We measure directly both the real and imaginary parts of the sub-band-gap conductivity σ in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ in the 15–80 ${\mathrm{cm}}^{\mathrm{-}1}$ frequency range using coherent time-domain terahertz spectroscopy. We observe a peak in the real part ${\mathrm{\sigma }}_1$ similar to the coherence peak expected in an s-wave BCS superconductor. The absence of such a peak in NMR relaxation-rate experiments suggests that its origin lies in a strongly temperature-dependent inelastic-scattering rate rather in coherence factors.

We present new evidence that a cross flow of sublattices of helical flux lines takes place when a current is induced with a component along the flux lines.

Local-magnetic-field distributions in the superconducting mixed state of high-${\mathit{T}}_{\mathit{c}}$ cuprates were measured by muon-spin rotation (μSR) in the absence of applied driving forces. In this first analytical treatment, a critical-state model, incorporating an anisotropic penetration depth [${\lambda }_{\mathit{a}\mathit{b}}$(0)=1300±100 Å in the basal plane], reveals the strong influence of local pinning on the characteristic shape of low-temperature μSR Fourier spectra for ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$. The linewidths are found to be nearly independent of flux density, indicative of noncollective or local pinning.

We introduce a new stochastic dynamics which corresponds to a class of critical nonequilibrium charge-carrier statistics chemical-reaction and epidemic models. The evolution rules depend continuously on a certain parameter, 0≤y≤1, in such a way that the bulk dynamics is the same for all y values. However, the finite-size relaxation time scales are found to depend on y, and show an interesting crossover reminiscent of a multicritical point and described by a scaling form.

The concept of a nonequilibrium potential is applied to dynamical systems with fractal attractors or repellers. In particular, we study the case of the Feigenbaum attractor in one-dimensional maps and the case of hyperbolic strange attractors or repellers in two-dimensional maps. The potential-height distribution of the latter is shown to exhibit multifractal features; that of the Feigenbaum attractor is characterized by a single number, the universal exponent for noisy period-doubling bifurcations.

We study the evolution of the nonlinear O(N) σ model in an expanding universe in the large-N approximation, where one can solve exactly for the scaling field distribution, incorporating the full nonlinear dynamics. This provides a valuable analytic approximation for theories of cosmic global monopoles (N=3), global texture (N=4), and ‘‘nontopological texture’’ (N>4). The analytic results are compared with those from numerical simulations, with good agreement. The probability distribution for mass fluctuations in the linear regime is positively skewed, providing a distinctive test for the theory.

A manifestly Lorentz-covariant local quantum field theory for dyons is shown to be possible by representing certain degrees of freedom of matter in dual form, viz., the phase by an antisymmetric tensor. For each magnetic matter field there is a ${\mathit{Z}}^4$ local gauge invariance. This implies Dirac quantization conditions. The transformation to the dual representation also reverses the roles of matter and gauge fields, and of Noether and and topological currents.

Supersymmetric QCD with large matter representations is analyzed in the ladder approximation. In the nonrunning limit we find that chiral-symmetry breaking takes place when the coupling constant exceeds a critical value equal to 3/2 times the critical coupling of ordinary QCD. Fermionic condensation induces a nonzero scalar mass, which, in turn, stabilizes the broken-symmetry phase. The relevance of our results to supersymmetry breaking in realistic theories is discussed.

We calculate all radiative corrections to one-loop order for the main decay of the top quark, t→b+W, in the standard model, retaining exact dependence on all masses. For ${\mathit{m}}_{\mathit{t}}$=150 GeV and ${\mathit{M}}_{\mathit{H}}$=100 GeV we find a -2.9% (-6.9%) correction with a very weak dependence on the Higgs-boson mass, in renormalization schemes that use α, ${\mathit{G}}_{\mathit{F}}$, and ${\mathit{M}}_{\mathit{Z}}$ (${\mathit{G}}_{\mathit{F}}$, ${\mathit{M}}_{\mathit{W}}$, and ${\mathit{M}}_{\mathit{Z}}$) as input parameters. Out of the above results, -8.5% is due to QCD. The ${\mathit{m}}_{\mathit{t}}$ and ${\mathit{M}}_{\mathit{H}}$ dependence is given up to 300 and 1000 GeV, respectively. The inadequacy of a leading ${\mathit{m}}_{\mathit{t}}$ calculation is pointed out.

We present a resolution to the problem of the helicity structure of tensor mesons produced in ψ→γT. The new feature is that the diagrams are separated according to their different topologies, the corresponding helicity structures being analyzed for each diagram. Different topologies appear to dominate the ${\mathit{f}}_2$(1270,1525) qq¯ production and the θ/${\mathit{f}}_2$(1720) production, suggesting that the θ production samples large transverse momentum ${\mathit{p}}_{\mathit{t}}$ in the ψ wave function.

Using the CUSB-II detector at the Cornell Electron Storage Ring, we have made precision measurements of the electric dipole transition rates from Υ″ to${\mathrm{\chi }}_{\mathrm{b}}^{\prime }$, which are in excellent agreement with theory. The fine-structure splitting is found to beM(${\mathrm{\chi }}_{\mathrm{b}2\mathrm{}}^{\prime }$)-M(${\mathrm{\chi }}_{\mathrm{b}1\mathrm{}}^{\prime }$=13.5±0.4±0.5 MeV and(${\mathrm{\chi }}_{\mathrm{b}1\mathrm{}}^{\prime }$)-M(${\mathrm{\chi }}_{\mathrm{b}0\mathrm{}}^{\prime }$)=23.2±0.7±0.7 MeV, leading to a ratioR=0.584±0.024±0.02. The fine structure measures the relative contributions of the spin-orbit interaction a=9.5±0.2±0.1 MeV and tensor interaction b=2.3±0.1±0.1 MeV. We also find that the long-range confining potential transforms as a Lorentz scalar.© 1991 The American Physical Society

We present a measurement of neutrino tridents, muon pairs induced by neutrino scattering in the Coulomb field of a target nucleus, in the Columbia-Chicago-Fermilab-Rochester neutrino experiment at the Fermilab Tevatron. The observed number of tridents after geometric and kinematic corrections, 37.0±12.4, supports the standard-model prediction of 45.3±2.3 events. This is the first demonstration of the W-Z destructive interference from neutrino tridents, and rules out, at 99% C.L., the V-A prediction without the interference.

The energy distribution of protons from many reaction channels were studied following the fusion of ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ and ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$ at 110 MeV. Channel-dependent and angular-momentum-dependent effects were clear, but no evidence was found for structural effects within any final nucleus, contrary to recent claims.

Multiphoton detachment from ${\mathrm{H}}^{\mathrm{-}}$ has been studied using linearly polarized light for photon energies from 0.15 to 0.39 eV. Characteristic threshold structures were observed. The electron detachment rates are estimated at peak laser intensities from 2 to 12 GW/${\mathrm{cm}}^2$ in the laboratory frame. The two-photon detachment data reveal intensity-dependent shifts of the ${\mathrm{H}}^{\mathrm{-}}$ electron detachment threshold energy.

Aggregation in deterministic flows is simulated using a chaotic flow, a regular flow, and a flow with both regular and chaotic regions. Whenever particles of diameter ${\mathit{d}}_0$ touch each other, they form rigid clusters that move as a unit. The model produces fractal aggregates whose morphology can be controlled by tuning flow parameters.

We present results of optical experiments which demonstrate that in a strongly scattering medium containing resonant scatterers the velocity for electromagnetic energy may differ by an order of magnitude from the phase velocity. We derive a microscopic theory that yields an expression for this velocity. Discrepancies are removed, and excellent agreement is found between experiment and theory.

We report the use of a photoionization electron source to pump a 116-nm laser in the Werner band (C ${}_{}{}^1{}_{}{}^{}\mathrm{\Pi }_{\mathit{u}}^{}{}_{}{}^{}$→X ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathit{g}}^{+}{}_{}{}^{}$) of molecular hydrogen. The laser is pumped by free electrons which are created by photoionizing molecular hydrogen with soft x-rays from a traveling-wave laser plasma. We show that even though the free electrons have an average temperature of ∼10 eV, the lasing hydrogen molecules retain an ambient temeprature of ∼0.01 eV. This allows an extrapolated small-signal gain of exp(43), with a 1064-nm pumping energy of 580 mJ in 200 psec.

Details of the process by which ionized pellet ablatant increases the density in a tokamak have been observed for the first time. For mild ablation rates, this process is not simply diffusive. Instead, the ionized ablatant takes the form of a temporally symmetric pulse which propagates over large distances along magnetic-field lines. This pulse has been observed both interferometrically and with an x-ray camera. These observations are not in qualitative agreement with models which predict a steplike densification of the plasma by the pellet.

We use quantum Monte Carlo techniques to study a one-dimensional, disordered, interacting, Bose Hamiltonian. The effect of disorder on the Mott-insulator portion of the phase diagram is determined. We observe the destruction of superfluidity by disorder at incommensurate densities, for the first time demonstrating the emergence of a ‘‘Bose-glass’’ phase. In addition to these strong-coupling phases, we observe an unanticipated re-entrance into an Anderson-type localized regime for weak couplings.

We examine the time evolution of a weakly interacting Bose gas in the course of the Bose-Einstein phase transition and show that, in contrast with previous claims in the literature, the relevant time scale for the appearance of the condensate is finite and, under the conditions we consider, of O(ħ/${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$), which is very small compared to the characteristic lifetime of the system due to inelastic collisions in the gas.