The coupling of density and spin-density fluctuations to the single-particle excitations in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ at zero temperature is studied. The present results exhibit a sharply peaked effective mass close to the Fermi surface, caused by the attractive quasiparticle interaction in the spin-1 particle-hole channel. The coupling to the zero-sound mode causes a secondary maximum around $2k_{\mathrm{F}}$. This mechanism can reconcile the rapid variation of the specific heat with a well-defined zero-sound mode at large momenta.

Plasma resonances have been observed in a two-dimensional sheet of positive ions held just under the surface of liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. The ion effective mass deduced from the resonant frequencies is found to be weakly temperature dependent, increasing linearly with temperature. Strong nonlinear effects are observed in the wave propagation. The ponderomotive force of the plasma wave induces a static $2k$ distortion of the charge density, which produces large shifts of the resonant frequency with drive amplitude.

The atomic geometry of $\mathrm{Cu}\mathrm{}\left\{001\right\}c(2\mathrm{}\times 2)$-Cl has been determined by surface extended x-ray-absorption fine structure to consist of a simple Cl overlayer in fourfold Cu hollows with a (2.37 ± 0.02)-Å bond length. With use of this geometry, self-consistent electronic structure calculations were performed and compared with angle-resolved photoemission data, giving excellent agreement for the position and dispersion of several Cl-induced surface states and resonances. These results have resolved previously reported discrepancies for the $\mathrm{Ag}\mathrm{}\left\{001\right\}c(2\mathrm{}\times 2)$-Cl system.

For isotropic, centrosymmetric, ferroelectric systems described by an $n>\mathrm{}1\mathrm{}$ order parameter $\overrightarrow{\mathrm{p}}$, a new term in the Hamiltonian density, $w[{\overrightarrow{\mathrm{p}}}^2\nabla ·\overrightarrow{\mathrm{p}}-\overrightarrow{\mathrm{p}}·\nabla ({\overrightarrow{\mathrm{p}}}^2\left)\right]$, is allowed. It is relevant, in the renormalization-group sense, in $d=4-\epsilon$ dimensions, when $n<\mathrm{}{n}_0=4-\frac{11\epsilon }{4}+O\left({\epsilon }^2\right)$. For $n<\mathrm{}{n}_0$, no stable fixed points exist. It is concluded that simple ferroelectric and ferromagnetic systems are not in the same universality class and thus do not necessarily exhibit identical critical behavior.

Infrared-laser photoacoustic spectroscopy has been applied to surface studies at submonolayer coverage under ultrahigh-vacuum conditions. The adsorption of S${\mathrm{F}}_6$ on silver films at 90 K was investigated. Simultaneous x-ray photoelectron measurements show that 0.1 of a monolayer can be readily detected with the photoacoustic technique. Photoacoustic spectra from submonolayer to multilayer surface coverages have been obtained and show characteristic S${\mathrm{F}}_6$ vibrational bands.

Inelastic neutron scattering has been used to study the spin dynamics of $\mathrm{Li}{\mathrm{Tb}}_p{\mathrm{Y}}_{1-p}{\mathrm{F}}_4$, an induced-moment ferromagnet in which the lowest crystal-field levels of the ${\mathrm{Tb}}^{3+}$ ions are two singlets. For $p=0.38\mathrm{}$, magnetic excitons are observed which obey the dispersion relation given by mean-field theory. In addition, there is a resolution-limited peak at zero energy transfer which is not accounted for within conventional mean-field theory. For $p=0.97\mathrm{}$, this central peak is also observed, but no magnetic excitons are seen.

Temperature-dependent Hall-effect measurements on two Cr-doped GaAs samples show a dominant center at $E_1=0.324-1.4\mathrm{}\times {10}^{-4\mathrm{}}T$ eV, with respect to the valence-band edge. By comparison with secondary-ion mass spectroscopy measurements of the Cr concentration, and recent EPR measurements of the ${\mathrm{Cr}}^{2+}$, ${\mathrm{Cr}}^{3+}$, and ${\mathrm{Cr}}^{4+}$ concentration in several samples, it is shown unambiguously that this energy describes the ${\mathrm{Cr}}^{4+}$ → ${\mathrm{Cr}}^{3+}$ transition. This is the first conclusive evidence for a charge-state transition involving ${\mathrm{Cr}}^{4+}$ in GaAs.

The capability of a He-scattering experiment to detect the migration of adsorbed molecules on monocrystal surfaces is demonstrated for the first time. CO migration and island formation can be investigated at very low coverages because of the very large cross section for He scattering of CO adsorbed on Pt(111). The activation energy for CO migration on Pt(111) is found to be 7 kcal/mole. No island formation at low CO coverages on a defect-free Pt(111) surface was detected in the temperature range 120-400 K.

The electric quadrupole hyperfine interaction at indium surfaces has been detected with use of the perturbed angular correlation technique. The electric-field gradient at the surface is found to be $\left|V_{\mathrm{zz}}\right(\mathrm{surface}\left)\right|=5.6\left(6\mathrm{}\right)\mathrm{}\times {10}^{17}$ V/${\mathrm{cm}}^2$ with an orientation perpendicular to the surface. This is a drastic enhancement of the field gradient compared with bulk by a factor $\left|\frac{V_{\mathrm{zz}}\left(\mathrm{surface}\right)}{V_{\mathrm{zz}}\left(\mathrm{b}\mathrm{u}\mathrm{l}\mathrm{k}\right)}\right|\approx 4$. The result is discussed in a simple one-Wigner-Seitz-cell model.

One of several contradictory existing results for the time a tunneling particle interacts with its barrier is confirmed, by considering tunneling through a time-modulated barrier. At low modulation frequencies the traversing particle sees a static barrier. At high frequencies the particle tunnels through the time-averaged potential, but can do it inelastically, losing or gaining modulation quanta. The transition between the two regimes yields $\int \mathrm{dx}{\left[\frac{m}{2(V-E)\mathrm{}}\right]}^{\frac{1}{2}}$ for the traversal time.

The influence at criticality of a "far" wall on the order parameter close to a "near" wall as discussed by Fisher and de Gennes is recast in a form more suitable for microscopic analysis. An argument beyond ordinary scaling is presented which yields the perturbation exponent $d^{*}=d$ for longitudinal perturbations and ${d_{\perp }}^{*}=d-1\mathrm{}$ for transverse perturbations. These results are consistent with explicit renormalization-group computations to $O(\epsilon =4-d)$ which involve solution of the nonlocal equation of state.

The intermittent-type chaos occurring in rf- and dc-current-driven Josephson junctions is investigated. A simple physical model is proposed and is used for an analytic calculation of the power spectrum. The latter has a broadband part which decays algebraically at high frequencies. Comparison with numerical results shows good agreement. Further application and generalizations of this approach are outlined.

It is shown that the reduced large-$N_c$ Hamiltonian can reproduce not only the Wilsonloop expectation values but also the glueball spectrum of the infinite-volume lattice gauge theory. With the addition of fermions, the resulting quantum matrix model can potentially be used to calculate a large set of strong-interaction features.

The exchange of gluons in the initial or final state of hard processes is shown to build a $Q^2$-dependent phase shift which may be observed as an oscillatory scaling violation in infrared-dominated processes. The non-Abelian structure of the phase can affect cross sections for experiments as diverse as $\mathrm{pp}$ elastic scattering at fixed angle and dilepton production in hadronic collisions, resulting in oscillatory components. These oscillations may already have been detected.

The charged-hadronic multiplicity distribution for high-mass diffractive states is derived from the distribution for $e^{+}{e}^{-}$ annihilation, and is compared with a recent experiment. The result involves no parameters.

The existence of $P$-wave $b\overline{b}$ bound states ${{\chi }_b}^{\prime }$ is demonstrated by observation of photons from the transition ${\Upsilon }^{\prime \prime }\to \gamma +{{\chi }_b}^{\prime }$ in the inclusive photon spectrum from ${\Upsilon }^{\prime \prime }$ decays. The center of gravity of the observed photon energies is 98 MeV and the branching ratio for the transition of the ${\Upsilon }^{\prime \prime }$ to the ${{\chi }_b}^{\prime }$ states is (34±3)% (statistical).

Evidence is presented for the production of ${{\chi }_b}^{\prime }$ states ($2{}_{}{}^3{}_{}{}^{}P$) in the reaction ${\Upsilon }^{\prime \prime }\to \gamma {{\chi }_b}^{\prime }\to \gamma \gamma {\Upsilon }^{\prime } (or \gamma \gamma \Upsilon )$ with subsequent decay of the ${\Upsilon }^{\prime }\left(\Upsilon \right)$ into $e^{+}{e}^{-}$ or ${\mu }^{+}{\mu }^{-}$. The data were obtained with the nonmagnetic CUSB detector at the Cornell $e^{+}{e}^{-}$ storage ring. Observed were 14 (15) events consistent with the transition to the ${\Upsilon }^{\prime }\left(\Upsilon \right)$ with an estimated background of 1.7 (1.3) events. The lower-energy photons cluster around 100 MeV, implying $M\left({\Upsilon }^{\prime \prime }\right)-M\left({{\chi }_b}^{\prime }\right)\sim 100$ MeV. A single-line interpretation of the lower-energy photon spectrum is disfavored.

The authors have detected 1203 events of the reaction ${\pi }^{-}p\to \varphi \varphi n$ at 22 GeV/c, and have performed a partial-wave analysis of the $\varphi \varphi$ system. The results are well represented by two resonances, both with quantum numbers $I^G=0^{+}$, $J^{\mathrm{PC}}=2^{++}$. The breakdown of the Okubo-Zweig-Iizuka suppression observed in this reaction is naturally explained in the context of QCD if these states are considered to be glueball candidates.

A strong $1^{+}$ state which has a large component of the $\pi \left({h_{\frac{11}{2}}}^{-1\mathrm{}}{h}_{\frac{9}{2}}\right)$ configuration was found at 5.845 MeV of excitation in ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ from the reactions ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}(p, p^{\prime })$ and ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}(d, {}_{}{}^3{}_{}{}^{}\mathrm{He})$. The 4.841-MeV state which was recently suggested to be of $1^{+}$ is shown to be a $1^{-}$ state.

This paper reports the observation of collisionally induced spectral narrowing of the longitudinal relaxation linewidth with use of the nearly degenerate four-wave mixing process. A theoretical model, which takes into account strong velocity-changing collisions for the population and phase-interrupting collisions for the optical coherence, leads to a good qualitative understanding of this new effect.