An extension of the Hubbard model, introduced by Montorsi and Rasetti, is studied using exact diagonalization and quantum Monte Carlo methods. Our numerical results for the static and dynamic properties of one- and two-dimensional systems strongly suggest the occurrence of a metal-insulator transition.

The general magnetic response function χ(Q,ω) of a superconducting (${\mathit{T}}_{\mathit{c}}$=33 K) single crystal of ${\mathrm{La}}_{1.86}$${\mathrm{Sr}}_{0.14}$${\mathrm{CuO}}_4$ is measured using inelastic neutron scattering. There is no gap in the magnetic fluctuation spectrum of the normal state. Superconductivity reduces χ’’(Q,ω) in progressively greater proportion as ħω decreases below 6 meV. The superconducting transition coincides with a sharp maximum in the temperature-dependent χ’’(Q,ω) for ħω=4 meV and Q fixed at the incommensurate wave vector characterizing the magnetic fluctuations in the normal state.

We have studied the ESR properties of three compensated n-type Si:P,B samples ner the metal-insulator transition covering the low-temperature regime from 30 mK to 10 K. We find that both the susceptibility and the ESR linewidth increase dramatically as the temperature is lowered, and in the metallic Si:P,B samples the susceptibility increase is more than in similar uncompensated Si:P samples. We compare results for the insulating phase with numerical calculations, then discuss the metallic region in light of various theoretical models for the low-temperature thermodynamic behavior.

The transition from a fluid to solid state is investigated by studying the viscosity of concentrated noncolloidal monodisperse and bidisperse suspensions of hard spheres. The notion of a range of fluidity limits, analogous to glass transition densities, is introduced. Lower bounds to this range are determined and compared with dry random close packings of the spheres. The ratio of the random-close-packing fraction to the lower bound fluidity limit of the bidisperse suspensions is found to be the same as for monodisperse spheres, a constant of 1.19, suggesting a random structure at this limit.

We use a pulsed electric field to study transient electric birefrigence in ‘‘living polymers.’’ The decay of birefringence is nonexponential and depends on the pulse width of the electric field. The relaxation is modeled in terms of the collective modes of the individual polymers with Zimm dynamics. Using this model, we find that the living polymer behaves as a flexible chain even at very low concentrations. Two distinct growth regimes are found for the polymer contour length L¯ as a function of surfactant concentration φ. In the very dilute regime L¯ increases as √φ , and levels off at higher concentrations.

Experiments on the field-free transient electric birefrigence decay of dilute solutions of monodisperse DNA fragments whose length L is much larger than the persistence length ${\mathit{L}}_{\mathit{p}}$ are reported. They reveal that the highly nonexponential decay is independent of L. We present a simple theoretical model which includes the internal rigidity of the polymer, explains the qualitative results of the measurements, and describes quantitatively the dynamics of long semiflexible polymers such as DNA, providing an accurate determination of the persistence length.

The tunneling in a double-well potential with dissipation is formulated without referring to instantons. This formulation, in the domain of small dissipation, goes beyond the instanton approximation. Several applications of this formulation for the Ohmic case are presented, and we show that the virtual mixing of excited states induced by dissipative interactions tends to enhance the tunneling rate. This enhancement effect becomes dominant for a super-Ohmic case.

Einstein evolution equations are written as a hyperbolic system of balance laws. A harmonic time coordinate is used with zero shift vector (harmonic slicing). The principal part of the evolution system reduces to a set of uncoupled wave equations in first order form. The relevance for three-dimensional numerical relativity of both the harmonic slicing and the resulting evolution system is stressed.

Working in the context of spontaneously broken gauge theories, we show that the magnetically charged Reissner-Nordström solution develops a classical instability if the horizon is sufficiently small. This instability has significant implications for the evolution of a magnetically charged black hole. In particular, it leads to the possibility that such a hole could evaporate completely, leaving in its place a nonsingular magnetic monopole.

We present a measurement of the inclusive jet cross section in p¯p collisions at √s =1.8 TeV at the Fermilab Tevatron using the Collider Detector at Fermilab. Good agreement is seen with the predictions of recent next-to-leading-order [O(${\mathrm{\alpha }}_{\mathit{s}}^3$)] QCD predictions. The dependence of the cross section on clustering cone size is reported for the first time. An improved limit on ${\mathrm{\Lambda }}_{\mathit{c}}$, a term characterizing possible quark substructure, is set at 1.4 TeV (95% C.L.).

We demonstrate that emission of bosons from a dispersed collection of droplets will produce a two-tiered correlation function reflecting the size of the droplets as well as their spatial distribution. We argue that this two-tiered correlation can signal the existence of a mixed phase of QCD plasma and hadronic gas in relativistic nuclear collisions, and propose that positive kaons will be the best indicators of this structure.

Angular distributions of cross sections and tensor analyzing powers have been measured for ground state transitions in ${}_{}{}^{95}{}_{}{}^{}\mathrm{Mo}$(d,t${)}^{94}$Mo, ${}_{}{}^{119}{}_{}{}^{}\mathrm{Sn}$(d,t${)}^{118}$Sn, and ${}_{}{}^{149}{}_{}{}^{}\mathrm{Sm}$(d,t${)}^{148}$Sm for deuteron energies below the Coulomb barrier. Exact, finite-range distorted-wave Born approximation analyses of these data have been made using both the S- and D-state amplitudes for the three-nucleon systems. A best fit of the data gives ${\mathit{D}}_2$(t)=-0.217±0.010 ${\mathrm{fm}}^2$ and ${\mathrm{\eta }}_{\mathit{t}}$=-0.043±0.002, significantly lower than previous experimental determinations, and in agreement with a recent theoretical prediction of ${\mathrm{\eta }}_{\mathit{t}}$=-0.046±0.001.

We report the results of an experiment designed to search for superheavy isotopes of hydrogen. Based on the centrifugation of sea water, followed by atomic spectroscopy, it is sensitive to large masses from ${10}^4$ up to ${10}^8$ GeV/${\mathit{c}}^2$. The relative abundance of such hypothetical atoms, compared to ordinary hydrogen, is found to be less than about 6×${10}^{\mathrm{-}15}$. This may be used to exclude charged dark matter particles between ${10}^4$ and ${10}^7$ GeV/${\mathit{c}}^2$.

The 1S ground-state Lamb shift in atomic hydrogen has been measured to an accuracy of 1.3 parts in ${10}^5$ by directly comparing the optical frequencies of the 1S-2S and the 2S-4S,4D two-photon transitions. The result, 8172.82(11) MHz, agrees with the theoretical prediction of 8172.94(9) MHz and rivals measurements of the 2S Lamb shift as a test of QED for a bound system. A comparison of the 2S-4S and 2S-4D intervals yields a 4S Lamb shift of 131.66(4) MHz.

In a Stern-Gerlach experiment, the spatial trajectory of a particle traversing a magnetic-field gradient serves as a detector of its spin state. This paper considers the implications of adding a harmonic restoring potential to the Stern-Gerlach experiment. The behavior of such systems is shown to resemble that of a ‘‘folded’’ Stern-Gerlach experiment, in that the linear spatial trajectory of the Stern-Gerlach experiment becomes folded into the cyclic phase-space trajectory of the oscillator. Under appropriate circumstances, the coupling between the spin and the oscillator is shown to be sufficiently strong that it might find practical application as the basis of a magnetic resonance imaging device.

We carry out a topological analysis on an experimental data set from the laser with saturable absorber. This analysis is based on the topological organization of low period orbits extracted from chaotic time series data. This allows us to determine for the first time that previously proposed models are compatible with the data.

An investigation of the spectral response of a small collection of two-state atoms strongly coupled to the field of a high-finesse optical resonator is described for mean number N¯≤10 atoms. For weak excitation, a coupling-induced normal-mode splitting is observed even for one intracavity atom, representing a direct spectroscopic measurement of the so-called vacuum Rabi splitting for the atom-cavity system.

The acceleration of electrons using a laser and a static electric field perpendicular to the former is considered. The coupled particle-field equations are reduced to a second-order nonlinear inhomogeneous equation which determines the trajectory of the particle. The particle energy equation is considered and found to exhibit fold catastrophes. At these catastrophes, which may occur whenever the wave phase is nπ if the direction of the applied weak field is reversed at (2n-1)π/2, further net acceleration occurs. The trajectory equation is found amenable to analytical treatment.

The coherent effect in spontaneous emission has been observed in a low-energy, low-gain, waveguide-mode free-electron laser. The intensity of the radiation was more than ${10}^3$ times stronger than that of the incoherent radiation. This enhancement is due to coherent radiation emitted by the electron beam, which is randomly modulated by micropulses whose typical pulse width (∼35 ps) is comparable to the radiation wavelength. The measured power tends towards a quadratic dependence on the electron-beam current.

Direct measurements of velocity-space transport coefficients, which may be interpreted as the Fokker-Planck coefficients, in the direction of the confining magnetic field are reported. The measurements were made using the techniques of laser-induced fluorescence and optical tagging, which allow ions to be followed in phase space. The results show that in a plasma with nearly only thermal fluctuations the data agree well with test particle calculations made by N. Rostoker for classical collisional processes. In the presence of larger-amplitude drift-wave fluctuations there is a pronounced enhancement of the velocity diffusion coefficient at a velocity less than the thermal speed.