Raman measurements on a superconducting superlattice with a 2:1 sequence of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ and ${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ layes revealed a new line in the spectral region around 320 ${\mathrm{cm}}^{\mathrm{-}1}$, which is interpreted as a combination of odd lattice vibrations on the Y sublayers with a partial admixture of an even parity mode on the Pr sublayer, becoming Raman active in the superstructure. Its eigenvector is similar to a phonon from the interior of the Brillouin zone of the original lattice. No superconductivity-induced self-energy effects are observed for this phonon, although the IR-active counterparts of the individual Y sublayer vibrations in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ exhibit those effects.

The staggered magnetization of the Heisenberg antiferromagnet in two dimensions can be systematically approximated by a 1/N expansion. Cancellation between self-energy diagrams leads to a Luttinger-like theorem for the ground state. We prove (for a smooth enough self-energy) that the long range order of mean field theory (N=∝) survives corrections to all orders of 1/N. Divergences of this series provide a new route to the disordered phases of quantum antiferromagnets.

It is shown that site disorder induces noncoplanar states, competing with the thermal selection of coplanar states, in the nearest neighbor, classical Kagomé Heisenberg antiferromagnet. For weak disorder, it is ofund that the ground state energy is the sum of energies of separately satisfied triangles of spins. This implies that disorder does not induce conventional spin glass behavior. A transformation is presented, mapping ground state spin configurations onto a folded triangular sheet (a new kind of ‘‘spin origami’’) which has conformations similar to those of tethered membranes.

Several centers that involve Pt and H have been introduced into n-type Si and studied by electron paramagnetic resonance and vibrational spectroscopy to provide unique structure-sensitive data for an H-passivated deep level impurity. Through the observation of Pt-H and -D hyperfine interactions, a new Pt-${\mathrm{H}}_2$ complex has been identified. This defect is still electrically active and its level has been located. Several vibrational bands due to Pt and H related centers were also found. H stretching bands at 1888.2 and 1889.5 ${\mathrm{cm}}^{\mathrm{-}1}$ are assigned to the different charge states of the Pt-${\mathrm{H}}_2$ defect.

Ultrafast relaxation processes in poly(p-phenylenevinylene) and its oligomers are investigated using femtosecond luminescence spectroscopy. A quasi-instantaneous luminescence rise and the absence of luminescence near the excitation energy indicate very rapid vibronic relaxation. The subsequent transient redshift of the spectra is attributed to ultrafast energy relaxation of optical excitations within an inhomogeneously broadened density of states.

We study the front formed by the reaction mA+nB→ [inert] between opposing currents of A and B, as a function of current density J(->0), diffusion constant D, and reaction constant k. We show that the critical dimension is ${\mathit{d}}_{\mathit{c}}$=2/(m+n-1), above which mean-field theory is valid. Below ${\mathit{d}}_{\mathit{c}}$,J→0 is equivalent to k→∞. Consistency arguments give exact predictions for the scaling exponents when d<${\mathit{d}}_{\mathit{c}}$. Our results apply also to the case of a front that moves or grows with time, or where the diffusion constants differ.

By using low-angle static light scattering we study the aggregation of dense colloidal solutions under diffusion limited (DLCA) and reaction limited cluster aggregation (RLCA) conditions. While it has been recently shown that DLCA surprisingly leads to a semiordered state, we show here that any order is lost under RLCA conditions. While in the first case a scattering peak at finite wave vector q is observed, in the second the scattered peak always occurs at q=0.

We classify all generalized symmetries of the vacuum Einstein equations in four spacetime dimensions. They consist of constant scalings of the metric, and of the infinitesimal action of generalized spacetime diffeomorphisms. Our results rule out a large class of possible ‘‘observables’’ for the gravitational field, and suggest that the vacuum Einstein equations are not integrable.

We determine the entropy-per-baryon content of the central reaction region in terms of the charged particle multiplicity. We study the consistency of our findings with recent data on strange antibaryon production at 200A GeV in S-A collisons (A∼200) assuming formation of a central fireball. Hadron gas models which do not invoke strong medium modifications of hadron masses do not provide enough entropy and are inconsistent with the combined experimental results. In contrast the quark-gluon plasma hypothesis explains them naturally.

Velocity distributions and fragment-fragment correlation functions have been measured as a function of charged particle multiplicity for ${}_{}{}^{129}{}_{}{}^{}\mathrm{Xe}$${+}^{\mathrm{nat}}$Cu collisions at E/A=50 MeV. With increasing multiplicity, the velocity distributions evolve from a pattern exhibiting targetlike and projectilelike sources to an anisotropic distribution centered near the projectile-target center-of-mass velocity. Emission times extracted from the correlation functions are consistent with the decay of a projectilelike source in low multiplicity collisions, but the time scale for fragment emission in the high multiplicity collisions is too short for a separation of sources to occur.

Fission dynamics of hot nuclei have been investigated using the two-dimensional Langevin equation. Including particle evaporation in the continuous limit, prescission multiplicities of neutrons, protons, and α particles have been calculated. Both the calculated number of prescission neutrons and the average total kinetic energy of fission fragments are consistent with experimental values when one-body dissipation is assumed. Unusually strong hydrodynamical two-body viscosity also reproduces the experimental neutron multiplicity, but it significantly underestimates the average kinetic energy.

The bond energy and electronic properties of a multicomponent fullerene ${\mathrm{C}}_{12}$${\mathrm{B}}_{24}$${\mathrm{N}}_{24}$ have been calculated by using a self-consistent field (SCF) molecular-orbital (MO) method based on the non-local-density-functional formalism. The structure of ${\mathrm{C}}_{12}$${\mathrm{B}}_{24}$${\mathrm{N}}_{24}$, which is optimized by a non-SCF-MO method with Harris functional approximation, is distorted by at most 5% from that of ${\mathrm{C}}_{60}$. The electrons of ${\mathrm{C}}_{12}$${\mathrm{B}}_{24}$${\mathrm{N}}_{24}$ are localized on the N atoms, so that the network of π electrons of ${\mathrm{C}}_{60}$ is broken and the bond energy of ${\mathrm{C}}_{12}$${\mathrm{B}}_{24}$${\mathrm{N}}_{24}$ is about 0.7 eV/atom smaller than that of ${\mathrm{C}}_{60}$. The energy gaps between the highest-occupied MO and the lowest-unoccupied MO are 2.44 (${\mathrm{C}}_{12}$${\mathrm{B}}_{24}$${\mathrm{N}}_{24}$) and 1.63 eV (${\mathrm{C}}_{60}$).

The natural decay rates of the 1${\mathit{s}}_5$[${\mathit{np}}^5$(n+1)s${,}^3$${\mathit{P}}_2$] metastable state of argon and krypton are experimentally determined using a magneto-optical trap. The method to determine the lifetime of a metastable state from a fluorescence decay measurement is described. The obtained lifetimes are ${38}_{\mathrm{-}5}^{+8\mathrm{}}$ s for argon and ${39}_{\mathrm{-}4}^{+5\mathrm{}}$ s for krypton.

The first direct experimental observation of an intramolecular reaction path going through a transition state is reported. The results of a Coulomb explosion imaging experiment for cold ${\mathrm{CH}}_4^{+}$ molecular ions show the intermediate conformations between permutationally equivalent structures of this molecule. Comparison with a theoretical prediction of the corresponding reaction path is given and the agreement is satisfactory.

We prove the existence (as ħ→0) of the singular continuous (‘‘Cantor-like’’) spectrum for the Fermi-Ulam model on a torus for a certain phase-space domain and an irrational (diophantine) ratio of the minimum distance between the walls to the oscillation amplitude of the moving wall. Varied scenario of the stabilizing and destabilizing spectral transitions are observed in the limit of vanishing Planck’s constant, and their relation with the qualitative changes in the quasienergy eigenstates is brought out.

A new method of using an additional laser for electron-beam conditioning in free-electron lasers and synchrotrons is proposed. Theoretical analysis and calculations are presented, showing that the axial energy spread of electrons due to their betatron motion in undulators can be dramatically reduced by interacting with a quasi-${\mathrm{TEM}}_{10}$ mode Gaussian optical beam. It is required that the electrons be pre-bunched over half an optical cycle in advance.

The first observation of coherent backscattering (CB) in an optically anisotropic, homogeneous (liquid crystal) medium with long-range dielectric tensor fluctuations is reported. The results are compared with previous CB experiments in heterogeneous, centrosymmetric, isotropic scattering media and with theoretical predictions for isotropic, noncentrosymmetric media. Fundamental differences and analogies between these cases are observed and explained. Other liquid crystal phases of potential interest in the localization of electromagnetic modes are introduced briefly.

In a Langmuir monolayer of NBD-stearic acid, long rods (∼1 mm long, ∼20 μm wide) of a two-dimensional solid phase are obtained at the liquid-solid phase transition. A rod bent in the plane of the water under the effect of a bending stress breaks after a clearly defined delay. This delay is a function of the strain applied to the rod and of the temperature. For large deformations, a second mode of breaking appears and coexists with the previous one: A fraction of the rods break instantaneously while other break after a delay.

The energy spectral density of axial velocity components in a rotating Couette-Taylor system was obtained using a Fourier transform of axial velocity distributions. As this configuration has well defined spatial periodicity, strong peaks appear on the spectrum corresponding to Taylor vortex flow and wavy vortex flow modes and their harmonics. A continuous background shows an exponential decay with wave number, which supports the results of the numerical simulations. A variation in the decay rate at reduced Reynolds number shows a maximum at ${\mathit{R}}^{\mathrm{*}}$≊22. At this Reynolds number, an energy exchange between the first and second harmonics was observed.

We report an experimental study of the onset of convection in ethanol/water mixtures confined in a circular cell of radial aspect ratio of 11.6. The initial bifurcation was to radial traveling waves; however, the linear state typically gave way to a nonlinear one in which convection alternately focused along one cell diameter and then another roughly perpendicular to the first. After a number of cycles, this state frequently contracted radially to a localized pulse of traveling-wave convection very similar to those observed in 1D geometries. The pulses we observed either decayed back to pure conduction or grew to fill or nearly fill the cell.