It has been often claimed that the formation of a Kondo ground state requires a minimum particle radius of r≫${\mathrm{\zeta }}_{\mathit{R}}$=ħ${\mathit{v}}_{\mathit{F}}$/Δ, where Δ=${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{K}}$ is the width of the Kondo resonance. We suggest that this minimum size is an artifact of the high symmetry of the sphere for which these calculations have been performed. A similar argument applies to other resonances. This question of minimal size is investigated for a Friedel resonance in two geometries, a sphere and a parallelepiped with a noninteger ratio of its edge lengths. A numerical calculation shows that the minimum linear size of the sample can be much smaller than ${\mathrm{\zeta }}_{\mathit{R}}$.

The magnetoconductance of a 3D ballistic Ag point contact is found to show reproducible fluctuations with rms amplitude 2 orders of magnitude lower than usually observed in diffusive systems and insensitive to temperature up to T=5 K. This is explained by quantum interference of electrons, the length scale of the trajectories of interfering waves being the elastic mean free path. For an applied voltage across the constriction, the observed magnetofingerprint changes asymmetrically under bias reversal with a roughly constant rms amplitude. At higher voltages (V≥10 mV), electron-phonon coupling destroys the phase coherence and a strong decrease in fluctuation amplitude is observed.

We have used femtosecond time-resolved optical-absorption spectroscopy to study the dynamics of high-field parallel transport of electrons in GaAs quantum wells. We have directly observed the ballistic acceleration of electrons from the band edge by measuring the nonequilibrium transient energy distribution of the electrons. The ballistic acceleration occurs on a 150-fs time scale.

Nonequilibrium populations of both confined and interface phonons generated by picosecond laser pulses in a series of GaAs/AlAs quantum wells have been studied by time-resolved picosecond Raman scattering as a function of well width. The dependence of the nonequilibrium phonon populations on well width is found to be sensitive to the theoretical model which is used to describe the electron-phonon interaction. Our data disagree with the macroscopic model of electron-phonon interaction, but they are in excellent agreement with the microscopic model proposed recently by Huang and Zhu.

The magnetic susceptibility of ferromagnetic hcp cobalt has been calculated based on itinerant-electron theory. For wave vectors along the c axis, well-defined acoustic and optic spin-wave excitations were found to exist out to the zone boundary. In the basal plane, spin waves were found to exist up to energies of about 300 meV before disappearing into the Stoner continuum. These spectra are considerably less complex than expected and are in broad agreement with preliminary neutron-scattering data.

It is shown that the commutative magnetic translational symmetry of the effective Hamiltonian for a Bloch electron in a magnetic field is, in a well-defined sense, a subgroup of the translation group for zero magnetic field. Based on this group-subgroup relationship a proof is presented for the splitting of an energy band into N magnetic subbands in the framework of the effective Hamiltonian approximation.

The possibility of using temperature-dependent angle-resolved-photoemission data to derive an effective ${\mathrm{\alpha }}^2$F for high-temperature superconductors via a Nambu-Eliashberg inversion is investigated. On analyzing data of Dessau et al. on Bi 2:2:1:2, we find a sharp bosonic peak at low energy (at approximately 10 meV).

Improved experimental conditions enabled us to increase the signal-to-noise ratio of the photoemission spectra for the superconducting state of ${\mathrm{Bi}}_2$${\mathrm{Ca}}_2$${\mathrm{SrCu}}_2$${\mathrm{O}}_8$, taken with high angular and energy resolution. This also enabled us to reveal a pronounced minimum that separates the two basic features of this spectrum, the narrow quasiparticle excitation peak and the still controversial broad band at lower kinetic energies. The minimum is approximately 3Δ below the Fermi level.

We report a novel magnetic-field dependence of the critical-current density ${\mathit{J}}_{\mathit{c}}$ in artificially grown Ge/Pb superlattices. In these layered structures ${\mathit{J}}_{\mathit{c}}$ exhibits an unexpected minimum as a function of perpendicular applied field, unlike any ever observed in other superconductor systems. The systematic evolution of this minimum as a function of layer thicknesses, temperature and pinning strength is related to either flux-lattice decoupling or melting.

The temperature dependences of the transient photoinduced conductivity in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ at different light intensities (${\mathit{I}}_{\mathit{L}}$) and of the doping-induced conductivity at different δ are similar, indicative of ‘‘photodoping.’’ For δ≊0.7, signatures of the photoinduced transition to metallic behavior are observed at ${\mathit{I}}_{\mathit{L}}$>5×${10}^{15}$ photons/${\mathrm{cm}}^2$; the deep resistivity minimum below 100 K, reminiscent of the onset of superconductivity in granular superconductors and in inhomogeneously doped samples, is interpreted in terms of phase separation and metallic-droplet formation. A longitudinal magnetic field (≤0.5 T) reduces both the resistivity minimum and the superlinear contribution to the transient photoconductance. For δ≊0.6, the lifetime of the photoexcited state is enhanced by nearly 3 orders of magnitude at high excitation levels, indicative of metastability.

The surface and bulk tortuosities of sintered ${\mathrm{Al}}_2$${\mathrm{O}}_3$ ceramic samples have been measured using third- and fourth-sound propagation in superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ as a probe. Near the pore-space percolation threshold the index of refraction of both sound modes is found to increase rapidly as the porosity is lowered, to much higher values than previously observed. The bulk tortuosity exceeds the surface tortuosity for porosities below 30%, a reversal of the behavior observed at high porosity.

Experimental evidence of ultrasonic emission from volcanic rocks has been produced for the first time by a survey of the Strombolian activity. The statistical analysis of the recorded acoustic wave bursts show that the notion of self-organized criticality applies well to the mechanism responsible for the observed emission phenomenon.

It is shown that the conventional Bloch-Kirchhoff description of NMR-laser activity needs the inclusion of a nonlinear relaxation term for the transverse magnetization in order to account for the experimental observation. The validity of the modified equations is demonstrated by comparing in a two-parameter space the Poincaré sections and periodic-orbit structures of model and experiment. All unstable orbits up to order nine have been extracted and employed to obtain an approximation to a generating partition.

A detailed study of showers arriving from the direction of Scorpius X-1 has shown strong evidence for emission of ultrahigh-energy radiation from Scorpius X-1 during the period 4 March–2 May 1986. 178 showers were observed in the source bin againat an expected background of 122.9. The chance probability for observing this excess has been estimated from simulations to be less than ${10}^{\mathrm{-}3}$. The flux observed during this period was (6.4±1.6)×${10}^{\mathrm{-}12}$ ${\mathrm{cm}}^{\mathrm{-}2}$ ${\mathrm{s}}^{\mathrm{-}1}$ at energies >2.5×${10}^{14}$ eV, corresponding to a luminosity of 2×${10}^{35}$ ergs ${\mathrm{s}}^{\mathrm{-}1}$.

We examine the formation of nontopological solitons in a Yukawa theory with N fermions and a single scalar field. We solve the theory to leading order 1/N, for any value of the Yukawa coupling g. We find that the full quantum theory supports nontopological soliton solutions, and that the quantum solitons differ significantly from those in the classical theory. For large g, the energy scales with g, while the radius deceases as 1/g. Thus, quantum corrections invalidate the classical picture of tightly bound fermions inside deep bags.

We use chiral Lagrangians to study the nonresonant production of longitudinally polarized vector bosons through gluon fusion at pp colliders. We compute the contributions induced by loops of colored pseudo Goldstone bosons and colored fermions. We find that the resulting cross sections potentially dominate the standard-model predictions and provide an important probe of the electroweak-symmetry-breaking sector.

It is shown that the unitarized P-wave Nambu-Goldstone scattering amplitude, constructed from the one-loop chiral perturbation theory by the K-matrix method, does not have a resonant behavior, but has instead one nearby pair of complex-conjugate poles on the physical sheet and should be rejected. This result is in constrast with other unitarization schemes such as the Padé, the inverse amplitude, and the N/D methods, which produce a P-wave resonance and have the correct analytic property.

Using generalized factorization theorems, we calculate the leading single transverse spin asymmetry for high transverse-momentum direct-photon production in pp collisions, in terms of partonic matrix elements. The leading contribution comes from a ‘‘twist-3’’ parton distribution, involving the correlation between quark fields and the gluonic field strength. With simple assumptions for this matrix element, the asymmetry increases with ${\mathit{x}}_{\mathit{F}}$, naturally giving effects of 10% or more at large ${\mathit{x}}_{\mathit{F}}$.

We discuss the production of doubly strange ΛΛ hypernuclei in ${\mathrm{\Xi }}^{\mathrm{-}}$ atomic capture. The two-body ${\mathrm{\Xi }}^{\mathrm{-}}$${+}^6$Li${\to }_{\mathrm{\Lambda }\mathrm{\Lambda }}^6$He+n reaction is a relatively favorable case, with a yield of order 3%. For heavier p-shell targets, such as ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$, the (1${\mathit{s}}_{\mathrm{\Lambda }}$${)}^2$ ground-state yields in the ${\mathrm{\Xi }}^{\mathrm{-}}$${+}^{\mathit{A}}$Z${\to }_{\mathrm{\Lambda }\mathrm{\Lambda }}^{\mathit{A}}$(Z-1)+n reaction are suppressed, and states of (1${\mathit{s}}_{\mathrm{\Lambda }}$1${\mathit{p}}_{\mathrm{\Lambda }}$) structure, coupled to an excited nuclear core, are preferentially populated.

We report the first observation of magnetic octupole decay in the x-ray spectrum of a highly charged ion. Detailed analyses of the low-energy region of the n=3→4 spectra of nickel-like ${\mathrm{Th}}^{62+}$ and ${\mathrm{U}}^{64+}$ show an intense line at 2558.7±0.2 and 2689.3±0.2 eV, respectively, that is attributed to the transition (3${\mathit{d}}^9$4s${)}_{\mathit{J}=3\mathrm{}}$→(3${\mathit{d}}^{10}$${)}_{\mathit{J}=0\mathrm{}}$. Calculations indicate that the line is almost exclusively excited by indirect processes such as radiative cascades and electron capture.