When the Griffiths theory of tricritical points is extended to the eighth power of the order parameter, certain composition differences preserve the limiting behavior far from the tricritical point, an unexpected result that had been overlooked in earlier van der Waals model calculations. New calculations show that, along a path of constant tricritical composition and density and increasing temperature, a quasibinary system goes from two phases to one at the tricritical point, with no appearance of the third phase.

The magnitude of backward-wave phonon echoes at 17.3 GHz in $\alpha$-quartz was found to depend strongly on the presence of structural defects introduced by thermal quenching of the crystal. In addition, the linear dependence of echo magnitudes on both acoustic and microwave powers suggested that the effect of the defects was to introduce large static strains which enhanced the intrinsic mechanisms.

The momentum density of the quantum solid hcp ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ has been measured at 1.70 K at the molar volumes 18.20 and 19.45 ${\mathrm{cm}}^3$. The measurements were made with pulsed incident neutrons of energy 505 meV, and with momentum transfers in the range of 12 to 22.5 ${\mathrm{\AA }}^{-1\mathrm{}}$. The momentum density is obtained from the inelastic-scattering spectrum at each of eight scattering angles. The measured distribution is Gaussian and the derived kinetic energies are larger than proposed by existing theories.

This Letter reports the first application of an ESR-tagged magnetic circular dichroism measurement to a paramagnetic deep-level defect in a semiconductor. In semi-insulating GaAs two new absorption bands are found at 1.05 and 1.29 eV. Both bands are identified as intracenter electronic transitions of the As-antisite defect. The analysis of the absorption and concentration data implies that the "dominant electron trap" ($\mathrm{EL}2$) in GaAs is not the As antisite.

Using time-resolved luminescence imaging, the authors have measured extremely large mobilities ($\mu \sim {10}^7$ ${\mathrm{cm}}^2$/V-s) and diffusion constants ($D\sim {10}^3$ ${\mathrm{cm}}^2$/s) of paraexcitons in ${\mathrm{Cu}}_2$O at liquid-helium temperatures. Both $\mu$ and $D$ depend strongly on temperature, indicating that phonons are the dominant scattering mechanism, even down to 1.2 K. Clues to the origin of this remarkably weak exciton-phonon scattering are provided by an unusual temperature dependence and a marked stress dependence of the mobility.

Two-dimensional weak-coupling Fröhlich polarons are investigated theoretically in the presence of an external magnetic field normal to the two-dimensional plane. For a weak magnetic field (${\omega }_c\ll {\omega }_{\mathrm{LO}}$), the cyclotron mass approaches the zero-field polaron effective mass, whereas for large fields (${\omega }_c\gg {\omega }_{\mathrm{LO}}$), the cyclotron mass approaches the bare band mass from below. In the resonance situation (${\omega }_c\approx {\omega }_{\mathrm{LO}}$), the cyclotron mass shows an enhanced polaronic renormalization effect. Connection is made with recent data on magnetoabsorption in semiconductor space-charge layers, explaining why polaron effects show up only in some experiments.

Thin graphite films consisting of up to 25 atomic layers are studied with self-consistent local-density all-electron theory and model-potential calculations. Unoccupied surface states confined in the direction of the $c$ axis are predicted at 3.8 eV above the Fermi energy. They are split off from the bulk free-electron interlayer band. These results provide a convincing interpretation of the surface states found in recent inverse photoemission experiments by Fauster et al.

The time decay of the thermoremanent magnetization (${\sigma }_{\mathrm{TRM}}$) has been measured in 1.0% Cu: Mn and 2.6% Ag: Mn spin-glasses. It is shown that ${\sigma }_{\mathrm{TRM}}$ is neither an algebraic nor a logarithmic function of time, but it is found that ${\sigma }_{\mathrm{TRM}}$ can be characterized by a "stretched" exponential: ${\sigma }_{\mathrm{TRM}}={\sigma }_0\exp [-\frac{C{\left(\omega t\right)}^{1-n}}{\mathrm{}(1-n)\mathrm{}}]$. Similar time dependences appear in the disorder-diffusion theory of Grassberger and Procaccia and the cooperative-relaxation theory of Ngai, but neither theory in its present form is directly applicable to spin-glasses.

The theory of transport in nonlinear dynamics is developed in terms of "leaky" barriers which remain when invariant tori are destroyed. A critical exponent for transport times across destroyed tori is obtained which explains numerical results of Chirikov. The combined effects of many destroyed tori lead to power-law decay of correlations observed in many computations.

A new type of "crisis" is found in coupled logistic maps. In this crisis several chaotic attractors undergo cyclic collisions with their basin boundaries and merge into a bigger one, whose chaotic dynamical behavior exhibits cyclic transitions among its components with a well defined overall period.

The role of symmetry is examined in systems displaying period-doubling instabilities. It is found that symmetric orbits will not undergo period doubling except in extraordinary cases. Such exceptional cases cannot occur in a large class of systems, including the sinusoidally driven damped pendulum and the Lorenz equations.

Sinai billiard systems are studied numerically and analytically. Let $h$ be the Kolmogorov-Sinai entropy, $〈\tau 〉$ the mean free time of the point mass, and $\epsilon$ the scaling factor of the size of convex scatters (so that $\epsilon \to 0$ implies vanishing scatters). The following universal behavior is conjectured for any periodic Lorentz model in $d(>~2)$-space: ${\lim }_{\epsilon \to 0}h\frac{〈\tau 〉}{(-\ln \epsilon )}=d.\mathrm{}$

A new mechanism for generating an ultralight Dirac neutrino is given. A discussion is given of how it might possibly arise in a supergrand unified theory (SUSY GUT) with a geometric type of a hierarchy. The neutrino is scaled in mass relative to its charge-$\frac{2}{3}$ family member by $\frac{M_{\mathrm{SUSY}}}{M_{\mathrm{GUT}}}$.

A new theoretical value $a_{\mu }=11\mathrm{}659202\left(20\right)\mathrm{}\times {10}^{-10\mathrm{}}$ is reported for the muon anomalous moment, based on the first complete calculation of the ${\alpha }^4$ QED term and improvements of the ${\alpha }^3$ QED term and various hadronic contributions. The remaining error is mostly due to the experimental inputs needed for evaluation of the hadronic vacuum polarization effect. Further improvement of this error seems possible. Thus the electroweak theory may be tested at the one-loop level if measurement of $a_{\mu }$ is improved by an order of magnitude.

This Letter reports the results of a search for neutron-antineutron transitions in oxygen, using a water Cherenkov detector of fiducial mass 3300 metric tons located 1570 m (water equivalent) underground. Two independent methods yield 90% -confidence lower limits of 1.7 × ${10}^{31}$ and 2.4 × ${10}^{31}$ yr, respectively, for the partial lifetime of this process. According to recent calculations the latter result corresponds to a lower limit in the range 2.7 × ${10}^7$ to 1.1 × ${10}^8$ sec for the free-neutron oscillation time.

It is suggested that the observed difference between deep-inelastic structure functions from iron and deuterium targets comes about because two of the quarks in a nucleon are tightly bound in a diquark. It suffices for reproducing the data to assume that the diquark radius is enhanced by 10%-45% in a dense nucleus because of the disturbance from the surrounding nucleons. It is suggested how such a change of the diquark scale could be distinguished from the recently proposed change in the scale of the whole nucleon.

The deep-inelastic electron-scattering cross sections per nucleon ${\sigma }_A$ for D, He, Be, C, A1, Ca, Fe, Ag, and Au were measured in the kinematic range $0.09<~x<~0.9$ and $2<~Q^2<~15$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$ with use of electrons with energies ranging from 8 to 24.5 GeV. The ratio $\frac{{\sigma }_A}{{\sigma }_{\mathrm{D}}}$ is consistent with unity in the range $0.1<x<\mathrm{}0.3\mathrm{}$. For $0.3<x<\mathrm{}0.8\mathrm{}$, the ratio decreases logarithmically with atomic weight $A$, or linearly with average nuclear density. No $Q^2$ dependence in the ratio was observed over the kinematic range of the data.

The absorption cross sections and the differential elastic-scattering cross sections of antiprotons on carbon, aluminum, and copper nuclei were systematically measured at six beam momenta between 470 and 880 MeV/c. From these data, the antiproton-nucleus optical potential was derived for the first time.

Elastic scattering of 180-MeV ${\pi }^{+}$ and ${\pi }^{-}$ on ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and tritium has been measured from ${\theta }_{\pi }(\mathrm{c}.\mathrm{m}.\mathrm{})=44\mathrm{}° \mathrm{to} 96°$. The superratio $R\equiv \frac{\left[\frac{d\sigma ({\pi }^{+}+{}_{}{}^3{}_{}{}^{}\mathrm{H})}{d\sigma ({\pi }^{-}+{}_{}{}^3{}_{}{}^{}\mathrm{He})}\right]}{\left[\frac{d\sigma ({\pi }^{+}+{}_{}{}^3{}_{}{}^{}\mathrm{He})}{d\sigma ({\pi }^{-}+{}_{}{}^3{}_{}{}^{}\mathrm{H})}\right]}$ is found to vary with angle reaching a maximum of 1.31±0.09 at ${\theta }_{\pi }=65\mathrm{}°$. The direct Coulomb effects do not account for the observed differences of $R$ from 1.0 which is the expected value on the basis of charge symmetry.

Total cross sections and astrophysical $S$ factors of the radiative capture reactions $\alpha ({}_{}{}^3{}_{}{}^{}\mathrm{He},\gamma ){}_{}{}^7{}_{}{}^{}\mathrm{Be}$ and $\alpha (t,\gamma ){}_{}{}^7{}_{}{}^{}\mathrm{Li}$ are investigated by the resonating-group method. The theoretical value of the $S$ factor at $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}=10\mathrm{}$ keV is predicted to be 0.508 keV·b without the use of any artificial parameters. An interesting difference between the mirror reactions is found and theoretically explained.