The antiferrodistortive phase transitions of some perovskite crystals, driven first order by strongly cubic order-parameter fluctuations, are shown to become continuous at finite anisotropic stresses favoring ordering of one order-parameter component. The associated critical and tricritical behavior will be that of one of a spectrum of uniaxial Lifshitz systems, its precise character reflecting the form of the anisotropy in the soft-mode dispersion. Our predictions are in substantial accord with recent experiments on RbCa${\mathrm{F}}_3$.

We have investigated the ${O_h}^1$ to ${D_{4h}}^{18}$ structural phase transition in RbCa${\mathrm{F}}_3$ under near-[100] stress $\sigma$, by monitoring the electron-paramagnetic resonance lines of ${\mathrm{Gd}}^{3+}$ ($S=\frac{7}{2}$) on a ${\mathrm{Ca}}^{2+}$ site. We find that the first-order transition at zero stress becomes second order at ${\sigma }_t=1.9\mathrm{}\pm 0.3$ kg/${\mathrm{mm}}^2$ and $T_t=195.3\mathrm{}\pm 0.1$ K. At this tricritical point, and up to $\sigma =4.8\mathrm{}\pm 0.3$ kg/${\mathrm{mm}}^2$, the exponent of the temperature dependence of the rotational order parameter is $\beta =0.18\mathrm{}\pm 0.02$.

A phase transformation is observed in scandium metal at a pressure of approximately 170 kbar as seen from a rise of the electrical resistance by a factor of 4. The high-pressure phase (Sc II) is a superconductor. $T_c$ is found to increase with pressure from ≃ 50 mK at 186 kbar to ≃ 0.35 K at 210 kbar. Experimentally, sintered diamond anvils have been used in a six-lead Bridgman setup with internal manometer.

First-principles Hartree-Fock calculations are reported for binding of a hydrogen atom at high-symmetry sites of low-index faces of Ni surfaces (using an ${\mathrm{Ni}}_{20}$ cluster as a model). Chemisorption energies increase with ligancy of the binding site, with bond energies of 1.6, 2.8, 3.2, and 3.0 eV for onefold, twofold, threefold, and fourfold sites, respectively. A reverse trend is found for ${\omega }_e$ with values of 275, 170, 150, and 73 meV for ligancies of 1, 2, 3, and 4, respectively.

The electronic properties of H chemisorbed on the Pd(111) surface are studied using a self-consistent pseudopotential mixed-basis method. The H atoms are predicted to prefer a threefold hollow over the top site. Strong bonding between hydrogen and the substrate results in (a) dramatic changes in the surface electronic structure and (b) the formation of bonding and antibonding adsorbate states. The combined effects of both accurately reproduce and explain the salient features of recent photoemission measurements.

A fit to the integrated rate and $3\pi$ mass distribution of the decay mode ${\tau }^{+}\to {\pi }^{+}{\pi }^{+}{\pi }^{-}{\overline{\nu }}_{\tau }$ has been performed using hard-pion current algebra, including the approximate imposition of unitarity. We find that the existence of an $A_1$ with low mass ($m_A=1.1\mathrm{}$ GeV) and narrow width [${{\Gamma }_A}^{\mathrm{tot}}=150\mathrm{}$ MeV, $\Gamma (A_1\to \rho \pi )=100\mathrm{}$ MeV] is consistent with the data if there is an appreciable non-$\rho \pi$ contribution to the matrix element $〈3\pi \left|A_{\mu }\right|0\mathrm{}〉$. Such a contact term is predicted to exist by current algebra.

Parity nonconservation is observed in the $6{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}-7\mathrm{}{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ transition in thallium. Absorption of circularly polarized 293-nm photons by $6{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ atoms in an $E$ field results in polarization of the $7{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ state through interference of Stark $E1\mathrm{}$ amplitudes with $M1\mathrm{}$ and parity-nonconserving $E1\mathrm{}$ amplitudes $\mathfrak{M}$ and ${\mathcal{E}}_p$. Detection of this polarization yields the circular dichroism $\delta =+(5.2\mathrm{}\pm 2.4)\times {10}^{-3\mathrm{}}$, which agrees in sign and magnitude with theoretical estimates based on the Weinberg-Salam model.

We present the first results from an isobar-model phase-shift analysis of the $\eta {\pi }^{+}{\pi }^{-}$ system. The $D\left(1275\right)\mathrm{}$ meson is established as a narrow ($\Gamma \sim 10$ MeV) $I{J}^P={01}^{+}$ resonance. Beneath the $D$ is a broader ($\Gamma \sim 70$ MeV) $I{J}^P={00}^{-}$ state which is a candidate for the radially excited $\eta$. The data are consistent with the existence of another 0$0^{-}$ object of comparable width near 1.4 GeV.

Precise measurements at$t=0\mathrm{}$ of the $K_{L}p\to K_{S}p$ amplitude (modulus and phase) were made. Over 50000 $K_{\pi 2}$ decays along with normalizing $K_{\mu 3}$ events were detected behind a 7.2-m-long liquid-hydrogen regenerator. The momentum dependence of the modulus and phase are presented, and the results are combined with those of other experiments to extract the relevant parameters of $\omega$ exchange.

The ratio of the matrix elements for two competing multipoles ($E2\mathrm{}$ and $M1\mathrm{}$) for the 129-keV transition in ${}_{}{}^{191}{}_{}{}^{}\mathrm{Ir}$ was found to have an imaginary part with a phase angle of (-4.7±0.3)×${10}^{-3\mathrm{}}$ rad. This time-reversal-like phase shift may be attributed, at least in part, to quantum electrodynamical effects in the atomic final state. The observed phase shift, however, is somewhat larger than the atomic final-state phase calculated by Goldwire and Hannon.

High-spin spectra are determined by approximately projecting states of good angular momentum from cranked Hartree-Fock-Bogoliubov (CHFB) wave functions. For each $J$ the projected energy is $E_{\mathrm{PROJ}}\approx E_{\mathrm{CHFB}}-\frac{{\left(\Delta \overrightarrow{\mathrm{J}}\right)}^2}{2{\mathcal{I}}_{\mathrm{CHFB}}}$, where the moment of inertia $\mathcal{I}$ and the fluctuation $\Delta \overrightarrow{\mathrm{J}}$ are spin dependent. For ${}_{}{}^{168,170}{}_{}{}^{}\mathrm{Yb}$ and ${}_{}{}^{174}{}_{}{}^{}\mathrm{Hf}$ the projected $\mathcal{I}$ is less than the CHFB value for all $J$. Consequently approximate projection increases all yrast excitation energies for these nuclei.

In coincidence experiments using mica and glass track detectors four- and five-pronged events have been observed in 1785-MeV ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$+${\mathrm{U}}_{\mathrm{nat}}$ and 1535-MeV ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$+${\mathrm{U}}_{\mathrm{nat}}$. The events are shown to be due to multiple sequential fission. Angular distributions are presented.

New differential cross section and analyzing-power data for 800-MeV $p_{\mathrm{pol}}$+${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$,${}_{}{}^{116,124}{}_{}{}^{}\mathrm{Sn}$ inelastic scattering to the first $2^{+}$ states are presented. A distorted-wave Born-approximation analysis which utilizes collective form factors and includes deformation of the spin-orbit potential is shown to provide a reasonable description of the analyzing-power data.

Evidence is presented for the excitation of giant multipole resonances in inelastic scattering of 315-MeV ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ ions on ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$, and ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ targets. Highly excited modes around 20 MeV of excitation energy in ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ exhaust a large fraction of the energy-weighted isoscalar $E3\mathrm{}$ and $E5\mathrm{}$ sum-rule strengths.

A molecular O${\mathrm{H}}^{+}$ beam has been produced in a Pelletron accelerator. The Coulomb explosion of 10-MeV O${\mathrm{H}}^{+}$ in thin carbon foils was studied by measuring energy and angular distributions of protons in simultaneous coincidence with the emerging oxygen charge states. A simple model of effective charge illustrates the use of such coincidence measurements to provide information about the electronic screening of moving ions in a solid, and about the formation of emergent charge states.

For center-of-mass electron energies ≤0.08 eV (∼1000 K) nearly all electron-ion dissociative recombination cross sections thus far measured by us follow the $E^{-1\mathrm{}}$ dependence predicted by theory. This corresponds to a ${T_e}^{-0.5\mathrm{}}$ temperature dependence of the reaction-rate coefficient.

A large fraction of the number of positrons observed in collisions of ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ + ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$, ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ + ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$, and ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ + ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ at relative velocities $\frac{v}{c}=0.11\mathrm{}$ and for distance of closest approach $R_{min}<40\mathrm{}$ fm can be assigned to pair creation of nonnuclear origin, induced by the time-varying Coulomb fields of the projectile and target charges. An order-of-magnitude increase in the positron yield is observed by increasing the united-atom charge from 164 to 184.

A large-amplitude ion-acoustic cylindrical soliton launched in an axisymetric cylindrical system collapses at the center of the cylinder. The highly nonlinear nature of the interaction is observed after the collapse, showing that many isolated pulses are emitted abruptly and propagate out without clear evolution of their wave forms.

Using a diamond indentor, diamond anvil technique along with nonshorting interdigitated electrodes produced on the anvil by lithographic processes, we have shown that the electrical resistance of a xenon sample at 32°K drops from ${10}^{13}$ Ω (or greater) to about ${10}^4$ Ω at 0.33 Mbar. Further studies using a second method reveal that the resistivity has dropped to below ${10}^{-1\mathrm{}}$ Ω cm and possibly much lower.

We show that metallic glasses are stabilized by the close matching between the minima in the interionic potentials and the maxima of the partial pair-distribution functions. We use an ab initio pseudopotential method to calculate the interionic potentials for transition-metal-free glasses (Mg-Zn, Ca-Mg, and Ca-Al). The amorphous structure is determined using cluster relaxation and thermodynamic variational techniques. Our theory demonstrates that there is a close relation between glass formation and the formation of topologically close-packed intermetallic compounds.