyclotron heating with the wave cutoff region in the plasma core. Threshold power for the H-mode transition can be as low as 120 kW, which is the lowest threshold power observed in spite of the peripheral power deposition. These experimental results show clearly that the H mode can be produced without additional heating of the plasma core, and provide direct confirmation that the H-mode mechanism is closely related to an increase in the plasma edge electron temperature.

Using the Ginzburg criterion, we demonstrate that the width of the critical region associated with the smectic-A–smectic-C transition is dependent on the observable studied. This affords an explanation of the results obtained for terephthal-bis-p-p’-butylaniline, which shows a specific-heat behavior consistent with a Landau-type model, unlike that of the elastic constants. This analysis could apply to other phase transitions.

In a Monte Carlo simulation of a three-dimensional lattice-gas model of the adsorption of a liquid film on a substrate, we have observed the thick-film–thin-film (prewetting) transition, located the prewetting critical point, and determined its nature via finite-size scaling. We find this critical point to be two-dimensional Ising type, confirming the conjecture that surface phase transitions out of bulk coexistence should belong to this universality class.

Using torque magnetometry, the superconducting anisotropy of a single crystal of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_1$${\mathrm{Cu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ has been measured in the temperature range 77.5<T<83 K ($T_c$=85 K). The data provide direct experimental evidence for a giant anisotropy. A London analysis provides an excellent description of the results and establishes a temperature-independent value of 3×${10}^3$ for the superconducting effective-mass anisotropy.

We present the first theoretical calculations of the electronic structure of long (200 rings) linear chains of polyaniline, ranging in composition from leucoemeraldine to emeraldine, allowing for compositional disorder in that the sequence of quinoid-benzenoid groups is random. We show that random protonation of the disordered polymers may induce p-type conductivity: This process pulls the Fermi energy down into the valence band, past localized band tails, to extended states. The effect is only seen if disorder is taken into account.

The XXZ spin-1 chain is studied by means of a physically motivated variational approach. An effective Hamiltonian for the dynamics of spin-0 defects in antiferromagnetic environments (domain walls) is obtained, and its ground-state properties are analyzed. An order-disorder transition is found with critical behavior corresponding to the 2D Ising model universality class, in complete agreement with Haldane conjecture. The approximations involved in this variational scheme are carefully tested using known Monte Carlo results. In particular, excellent agreement with the recently calculated excited states at the Heisenberg (isotropic) point is found, clarifying the nature of these excitations.

Low-frequency resistance fluctuations in thin films of ${\mathrm{Cu}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$ (0.045≤x≤0.195) were found to grow dramatically as the films were cooled through the magnetic freezing temperature. The temperature dependence, magnetic field dependence, x dependence, and magnitude of the noise indicated it came from spin fluctuations, probably coupled to resistivity mainly via universal conductance fluctuations. Theoretical problems are discussed, as are future experiments on fundamental properties of spin glasses.

The effect of spin-orbit (SO) scattering in disordered mesoscopic systems is studied. In one dimension it is shown that any spin-independent thermodynamic or transport quantity can be expressed in terms of the same quantity in the absence of SO scattering, but with an effective magnetic flux. This leads to universal SO reduction factors of the harmonics. In particular, the conductance and its fluctuations are interrelated, within and beyond weak-localization theory. Generalizations to higher dimensions are discussed.

The I-V characteristics of two serially coupled small tunnel junctions (about ${10}^{\mathrm{-}18}$–${10}^{\mathrm{-}19}$ F capacitances) are measured at 4 K. The junctions are formed using a scanning-tunneling microscope to probe a metal droplet deposited on an oxidized metal substrate. Sharply defined Coulumb steps due to single-electron dynamics, oxide polarization, and nonlinear (voltage dependent) tunneling rates are observed. The results show very good quantitative agreement with theoretical calculations based on the semiclassical picture.

We study binary one-dimensional exchange-mediated A+B→0 reactions in the absence of diffusion. Using the Kirkwood superposition approximation to decouple the infinite hierarchy of many-particle densities a closed set of integrodifferential equations is derived. A comparison to simulation calculations shows that the approximation reproduces the correct long-time asymptotic decay of reactant concentration as well as the correct spatial correlations of the segregation pattern.

The unstable-neutrino expalantion for the reheating of the Universe, which accounts for the cosmic-background-radiation distortion, requires nonelectromagnetic fast decay of neutrinos and a small branching ratio to the channel including electromagnetically interacting particles. We show that there exists a simple particle-physics model which satisfies the conditions required for the reheating within the known family of particles (except for a massive neutrino). In this model the reheating is caused by the decay of ν’→ν$e^{+}$$e^{\mathrm{-}}$ and the dominant decay mode is given by ν’→ννν¯.

We derive an identity that relates the fermion electric dipole moment from weak CP-violation effects to the strong CP parameter θ. In the absence of Peccei-Quinn-type symmetries, we find that requiring θ naturally small generally implies that, for a large class of models, the dominant contribution to the neutron electric dipole moment $d_n$ comes from strong CP violation rather than directly from weak CP-violation effects.