The influence of three-wave interaction on stability of ballooning modes is determined in the presence of magnetic shear and a fully toroidal large-aspect-ratio field geometry. From the ideal two-fluid equations, possibilities for nonlinear instability of the explosive type are established.

The commensurate charge-density-wave state of $2H$-${\mathrm{TaSe}}_2$ can have hexagonal ($H$) or orthorhombic ($O$) symmetry. With use of a Landau theory it is shown that near the boundary between $H$ and $O$ states a discommensuration splits into two $H-O$ interfaces and thereby greatly lowers its energy. This leads to a large suppression of the incommensurate-commensurate transition temperature and a reentrant commensurate state under pressure.

The fine-structure constant $\alpha$ has been determined from precision measurements of quantized Hall resistances $R_{\mathrm{H}}$ of three different GaAs-${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ heterostructures. The result, ${\alpha }^{-1\mathrm{}}=137.035\mathrm{}968\left(23\right)\mathrm{}$ (0.17 ppm), is in excellent agreement with the 0.11-ppm value obtained from the gyromagnetic ratio of the proton, ${{\gamma }_p}^{\prime }$, and $\frac{2e}{h}$ via the Josephson effect. Our $R_{\mathrm{H}}$ value can be combined with ${{\gamma }_p}^{\prime }$ and $\frac{2e}{h}$ to yield a more accurate value of ${\alpha }^{-1\mathrm{}}$ independent of the ohm: ${\alpha }^{-1\mathrm{}}=137.035\mathrm{}965\left(12\right)\mathrm{}$ (0.089 ppm).

The conduction-electron-limited mean free path of long-wavelength longitudinal phonons in tungsten is calculated on the basis of an empirical deformation-potential description of the electron-phonon interaction. Good agreement with acoustic attenuation data of Jones and Rayne demonstrates the importance of the inclusion of renormalization effects when the deformation potential is constructed from the strain response of the Fermi surface. Lifetime broadening of the phonon spectrum is discussed.

The conductance of Au-doped Si single crystals was measured during irradiation with 30-nsec $Q$-switched ruby-laser pulses at energy densities above the Si melting threshold (∼0.9 J/${\mathrm{cm}}^2$). The sample conductance is determined primarily by the thickness of the molten layer so that the solid-liquid interface velocity can be found from the current transient. The interface velocity during crystallization was found to be 2.7±0.1 m/sec, in close agreement with calculated values based on a heat-flow model.

Optical detection of magnetic resonance is reported for the 0.97-eV luminescence in neutron-irradiated silicon. The resonance is of an excited triplet ($S=1\mathrm{}$) state of the defect, which is not the radiative state, known to be a singlet ($S=0\mathrm{}$). The spectrum is unusual in that it is characteristic of a statically distorted defect (from $C_{3v}$ to $C_{1h}$), but with residual dynamic tunneling effects where random strain stabilizes mixtures of the static $C_{1h}$ distortions. Vacancy-related models previously suggested for the defect are tentatively ruled out.

Observation of electron paramagnetic resonance signals at multiples (up to 7) of the conventional magnetic resonance field are reported in the case of the system GaP: ${\mathrm{Cr}}^{+}$ at low temperatures. These signals are shown to be due to multiphoton transitions. The physical reason for the observation of these resonances is a long relaxation time consistent with the physical nature of this paramagnetic center.

The response of a one-dimensional charge-density wave weakly pinned by impurities to perturbations which change the $Q$ vector is analyzed. From Monte Carlo calculations $Q$ is found to lag behind the equilibrium value and the response is different for perturbations which move $Q$ away from or towards equilibrium. $Q$ changes by addition or removal of phase solitons and the metastability arises from the energy barriers for these processes. The behavior is similar to the pressure-induced hysteresis of $Q$ observed in Cr.

The change in the phason spectrum on going from the "incommensurate plane wave" to the "multisoliton lattice" modulation limit has been observed via phason-induced ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$ spin-lattice relaxation in the incommensurate phase of ${\mathrm{Rb}}_2$${\mathrm{ZnCl}}_4$. The observed temperature dependence of ${T_1}^{-1\mathrm{}}$ close to the "lock-in" transition reflects the temperature dependence of the "acousticlike" branch of the phason spectrum in a multisoliton lattice.

It is shown that scattering of polarized low-energy electrons can be used to study magnetic critical behavior of well-characterized, free surfaces. For the topmost layers of the Ni(100) surface of a single-crystal magnetic circuit it is found that, in the temperature range $0.008<~1-\frac{T}{T_{\mathrm{C}}}<~0.1$, the magnetization decreases with the critical exponent ${\beta }_1={0.825}_{-0.040\mathrm{}}^{+0.025\mathrm{}}$, possibly indicating $\mathrm{XY}$ coupling at the Ni surface.

Residual resistivities of ${\mathrm{Ag}}_c{\mathrm{Pd}}_{1-c}$ alloys for $0.2<~c<~0.8$ were calculated from first principles using the Korringa-Kohn-Rostoker coherent-potential approximation and yielded quantitiative agreement with experiment with no adjustable parameters. Some aspects of Mott's $\mathrm{sd}$ model are confirmed, but the $s$ sheet velocities and lifetimes vary in a surprising manner with concentration because of the energy dependence of the disorder self-energy at the top of the $d$-band complex. These variations are analogous to electron-phonon mass and lifetime renormalization.

It is shown that a massless Goldstone field produced at a cosmological phase transition near $T\sim {10}^{17}$ GeV can generate density fluctuations sufficient to explain the galaxy formation.

A new type of universality associated with the onset of a deterministic diffusion for systems described by iterative one-dimensional maps is reported. The diffusion coefficient $D$ plays the role of an order parameter with a universal critical exponent. For the presence of external noise, the existence of a universal scaling function $d$ is shown. An analytic expression is derived for $d$ which is in good agreement with results of a numerical experiment.

The London equations, which were proposed as restrictions to conventional classical electromagnetism in order to explain the Meissner-Ochsenfeld effect in superconductors, are derived from familiar classical action integrals. The equations of motion, from which the London equations follow, should apply to some collisionless plasmas. Space and thermonuclear applications are timely and significant.

The capture of $\alpha$ particles by deuterium has been observed by using a magnetic analysis technique to detect the recoiling ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ ions. Measurements of the cross section down to 1 MeV in the center-of-mass system can be interpreted accurately in terms of a direct-capture model, and it is found that production of ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ in the big bang is 5 times smaller than has been assumed.

Pion production in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^3{}_{}{}^{}\mathrm{He}$ collisions has been measured at energies below the threshold for production in free ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-nucleon reactions, with use of the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ beam from the Orsay synchrocyclotron. Discrete states of the recoil nucleus, corresponding to the coherent process ${}_{}{}^3{}_{}{}^{}\mathrm{He}$(${}_{}{}^3{}_{}{}^{}\mathrm{He}$,${\pi }^{+}$) ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$, have been observed with sizable cross sections at 268.5- and 282-MeV kinetic energy. The ground-state data are consistent with pionic atom information.

The reactions of ${}_{}{}^{144}{}_{}{}^{}\mathrm{Sm}$ on ${}_{}{}^{144}{}_{}{}^{}\mathrm{Sm}$ and ${}_{}{}^{154}{}_{}{}^{}\mathrm{Sm}$ on ${}_{}{}^{154}{}_{}{}^{}\mathrm{Sm}$ have been studied at energies 30% in excess of the barrier. Whereas the number of exchanges nucleons is similar in both reactions, the number of exchanged protons is considerably larger at small energy losses in the ${}_{}{}^{144}{}_{}{}^{}\mathrm{Sm}$ system. On the basis of the shell-corrected liquid-drop potential energy surface these observations are attributed to the closed $N=82\mathrm{}$ neutron shell which, for ${}_{}{}^{144}{}_{}{}^{}\mathrm{Sm}$, hinders the neutron exchange and leads to a preferential transfer of protons.

Unambiguous $S$ matrix elements (resonant and background contributions) have been extracted from data on the reaction ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, (${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$,${\alpha }_0$)${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$. Assignments of $J^{\pi }={10}^{+}, 8^{+}, \mathrm{and} 8^{+}$ have been made to resonances at $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}=15.8, 15.9, \mathrm{and} 16.1$ MeV, respectively: Nonresonant amplitudes result from a narrow angular momentum window about the $l=10\mathrm{}$ grazing partial wave; at 90° these dominate the reaction. Systematic behavior of the $8^{+}$ resonances in this and other heavy-ion systems suggest that they may not be of binary character and that $\alpha$-particle substructure may play a role.

The shapes of angular distributions in reactions ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, $\alpha$) to the $J^{\pi }=0^{+}$ ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ ground state are studied from $E\left({}_{}{}^{16}{}_{}{}^{}\mathrm{O}\right)=28.5\mathrm{} \mathrm{to} 100$ MeV. The position of the first minimum (and maximum) of these oscillatory angular distributions is expected to be sensitive to the presence of a low-angular-momentum cutoff in the compound-nucleus formation cross section. No evidence for a low-$L$ cutoff is found even at energies well beyond the predicted threshold.

A perturbation-expansion approach is used to examine the sign of the orientation vector as a function of scattering angle and projectile charge. It is shown that for small angles, the sign of the orientation vector is different for oppositely charged projectiles consistent with the prediction of the classical grazing model. At large angles, on the other hand, the orientation vector for oppositely charged projectiles is shown to have the same sign.