A new thermal instability in a lyotropic uniaxial nematic liquid crystal is described. A roll structure with the axes perpendicular to the initial orientation of the director appears when the vertical thermal gradient starts to decrease. The wavelength of the roll structure is about 10 times smaller than the sample thickness. The relaxation of the roll structure to the initial planar geometry is studied. A mechanism is suggested based on the change in the size of the micelles as a function of the temperature.

A transition of brittle to ductile propagation of a sharp crack in a crystal is observed by molecular-dynamics simulation and the transition mechanism shown to be the emission of dislocations at the crack tip and activation of specific slip systems appropriate to the crystal structure. Implications for extension of current theories are discussed.

We show that the high-temperature Ni(001) surface undergoes a large vertical expansion. Low-energy electron diffraction experiments indicate that the surface thermal-expansion coefficient ${\mathrm{\alpha }}_{\mathrm{surf}}$ increases rapidly between 900 and 1300 K, reaching a value nearly 20 times larger than in the bulk. These results directly confirm that a large anharmonicity in the interatomic potential develops on metal surfaces and that it may be a precursor to various surface disordering transitions.

A recent theory of the magnetic-field-induced spin-density wave (FISDW) transitions observed in the Bechgaard Salts (TMTSF${)}_2$X emphasizes the role of the superconducting pairing interaction in FISDW formation. No superconductivity is observed in (TMTSF${)}_2$${\mathrm{NO}}_3$, unlike other metallic Bechgaard salts. Our study of (TMTSF${)}_2$${\mathrm{NO}}_3$ in pressure up to 13 kbar, temperature down to 0.5 K, and magnetic field up to 30 T shows no sign of FISDW. The ‘‘rapid’’ magnetoresistance oscillations observed but unexplained in other Bechgaard salts are also observed here.

Zero-field muon-spin-resonance and lower-critical-field data are presented for a wide range of Th concentrations in ${\mathrm{U}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Th}}_{\mathit{x}}$${\mathrm{Be}}_{13}$, spanning the region where both a superconducting and a second, lower-temperature phase transition are observed. Overall T-x phase boundaries are assigned and discussed according to the nature of the lower phase transition. Arguments for associating the lower phase with a possible magnetic (time-reversal–violating) superconducting state are given.

Electrorheological (ER) fluids are colloidal suspensions of highly polarizable particles in a nonpolarizable solvent. In a strong electric field, the behavior of the fluid is dominated by the dipolar interactions between the colloidal particles. We address problems of structure formation and long-time relaxation in an ER fluid. When a high electric field is applied to a fluid, dielectric columns oriented parallel to the field rapidly form. The relaxation of these columns is a much slower process. We calculate the shape of the columns formed and the time scale for the relaxation of the columnar structure.

The transition from periodic to quasiperiodic spiral-wave rotation is studied numerically by the pseudospectral method in a two-variable model of excitable kinetics. Quasiperiodic behavior originates from a supercritical Hopf bifurcation of one branch of circularly rotating spiral-wave solution. The secondary frequency is strongly determined by the presence of a second nearby branch of solution rotating about a larger hole radius. Scaling laws consistent with numerical results are proposed for the spiral-tip orbits in the critical region.

The velocity and angular velocity autocorrelation functions obtained by molecular-dynamics simulations of liquids composed of hard spheroids indicate the existence of damped oscillations with periods as long as 40 times the mean time between molecular collisions. In addition, little correlation is found between successive collisions. This suggests that the damped oscillations are not due to semicoherent molecular librations in cells.

We evaluate the Fourier path integral for an optical polaron by the method of partial averaging. Higher Fourier coefficients are taken into account by averaging with Feynman’s trial action, whereas the first N coefficients are treated explicitly by numerical integration. This method removes the Coulomb singularity in the effective action, yields lower bounds to the partition function, and systematically improves Feynman’s variational results. We present numerical results for the exact ground-state energy for small and intermediate values of the coupling constant.

Transient order→chaos transitions of two ${\mathrm{Ba}}^{+}$ ions are observed where the duration of the chaotic state increases smoothly with an increasing Mathieu control parameter q. The transient regime precedes the transition point ${\mathit{q}}_{\mathit{c}}$ for stationary chaos, which is well defined and reproducible. The perturbative step that initiates chaos takes place on a time scale of 1 min, far longer than previous estimates, and is due to a single ${\mathrm{H}}_2$-${\mathrm{Ba}}^{+}$ collision (${\mathrm{H}}_2$ pressure 5×${10}^{\mathrm{-}11}$ Torr). In contrast to random collision events, chaos can be initiated deterministically by a new pulse technique. These observations resolve earlier anomalies and challenge computer simulations.

It is shown that composites formed from charged one-dimensional objects and vortices in a multiply connected three-dimensional space which is invariant under O(2) rotations can have fractional angular momentum.

A general mechanism, based on very simple considerations of spin, is proposed, which has the effect of enhancing the neutrino magnetic dipole moment relative to the neutrino mass.

We have measured the three form factors governing the decay ${\mathit{D}}^{+}$→K¯ ${}_{}{}^{\mathrm{*}0}{}_{}{}^{}$${\mathit{e}}^{+}$${\nu }_{\mathit{e}}$, observed in Fermilab photoproduction experiment E691, using the complete decay distribution of the data. The results are ${\mathit{A}}_1$(0)=0.46±0.05±0.05, ${\mathit{A}}_2$(0)=0.0±0.2±0.1, and V(0)=0.9±0.3±0.1 for the two axial-vector and the vector form factors, respectively. The K¯ ${}_{}{}^{\mathrm{*}0}{}_{}{}^{}$ mesons have a ratio of longitudinal to transverse polarization of 1.$8_{\mathrm{-}0.4}^{+0.6\mathrm{}}$±0.3. These results are significantly different from values predicted by a number of different models.

We have measured for the first time the decay rate for ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$→4γ from rest, by using a multi-γ-ray spectrometer. The branching ratio R=${\lambda }_{\mathit{S}}^{4\gamma }$/${\lambda }_{\mathit{S}}^{2\gamma }$, where ${\lambda }_{\mathit{S}}^{4\gamma }$ and ${\lambda }_{\mathit{S}}^{2\gamma }$ are the 4γ and 2γ rates, respectively, was measured to be R=[1.30±0.26(stat)±0.16(system)]×${10}^{\mathrm{-}6}$, consistent with the lowest-order QED calculation.

We consider self-induced transparency beyond the slowly varying envelope approximation. For a carrier wave of given frequency, we show that the possible steady-state pulse velocities form a discrete set, qualitatively changing the previous picture of self-induced transparency. The selection mechanism cannot be seen in a perturbative expansion about the slowly varying envelope approximation.

A classical two-level system has been realized by coupling two propagation or two polarization modes of an optical ring resonator. This system can be driven by periodic modulation of either the coupling or the bare mode frequencies, at sufficient strength to violate the rotating-wave approximation (RWA) on resonance. Landau-Zener transitions, Rabi oscillation with non-RWA signature, and Autler-Townes doublets have been observed.

The photonic band structure of a face-centered-cubic lattice of spheres is calculated using the plane-wave method for Maxwell’s equations. Comparisons with the available experimental results show rather good agreement, except that we do not find a true gap for this configuration. This we believe is due to symmetry reasons at the W point.

We examine the propagation of electromagnetic waves in periodic dielectric structures by solving the vector Maxwell equations with the plane-wave method. Contrary to experimental reports, as well as results of scalar-wave calculations, we do not find a true gap extending throughout the Brillouin zone in the fcc structure. However, there is a depletion in the photon density of states, seemingly a remnant of the Mie resonance, giving rise to a pseudogap in the spectrum that is quite strong for dielectric-sphere packing fraction β∼0.3–0.4. An effect analogous to the Borrmann effect in x-ray diffraction is predicted, where certain photon modes will propagate an anomalously long distance before getting absorbed.

The extreme compressibility of a fluid near its liquid-vapor critical point significantly alters its dynamic response to temperature changes. Adiabatic processes allow a fluid of constant volume to thermalize in a matter of seconds, compared with the hours or days that thermal diffusion would require. Moreover, the adiabatic transient is faster as the temperature of the fluid approaches the critical temperature. We present experimental measurements of turbidity in critical xenon which show the fast time scale of the adiabatic transient and the phenomenon of critical speeding up.

We compute the ground-state structure and collective-excitation energies of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ droplets at zero temperature. The collective excitations are described by a generalized Feynman theory with inputs of exact one- and two-body densities sampled from a second-order diffusion Monte Carlo algorithm. The monopole transition density shows a pronounced shell structure not accountable by simple Jastrow-type variational trial functions.