The oxygen isotope shift of ${\mathit{T}}_{\mathit{c}}$ in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ has been measured for samples containing ∼95% ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ in the ${\mathrm{CuO}}_2$ plane [O2,O3] sites and primarily ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ in the apical [O4] and chian [O1] sites. The ${\mathit{T}}_{\mathit{c}}$ of the site-selectively substituted samples is higher than that of the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ control samples by 0.10–0.14 K, in contrast to the decrease in ${\mathit{T}}_{\mathit{c}}$ of 0.20–0.23 K for complete ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ substitution at all sites. Our results inicate that the phonon-mediated contribution to the pairing mechanism in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ involves the apical [O4] sites, and not just the ${\mathrm{CuO}}_2$ plane [O2,O3] sites.

The temperature dependence of the in-plane penetration depth ${\lambda }_{\mathrm{\parallel }}$ and surface resistance ${\mathrm{R}}_{\mathrm{s}}$ at 9.6 GHz were measured for high-quality ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_4$ thin films and single crystals. Both${\lambda }_{\mathrm{\parallel }}$(T) and ${\mathrm{R}}_{\mathrm{s}}$(T) are well described within the BCS framework over the entire temperature range ${\mathrm{belowT}}_{\mathrm{c}}$, yielding an energy gap ratio 2Δ/${\mathrm{k}}_{\mathrm{B}}$${\mathrm{T}}_{\mathrm{c}}$=4.1±0.2 and the London penetration depth${\lambda }_{\mathrm{L}\mathrm{\parallel }}$(0)∼1000 Å. These results indicate that the electrodynamics of this n-type cuprate superconductor are consistent with a single-gaps-wave BCS behavior. © 1993 The American Physical Society

The observation of magnetic flux noise in superconducting thin-film flux transformers of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ enables us to measure a very small, frequency-dependent resistance ${\mathit{R}}_{\mathit{v}}$(f) in the films, caused by vortex motion. In static magnetic fields below ${10}^{\mathrm{-}6}$ T and at voltages down to ${10}^{\mathrm{-}20}$ V ${\mathrm{Hz}}^{\mathrm{-}1/2}$,${\mathit{R}}_{\mathit{v}}$ is proportional to the frequency f at which the measurement is made, is approximately independent of temperature, and decreases with increasing static current in the transformer. This behavior cannot be explained by certain currently accepted models of dissipation arising from vortex motion, but is consistent with a model in which independent vortices hop between two potential wells in a confined region.

I summarize results from a numerical study of spherically symmetric collapse of a massless scalar field. I consider families of solutions, scrS[p], with the property that a critical parameter value, ${\mathit{p}}^{\mathrm{*}}$, separates solutions containing black holes from those which do not. I present evidence in support of conjectures that (1) the strong-field evolution in the p→${\mathit{p}}^{\mathrm{*}}$ limit is universal and generates structure on arbitrarily small spatiotemporal scales and (2) the masses of black holes which form satisfy a power law ${\mathit{M}}_{\mathrm{BH}}$∝‖p-${\mathit{p}}^{\mathrm{*}}$${\mathrm{\Vert }}^{\gamma }$, where γ≊0.37 is a universal exponent.

We prove the bound ${\mathit{m}}_{\mathit{s}}$(β)≥ √3 σ(β) between the spontaneous staggered magnetization ${\mathit{m}}_{\mathit{s}}$(β) and the long range order parameter σ(β) in the quantum Heisenberg antiferromagnets. The same bound has been known for the Heisenberg ferromagnets from the works of Griffiths, and of Dyson, Lieb, and Simon, but extensions to the antiferromagnets had been lacking for more than two decades. When combined with the results on nonvanishing σ(β) from the Dyson-Lieb-Simon method, our bound proves the existence of symmetry breaking in the physically natural equilibrium states and ground states obtained by applying an infinitesimal symmetry breaking field.

We have studied the relaxation of off-center ${\mathrm{Li}}^{+}$ ions in ${\mathrm{KTaO}}_3$ by dielectric measurements on 1% and 1.5% crystals, from 20 Hz to 2 MHz and from 4 to 200 K. The shape of the ɛ’’ vs ɛ’ Argand diagrams demonstrates the existence of a distribution of relaxation times. In analogy with spin glasses, a new expression is proposed for the analysis of these diagrams, a strophoidal function, which leads to a distribution function D(θ) decreasing for large θ as ${\mathrm{\theta }}^{\mathrm{-}(1+\mathrm{\alpha })}$ with 0<α<1. The most probable relaxation time ${\mathrm{\theta }}_{\mathrm{mp}}$ of the distribution follows an Arrhenius law with a barrier height close to 950 K.

Classical transition-rate theory provides analytic techniques for computing the asymptotics of a weakly perturbed particle’s mean residence time in the basin of attraction of a metastable state. If the dynamics of the particle are derivable from a potential, it typically escapes over a saddle point. In the nonpotential case exit may take place over an unstable point instead, leading to unexpected phenomena. These may include an anomalous pre-exponential factor, with a continuously varying exponent, in the residence time asymptotics. Moreover, the most probable escape trajectories may eventually deviate from the least-action escape path.

We develop a novel fragmentation model in which the fragmentation cascade is randomly inactivated according to the homogeneous inactivation probability function. For the multiplicative breakup kernel ${\mathit{scrF}}_{\mathit{i},}$j=(ij${)}^{\mathrm{\alpha }}$, we discuss the existence of scaling solutions as t→∞ in different regions of the phase diagram. We also study fluctuations in the cluster-size distribution and find an intermittent behavior in the infinite-cluster phase and at the transition line. Finally, we discuss the relevance of this model for nuclear multifragmentation.

Correlations are shown to arise in nonidentical mixed-particle pairs like ${\mathit{k}}^0$k¯ ${}_{}{}^0{}_{}{}^{}$ when observed in decay modes like ${\mathit{k}}_{\mathit{s}}$${\mathit{k}}_{\mathit{s}}$ in multiparticle final states containing many partial waves. No enhancement is found in any single partial wave. All partial wave analyses of s-wave threshold resonances ${\mathit{a}}_0$ and ${\mathit{f}}_0$ should give the same results for all decay modes. The effect may be useful in CP-violation experiments where ${\mathit{B}}^0$-B¯ ${}_{}{}^0{}_{}{}^{}$ pairs are inclusively produced and their Einstein-Podolsky-Rosen–correlated decays into ψ${\mathit{K}}_{\mathit{S}}$ and leptonic modes are observed. The CP-violating lepton asymmetry is enhanced by a factor of 2 in the kinematic region where Bose enhancement occurs.

Inclusive J/ψ and ψ(2S) production has been studied in p¯p collisions at √s =1.8 TeV using 2.6±0.2 ${\mathrm{pb}}^{\mathrm{-}1}$ of data taken with the Collider Detector at Fermilab. The products of production cross section times branching fraction were measured as functions of ${\mathit{P}}_{\mathit{T}}$ for J/ψ→${\mathrm{\mu }}^{+}$${\mathrm{\mu }}^{\mathrm{-}}$ and ψ(2S)→${\mathrm{\mu }}^{+}$${\mathrm{\mu }}^{\mathrm{-}}$. In the kinematic range ${\mathit{P}}_{\mathit{T}}$>6 GeV/c and ‖η‖≤0.5 we get σ(p¯p→J/ψ X)B(J/ψ→${\mathrm{\mu }}^{+}$${\mathrm{\mu }}^{\mathrm{-}}$) =6.88±0.23(stat${)}_{\mathrm{-}1.08}^{+0.93\mathrm{}}$(syst) nb, and σ(p¯p→ψ(2S)X)B(ψ(2S)→${\mathrm{\mu }}^{+}$${\mathrm{\mu }}^{\mathrm{-}}$) =0.232±0.051(stat${)}_{\mathrm{-}0.032}^{+0.029\mathrm{}}$(syst)nb. From these values we calculate the inclusive b-quark production cross section.

A new type of resonance appears to have been discovered in the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$${(}^{12}$C${,}^{12}$C($0_2^{+}$)${)}^{12}$C($0_2^{+}$) reaction at ${\mathit{E}}_{\mathrm{c}.\mathrm{m}.\mathrm{}}$=33 MeV. We suggest that it is formed coherently from nearly degenerate resonances with different l values. Taking the individual resonances to be members of a cluster rotational band with 2n+l=const, the coherence can be shown to follow from Levinson’s theorem. The new kind of resonance does not have a definite angular momentum but corresponds to an approximate shape eigenstate.

An unusual low-energy component has been observed in the spectrum of equilibriumlike intermediate-mass fragments (IMFs) emitted from systems formed in high-linear-momentum-transfer reactions of 270-MeV ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ with ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$. The properties of this component include strong focusing of IMFs at an angle perpendicular to the fission fragment separation axis, kinetic energies about one-half those for normal IMFs, and a preference for neutron-excess fragments. This behavior is consistent with emission of IMFs from the neck region of a fissioning nucleus.

We study discrete parallel dynamics of a fully connected network of nonlinear elements interacting via long-range random asymmetric couplings under the influence of external noise. Using dynamical mean-field equations, which become exact in the thermodynamical limit, we calculate the activity and the maximal Lyapunov exponent of the network in dependence of a nonlinearity (gain) parameter and the noise intensity.

We describe a new version of the well-known gedanken experiments to determine which of the two paths a particle took in a two-beam interference experiment. In this version the presence of a geometric phase can be deduced from the requirement that the interference pattern must disappear as a result of the welcher-weg (which path) information.

Recent experiments on an optical oscillator with photorefractive gain have shown that the output field undergoes periodic or chaotic alternations among a small number of patterns. The above features are explained by a dynamical model, based only on symmetry arguments, and hence applicable to a variety of physical situations.

We report the first observation of sideband mode competition in a gyrotron with an overmoded open waveguide resonator. Experiments were conducted in a gyrotron operating in the ${\mathrm{TE}}_{16,2}$ mode at 146 GHz. Up to 600 kW of power was generated in 3-μs pulses at 80 kV beam voltage and up to 40 A beam current. A pair of sidebands was excited corresponding to the ${\mathrm{TE}}_{15,2}$ and ${\mathrm{TE}}_{17,2}$ modes offset by ±5 GHz. A multimode, multifrequency, self-consistent time-dependent code predicts the excitation of the sidebands with good accuracy.

We report an imbibition experiment that involves water displacing air in a Hele-Shaw cell filled with glass beads. We varied the flow rate such that the capillary number (${\mathit{C}}_{\mathit{a}}$) spans the range ${10}^{\mathrm{-}5}$<${\mathit{C}}_{\mathit{a}}$<${10}^{\mathrm{-}2}$. The invasion front exhibits self-affine roughness and the total width ${\mathit{W}}_{\mathrm{tot}}$ increases with decreasing ${\mathit{C}}_{\mathit{a}}$. A heuristic argument suggests that ${\mathit{W}}_{\mathrm{tot}}$∝${\mathit{C}}_{\mathit{a}}^{\mathrm{-}2\mathrm{\alpha }/(2\mathrm{}\mathrm{\alpha }+1)}$, where α is the roughness scaling exponent. We also find that ${\mathit{W}}_{\mathrm{tot}}$ exhibits large fluctuations with time and the effective value of α varies over a wide range (0.65–0.91). These results suggests that the behavior of interfaces in random media is dominated by the existence of many metastable states in much the same way as the domain walls in the random-field Ising model.

The first quantitative measurement of photoabsorption in the region determining the Rosseland and Planck mean opacities is obtained for a well-characterized, radiatively heated iron plasma using new techniques and instrumentation. The plasma density and temperature are simultaneously constrained with high accuracy, allowing unambiguous comparisons with opacity models used in modeling radiative transfer in equilibrium astrophysical and laboratory plasmas. The experimental Rosseland and Planck group means are constrained to an accuracy of 15%.

A high brightness, Raman amplified KrF laser has been used to irradiate solid targets with 12 ps laser pulses at intensities above ${10}^{15}$ W/${\mathrm{cm}}^2$ without the presence of a preformed plasma caused by low level amplified spontaneous emission prepulse. Time-resolved x-ray spectroscopy of the K-shell emission from aluminum was used to infer electron densities in excess of ${10}^{23}$ ${\mathrm{cm}}^{\mathrm{-}3}$ at temperatures of several hundred electronvolts.

The use of a coherent field-emission electron source in transmission electron microscopy is combined with phase retrieval by digital processing of a focal image series. For the first time, a dramatic improvement of the high-resolution performance of the electron microscope beyond the usual ‘‘point-to-point’’ resolution has been realized: Experiments on a 200-kV microscope with a point resolution of 0.24 nm reveal reconstructed information down to 0.14 nm. Examples are shown in the field of high-${\mathit{T}}_{\mathit{c}}$ superconductors and ferroelectric oxides. The oxygen sublattice in these structures is revealed.