Evidence for the presence of alpha-particle clustering in ${}_{}{}^{218}{}_{}{}^{}\mathrm{Ra}$ comes from a number of observables: binding energy, $S_{2n}$, $Q_{\alpha }$, $E_x\left(J^{\pi }\right)$, ${{\theta }_{\alpha }}^2$, and $F_{\alpha }$. That this clustering is a signature for the new dipole collectivity suggested by Iachello and Jackson follows from observation of simultaneous enhancement of selected $E1$, $E2$, and $E3$ deexcitation transition matrix elements; of these the $E1$ enhancement is most pronounced as would be expected for dipole collectivity.

Instabilities in a single-mode, Fabry-Perot, predominantly inhomogeneously broadened xenon laser show progressions through increasingly complicated periodic behavior, ultimately reaching a deterministic chaotic behavior. Period doubling, two-frequency, and intermittency routes to chaos have been observed.

The electron-induced characteristic x-ray emission from a noncentrosymmetric crystal shows a pronounced intensity difference when the Bragg condition is fulfilled for the $G$ and $-G$ reflections, respectively, along a polar direction. This two-beam effect is demonstrated fro GaAs and InP, and is a direct method of obtaining information about the phase of a structure factor and determining the sense of a polar direction.

NMR pulse techniques have been used to study the orientational ordering of ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}_2^{}{}_{}{}^{}$ molecules physisorbed on graphite. A small hysteresis (~2.5%) and small discontinuity in the evolution of the NMR echo amplitudes which depend directly on the local order parameters indicates that the order-disorder transition is first order.

The principle of conservation and transferability of chemical bonds explains the recent discovery by extended x-ray-absorption fine-structure measurements of two unequal anion-cation bond lengths $R_{\mathrm{AC}}$ and $R_{\mathrm{BC}}$ in $A_x{B}_{1-x}C$ zinc-blende semiconductor alloys despite the close adherence of the lattice constant to the average value (Végard rule). This bond alternation, manifested as a structural distortion to a local chalcopyrite coordination around the anions, explains also most of the observed optical bowing in semiconductor alloys.

Superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films adsorbed on porous Vycor glass have been studied in the low-density limit where the superfluid interparticle spacing is more than an order of magnitude greater than the atomic hard-core diameter. For the lowest transition temperatures observed, the influence of critical fluctuations near $T_c$ is much reduced and a range of crossover to behavior more characteristic of the dilute Bose gas is observed.

The effect of isobarically heating solid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ at high density is investigated with use of constant-pressure molecular-dynamics calculations and a realistic interatomic pair potential. At $P\sim 16$ GPa the observed sequence of stable phases with increasing temperature is fcc→bcc→liquid. The presence of a new, thermally stabilized, bcc phase for high-density solid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ is thus confirmed.

The growth of ordered domains following a quench of a system from a temperature $T$ in the disordered phase to a state below the critical temperature $T_c$ is simulated by Monte Carlo computations. For a square-lattice gas with (fourfold-degenerate) 2 × 1 order the domain size $L\left(t\right)\mathrm{}$ grows with time $t$ as $L\left(t\right)\mathrm{}\propto t^x$, $x\cong 0.35$ (density conserved) and $x=\frac{1}{2}$ (density not conserved), respectively. Quenches to $T_c$ and to $T=0\mathrm{}$ are also discussed.

Backscattering yields of 1.00-MeV ${\mathrm{He}}^{+}$ ions on $1T$-Ta${\mathrm{S}}_2$ are reported from room temperature to 120 K. The temperature dependence of the backscattering yield exhibits the softening of an acoustic mode and an optical mode. An anomalously large peak in the backscattering yield with large hysteresis was observed around the nearly commensurate to commensurate transition temperature, indicating existence of a disordered state. A small peak around 280 K was observed on heating, indicating existence of a new transition in this material.

High-resolution electron-energy-loss spectroscopy has verified the existence of two fundamentally different types of adsorbed acetone, ${\eta }^1$-bonded acetone on Pt(111) and ${\eta }^2$-bonded acetone on Ru(001). On a Ru(001) surface, the Lewis acidity of which has been increased by the presence of oxygen adatoms, the ${\eta }^1$ and ${\eta }^2$ forms of acetone coexist. The ${\eta }^1$-bonded acetone desorbs molecularly, whereas ${\eta }^2$-bonded acetone is a precursor to dissociation on the surface.

It is shown that the macroscopic and microscopic dielectric responses to a constant electric field can be directly dealt with by a method which is general, alternative to linear-response theory, and not restricted to linear effects. The basic features of the theory are discussed; explicit calculations of the macroscopic dielectric constant, miccroscopic local fields, and transverse effective charges are presented for Ge and GaAs within the local-density functional theory.

This Letter reports measurements down to 3 mK of the magnetic field dependence of the resistance and critical current of a weakly coupled periodic array of Josephson junctions. Below 700 mK the resistance is an oscillatory function of field. The temperature dependence of the resistance for integral numbers of flux quanta per cell suggests the existence of a Kosterlitz-Thouless transition involving the unbinding of defects in the flux lattice.

Fluxon propagation profiles for Josephson transmission lines have been directly measured. Single-input-pulse dissociation into multifluxon as well as velocity-dependent wave forms have actually been observed. The experimental results agree well with the theoretical results if a large rf loss term is assumed.

The excitonic absorption at the direct gap in Ge is measured as a function of the density and temperature of excitations at the indirect gap. The spectra are analyzed by means of a many-body theory which is valid for arbitrary free-carrier densities. The critical densities for the ionization are determined for temperatures from 8.5 to 60 K and compared with other predictions.

The authors treat electronic transport in disordered systems, including the electron-phonon interaction, on both sides of a mobility edge ${\epsilon }_c$ on the same footing. By application of a mode-coupling approximation self-consistent equations are derived from which one gets the matrices describing density relaxation due to both coherent tunneling processes and hopping processes. The resulting conductivity interpolates between a metallic behavior well above and a hopping behavior well below ${\epsilon }_c$.

The metal-insulator transition in the magnetic semiconductor ${\mathrm{Gd}}_{3-x}{v}_x{\mathrm{S}}_4$, where $v$ stands for vacancies, has been studied by tuning through the transition with the application of a magnetic field at low temperatures. For two samples with $x=0.321\mathrm{} \mathrm{and} 0.325$ the transition is continuous. $\sigma (T\to 0)$ is linear in $H-H_c$ which implies that $\sigma (T\to 0)\propto (E-E_c)$, where $H_c$ is a critical field and $E_c$ is the mobility edge. This is consistent with new scaling theories of both localization and interaction effects.

Neutron polarization analysis is combined with small-angle scattering for the study of a concentrated cobalt ferrofluid. The geometry of the cobalt particle core and surrounding surfactant layer has been determined and an anomalous magnetic layer found at the cobalt surface. The interparticle structure factor, measured for the first time in a magnetically aligned ferrofluid, agrees well with a recent theory. Magnetic fluctuations, measured as a function of applied field, are found to be predominantly paramagnetic.

First photoemission spectra are reported for the valence bands and core levels of quench-condensed ${\mathrm{Au}}_x{\mathrm{Sn}}_{100-x}$ ($0<~x<~100$) films in the amorphous and the crystalline state. It is found that (1) the valence-band spectra for the $20<~x<~80$ region, measured just after condensation, exhibit a decrease in intensity towards the Fermi energy $E_{\mathrm{F}}$ which can be understood in terms of a minimum in the density of states near $E_{\mathrm{F}}$ and (2) the minimum near $E_{\mathrm{F}}$ is most pronounced at an alloy concentration where the highest stability for the amorphous phase has been found.

By minimization of a Hamiltonian with respect to its interaction parameters, new Hamiltonians are generated. The same minimization procedure is carried out with the free energy of the initial Hamiltonian, and it is proven that in the thermodynamic limit the minimized free energy is the free energy of the minimized Hamiltonian. This method is illustrated with two-dimensional Ising models generalized to include both nearest-neighbor bonds and infinite-range bond-bond interactions. New critical behavior is observed.

In contrast to some earlier studies, good agreement has been found between the asymptotic random-walk exponent for particles restricted to the occupied sites of a simplecubic lattice, and the dynamical scaling prediction of Gefen, Aharony, and Alexander. For the first time, up to ${10}^7$ steps were used in the Monte Carlo study, which moreover employed larger lattices (${180}^3$) than earlier studies.