Neutron diffraction, heat capacity, and susceptibility measurements on dilute YGd single crystals (1.5 to 4.4 at.% Gd) show long-range helical magnetic ordering at low temperatures. A periodic incommensurate spin structure with the Gd moments in the basal plane and a propagation wave vector of ${0.28c}^{*}$ is found. A semicusplike behavior is observed in the magnetic specific heats with the maxima at similar temperatures as the susceptibility $\frac{d\left(\mathrm{XT}\right)\mathrm{}}{\mathrm{dT}}$ peaks $\mathrm{H}\perp \mathrm{c}$ axis) and the onset of Bragg scattering. These results are quantitatively interpreted within a helical spin-density-wave stabilization of the conduction electrons.

We study the dynamical behavior of novel computing arrays which can accurately detect moving spots in one dimension in the presence of noise, while being able to operate reliably with temporary failures in their units. Their response to a purely fluctuating background without spuriously detecting nonexistent motion can be accounted for by the dynamics of a simple spin model.

Measurements on the Belousov-Zhabotinskii reaction in a stirred flow reactor reveal, beyond a period-doubling cascade, a sequence of periodic states that is different from the "U (universal) sequence" found in recent experiments on diverse systems. Particular periodic states occur in three different parameter ranges rather than one range as in the U sequence. However, the order of periodic states found here is in accord with a one-dimensional single-parameter map constructed from the laboratory data.

The stability at finite wave numbers of a hard-sphere fluid subjected to a large shear is examined by use of hard-sphere kinetic theory. For a fixed density, for molecular-scale wave numbers, and for a critical shear rate the fluid is found to become unstable with respect to density waves in the direction of the velocity gradient. The critical shear rates obtained are close to those found by Erpenbeck in his nonequilibrium molecular-dynamics simulations of shear-induced ordering.

A model is proposed for the evolution of the profile of a growing interface. The deterministic growth is solved exactly, and exhibits nontrivial relaxation patterns. The stochastic version is studied by dynamic renormalization-group techniques and by mappings to Burgers's equation and to a random directed-polymer problem. The exact dynamic scaling form obtained for a one-dimensional interface is in excellent agreement with previous numerical simulations. Predictions are made for more dimensions.

It is found that the effective Hamiltonian for nuclear rotation in a diatom is equivalent to that of a charged particle in a background magnetic-monopole field. In certain cases, half-integer orbital angular momentum or non-Abelian fields occur. Furthermore, the effects of magnetic-monopole-like gauge fields can be experimentally observed in spin-resonance experiments with variable magnetic fields.

The usual configuration-space semiclassical approximation to wave equations (e.g., Schrödinger's equation) needs refinement at caustics. When expressed in a phase-space coherent-state representation, such equations admit a semiclassical solution that properly captures the effects of caustics. Transformation of this expression to the configuration-space representation yields a global, uniform, semiclassical approximation.

The low-energy threshold behavior and Levinson's theorem are derived for general, not necessarily rotationally symmetric, two-dimensional scattering systems, with zero-energy resonances and zero-energy bound states explicity taken into account. Surprisingly, $s$-wave-type zero-energy resonances do not contribute at all to Levinson's theorem, while $p$-wave-type zero-energy resonances each contribute a term - $\pi$, exactly like (zero-energy) bound states. Some consequences of this for, e.g., the Witten index in certain supersymmetric quantum-mechanical models are briefly discussed.

We use the quantum Bethe-Ansatz method to compute the free energy of the classical sine-Gordon system. Previous attempts to do this have failed because the number of coupled integral equations to be solved to find the free-energy diverges in the classical limit. We present a transformation, extending a method of Maki, which reduces this divergent set to only two coupled equations, for the densities of solitons and anharmonic phonons. These equations can be solved iteratively in the temperature $t$ and the soliton density $e^{\frac{-1\mathrm{}}{t}}$ to give a double series for the free energy. This series coincides to high order with classical transfer-matrix results.

For a general quantum mechanical system undergoing a process, it is shown that one can tell from measurements on this system whether or not the system is characterized by quantum numbers the existence of which is unknown to the observer, even though the detection equipment used by the observer is unable to distinguish among the various possible values of the "secret" quantum number and hence always averages over them.

An approach to multiloop string path integrals is presented which expresses them in terms of Green's functions on triply connected domains or "pants."

It is shown that the exchange of gauge bosons between fermions in a real representation (e.g., the adjoint) produces an attractive force that binds identical ferminos into spin-zero Cooper pairs. The condensation of these pairs in the vacuum produces a Majorana mass for the fermions. The associated breaking of fermion number results in a composite Goldstone boson.

The $\beta$-decay asymmetry of the free neutron is measured by use of a beam of polarized neutrons and a long solenoidal $\beta$ spectrometer with $4\pi$ solid angle for electron detection. The asymmetry parameter corrected for recoil and weak magnetism is $A_0=-0.1146\mathrm{}\pm 0.0019$, implying $\frac{g_A}{g_V}=-1.262\mathrm{}\pm 0.005$ for the ratio of the axial-vector to the vector weak-coupling constants.

The silyl radical Si${\mathrm{H}}_3$ has been detected in a silane-discharge plasma through the observation of the ${\nu }_2$ band by infrared diode-laser spectroscopy. The band was observed to consist of two inversion-doubling components, $1^{-}$ ← $0^{+}$ and $1^{+}$ ← $0^{-}$, and the analysis of the observed spectrum yielded molecular constants in the ${\nu }_2=0\mathrm{} \mathrm{and} 1$ states. The barrier height to the inversion has been calculated from the observed band origins to be 1868 ${\mathrm{cm}}^{-1\mathrm{}}$. The same analysis yielded the height of the pyramid to be 0.465 Å, which was combined with the observed constant $B_0$ to calculate $r\left(\mathrm{SiH}\right)=1.468\mathrm{}$ Å and $\theta \left(\mathrm{HSiH}\right)=110.5\mathrm{}°$.

The stimulated Raman scattering from a micrometer-size C${\mathrm{Cl}}_4$ droplet exhibited up to fourteenth-order Stokes peaks and multiorder combination Stokes emission without detectable anti-Stokes emission. Such results are explained qualitatively by the morphology-dependent resonances of a droplet which acts as an optical cavity, providing high optical feedback and supporting enhanced internal fields at the various Stokes shifts.

The surface-phonon dispersion curves of Al(110) have been calculated with force constants determined from first-principles self-consistent total-energy calculations. Relaxation of the surface layers produces changes in the surface force constants which have a significant influence on the surface-phonon frequencies near the zone boundary.

Several theoretical concepts such as two-phonon bound states, off-center ions, and Fermi resonances have been suggested to explain the strong anomalies in the phonon spectra of copper halides, with no conclusive results. We take into account the anharmonic coupling of the $TO\left(\Gamma \right)$ phonons with the two-phonon states and calculate the line shapes in the framework of an anharmonic valence-shell model. We show that the anomalies are Fermi resonances; other mechanisms need not be invoked.

Spin-polarized atomic hydrogen gas has been compressed to densities > ${10}^{18}$/${\mathrm{cm}}^3$ with liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ at temperatures from 0.24 to 0.67 K in magnetic fields up to 7.5 T. We find that the critical pressure, at which recombination ultimately becomes a thermally triggered explosion, goes through a maximum as a function of temperature. The explosion pressure increases with decreasing sample size and with increasing field and nuclear polarization.

We propose a theory of shear-induced melting of colloidal crystals based on a nonequilibrium generalization of first-order freezing theory. The results agree qualitatively with experiment.