The blue bronze ${\mathrm{K}}_{0.3}$${\mathrm{MoO}}_3$ has been irradiated by a pulsed infrared laser beam with a photon energy corresponding to the single-particle gap. In the experiments, performed at T=4.2 K, no direct photocurrent due to normal carriers was observable; however, we found a unique phenomenon, photoinduced collective electron transport. We argue that the electrostatic screening by the optically excited single electrons initiates the sliding of the charge-density-wave condensate, and it is this collective motion which dominates the photocurrent.

We have studied universal conductance fluctuations in the nonlocal magnetoresistance of ${\mathit{n}}^{+}$-GaAs wires. The Lee-Stone correlation field Δ${\mathit{B}}_{\mathit{c}}$ increases by a factor of 5 as the magnetic field increases from 0 to 12 T but the amplitude of the fluctuations does not change. It is possible to explain the increase of Δ${\mathit{B}}_{\mathit{c}}$ quantitatively in terms of a variation of the size of the phase coherence length. However, this should also give rise to a large change in the fluctuation amplitude, which is not observed. This implies that the universal scaling of the conductance fluctuations is not valid in high magnetic fields.

We prove that for noninteracting electrons at zero temperature, an insulator exhibits finite Hall resistivity in the dc limit. More generally, we present simple macroscopic arguments that this behavior is generic for all insulators. We therefore conclude that the zero-temperature Hall coefficient is not a measure of the number of mobile carriers.

${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ NMR measurements on (${\mathrm{Pb}}_{0.75}$${\mathrm{Cu}}_{0.25}$)${\mathrm{Sr}}_2$ (${\mathrm{Ca}}_{0.5}$${\mathrm{Y}}_{0.5}$)${\mathrm{Cu}}_2$${\mathrm{O}}_7$ and Y${\mathrm{Ba}}_2$${\left({\mathrm{Cu}}_{0.9}{\mathrm{Co}}_{0.1}\right)}_3$${\mathrm{O}}_7$ show that the shifts have a similar value and temperature dependence to that of Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.48}$. All three have a $T_c$ of 45 K. This behavior parallels that found from a comparative NMR study of Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$ and Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.8}$ both with $T_c\approx 80$ K. The common behavior of ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ shifts in these different double layered superconductors strongly suggests that $T_c$ and the spin susceptibility are intimately related in systems where the changes in $T_c$ result from differences in hole concentration.

Flux creep in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ crystals exhibits two different regimes as a function of time, as well as of temperature and magnetic field. The short-time, low-temperature regime has a peak in current density versus field, which is enhanced by irradiation defects. The long-time, high-temperature regime has a monotonic and sharp falloff (step) in current density versus magnetic field, which is suppressed by irradiation defects. The former is identified with bulk pinning, and the latter with a surface barrier.

It is shown that the interactions in a strongly correlated quasi-two-dimensional electron gas with the Fermi level lying at a saddle point (Van Hove singularity) in ${\mathrm{\varepsilon }}_{\mathit{k}}$ can give rise to superconductivity by exchange of excitations with a characteristic low electronic energy scale ${\mathit{E}}^{\mathrm{*}}$. Application to cuprate superconductors shows that this mechanism along with a conventional electron-phonon interaction can explain the high ${\mathit{T}}_{\mathit{c}}$’s and the anomalous behavior of the isotope shift as a function of doping.

Anomalous resistance oscillations have been observed in meso- scopic superconducting Al loops. As for the classical Little-Parks effect, the oscillation period is governed by the superconducting fluxoid quantization. The oscillation amplitude shows, however, an unusual magnetic-field and current dependence for low flux quanta, which is completely different from the Little-Parks oscillations.

We show that first-principles electronic structure calculations of silicon wires with diameters up to ∼1.5 nm support the idea that quantum confinement and surface effects are responsible for the luminescence in porous silicon. Instead of the indirect gap of crystalline bulk silicon, the band structure of these wires exhibits a direct gap at k=0. The imaginary part of the dielectric function, polarized in the direction of the wire, shows a peak in the visible range. The dependence of this feature on wire size is analyzed and correlated to experimental luminescence spectra.

We have investigated the polarization-dependent two-photon absorption in GaAs/AlGaAs quantum wires. The anisotropic selection rules of the multiphoton absorption process are exploited to study the one-dimensional 2p exciton states and the transitions between quantum wire subbands of different quantum numbers (Δ${\mathit{n}}_{\mathit{y}}$≠0 selection rule). The deviations from the selection rules derived for the strict one-dimensional case are discussed, and depend on the actual quasi-one-dimensional character of the excitonic wave functions.

We derived the impact ionization and Auger recombination rates within the framework of the six-band Kane model. The theory involves the electron distribution function and average electron energy which we computed using Monte Carlo simulations, along with the electron drift velocity. The excess carrier concentration is then determined from the balance equation. The results of the calculation are in good agreement with experimental data for HgCdTe at 77 K.

We examined band discontinuities in AlAs-Ge-GaAs(001) and GaAs-Ge-AlAs(001) single-quantum-well structures, as well as individual isolated Ge-GaAs(001), Ge-AlAs(001), GaAs-Ge(001), and AlAs-Ge(001) heterojunctions. We found that well-defined in- equivalent neutral interfaces are established in III-V/IV/III-V structures for Ge coverages as low as 1–2 monolayers. Deviations from the transitivity and commutativity rules of heterojunction behavior reflect inequivalent local interface environments rather than charged interfaces.

I propose a numerical simulation algorithm for statistical systems which combines a microcanonical transfer of energy with global changes in clusters of spins. The advantages of the cluster approach near a critical point augment the speed increases associated with multispin coding in the microcanonical approach. The method also provides a limited ability to tune the average cluster size.

We construct a solution of the classical equations of motion arising in the low-energy effective field theory for heterotic string theory. This solution describes a black hole in four dimensions carrying mass M, charge Q, and angular momentum J. The extremal limit of the solution is discussed.

Muon neutrinos produced as a result of cosmic-ray interactions with the atmosphere are used to search for ${\nu }_{\mathrm{\mu }}$ oscillations into ${\nu }_{\mathrm{\tau }}$ by comparing the measured rate of upward-going muons in the Irvine-Michigan-Brookhaven detector with the expected rate. In addition, the ratio of upward-going muons which stop in the detector to those which exist is used to search for deviations from the expected energy spectrum. This latter technique is free of flux and cross-section normalization uncertainties. No evidence for oscillations is found. 90% C.L. limits on δ${\mathit{m}}^2$ are derived in the range (1–2)×${10}^{\mathrm{-}4}$ ${\mathrm{eV}}^2$ for ${\sin }^2$2θ>0.5.

We study the renormalization-group evolution of gauge coupling constants including mass threshold effects of both light and heavy particles, in the minimal supersymmetric SU(5) grand unified model assuming supersymmetry breaking at ≲1 TeV. We show that a certain combination of superheavy masses ${\mathit{M}}_{\mathit{V}}^2$${\mathit{M}}_{\mathrm{\Sigma }}$ is tightly constrained, while the mass of the color-triplet Higgs boson, ${\mathit{M}}_{\mathit{H}}$, can vary from 2×${10}^{13}$ to 2×${10}^{17}$ GeV. With a relatively heavy color-triplet mass (${\mathit{M}}_{\mathit{H}}$∼${10}^{17}$ GeV), the present nucleon-decay experiments still allow superparticles below 1 TeV. The importance of a precise measurement on ${\mathrm{\alpha }}_3$ is stressed.

The large-mass limit of QCD uncovers symmetries that are not present in the QCD Lagrangian. These symmetries have been applied to physical (finite mass) systems, such as B and D mesons. We explore the validity of this approximation in the ’t Hooft model (two-dimensional QCD in the large-N approximation). We find that the large-mass approximation is good, even at the charm mass, for form factors, but it breaks down for the pseudoscalar decay constant.

The effective action for quark-antiquark resonances is derived in the scenario in which the ${\mathrm{U}}_{\mathit{Y}}$(1) interaction is responsible for the large t-b splitting. It is shown that there is a clear signature for the resonances to be seen at the Superconducting Super Collider. It is also shown that while flavor-changing neutral currents (FCNCs) in K meson physics are naturally suppressed in the scenario, there can be interesting enhancements of FCNCs (due to these resonances) in B meson physics.

The first measurements of forward multijet rates in deep-inelastic lepton scattering are presented. Data were taken with a 490-GeV muon beam incident on a hydrogen target. The jets were defined using the gade algorithm. The measured rates are presented as a function of the jet resolution parameter ${\mathit{y}}_{\mathrm{cut}}$, and as a function of the virtual-photon–proton center-of-momentum energy W, in the range 13≤W≤33 GeV. Comparisons are made to the predictions of the Lund Monte Carlo programs and good agreement is obtained when QCD corrections are included in the model.

The E802 Spectrometer at the Brookhaven Alternating Gradient Synchrotron has been used to measure the correlations in relative momentum between like-sign pions emitted in central Si+Al and Si+Au collisions at 14.6A GeV/c. Data are presented in terms of the correlation function for both identified ${\mathrm{\pi }}^{\mathrm{-}}$ and ${\mathrm{\pi }}^{+}$ pairs near the nucleon-nucleon center-of-mass rapidity. All parametrizations of the correlation function are consistent with a spherically symmetric source of rms radius ${\mathit{R}}_{\mathrm{rms}}$≊3.5±0.4 fm and lifetime τ∼2 fm/c.

Calculations are presented for the Gamow-Teller strength distribution in the ${\mathrm{\beta }}^{+}$ decay of ${}_{}{}^{37}{}_{}{}^{}\mathrm{Ca}$ and compared to results extracted from recent beta decay data and from (p,n) data on the analog nucleus ${}_{}{}^{37}{}_{}{}^{}\mathrm{Cl}$. The strength distribution is shown to be sensitive to the Hamiltonian, and comparison with experiment indicates a need for further improvement in the 1s0d shell Hamiltonian. It also indicates that the quenching of the Gamow-Teller operator in the upper part of the 1s0d shell is similar to that deduced from previous analyses.