Approaching the Curie temperature of ${\mathrm{LiTbF}}_4$ ellipsoids of thicknesses between 0.5 and 7 mm up to ${\mathit{T}}_{\mathit{c}}$-T=3×${10}^{\mathrm{-}4}$${\mathit{T}}_{\mathit{c}}$, the first clear evidence for critical fluctuations and macroscopic size effects on the domain-wall relaxation is presented. An analog to the Landau-Lifshitz model proposed recently and additional optical data on the domain width suggest associating both phenomena with the existence of linear (Bulaevskii-Ginzburg) walls and with domain branching near the surface.

We study a version of the uniform resonating-valence-band state in which fermions and spinless bosons are coupled by a gauge field. We show that above the Bose-Einstein temperature, the boson inverse lifetime due to scattering by the gauge field is of order kT, which suppresses the condensation temperature and leads to a linear T resistivity. The Hall number is proportional to the hole density and temperature dependent. The single-particle spectral weight exhibits a continuum plus a broadened peak. The ‘‘Fermi surface’’ area satisfies Luttinger’s theorem. Comparison with the anomalous normal-state properties of oxide superconductors is made.

A general scaling approach to pinning and response in weakly disordered systems is developed that considers pinning at arbitrary high energy barriers. These are a consequence of a disordered T=0 renormalization-group fixed point which characterizes the condensed phase. Application to flux creep in superconductors yields a creep velocity ν(j)∝exp-${\mathit{Cj}}^{\mathrm{-}\mathrm{\mu }}$ where μ is related to the roughness exponents ζ of the flux-line lattice. We argue that ζ=0(log) and μ=(d-2)/2 as for charge-density waves.

A novel resonant ultrasound technique is used to measure all elastic constants and their temperature dependence in single-crystal ${\mathrm{La}}_{1.86}$${\mathrm{Sr}}_{0.14}$${\mathrm{CuO}}_4$. The in-plane shear modulus ${\mathit{c}}_{66}$ drops by nearly half upon cooling to 227 K, an effect attributed to order-parameter fluctuations (of 2D Gaussian character) near the tetragonal-orthorhombic phase boundary. Implications for current models of the structural phase transition are discussed, as well as the effect of domains in the low-temperature orthorhombic state.

A method for constructing a variety of probabilistic lattice-gas cellular automata for chemically reacting systems is described. The microscopic reactive dynamics give rise to a general fourth-order polynomial rate law for the average particle density. The reduction of the microdynamical equations to a discrete or continuous Boltzmann equation is presented. Connection between the linearized Boltzmann equations and a reaction-diffusion macroscopic equation is discussed. As an example of the general formalism a set of cellular automata rules that yield the Schlögl phenomenological model is constructed. Simulation results are presented.

The eigenfrequencies of resonance cavities shaped as stadium or Sinai billiards are determined by microwave absorption. In the applied frequency range 0–18.74 GHz the used cavities can be considered as two dimensional. For this case quantum-mechanical and electromagnetic boundary conditions are equivalent, and the resonance spectrum of the cavity is, if properly normalized, identical with the quantum-mechanical eigenvalue spectrum. Spectra, containing up to a thousand eigenfrequencies, are obtained within minutes. Statistical properties of the spectra as well as their correlation with classical periodic orbits are discussed.

A new class of exact solutions to Einstein’s field equations with a perfect-fluid source is presented. The solutions describe spatially inhomogeneous cosmological models and have a realistic equation of state p=ρ/3. The properties of the solutions are discussed. The most remarkable feature is the absence of an initial singularity, the curvature and matter invariants being regular and smooth everywhere. We also present an alternative interpretation of the solution as a globally regular cylindrically symmetric space-time.

Time-dependent solutions of 2+1 gravity are found for N particles on a sphere with g handles. The theory is reduced to a constrained Hamiltonian system with a finite number of global observables. These include three-vectors that give the spacetime position of each particle, and others that are related to the Teichmüller parameters. With these global observables, the dynamical equations (${\mathit{d}}^2$${\mathit{x}}^{\mathit{i}}$${\mathit{dt}}^2$=0) decouple. The cases of the torus and uniformly expanding sphere with N sources are shown to agree with solutions from the metric formulation.

We report on the observation of ψ mesons from Υ(4S) decays which are too energetic to come from B mesons. These events provide the first evidence for non-BB¯ decays of the Υ(4S). The measured rate is B[Υ(4s)→ψX]=(0.22±0.06±0.04)% for ψ momentum above 2 GeV/c.

We have examined vortex solutions in (2+1)D Chern-Simons-Higgs theory which has no usual Maxwell term. It is shown that the Bogomol’nyi-type equations can be derived for a simple sixth-order Higgs potential and corresponding general n-vortex solutions should contain 2n free parameters. Various characteristics of Chern-Simons vortices are discussed briefly.

We study vortex solutions in an Abelian Chern-Simons theory with spontaneous symmetry breaking. We show that for a specific choice of the Higgs potential the vortex satisfies a set of Bogomol’nyi-type, or ‘‘self-duality,’’ equations.

We use covariant string field theory to explore many-body string physics. A candidate nonperturbative vacuum is identified via a level-truncation scheme. The breaking of internal and Lorentz symmetries is investigated. We show that tree-level string physics exhibits similarities to loop-level particle physics. The effective string coupling runs at tree level, inducing asymptotic freedom. This is a feature of any string theory. The running coupling has significant effects on the spectrum in the nonperturbative vacuum, including the disappearance of states.

The production rate for soft dileptons in a hot quark-gluon plasma is computed to leading order in the QCD coupling constant through a systematic resummation of perturbation theory. The partial rate from the annihilation and decay of soft quarks and antiquarks exhibits unexpected structure—sharp peaks due to Van Hove singularities. These peaks are overwhelmed by the partial rate from processes involving hard quarks and gluons. The total rate for soft dileptons is larger than the naive one-loop prediction by orders of magnitude.

The Michigan State University 4π array has been used to select central-impact-parameter events from the reaction ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$${+}^{51}$V at incident energies from 35 to 85 MeV/nucleon. The event shape in momentum space is an observable which is shown to be sensitive to the dynamics of the fragmentation process. A comparison of the experimental event-shape distribution to sequential- and simultaneous-decay predictions suggests that a transition in the breakup process may have occurred. At 35 MeV/nucleon, a sequential-decay simulation reproduces the data. For the higher energies, the experimental distributions fall between the two contrasting predictions.

The ratio of transverse and longitudinal response functions is investigated for the quasielastic reaction ${\mathrm{Li}}^6$(e,e′p). The ratio itself and its dependence on the missing energy cannot be explained in the distorted-wave impulse approximation. It is proposed that a reaction mechanism other than quasielastic one-body knockout plays a significant role. © 1990 The American Physical Society

A model accounting for the time dependence of thermal shape fluctuations in hot nuclei and their effects on the giant dipole resonance (GDR) is presented. Experimental data for Er and Sn isotopes are compared with theoretical calculations, and evidence for the motional narrowing of the GDR is investigated.

The measured rate coefficient α for ${\mathrm{Xe}}_2^{+}$+${\mathrm{Cl}}^{\mathrm{-}}$→${\mathrm{XeCl}}^{\mathrm{*}}$+2Xe (and similar processes) is far higher than had been expected. The exciting experimental results are reminiscent of assisted mutual neutralization, but this cannot occur. Monte Carlo simulation that allows for the ${\mathrm{Cl}}^{\mathrm{-}}$-${\mathrm{Xe}}^{+}$, ${\mathrm{Cl}}^{\mathrm{-}}$-Xe, and ${\mathrm{Xe}}^{+}$-Xe forces shows that the rotational and vibrational modes of ${\mathrm{Xe}}_2^{+}$ tend to be excited by the passage of ${\mathrm{Cl}}^{\mathrm{-}}$ through perihelion. Because of this electrostatic tidal action the orbit contracts and the internal energy of ${\mathrm{Xe}}_2^{+}$ increases, leading to dissociation. The measured values of α are reproduced satisfactorily.

We have observed coherence among the four magnetic sublevels of freely precessing ${}_{}{}^{21}{}_{}{}^{}\mathrm{Ne}$ for times of 4.5 h. This has allowed direct detection of the quadrupole component of coherence relaxation and a test of linearity of quantum mechanics. The quadrupole component of relxation is evident in the different relaxation rates for the three lines of the NMR spectrum. Nonlinear corrections to quantum mechanics are found to be less than 1.6×${10}^{\mathrm{-}26}$ of the binding energy per nucleon of ${}_{}{}^{21}{}_{}{}^{}\mathrm{Ne}$.

The electron-hydrogen scattering problem has been a nemesis to theoretical atomic physicists due to the fact that even the most sophisticated of theoretical calculations, both perturbative and nonperturbative, do not agree with experiment. The current opinion is that the perturbative approach cannot be used for this problem since recent second-order calculations are not in agreement with the experimental data and higher-order calculations are deemed impractical. However, these second-order calculations neglected second-order exchange. We have now added exchange to the second-order calculation and have found that the primary source of disagreement between experiment and theory for intermediate energies is attributable not to higher-order terms but to second-order exchange.

The curvature of the boundary is shown to alter the fluid’s slip length, which may even become negative as a result. Due to the mesoscopic curvature of surface roughness, the microscopically calculated and the macroscopically measured slip lengths can be quite different.