A detailed magnetotransport study of novel heterostructures comprised of epitaxial layers of semimetallic ErAs buried inside GaAs displays a cusplike anomaly in the magnetoresistance that is caused by spin-disorder scattering at the antiferromagnetic-paramagnetic phase boundary. The relatively low carrier concentration spares the Fermi surface from being cut by the broken symmetry and only spin disorder contributes to the anomaly.

An efficient Monte Carlo method is proposed to calculate the elementary excitation spectrum of quantum systems. The lowest energy with arbitrary momentum is obtained by the projector Monte Carlo method. This is applied to the spin-(1/2 and -1 Heisenberg antiferromagnetic chain with length 32. For the S=(1/2 case, the spectrum coincides completely with the spectrum of des Cloiseaux and Pearson. For the S=1 case, the spectrum has a gap at momentum π as was predicted by Haldane. The value of the gap coincides with the calculation of Nightingale and Blote. The spectrum satisfies a variational relation with the structure factor.

The muon-spin-relaxation rate $\sigma$ has been measured in sixteen specimens of high-$T_c$ cuprate superconductors (the 2:1:4, 1:2:3, 2:2:1:2, and 2:2:2:3 series). This has allowed us to study the magnetic field penetration depth $\lambda$ and thus the superconducting carrier density $n_s$ divided by the effective mass $m^{*}\left(\sigma \propto \frac{1}{{\lambda }^2}\propto \frac{n_s}{m^{*}}\right)$. A universal linear relation between $T_c$ and $\sigma (T\to 0)\propto \frac{n_s}{m^{*}}$ has been found with increasing carrier doping. In heavily doped samples, however, $T_c$ shows saturation and suppression with increasing $\frac{n_s}{m^{*}}$. This saturation starts at different values of $\frac{n_s}{m^{*}}$ for materials with different multiplicities of CuO planes.

We investigate the phenomenon of coagulation with constant feed-in of monomers at a single point. For spatial dimension d>4, the steady-state cluster concentration, c(r), obeys Laplace’s equation while for d<4, the steady-state concentration of clusters of mass k a distance r from the source scales as ${\mathrm{c}}_{\mathrm{k}}$(r)∼${\mathrm{k}}^{\mathrm{-}\mathrm{\tau }}$φ(${\mathrm{kr}}^{\mathrm{-}\mathrm{z}}$), with z=4-d, τ=(d-6)/(d-4) for d>2, and z=2, τ=1+d/2 for d<2. For the linear chain, we outline an exact solution for which ${\mathrm{c}}_{\mathrm{k}}$(r)∼${\mathrm{k}}^{\mathrm{-}3/2}$φ(μ), with φ(μ)∼${\mathrm{\mu }}^{5/2}$ for μ=k/${\mathrm{r}}^2$→0, c(r)∼${\mathrm{r}}^{\mathrm{-}1}$, and the number of clusters increases with time t as lnt. The effects of cluster drift and the presence of a sink are also considered.

The cubic nonlinear Schrödinger equation and some of its discretizations, one of which is integ- rable, are studied. Apart from the integrable version the discretizations produce chaotic solutions for intermediate levels of mesh (mode) refinement. Chaos disappears when the discretization is fine enough and convergence to a quasiperiodic solution is obtained. Details are given for finite difference calculations, although similar results are also obtained by Fourier spectral methods. Results regarding a forced nonlinear Schrödinger equation are briefly described.

A search is made for bursts of low-energy (20–60 MeV) neutrino interactions, signaling stellar core collapse, in a large underground water-Čerenkov detector. Over a period of 5.6 years (2.1 live years) no bursts were observed. A 90%-confidence-level (C.L.) upper limit to the rate of stellar collapse in the Galaxy is set at <1.5 ${\mathrm{yr}}^{\mathrm{-}1}$. A limit of <1.1 stellar collapses per year (90% C.L.) is set for a smaller volume of the Galaxy (10 kpc radius) which includes the Galactic center and >50% of the stars in the Milky Way.

Correlations between hadrons produced in the forward and backward hemispheres with respect to the collision axis have been analyzed using the relative mutual information $I_r$. This simple example provides a model-independent information-theoretical measure of correlations capable of generalization to any order. Evaluation of the F/B mutual information in the NA22 experiment gives $I_r$=0.02 (0.003) for a rapidity window of 2 for ‘‘all-charge’’ (‘‘negative-only’’) distributions. For the UA5 experiment at the CERN Collider we obtain the larger value of 0.1 for the pseudorapidity window of 4.

Using the Columbia University–Stony Brook detector at the Cornell Electron Storage Ring, we have measured $B_{\mu \mu }$, the branching fraction into muons, of the 1S, 2S, and 3S Υ mesons. We obtain ${\mathit{B}}_{\mathrm{\mu }\mathrm{\mu }}$(1S)=(2.61±0.09±0.11)%, ${\mathrm{\Gamma }}_{\mathrm{tot}}$(1S)=51.1±3.2 keV, ${\mathit{B}}_{\mathrm{\mu }\mathrm{\mu }}$(2S)=(1.38±0.25±0.15)%, ${\mathrm{\Gamma }}_{\mathrm{tot}}$(2S)=42.3±9.2 keV, ${\mathit{B}}_{\mathrm{\mu }\mathrm{\mu }}$(3S)=(1.73±0.15±0.11)%, and ${\Gamma }_{\mathrm{tot}\left(3\mathrm{}\mathrm{S}\right)=27.7\mathrm{}\pm 3.7}$ kev. We also derive, from these results, ${\alpha }_s$=0.174 and ${\Lambda }_{\mathrm{MS}\mathrm{¯}=157\mathrm{}}$ MeV, where MS¯ denotes the modified minimal-subtraction scheme.

The γ-ray spectrum in the giant dipole resonance (GDR) region associated with the reaction ${}_{}{}^{40}{}_{}{}^{}$Ar${+}^{70}$Ge at 10 MeV/nucleon has been measured in coincidence with residues of the heavy composite systems whose excitation energy was $E^{\mathrm{*}}$=230 MeV. From the statistical-model analysis, it is deduced that the GDR strength is consistent with 100% of the energy-weighted sum rule, the energy is 16±1 MeV while the width is ${13}_{\mathrm{-}2}^{+1\mathrm{}}$ MeV. This value is not very different from the one measured at $E^{\mathrm{*}}$=130 MeV, thus pointing to saturation effects in the damping of the GDR.

The special degeneracy which occurs in the harmonic-oscillator potential when two of the frequencies have ratios of small integer numbers is exploited. Favored shell energies occur for particle numbers which vary with the third frequency. Applications to realistic nuclear potentials show that this degeneracy leads to special shell effects for axial shapes (1:1 ratio) but also for the 2:1 ratio exemplified by the triaxial ground state of ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ and triaxial superdeformed high-spin isomers in the light Hf nuclei.

Nonlocal potentials based on angular-momentum projection operators can be transformed into local, yet angular-momentum dependent, pseudo-Hamiltonians by modifying the kinetic energy operator. Ionic pseudo-Hamiltonians of this type can replace core electrons in atomic calculations. Their use in Green’s-Function Monte Carlo simulations gives accurate electron affinities, ionization, and binding energies for second-row atoms and diatomics. This opens the way to quantum simulations of many condensed-matter systems.

The infrared divergence of the resonant Raman-Compton scattering has been studied in collisions of photons on atomic L electrons in the intermediate-momentum-transfer regime. Low-energy continua emitted by Zr atoms, excited, in the vicinity of the K edge, by the monochromatized x rays delivered by the LURE Synchrotron Radiation Facility, have been observed on vary thin targets and compared with the theoretically predicted infrared divergence of the Raman scattering. The characteristic change in shape of these continua has been studied on a wide energy range below the Zr K edge.

Classical mechanics is used to compute the low-resolution spectrum of ${\mathrm{Na}}_3$ in the classically chaotic regime. Excellent agreement between the numerical simulation and experiment enables an assignment of the experimental coarse-grained spectrum, which is based not on periodic orbits but on weakly unstable, reduced dimension, quasiperiodic motion. In contrast to previous work on similar systems, there is no need to use a semiclassical approximation based on the Gutzwiller summation formula.

We report the first observation of the high vibronic levels of the ${\mathrm{Na}}_3$ ground state, performed by stimulated emission spectroscopy. The highest cross sections of the process, which uses the $C^2$E’’ state as an intermediate state, are obtained for combinations of symmetric stretch and bending modes. These experimental results are of primary importance for the understanding of energy-level structures in systems exhibiting strong vibronic coupling.

The dielectronic recombination excitation function for He-like ${\mathrm{Ni}}^{26+}$ ions has been measured. The ${\mathrm{Ni}}^{26+}$ ions were created and held in an electron beam ion trap, and K x rays were detected. Several features were observed, including resonant excitation of ${\mathrm{Ni}}^{26+}$ x rays and the transition from dielectronic recombination to direct excitation at threshold. The cross section for the KLL dielectronic recombination resonances relative to radiative recombination has been measured, and agrees with theoretical cross sections calculated using the multiconfiguration Dirac-fock model.

Resonant laser excitation of Yb ions confined in an rf ion trap is found to result in perfect population trapping in the highly metastable ${}_{}{}^2{}_{}{}^{}\mathrm{F}_{7/2}^{}{}_{}{}^{}$ state. The lifetime of this level is measured to be greater than 8 days. Population and depopulation of this level under narrow- and broad-band laser excitation conditions is investigated and its potentiality for ultimate-resolution laser spectroscopy indicated.

Doubly excited states have been produced in Ar viii by double electron capture from cesium atoms by 90-keV ${\mathrm{Ar}}^{9+}$ ions. The transitions between 1${\mathrm{s}}^2$2${\mathrm{s}}^2$2${\mathrm{p}}^5$5g7l (l=3,...,6) 1${\mathrm{s}}^2$2${\mathrm{s}}^2$2${\mathrm{p}}^5$5g8l (l=4,...,7), and 1${\mathrm{s}}^2$2${\mathrm{s}}^2$2${\mathrm{p}}^5$5g9l (l=5,...,8) double Rydberg states have been observed by photon decays. The spectra have been analyzed using an independent-particle framework with a frozen-core approximation leading to a good agreement for transition energies.

The time evolution of Couette-Taylor flow has been measured for supercritical, almost tricritical, and subcritical bifurcations. The subcritical transitions near the critical point are dramatically different from the supercritical transitions. A simple relaxational dynamics equation together with a slightly asymmetric sixth-order Landau potential has been shown to fit the experimental data very well. The coefficients of the fit have been compared to the theoretical structure of the coefficients revealed earlier.

We present a new numerical method for calculating interference phenomena for waves propagating through random media. The model is applied to calculate probability distribution functions for the transmission P(T) in one and two dimensions, T being the transmission coefficient. The model reproduces the analytical predictions for one dimension, and yields new results for two-dimensional systems. The distribution function P(T) in two dimensions, in the diffusive regime, is found to be close to a Gaussian with a variance proportional to the mean, in agreement with the results of diagrammatic calculations. A crossover of the distribution to log-normal behavior typical for strong localization is obtained.

A simple optical model to realize a reversible, potentially error-free logic gate—a Fredkin gate—is discussed. The device dissipates no energy and makes use of the Kerr nonlinearity to produce intensity-dependent phase shifts. The analysis shows that quantum mechanics permits the operation of error-free logic gates which dissipate no energy. However, even though the device is nondissipative, error-free performance only occurs under particular operating conditions.