We have fabricated tunnel junctions between chemically etched single crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ and evaporated metal counterelectrodes which exhibit reproducible characteristics. Above the bulk critical temperature of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$, $T_c$, the conductance, G(V), has a linear dependence with voltage and has some asymmetry. Below $T_c$, additional structure associated with the superconductivity appears in G(V). At T≪$T_c$ there is a reproducible, finite, zero-bias conductance which suggests that there are states at the Fermi energy superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$. Junctions with Pb, Sn, Bi, Sb, PbBI, and Au counterelectrodes all show qualitatively similar behavior.

Magnetic susceptibility data are presented for the title compounds. All show negative curvature below ∼2$T_c$. Data for grain-aligned ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ are in excellent agreement with a new calculation of the superconducting fluctuation diamagnetism. We infer s-wave pairing and Ginzburg-Landau coherence lengths ${\xi }_{\mathrm{ab}}$(0)=13.6±0.8 Å and ${\xi }_c$(0)=1.23±0.19 Å. Both of the order-parameter bands are found to be essential in the fit. Part of the above negative curvature is inferred to arise from the normal-state background.

We have for the first time epitaxially grown superlattices of Y-Ba-Cu-O/Dy-Ba-Cu-O by dc magnetron sputtering onto ${\mathrm{SrTiO}}_3$ and MgO substrates. The wavelengths range from 24 (twice the c axis) to 300 Å and x-ray diffractograms show satellite peaks characteristic of multilayers for each wavelength. The 24-Å-wavelength sample consists of, on the average, alternate planes of Y and Dy. The multilayers are superconducting with ${\mathrm{T}}_{\mathrm{c}0}$’s between 85 and 89 K, transition widths of 2 K, and resistivity ratios of about 3.

The onset of chaotic behavior in a class of classical scattering problems is shown to occur in two possible ways. One is abrupt and is related to a change in the topology of the energy surface. The other arises as a result of a complex sequence of saddle-node and period doubling bifurcations. The abrupt bifurcation represents a new generic route to chaos and yields a characteristic scaling of the frac- tal dimension associated with the scattering function as [ln($E_c$-E${)}^{\mathrm{-}1}$${]}^{\mathrm{-}1}$, for particle energies E near the critical value $E_c$ at which the scattering becomes chaotic.

Critical behavior associated with intermittent temporal bursting accompanying the sudden widening of a chaotic attractor was observed and investigated experimentally in a gravitationally buckled, parametrically driven, magnetoelastic ribbon. As the driving frequency, f, was decreased through the critical value, $f_c$, we observed that the mean time between bursts scaled as ‖$f_c$-f${\Vert }^{\mathrm{-}\gamma }$. .AE

We present a procedure of practical use for representing functions of several variables as superpositions of functions of only one variable. We show how the procedure works when applied, for example, to the location of global minima. Our numerical examples are restricted here, for simplicity, to functions of two variables. The straightforward extension to functions of more variables will be discussed elsewhere.

Exact one-soliton and two-soliton solutions of generalized Calogero-Moser (gCM) equations are presented. We explain how these solutions describe the origin of the successive avoided crossings observed in the energy-level structure of bounded systems when a parameter is varied. A new statistical description, based on the grand-canonical ensemble for the gCM system, is developed for the study of the parametric properties of irregular spectra. The relationship with random matrix theory is discussed.

We show that under certain circumstances a simple quantum harmonic oscillator driven by a quantum current evolves to unique pure states even if started as a mixed state. In various limits, these states exhibit nonclassical properties such as sub-Poissonian statistics, or more interestingly resemble macroscopic superpositions.

We show that in a large class of gauge models that include the right-handed neutrino ${\nu }_R$, and an explicit U(1) factor contributing to the electric charge in the gauge group, quantization of electric charge follows from the requirement of anomaly cancellation and nonvanishing fermion masses only if the neutrino is assumed to be a Majorana particle. Three specific examples are presented: (i) standard model with ${\nu }_R$, (ii) left-right-symmetric models, and (iii) flipped SU(5)×U(1) model.

If an SU(2)-singlet quark (we call it D) with mass on the weak scale exists, for which there is some motivation, it can have a large decay to a normal quark plus a Higgs boson, e.g., D→b+$H^0$. If $M_D$≲150 GeV, this converts present-day hadron colliders from being unlikely to detect Higgs bosons into useful Higgs boson factories, &≲${\mathrm{M}}_{\mathrm{Z}}$.

We investigate thermal effects occurring when an inertial string vacuum is described in an accelerating frame. It is shown that there is a critical acceleration corresponding to a temperature equal to $T_H$/π, where $T_H$ is the Hagedorn temperature for a bath of free strings.

The leading T-conserving P-nonconserving (PNC) electromagnetic coupling to the nucleon or nucleus is known as the anapole moment. We evaluate the pion-cloud contribution to the nucleon anapole moment, and the enhancements in nuclei associated with meson-exchange currents and with the mixing of the nuclear ground state with opposite-parity excited states. We find that the anapole moment becomes the dominant PNC spin-dependent coupling in heavy nuclei.

An effective theory for low-energy nuclear interactions is proposed, based on results obtained from the 1/$N_c$ expansion of quantum chromodynamics. The Lagrangian is local in the meson sector, but in the baryon sector it is nonlocal both in the meson-baryon Yukawa coupling and in the baryon propagators. It satisfies an important consequence of the 1/$N_c$ expansion, the suppression of baryon loops. Our findings are then shown to support the traditional approaches in nuclear physics and, more especially, the relativistic nuclear many-body theories at baryon-tree level.

We calculate rates for two exothermic excited-state collision processes involving sodium at ultracold temperatures. We predict that the rate for fine-structure changing collisions exceeds that of radiative redistribution with release of enough energy to cause loss from current optical traps. A semiclassical treatment is used which accounts for the frequency dependence of absorption and spontaneous emission in midcollision.

We have studied the collisional loss rates for very cold cesium atoms held in a spontaneous-force optical trap. In contrast with previous work, we find that collisions involving excitation by the trapping light fields are the dominant loss mechanism. We also find that hyperfine changing collisions between atoms in the ground state can be significant under some circumstances.

The electric-dipole moment (edm) of the ground state of cesium has been measured using a two-laser method that does not require the presence of an external B field. The measured value ${\mathit{d}}_{\mathrm{Cs}}$=(-1.8±6.7±1.8)×${10}^{\mathrm{-}24}$ e cm implies that the electron EDM is ${\mathit{d}}_{\mathit{e}}$=(-1.5±5.5±1.5)×${10}^{\mathrm{-}26}$ e cm. This result represents more than an order-of-magnitude improvement over all previous limits.

The first detection of neutral heavy rare-gas hydrides and deuterides produced in a solid is reported. XeD, XeH, and KrD were generated in Xe and Kr-doped matrices of the solid hydrogens irradiated by 2-keV electrons. The fluorescence bands peak close to the wavelength value of the bound-free hydride B ${}_{}{}^2{}_{}{}^{}X$ ${}_{}{}^2{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ transition in the gas phase.

We present a theoretical analysis of multiphoton ionization through intermediate states shifted through resonance during a short intense pulse. The dynamical behavior is described in terms of a density matrix evolving under a realistic temporal and spatial pulse shape. Our results, on atoms and with realistic parameters, show structure similar to that observed in recent experiments and suggest further interesting phenomena.

We show that a semiconductor photodiode, or photoconductor, will exhibit a strong reactive response to optical radiation tuned to the transparent region just below the band gap. This response is due to the excitation of virtual electron-hole gas readily polarized by the dc electric field which can contribute a major change (>1) in the low-frequency dielectric constant. For ultrahigh speed signals, this nondissipative photoresponse can become similar to conventional dissipative photoresponse by real carriers excited above the band gap. The influence of this reactive response on zero-point electromagnetic fluctuations may aid the laboratory detection of Unruh-Davies-Fulling-DeWitt radiation.

A free-energy density functional for the inhomogeneous hard-sphere fluid mixture is derived from general basic considerations and yields explicit analytic expressions for the high-order direct correlation functions of the uniform fluid. It provides the first unified derivation of the most comprehensive available analytic description of the hard-sphere thermodynamics and pair structure as given by the scaled-particle and Percus-Yevick theories. The infinite-order expansion around a uniform reference state does not lead, however, to a stable solid, thus questioning the convergence of the density-functional theory of freezing.