We report the first infrared and optical measurements of the absolute conductivity of FeSi. Above 200 K the ac conductivity of FeSi resembles that of a dirty metal, while at low temperature most of the conductivity below 60 meV is depleted, consistent with an energy gap of that magnitude. Thus in contrast to what is expected from a picture based on simple thermal activation, the gap disappears at a temperature which is low relative to its size. Moreover, the spectral weight lost below the gap does not reappear just above the gap, but is instead distributed over an energy range of order eV.

The negatively charged exciton ${\mathit{X}}^{\mathrm{-}}$ is identified by its circular polarization properties in 1.7 K magentoabsorption spectra of CdTe/${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$Te multiple quantum wells modulation doped with electron concentrations ${\mathit{N}}_{\mathit{s}}$≊2×${10}^{10}$ to 1.5×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$. The binding energy of the second electron of ${\mathit{X}}^{\mathrm{-}}$ is 0.20 3D Rydbergs at 100 Å well width. The species ${\mathit{X}}^{\mathrm{-}}$ and the neutral exciton X exist in zero field for low ${\mathit{N}}_{\mathit{s}}$. At ${\mathit{N}}_{\mathit{s}}$=1.45×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$ a Fermi edge singularity is seen at B=0 but ${\mathit{X}}^{\mathrm{-}}$ appears with field, at filling factor ν=2, while X appears at ν=1.

The theory of inelastic electron resonant tunneling through a double-barrier nanostructure [Phys. Rev. Lett. 69, 3224 (1992)] is extended to establish a theory of phonon-assisted resonant magnetotunneling when a magnetic field is applied parallel to the tunneling current. At higher temperatures we have discovered an interesting characteristic oscillation of the width and height of the main peak in the I-V curve when the ratio ${\mathrm{\omega }}_0$/${\mathrm{\omega }}_{\mathit{c}}$ changes between integer (double resonant) and half integer (single resonant), where ${\mathrm{\omega }}_0$ is the LO phonon frequency and ${\mathrm{\omega }}_{\mathit{c}}$ is the cyclotron frequency in the well. If confirmed experimentally, our theoretical prediction provides an additional method to determine the electronic effective mass in the well.

Ballistic electron emission spectroscopy is used to investigate current attenuations in thin films of Pd/Si, from which the elastic and inelastic mean free paths are uniquely determined. The observed equality of transmission across Pd/Si(100) and Si(111) interfaces is attributed to interface scattering, on the basis of which a current transport model is developed that gives unprecedented agreement with experiment over a wide energy range.

The muon spin depolarization rate σ was measured in overdoped ${\mathrm{Tl}}_2$${\mathrm{Ba}}_3$${\mathrm{CuO}}_{6+\mathrm{\delta }}$. σ(T→0) was found to decrease proportional to the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ as doping δ is increased. In the framework of the clean-limit London model, σ(0)∼${\lambda }^{\mathrm{-}2}$∼${\mathit{n}}_{\mathit{s}}$/${\mathit{m}}^{\mathrm{*}}$, this implies that the depression of ${\mathit{T}}_{\mathit{c}}$ by overdoping is associated with a decrease of the superconducting condensate density ${\mathit{n}}_{\mathit{s}}$ in spite of the increasing normal-state carrier density. This can be largely accounted for in terms of strong pair breaking, which depresses both the condensate density and ${\mathit{T}}_{\mathit{c}}$ with increased doping.

We report the magnetic properties of a new class of materials: ${\mathrm{Ni}}_9$${\mathrm{Te}}_6^{\mathit{n}+}$ and ${\mathrm{Co}}_6$${\mathrm{Te}}_8^{\mathit{n}+}$ with n=0, 1, 2. These cluster compounds, which can be charged by chemical means from neutral to 2+, provide a unique and novel way to change the Fermi level. For most charge states we observe quenching of the spin and orbital moments at low temperatures, accompanied by a large value for the temperature independent susceptibility of the ground state. The generic presence of low-energy magnetic excitations in these compounds indicates that these systems exhibit strong electron correlations and form mesoscopic analogs of the mixed-valence–heavy-fermion compounds.

The generalization abilities of neural networks for inferring a rule on the basis of examples can be characterized by the convergence of the learning error to the generalization error with increasing size of the training set. Using the replica technique, we calculate the maximum difference between training and generalization error for the ensemble of all perceptrons trained by a teacher perceptron and the maximal generalization error for the perceptrons that have a training error equal to zero. The results are compared with the rigorous bounds provided by the Vapnik-Chervonenkis theorem.

We consider a Brownian particle in a periodic potential under heavy damping. The second law forbids it from displaying any net drift speed, even if the symmetry of the potential is broken. But if the particle is subject to an external force having time correlations, detailed balance is lost and the particle can exhibit a nonzero net drift speed. Thus, broken symmetry and time correlations are sufficient ingredients for transport.

We discuss a new mechanism generating long range temporal correlations in dynamical systems coupled to a source of white noise. The external noise induces dynamical events uncorrelated on a logarithmic time scale and produces a fluctuating output with ‘‘1/f’’ power spectrum. This behavior requires a complex phase space with many traps, which can arise due to strong cooperative effects. As a demonstration, we numerically analyze a system of many coupled degrees of freedom, which is externally driven and subject to a white noise perturbation. We find both Poisson statistics for events in log(time) and the 1/f power spectrum.

All three-manifolds are known to occur as Cauchy surfaces of asymptotically flat vacuum spacetimes and of spacetimes with positive-energy sources. We prove here the conjecture that general relativity does not allow an observer to probe the topology of spacetime: Any topological structure collapses too quickly to allow light to traverse it. More precisely, in a globally hyperbolic, asymptotically flat spacetime satisfying the null energy condition, every causal curve from ${\mathit{scrI}}^{\mathrm{-}}$ to ${\mathit{scrI}}^{+}$ is homotopic to a topologically trivial curve from ${\mathit{scrI}}^{\mathrm{-}}$ to ${\mathit{scrI}}^{+}$.

We report a measurement of the inclusive production of low-energy photons from pp¯ collisions at √s =1.8 TeV. A NaI calorimeter located near η=0 was used to measure the photon transverse momentum spectrum over the range 30≤${\mathit{p}}_{\mathit{t}}$≤5000 MeV/c. The data are compared to the photon production expected from known particle decays in an attempt to isolate a low-energy direct photon signal. We find no evidence for excess photons above those expected from hadron decays and place an upper limit of 16% (90% C.L.) on any excess direct photons in the ${\mathit{p}}_{\mathit{t}}$ range from 30 to 100 MeV/c.

We present the results of a detailed study of the effects of b tagging on the heavy top-quark signal and backgrounds for the modes of the dilepton plus two-high transverse-energy jets at the Fermilab Tevatron. The general characteristics of the heavy top-quark signal events are also discussed so that a comparison can be made between b tagging and imposing stringent kinematical cuts to eliminate backgrounds.

We have performed accurate G-matrix calculations for the hypernucleus ${}_{\mathrm{\Lambda }}{}^{16}{}_{}{}^{}\mathrm{O}$ using the one- and two-boson-exchange Jülich hyperon-nucleon potentials. A finite-nuclei Pauli operator defined in terms of oscillator wave functions is employed, and treated essentially exactly in solving the coupled ΛN and ΣN G-matrix equation. Numerical stability concerning the choice of the k-space mesh points for discretization and the accuracy of a finite-boundary approximation are examined. Our calculated spectrum for ${}_{\mathrm{\Lambda }}{}^{16}{}_{}{}^{}\mathrm{O}$ is in good agreement with experiments.

Multifragmentation has been measured for ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$${+}^{197}$Au collisions at E/A=100, 250, and 400 MeV. The mean fragment multiplicity increases monotonically with the charged particle multiplicity at E/A=100 MeV, but decreases for central collisions with incident energy, consistent with the onset of nuclear vaporization. Molecular dynamics calculations follow some trends but underpredict the observed fragment multiplicities. Including the statistical decay of excited residues improves the agreement for peripheral collisions but worsens it for central collisions.

We have developed a method, based on the use of B-spline basis sets and model potentials, for determining properties of systems with two or three electrons outside a polarizable closed-shell core. It is applied to the calculation of the electron affinity of Ca and the resulting value of 17.7 meV is in excellent agreement with the most recent experiments. It is found that the dielectronic core-valence interaction reduces the electron affinity by 39.5 meV.

Both polarized and depolarized light scatterings were observed in the isotropic phase of p-n-hexyl p’-cyanobiphenyl (6CB) with a new technique of optical beating spectroscopy, in which a Fabry-Pérot etalon was replaced with a spectrum analyzer. This technique is very useful for Rayleigh and Brillouin scattering over the range, at present, from ${10}^4$ to ${10}^9$ Hz with a resolution as high as 1 kHz if needed. The result in 6CB was correlated with the equation derived from the theories by de Gennes and also by Alms et al. The observed spectra were in a remarkable agreement with the theoretical prediction, giving strong support to the theories.

We have used a novel new spectrometer to make the first observation and measurement of electron capture from ${\mathit{e}}^{+}$-${\mathit{e}}^{\mathrm{-}}$ pair production in relativistic heavy ion collisions. For a 0.96 GeV/nucleon ${\mathrm{U}}^{92+}$ beam the cross section for a Au target is 2.19(0.25) b, and the target Z dependence is ${\mathit{Z}}_{\mathit{t}}^{2.8\pm 0.25}$. We also measured the energy and angular distributions of the of the positrons, and for comparison, the cross section for ${\mathit{e}}^{+}$-${\mathit{e}}^{\mathrm{-}}$ pair production without capture is 3.30(0.65) b.

We report evidence of a significant signal enhancement caused by fragmentation of heteronuclear metal clusters during the interface analysis using laser secondary-neutral-mass spectroscopy (LSNMS). Measurements have been performed in Ti-Pt-Ti thin film interfaces and pure Ti. The LSNMS signal of atomic Ti shows a pronounced peak in the interfaces. A comparison of the velocity distributions of Ti atoms in the Ti-Pt interface and in the pure Ti layer clearly shows that clusters, in particular TiPt clusters, are responsible for the interface peak.

Capture cross sections for circular Rydberg states have been measured as a function of the angle, φ, between ion velocity and angular momentum of the circular orbital. The system studied is 2.5 keV ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}_{}^{+}{}_{}{}^{}$ on Li(1${\mathit{s}}^2$,nlm) with n=25, l=n-1, and m=+l, where m is defined relative to a weak, external magnetic field. A strong dependence on φ is found. It is expected that studies such as the present will lead to an improved understanding of the three-body problem in the region of sufficiently large quantum numbers and impact velocities for classical physics to be accurate.

When a chemical wave encounters a boundary between two media with different wave speeds, refraction and reflection can occur. We demonstrate experimentally that the refraction of chemical waves obeys Snell’s law. Reflection of chemical waves has been observed for the first time. The waves do not show specular reflection, but rather exhibit a single reflection angle, equal to the critical angle.