Comparison of inclusive neutron and proton spectra from 90-MeV protons on ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$, ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$, ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$, and ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$ indicate that single nucleon-nucleon scattering dominates the highenergy regions of forward-angle continuum spectra. The ratios of cross sections observed agree with ratios expected for scattering of the incident proton by a bound target nucleon. For ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$, a plane-wave-impulse-approximation quasifree-scattering calculation fits forward-angle proton and neutron spectra simultaneously with the same normalization.

An extension of the unsymmetrized optical-potential formalism for two-fragment elastic scattering is found which fully incorporates the Pauli principle without the use of complicated projection operators. The discrete singularities of this optical potential are shown to be correlated with the physical resonance structure of the scattering amplitude.

A measurement based on the fast-atomic-beam separated-oscillatory-field method of sub-natural-linewidth spectroscopy gives, for the Lamb shift in hydrogen, $\mathcal{S}\mathrm{}(n=2\mathrm{})=1057.845\mathrm{}\left(9\right)\mathrm{}$ MHz. The result is not in good agreement with theory.

Cooling of a three-level system resulting from anti-Stokes scattering of electromagnetic radiation has been observed for the first time. A C${\mathrm{O}}_2$ laser was used to lower the temperature of samples of C${\mathrm{O}}_2$ and C${\mathrm{O}}_2$-Xe mixtures. Satisfactory agreement between theory and experiment was found. The technique's ability to deliver an extremely fast cooling pulse should find application in the study of nucleation dynamics.

With use of the Lie transform technique, a simple and compact but quite general expression is derived for the ponderomotive scalar and vector potentials ${\varphi }^{\mathrm{av}}$ and ${\overrightarrow{\mathrm{A}}}^{\mathrm{av}}$ in the covariant form $({\overrightarrow{\mathrm{A}}}^{\mathrm{av}},i{\varphi }^{\mathrm{av}})={A^{\mathrm{av}}}_{\mu }=-\frac{1}{2}〈F_{\mu \nu }{\zeta }_{\nu }〉$. The four-vector ${\zeta }_{\nu }$ is the particle excursion in four-dimensional space due to the rf electric and magnetic fields, $F_{\mu \nu }$ is the high-frequency electromagnetic tensor, and the brackets denote an average over the high-frequency phase $\overrightarrow{\mathrm{k}}·\overrightarrow{\mathrm{x}}-\omega t$.

The Fokker-Planck equation has been solved numerically in one spatial and two velocity dimensions in order to study thermal conduction in large temperature gradients occurring in laser-produced plasmas. The heat flow is an order of magnitude smaller than that predicted by the classical theory when the temperature scale length is a few electron mean free paths.

It is found that nonlinear magnetic islands are formed, in an inhomogeneous magnetized plasma, in the presence of a longitudinal current. These magnetic islands may cause an enhanced electron thermal conductivity in a turbulent plasma state.

A general theory is given for the quenched dilute $s$-state Potts and $n$-vector models in any dimension $d$. It is shown that for $T\to 0$ at the percolation threshold the Potts thermal exponent ${\nu }_T$ equals the percolation exponent ${\nu }_p$, implying a crossover exponent $\phi =1\mathrm{}$, for any $s$ and $d$. For the $n$-vector model ($n>\mathrm{}1\mathrm{}$), ${\nu }_T=\frac{{\nu }_p}{{\zeta }_R}$, where ${\zeta }_R$ is a resistivity critical exponent. Agreement with recent experiments for two-dimensional dilute Ising and Heisenberg systems is excellent.

A systematic study of mixed stacked organic charge-transfer compounds shows that these materials are either neutral or ionic and that they range from being near the neutral-ionic boundary to being far away. Under pressure, several neutral compounds near this boundary are found to undergo a reversible phase transition to an ionic ground state. This is the first observation of a neutral-to-ionic transition in any kind of material.

A self-consistent calculation of the potential-energy surface for a molecule (${\mathrm{H}}_2$) on a metal surface [Mg(0001)] is presented. The following experimentally observable adsorption features are identified and interpreted: (i) an activation barrier for molecular adsorption, (ii) a mobile molecularly chemisorbed state ("precursor state"), (iii) an activation barrier for dissociation (${E_a}^d$), and (iv) a strong dependence of ${E_a}^d$ on the adsorption-site geometry.

Measurements of diffuse x-ray scattering from $\alpha$-quartz single crystals after irradiation with 3×${10}^{18}$ fast neutrons per square centimeter are reported. The neutrons create defective regions of mean radius 10 Å, concentration 13.6×${10}^{-5\mathrm{}}$, and a volume change per region of 29 mean atomic volumes. The crystalline matrix is heavily strained by the defective regions and the symmetry and magnitude of the strain field is determined. These regions, which are most likely amorphous, contain the tunneling centers responsible for the recently observed phonon echoes.

Measurements have been made of the heat capacity of thin plates of vitreous silica between 0.35 and 1.6 K. In an adiabatic sample arrangement the time-resolved specific heat was measured over a time scale from 2 μsec to 0.5 msec. The observed time dependence of the heat capacity is in quite good agreement with the tunneling model, which predicts a logarithmic increase in time. In addition, a much stronger time dependence was observed above 1 K, which was interpreted as a decoupling of excess modes from the Debye phonons.

Striking changes have been observed in the photoemission energy-distribution curves from two phases of Pd overlayers on (110) Nb surfaces. In the commensurate (110) phase the Pd $4d$ band is found to have an edge about 1.5 eV below $E_{\mathrm{F}}$, similar to that of a noble metal. When the transition to the incommensurate Pd(111) phase occurs, the Pd $4d$ bands broaden and cross the Fermi level. As the density of Pd $4d$ states increases near $E_{\mathrm{F}}$, a dramatic increase in the hydrogen-uptake rate through the Pd is observed.

Graphite intercalation compounds have been studied by positron annihilation experiments. In donor compounds, a highly inhomogeneous charge distribution is detected by Coulomb repulsion of the positrons from the intercalant. In acceptor compounds, however, quasifree positron-anion systems are formed within the intercalant. This is used to identify the anionic species.

The two anomalous rare-earth ternaries, ${\mathrm{Eu}}_{1.2}$${\mathrm{Mo}}_6$${\mathrm{S}}_8$ and ${\mathrm{Eu}}_{1.2}$${\mathrm{Mo}}_6$${\mathrm{Se}}_8$, are investigated both resistively and magnetically under hydrostatic pressure up to ∼18 kbar. Superconductivity with an onset temperature of ∼11 K was observed abruptly at pressure above ∼7 kbar in ${\mathrm{Eu}}_{1.2}$${\mathrm{Mo}}_6$${\mathrm{S}}_8$. However, no superconductivity was detected in ${\mathrm{Eu}}_{1.2}$${\mathrm{Mo}}_6$${\mathrm{Se}}_8$ down to 1.2 K and up to 18 kbar. Several possibilities are discussed and contrasted.

Pressure-induced high-temperature superconductivity is observed in semiconducting ${\left({\mathrm{Sn}}_x{\mathrm{Eu}}_{1-x}\right)}_y{\mathrm{Mo}}_6{\mathrm{S}}_8$, where $0<~x<~0.1$ and $y=1.0\mathrm{} \mathrm{and} 1.2$, having a carrier concentration ≃${10}^{19}$/${\mathrm{cm}}^3$ at 4.2 K as determined from Hall-effect measurements. Above a threshold pressure ≃7 kbar, superconductivity appears with $\frac{d{T}_c}{\mathrm{dP}}\simeq 2$ K/kbar. The maximum superconducting temperature ($T_c\sim 10$ K), reached at ∼ 12 kbar, represents the highest pressure-induced $T_c$ in any semiconductor. For $P>~13$ kbar, the temperature-dependent resistance appears metallic.

Optical emission was observed from ${\mathrm{N}}_2^{+}$ molecules formed during ion bombardment of nitrogen-implanted silicon. A proposed two-body ionized-molecule formation mechanism which invokes inverse predissociation of the corresponding neutral, followed by a Franck-Condon ionization, quantitatively explains the data. The ionization step proceeds either by resonant electron tunneling to a vacant state in the nearby solid, or, for neutral molecule states above the first ionization limit, by autoionization.

The effect of uniaxial stress ($\sigma$) on the superconducting transition temperature ($T_c$), resistivity ($\rho$), and magnetothermopower of Al has been measured. It has been found that ${\left[\frac{\partial T_c}{\partial \sigma }\right]}_{\sigma =0\mathrm{}}=(9.6\mathrm{}\pm 0.5)\times {10}^{-11\mathrm{}}$ K/Pa. Further, it is found that an anomaly exists in $\frac{\partial T_c}{\partial \sigma }$ and $\frac{\partial \rho }{\partial \sigma }$ near $\sigma =0.5\mathrm{}$ GPa, and that the magnetothermopower oscillations associated with the $\alpha$ orbit in the third-zone electron surface disappear at $\sigma =0.54\mathrm{}$ GPa.

Starting from an atomistic sterical hindrance potential, translational and orientational motion of the N${\mathrm{O}}_2$ group in the deformable lattice are described. The reorientations of the N${\mathrm{O}}_2$ group in the paraelectric phase occur essentially by rotations around the crystallographic $\overrightarrow{\mathrm{c}}$ axis, in agreement with slow or zero-frequency experiments. It is explained why high-frequency optical methods lead to the incorrect conclusion that reorientations take place around the $\overrightarrow{\mathrm{a}}$ axis. The collective interaction of translations and rotations is discussed.

Radiative corrections to $\nu$-induced neutral-current scattering are computed for an arbitrary $G\supset {\left[\mathrm{SU}\mathrm{}\right(3\left)\right]}_c\otimes {\left[\mathrm{SU}\mathrm{}\right(2\left)\right]}_L\otimes \mathrm{U}\mathrm{}\left(1\right)\mathrm{}$. Application to the Georgi-Glashow SU(5) model with a single superheavy mass leads, for ${\Lambda }_{\overline{\mathrm{MS}}}$ (the mass scale of quantum chromodynamics) = 0.4 GeV, to the precise predictions ${{{sin}^2\theta }_{\mathrm{W}}}^{({\nu }_{\mu };l)}\mathrm{}\left(0\right)=0.2104\mathrm{}$ for ${\nu }_{\mu }$-lepton scattering at $q^2=0\mathrm{}$ and ${{{sin}^2\theta }_{\mathrm{W}}}^{(\nu ;h)}(-20\mathrm{} \mathrm{Ge}{\mathrm{V}}^2)=0.2098\mathrm{}$ for deep-inelastic $\nu$-hadron scattering. This result, together with our previous radiative corrections to $m_W{sin\theta }_{\mathrm{W}}$, gives the SU(5) predictions $m_W=84.36\mathrm{}$ GeV and $m_Z=94.91\mathrm{}$ GeV.