It is shown that the stability of a cluster can be substantially enhanced by changing its size and/or composition so as to take advantage of the electronic shell filling as well as close atomic packing. The interaction between two such clusters is found to be weak and can form the basis for synthesizing a new class of cluster-assembled crystals with uncommong properties.

Using an optical technique ultrafast vibrations of the surface of thin opaque films in the 0.1-THz range have been directly observed in the time domain. We propose a new type of phonon detection by monitoring the surface velocity, and demonstrate an exceptional sensitivity to atomic-scale inhomogeneities by observing an anomalous reflection from a buried film of monolayer order in thickness.

We studied the initial growth mode of Au on Ag(110) using first-principles total energy calculations. We found that a recently observed bilayer growth mode in this system is not energetically favorable and thus may not be an equilibrium process. The most favorable initial growth process up to 1 monolayer Au coverage is found to proceed via subsurface substitution, which is an interesting growth mode for a metal-on-metal system.

A theory of the perpendicular transport of electrons through metallic multilayers based on the Landauer-Büttiker formalism is presented and applied to the magnetoresistance of antiferromagnetically coupled magnetic multilayers. The contributions of contact potential, interface roughness, and bulk impurity scattering to the spin-selective transmission or spin-valve effect are described by a simple closed formula which, in the absence of spin-flip scattering, unites previous approaches.

We demonstrate that nonlinear dynamics is able to elucidate many details of Hall experiments in ballistic multiprobe conductors in semiconductor heterojunctions, if soft boundaries with a realistic confining potential are assumed. This gives rise to two additional plateaus above the last Hall plateau, which were observed experimentally, but had remained unexplained. We show that all three plateaus are caused by trajectories leaving the junction in the forward direction and that the generally accepted explanation of the last Hall plateau is invalid.

Nonequilibrium phonon pulses, generated by laser heating a metal film, have been used to probe small areas of an extended two-dimensional electron gas (2DEG) in a GaAs-AlGaAs heterojunction, at low temperature and in a quantizing magnetic field. The electron-phonon interaction was studied via the change in the two-terminal resistance of the 2D electron system. By scanning the phonon beam over the entire area of the 2DEG, images of the effects were made. We show that our images are in qualitative agreement with theoretical descriptions of the quantum Hall effect which include electron states confined near the edges of the 2DEG.

We have measured the normal-state susceptibility χ for both ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3$${\mathrm{Cu}}_{60}$ and the specific heat jump ΔC at the superconducting ${\mathit{T}}_{\mathit{c}}$ for ${\mathrm{K}}_3$${\mathrm{C}}_{60}$. Within a phonon-mediated pairing model, the combined effects of χ and ΔC are shown to give strong support to pairing via high-energy modes (>1000 K) associated with intramolecular vibrations. In addition, the small ΔC along with the known upper critical field slope provide evidence for an extreme dirty-limit superconductor, with mean free path l≊10 Å.

A fast magneto-optical technique is used to explore flux dynamics in the optically driven nonequilibrium state of type-I superconducting Pb films. It is found that the penetration of magnetic flux through the nonequilibrium intermediate state can be dramatically faster than through the normal metal. Studies as a function of how far the system is driven from equilibrium, and of the effect of a static applied magnetic field, indicate that the observations reflect the dynamics of normal/superconducting interfaces, and are strongly dependent on the microscopic pattern of the intermediate state.

Recently an exact SU(2)⊗SU(2) symmetry for the half-filled Hubbard model has been elucidated but has not yet been properly incorporated in many analyses of this model. We compute the irreducible representations of the symmetry group, a necessary step for any consistent mean-field analysis. A proper mean-field theory valid for both negative- and positive-U Hubbard models is then presented. A by-product of the description is a systematic enumeration of the Lie group SU(8) of unitary canonical transformations that is a direct generalization of the SU(4) transformation in the theory of superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and the SU(2) Bogoliubov transformation in BCS theory.

We present a high-frequency electron spin resonance study of the magnetic transitions occurring between excited states in a Haldane spin system. We show that these transitions are induced between the low-energy magnon states, which develop at the boundary (q≊π) of the Brillouin zone. Our study provides therefore, the first evidence that, in the Haldane phase, the excitations associated with the quantum gaps correspond actually to a s=1 magnetic state.

We analyze interacting lattice fermions with anisotropic hopping amplitudes. Calculating the linear response of the momentum distribution function, we find that one-dimensional Luttinger liquid phases are unstable with respect to arbitrarily small transverse hopping, even if the interaction is strong. This instability is expected to signal the formation of states with broken symmetry in the strongly anisotropic regime.

We show spectroscopically that the origin of optical gain and laser emission in (Zn,Cd)Se/ZnSe quantum wells at blue-green wavelengths is of excitonic nature. This circumstance derives from the large enhancement in the exciton binding and its oscillator strength which occurs in the quasi-2D case, so that an exciton gas is stable against ionization by optical phonons up to room temperature and that gain in the context of partial phase-space filling can develop at pair densities below the onset to an electron-hole plasma.

We report on the first successful two-electron coincidence experiment at solid surfaces in a back-reflection geometry. We found that two-electron coincident electron emission events exist, and that they require a certain threshold energy of the primary electron (relative to the Fermi level) which is not equal to twice the work function of the sample surface [W(100)] as one might expect from the energy conservation law. We give evidence that we observe at low primary energy single electron-electron scattering events between the primary electron and a valence-band electron and give a semiquantitative estimate of the relative probability of such events.

Using a multiple light scattering technique and a stabilizing counterflow (i.e., a fluidized bed), we measured the average sedimentation velocity, its variance, and the short-time self-diffusion coefficient in a concentrated hard-sphere suspension. Many-body hydrodynamic interactions slow both the Brownian diffusion and mean sedimentation and provide a novel mechanism for long time dispersion.

It is known that statistical properties of the energy levels for various billiards on a constant-negative-curvature surface do not follow the universal random-matrix predictions. We show that nongeneric behavior of the systems investigated so far originates from the special arithmetic nature of their tiling groups, which produces an exponentially large degeneracy of lengths of periodic orbits. A semiclassical study of the two-point correlation function shows that the spectral fluctuations are close to Poisson-like ones, typical of integrable systems.

We introduce and study a new class of random sequential adsorption models which we call the coordination models. These exhibit longer-range interactions than any local models studied to data and are intractable to any analytic techniques heretofore developed. We present computer simulation results and derive a simple mean-field theory which agrees closely with the simulations.

We make the simple observation that, because of global-symmetry-violating higher-dimension operators, as for instance might be expected to be induced by Planck-scale physics, textures are generically much too short lived to be of use for large-scale structure formation.

Nambu-Goldstone modes are sensitive to the effects of physics at energies comparable to the scale of spontaneous symmetry breaking. We show that as a consequence of this the global texture proposal for structure formation requires rather severe assumptions about the nature of physics at the Planck scale.

We redo the quantization of the N=4 string, taking into account the reducibility of the constraints. The result is equivalent to the N=2 string, with critical dimension D=4 and signature (++--). The N=4 formulation has several advantages: The σ-model field equations are implied classically, rather than by quantum or β-function calculations; self-duality (chirality) is one of the super-Virasoro constraints; SO(2,2) covariance is manifest. This reveals that the theory includes fermions, and is apparently spacetime supersymmetric.

We calculate the two-jet inclusive cross section at order ${\mathrm{\alpha }}_{\mathit{s}}^3$ in QCD. Because the one-loop perturbative corrections are included, there is considerable improvement in theoretical accuracy compared to the Born-level calculation. We compare the predicted dependence of the cross section on the jet-jet scattering angle ${\mathrm{\eta }}_1$-${\mathrm{\eta }}_2$ to data from the CDF collaboration. We also discuss the dependence of the cross section on the jet-jet mass.