Reflectivity measurements, at different fixed frequencies, have been performed on ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ films as a function of temperature and magnetic field. Analysis of the data provides evidence for an order parameter 2Δ(0)≃3.3${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$ although the variation of this order parameter with temperature is significantly different from that predicted by the weak-coupling BCS model.

We present the first theoretical evidence for ferromagnetism in 4d overlayers on a layers on a nonmagnetic substrate. These predictions are based on ab initio, spin-polarized, electronic-structure calculations within the framework of the local-spin-density approximation. For Tc, Ru, Rh, and Pd overlayers chemisorbed on Ag(001), only Ru and Rh exhibited ferromagnetism. Several metamagnetic spin states were found for the Ru overlayers.

We report the temperature dependence of the magnetization hysteresis loops of high-quality ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ single crystals, each with a single unique orientation of twin boundaries. We have observed magnetization anisotropy in the temperature range 6–77 K for various magnetic fields applied normal to the c axis and parallel (stronger pinning) or perpendicular (weaker pinning) to the twin-boundary orientation, thus causing an anisotropic critical current density ${\mathit{J}}_{\mathit{c}}$. Magnetization-time-relaxation and Meissner-fraction measurements also support the above observation.

Femtosecond time-resolved and quasi-cw measurements of optical nonlinearities of type-II GaAs/AlAs quantum wells are reported. A pronounced blueshift of the heavy-hole exciton is observed, which develops on a 100-ps time scale after resonant interband excitation. The experimental observations are analyzed using many-body theory, explaining exciton blueshift and dynamical evolution in terms of electron-hole Coulomb enhancement, hole-phase-space filling, and hole-plasma cooling.

We report the first well-resolved band-edge luminescence from excitons confined in fully strained SiGe quantum wells grown on Si. At liquid-He temperatures the photoluminescence is due to shallow bound excitons, and in addition to a no-phonon line, phonon-assisted transitions involving TA phonons and Si-Si, Si-Ge, and Ge-Ge TO phonons are observed At higher temperatures the spectra are dominated by free-exciton luminescence. Quantum-confinement effects shift the observed free-exciton edge above the bulk strained band-gap energy, and also influence the relative intensities of the three TO-phonon replicas.

We consider here a single randomly protonated strand of the conducting polymer polyaniline and show explicitly that the disorder inherent in this system is described by the random-dimer model of Dunlap, Wu, and Phillips [Phys. Rev. Lett. 65, 88 (1990)] with off-diagonal disorder. We demonstrate exactly that the location in the energy band of the unscattered or conducting states of this model coincides with recent calculations of the location of the Fermi level in the protonated form of the polymer. We argue then that the random-dimer model is capable of explaining the insulator-metal transition in polyaniline.

We study the representation of static patterns and temporal sequences in neural networks with signal delays and a stochastic parallel dynamics. For a wide class of delay distributions, the asymptotic network behavior can be described by a generalized Gibbs distribution, generated by a novel Lyapunov functional for the determination dynamics. We extend techniques of equilibrium statistical mechanics so as to deal with time-dependent phenomena, derive analytic results for both retrieval quality and storage capacity, and compare them with numerical simulations.

We present the first realization of a measurement of a ‘‘weak value,’’ a concept recently introduced by Aharonov, Albert, and Vaidman (AAV). Our experiment uses a birefringent crystal to separate the two linear-polarization components of a laser beam by a distance small compared to the laser-beam waist. This ‘‘weak measurement’’ is followed by a strong measurement which translates the centroid of the beam by a distance far larger than the birefringence-induced separation. In addition, we present data corresponding to orthogonal initial and final states, for which the weak value is not defined. This interference effect may have application in the amplification and detection of weak effects.

A two-photon interference experiment has been carried out, based on an idea recently proposed by Croca, Garuccio, Lepore, and Moreira. The experiment is designed to test a prediction of the de Broglie guided-wave theory, according to which the waves have a physical reality in addition to yielding photon detection probabilities. The results clearly contradict the de Broglie theory, but are consistent with conventional quantum mechanics.

In this Letter, we present a theoretical investigation into the quantum-nondemolition properties of a system which utilizes intracavity second-harmonic generation. The results show a significant improvement over the quantum-nondemolition properties of other systems studied previously, and should be accessible to experiment.

Two quantum systems are identically prepared in different locations. An observer’s task is to determine their state. A simple example shows that a pair of measurements of the von Neumann type is less effective than a sequence of nonorthogonal probability-operator measures, alternating between the two quantum systems. However, the most efficient set of operations of that type that we were able to design falls short of a single combined measurement, performed on both system together.

Using both experimental and theoretical results, this Letter describes how low-energy, feedback control signals can be successfully utilized to suppress (laminarize) chaotic flow in a thermal convection loop.

Exact solutions of Einstein’s field equations are presented for the general case of two moving straight cosmic strings that do not intersect. The solutions for parallel cosmic strings moving in opposite directions, each with ${\gamma }_{\mathit{s}}$>(sin4πμ${)}^{\mathrm{-}1}$ in the laboratory frame show closed timelike curves (CTC’s) that circle the two strings as they pass, allowing observers to visit their own past. Similar results occur for nonparallel strings, and for masses in (2+1)-dimensional spacetime. For finite string loops the possibility that black-hole formation may prevent the formation of CTC’s is discussed.

It has recently been shown that hadrons containing a single heavy quark exhibit a new flavor-spin symmetry of QCD. We discuss the implications of this symmetry for the masses and strong decay widths of such hadrons.

It is shown that the nuclear Hamiltonian scrH separates rigorously into a monopole field ${\mathit{scrH}}_{\mathit{m}}$ and a multipole part ${\mathit{scrH}}_{\mathit{m}}$. ${\mathit{scrH}}_{\mathit{m}}$ is entirely responsible for saturation properties and can be treated phenomenologically with few parameters. When realistic interactions are used for ${\mathit{scrH}}_{\mathit{M}}$ in regions from the p shell to the N=82 isotones, shell-model calculations yield excellent spectroscopy and demand nuclear radii very close to the observed ones.

The electronic dephasing of large molecules in solution is investigated with three-pulse photon echoes generated by 6-fs optical pulses. The observed dephasing behavior, which is analyzed with a model based on a stochastic coupling of the molecules with their heat bath, is clearly shown to be non-Markovian. Moreover, the correlation function of the fluctuations asymptotically approaches an exponential with characteristic times varying from 20 to 80 fs, depending on the solvent environment.

We perform a two-photon interference experiment in a Michelson interferometer with photon pairs produced by spontaneous parametric down-conversion. The interferometer is set to a path difference much larger than the coherence length of the light field. Application of a time-resolved coincidence detection scheme reveals two-photon interference fringes with a visibility of 87%. This high visibility clearly demonstrates the nonclassical nature of the observed interference and should allow further spin-free tests of Bell’s inequalities.

It is shown that the time development of the electronic Berry phase predicted to accompany molecular pseudorotation around a conical degeneracy is accessible to measurement with pairs of vibrationally abrupt phase-controlled optical pulses. In the simplest case, an electronic sign change resulting from the induced vector potential will manifest itself as a delay-dependent oscillation in the interference signal with twice the period of pseudorotational motion.

We report a measurement of the 3${\mathit{s}}_{1/2\mathrm{-}}$3${\mathit{p}}_{3/2}$ transition energy in Na-like ${}_{78}{}^{}{}_{}{}^{}\mathrm{Pt}_{}^{67+}{}_{}{}^{}$ ions of 653.44±0.02(stat) ±0.05(syst) eV. The x rays were observed from ions in an electron-beam ion trap. The uncertainty in our result corresponds to 1% of the total estimated quantum electrodynamic radiative contribution to this transition energy. This value differs significantly from extrapolations based on previous lower-Z data, and establishes a benchmark for calculations of high-Z multielectron radiative and relativistic effects.

We show a model laser system which operates by an electromagnetically induced interference. Provided that an inversion condition for the thermal radiation field is satisfied, the system lases without atomic population inversion in steady state. The system is pumped by incoherent radiation on the transition on which lasing occurs.