We report a direct observation, via electron energy spectroscopy, of lateral tunneling and lateral ballistic-electron transport in a two-dimensional electron gas (2D EG). This was accomplished through the use of a novel transistor structure employing two potential barriers, induced by 50-nm wide metal gates deposited on a GaAs/AlGaAs selectively doped heterostructure. Hot electrons with very narrow energy distributions (≃5 meV wide) have been observed to ballistically traverse 2D EG regions ≃170 nm wide with a mean free path of about 480 nm.

A quantum-mechanical calculation is made of charged-particle motion at a four-terminal junction of narrow wires in the presence of a magnetic field of arbitrary strength. The scattering probabilities strongly reflect the influence of quantum effects of the junction, including subband thresholds and virtual resonant states. The Hall resistance calculated from them may depart considerably from the classic wide-wire result. Physical features are related to the emergence of pinned Landau levels as the field strength increases.

Two-photon absorption spectroscopy was performed in GaAs quantum wells with photon energy ħω near half the band gap ${\mathrm{E}}_{\mathrm{g}}$. For the case of incident light polarized perpendicular to the confinement direction ε^⊥z^, the onset of two-photon absorption occurs at the 2P state of the lowest-energy heavy-hole exciton and the absorption increases linearly with 2ħω-${\mathrm{E}}_{\mathrm{g}}$. For ε^∥z^, the two-photon spectrum is stepped with superpositions of sharp 1S exciton features at odd ΔN light-hole–conduction-band transitions. These results are used to accurately determine exciton binding energy, successive subband energy splittings, and band offsets.

The lifetime of the zero-voltage state of a current-biased Josephson junction shunted by a transmission line has been measured in the thermal regime. As the length of the line is increased in situ, the lifetime is modulated in an oscillatory pattern. The amplitude, period, phase, and tailing off of the modulation is the first direct evidence for the oscillations of the particle in the well before escape which are implied by Kramer’s energy-diffusion model for the escape from a metastable state.

The standard Davydov Hamiltonian can be partially diagonalized using a method due to Eremko, Gaididei, and Vakhnenko. The complete Hamiltonian in this partially diagonalized form, however, includes a term omitted by these authors. Using this term in a first-order perturbation theory calculation results in an estimate of the lifetime of a Davydov soliton at finite temperatures. The lifetime at 300 K is on the order of ${10}^{\mathrm{-}12}$ s for parameters appropriate for the α-helical protein molecule. This is too short to be useful in biological processes.

Through an application of the quantum Monte Carlo technique, we investigate the thermal equilibrium properties of the one-dimensional model proposed by Davydov for the description of energy transport processes in the α helix. The deformation of the lattice about a single (moving) excitation is computed over a wide range of temperatures. We find that the model does admit a coherent structure at low temperatures, but that this structure is substantially destroyed above 7 K.

A long-standing problem for wide band-gap semiconductors is that of achieving good bipolar conductivity. I here show that this difficulty can be understood via solubility considerations of dopant incorporation. Reports of good conductivity in such materials are likely due to nonequilibrium impurity incorporation, and prior literature is reinterpreted to provide examples of this view.

The influence of the Lyapunov instability on a trajectory in phase space is investigated numerically for a Lennard-Jones fluid under a constant shear rate γ. The time dependence of the distance d(t,γ;Γ) between two individual trajectories, starting at the same phase point Γ, but moving either with or without an imposed shear, can be described by d(t,γ;Γ)=Aγ exp(t/${\mathrm{\tau }}_{\mathrm{L}}$), where ${\mathrm{\tau }}_{\mathrm{L}}$ is the inverse of the largest Lyapunov exponent of the system without shear. A possible explanation of this behavior and the consequences for the validity of linear-response theory are discussed.

We present the results of new theoretical work on surface ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ and ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ evolution in the oldest halo stars along with a new and refined analysis of the predicted primordial lithium abundance resulting from big-bang nucleosynthesis. This allows us to determine the constraints which can be imposed upon cosmology using primordial lithium and both standard big-bang and stellar-evolution models. This leads to a constraint on the baryon density today of $0.0044<\mathrm{}{\Omega }_b{h}^2<0.025\mathrm{}$ (where the Hubble constant is $100h$ km ${\sec }^{-1\mathrm{}}$ ${\mathrm{Mpc}}^{-1\mathrm{}}$), and impose limitations on alternative nucleosynthesis scenarios.

We present an analysis of the exclusive semileptonic decay mode $D^0$→$K^{\mathrm{-}}$$e^{+}$${\nu }_e$. We have measured the ratio of decay rates .11(syst), which corresponds to a $D^0$→$K^{\mathrm{-}}$$e^{+}$${\nu }_e$ branching fraction of (3.8±0.5±0.6)%. Combining our result with a measurement of the $D^0$ lifetime, we find .1±1.4)×${10}^{10}$ ${\mathrm{s}}^{\mathrm{-}1}$. We have also measured the vector form factor $f_{+}$(t) and find that it is consistent with the single-pole form where the pole mass &=2.11 GeV/$c^2$.

We have measured the polarization P, the analyzing power A, and the polarization transfer D of ${\Sigma }^0$’s produced inclusively by a polarized proton beam at 18.5 GeV/c. Our data cover a region of moderate $p_T$ (average 1 GeV/c) and Feynman x up to 0.75. We find agreement with a previous measurement of the ${\Sigma }^0$ polarization P. We observe nonzero values for A and D, but they are significantly smaller than predictions based on a simple parton-recombination model. We have extended this model to include finite transversity spin flips, which improves agreement with the data considerably.

The dilepton yield at invariant masses between 2$m_{\pi }$ and 1 GeV is shown to be reduced substantially if a baryon-rich quark-gluon plasma is formed in high-energy heavy-ion collisions. We suggest therefore to measure the total dilepton yield in this region of invariant masses as a function of the incident energy. The onset of a baryon-rich quark-gluon plasma is then signaled by a decrease in this excitation function.

We investigate the forward electron emission induced by fast antiprotons transmitted through thin carbon foils. The capture of electrons into wake-riding states is calculated in the second Born approximation. The transport through the foil is analyzed within the framework of classical stochastic dynamics. We find evidence that the usual cusp-shaped peak for positively charged ions is replaced by a broad peak due to emission of wake-riding electrons.

Two-photon absorption of light with nonclassical (squeezed) fluctuations is considered. Several unique effects are predicted for various possible regimes. These include a linear (instead of quadratic) growth of the absorption rate with the light intensity for weak fields, the possibility of a decreasing absorption with increasing intensity, and orders-or-magnitude differences between absorption rates for phase- and amplitude-squeezed beams of the same intensity.

The Fourier transform of the Hartley uv absorption spectrum in ozone is shown to have small recurrent features on time scales of typical molecular vibrations. These times are found to correspond closely to those of classical periodic or near-periodic orbits on a global excited-state potential-energy surface.

New techniques are demonstrated which measure, for the first time, collision-induced phase in macroscopic optical coherence. The shape of the phase versus time-delay curves obtained from the data yields the first information on imaginary velocity-changing collision kernels for optical radiators.

An analytical classical theory describing the dynamics of a Rydberg atom exposed to broadband-noise electromagnetic radiation is developed. The Fokker-Planck equation governing the diffusion excitation and ionization of the atom is derived. The classical diffusion of the atomic electron over Keplerian orbits is shown to be contributed by the noise radiation components, which are in first-order resonance with the Keplerian motion of the electron. The general properties of the diffusion excitation are studied, and the conditions of the applicability of the theory are delineated.

We describe a turbulent state characterized by the presence of topological defects. This ‘‘topological turbulence’’ is likely to be experimentally observed in nonequilibrium systems.

We present results of studies on shock-induced shifts in the K-photoabsorption edge of aluminum. Time-resolved x-ray spectroscopic measurements indicated maximum red shifts of ∼7 eV for compressions of ∼2.2 times normal density. The results are interpreted using hydrodynamic simulations which incorporated a new model for calculating the K-edge energy in a dense plasma.

The first experimental evidence of the excitation of an electron Bernstein wave by the nonlinear coupling of two microwave beams in a magnetized plasma is reported. The dependence of the excited wave on the electron density and magnetic field is found in excellent agreement with the dispersion relation of the Bernstein wave. The level measured with an electric probe is typically ${3.10}^{\mathrm{-}16}$ W for a mean incident power of 10 W. This value is about 10 dB below the theoretical value, but still within the uncertainty limit. The resonance width is consistent with the theory.