Magnetic properties of the two-dimensional Hubbard-model are inferred from results of Monte Carlo simulations. Lattice sizes up to 8×8 and temperatures down to $T=\frac{t}{20}$ ($t=\mathrm{hopping}$) were studied. The half-filled system is found to exhibit antiferromagnetic long-range order for all values of the Coulomb repulsion $U$. The low-temperature magnetic properties are found to be well described by spin-wave theory with renormalized local moment and spin-wave velocity. Numerical evidence presented suggests that when doped the system loses the long-range order immediately away from half filling.

We examine a new two-band model which allows the coexistence of a spin liquid and conduction sea. Exchange coupling between the two fluids leads to Kondo-spin compensation of the spin liquid, generating bound states between the spin liquid and conduction electrons. When this occurs, the spin and charge degrees of freedom decouple, forming a superconductor of a novel kind.

A far-infrared sphere resonance has been observed in superconducting ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_{4-y}$ isolated small particles at low temperatures. The frequency and strength of this electronic mode increase with increased Sr doping, decrease with applied magnetic field, and decrease as the temperature is raised; the resonance disappears above the superconducting transition temperature. This superconducting sphere resonance occurs at a frequency below the energy gap and is polarized along the $c$ axis of the particle.

We study theoretically and experimentally the effect of noise on Rydberg atoms passing through a strong coherent microwave field. We derive a master equation containing the coherent field exactly. Its solution reveals the presence of four universal dynamical regimes: (i) an initial classical regime, (ii) a subsequent coherent localized regime, (iii) a transition, induced by the noise, in which coherence and localization are destroyed, and (iv) the final regime, where equidistribution over the quasienergy states is reached.

We report results of numerical investigations of the quantum dynamics of a 1D system subject to a time-dependent perturbation with three incommensurate frequencies. These results demonstrate a transition from localized to extended states occurring at a critical value of the perturbation parameter. The dependence of the localization length and of the diffusion rate on this parameter near the critical point is analyzed and found to be in agreement with the predictions of renormalization-group theory.

The relaxation properties of a stochastic bistable system perturbed by a periodic low-frequency forcing term is investigated by means of analog simulation. The so-called stochastic resonance phenomenon is revealed under diverse experimental conditions. Its dependence on the parameters of both the periodic perturbation and the forced system is explained theoretically.

Transport in three-dimensional nonintegrable Hamiltonian flows is studied and the destruction of Kolmogorov-Arnold-Moser barriers in the presence of stochastic perturbations described. We extend the action principle to Hamiltonians with small noise, which provides a framework to determine universal scaling of characteristic times as a function of the noise.

Diffusion processes are studied which occur in three-dimensional media with traps at quenched random positions. For the first time, aspects of survival at intermediate times are derived, given that the concentration of traps is small. A first-order phase transition is found at zero concentration. The predictions are in good agreement with recent numerical data. They also have new implications for the Lifshitz band-edge singularity of the density of states of random binary mixtures, for a model of selfattracting polymers, and for the distribution of the number of distinct sites visited in a random walk.

A Monte Carlo algorithm is presented that updates large clusters of spins simultaneously in systems at and near criticality. We demonstrate its efficiency in the two-dimensional $\mathrm{O}\mathrm{}\left(n\right)\mathrm{}$ $\sigma$ models for $n=1\mathrm{}$ (Ising) and $n=2\mathrm{}$ ($x-y$) at their critical temperatures, and for $n=3\mathrm{}$ (Heisenberg) with correlation lengths around 10 and 20. On lattices up to ${128}^2$ no sign of critical slowing down is visible with autocorrelation times of 1-2 steps per spin for estimators of long-range quantities.

A Cu-Pb torsion balance placed next to a large lock on the Snake River permitted setting limits on the coupling constant for a possible intermediate-range material-dependent gravitational force. A 1.7 × ${10}^8$-kg perturbing mass of water could be turned on or off within 12 min and had a well-known distribution. No evidence for a fifth force was observed. The experiment is primarily sensitive to isospin coupling, for which it yields limits on the intrinsic coupling coefficient ranging from ${\alpha }_0=(-2.2\mathrm{}\pm 4.6)\times {10}^{-3\mathrm{}}$ at $\lambda =10\mathrm{}$ m to ${\alpha }_0=(-0.5\mathrm{}\pm 1.0)\times {10}^{-3\mathrm{}}$ for $\lambda \ge 200$ m.

Theories of gravitation in which a nonsymmetric metric couples minimally to the electromagnetic field violate the weak equivalence principle (WEP) by predicting that test bodies with internal electrostatic energy fall with different accelerations. This further supports Schiff's conjecture that the WEP implies the Einstein equivalence principle whose consequence is that gravity is described by a symmetric spacetime metric. Constraints on the nonsymmetric gravitational theory of Moffat are placed using the results of a recent free-fall Galilean test of the WEP.

I give an effective-action model for the vanishing of the cosmological constant, based on the qualitative Ansatz that the intermediate-energy gravitational action has a nonlinear dependence on the spacetime volume. This Ansatz scales consistently into the infrared, and by straightforward functional-integral methods implies the vanishing of the physical cosmological constant.

We present a new type of inflationary scenario based on metric formulations of gravity different from that of Einstein, e.g., a Brans-Dicke theory of gravity. Unlike previous inflation models, the inflationary phase transition can be completed via bubble nucleation. Hence, the fine tuning of an effective potential to obtain a slow-rollover transition is not required.

We describe numerical simulations of large-scale structure formation from cosmic strings and massive neutrinos. The linear power spectrum in this model resembles the cold-dark-matter power spectrum. Galaxy formation begins early, and the final distribution consists of isolated density peaks embedded in a smooth background, leading to a natural bias in the distribution of luminous matter. The distribution of clustered matter has a filamentary appearance with large voids and visually resembles the survey done by the Center for Astrophysics.

Analysis of the cumulative Fly's Eye data reveals an excess of air showers from the direction of Cygnus X-3 at energies above 0.5×${10}^{18}$ eV. No point source has previously been identified at such high energies. The probability of this excess arising as a chance clustering of isotropic cosmic rays is 6.5×${10}^{-4\mathrm{}}$. The inferred signal flux is (2.0±0.6)×${10}^{-17\mathrm{}}$ particles/${\mathrm{cm}}^2$ s. The Cygnus X-3 4.8-h periodicity is weakly present in the data.

Using the relativistic light-cone description of the two-nucleon system we argue that experimental confirmation of the recently observed structure in the deuteron magnetic form factor at $Q^2\approx 2$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$ would provide new independent evidence in favor of the dominant role of nucleonic degrees of freedom in the deuteron.

The longitudinal response function for quasielastic electron scattering from ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ and ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ at a momentum transfer $q=550\mathrm{}$ MeV is calculated in a relativistic random-phase approximation (RPA). The particle-hole response is calculated exactly in the finite system, while vacuum-polarization effects are included in a local-density approximation. The RPA result with vacuum polarization is about 25% below the Hartree response and comparable to Saclay data.

We present the first quantitative study of the final-state-interaction effect in the missing-energy spectrum measured in a coincidence ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}(e, e^{\prime }p)$ experiment in the dip region. It is found that the calculated cross section for the two-body type of final-state interaction is very small compared with the data. Furthermore, it decreases with increasing missing energy; this qualitative behavior does not agree with the data. From a qualitative investigation, the presence of the many-body mechanism involving more than two nucleons is suggested.

The cross sections for the fusion reaction ${}_{}{}^{128}{}_{}{}^{}\mathrm{Te}({}_{}{}^{12}{}_{}{}^{}\mathrm{C}, 3n){}_{}{}^{137}{}_{}{}^{}\mathrm{Ce}$ were measured from $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}=34.5\mathrm{} \mathrm{to} 50$ MeV by observing the decay of the ground ($J^{\pi }={\frac{3}{2}}^{+}$, $t_{\frac{1}{2}}=9.0\mathrm{}$ h) and isomeric (${\frac{11}{2}}^{-}$, 34.4 h) states in ${}_{}{}^{137}{}_{}{}^{}\mathrm{Ce}$. The isomer ratio decreases rapidly from a value of ≃34 at 47 MeV to an essentially constant value of ≃1.5 below 38 MeV. This confirms the prediction of an approximately constant value for the average angular momentum for fusion of heavy ions at energies well below the fusion barrier.

A single trapped ${}_{}{}^{198}{}_{}{}^{}\mathrm{Hg}_{}^{+}{}_{}{}^{}$ ion was cooled by scattering laser radiation that was tuned to the resolved lower motional sideband of the narrow ${}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}-{}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{}$ transition. The different absorption strengths on the upper and lower sidebands after cooling indicated that the ion was in the ground state of its confining well approximately 95% of the time.