The transition from nonmagnetic to magnetic behavior in transition-metal ferromagnets is studied by analysis of all possible volume evolutions of the variation of the total energy with magnetic moment. In contrast to a large body of previous work, magnetic transitions are shown to be necessarily singular, and usually multivalued and discontinuous. Self-consistent spin-polarized energy-band calculations for bcc nickel, fcc cobalt, bcc vanadium, and fcc iron are presented as examples supporting and illustrating the general conclusions.

Experimental densities of states of U${\mathrm{Si}}_3$ and U${\mathrm{Ir}}_3$ obtained by x-ray photoelectron spectroscopy and bremsstrahlung isochromat spectroscopy (BIS) are compared to fully relativistic self-consistent band-structure calculations. Very good agreement is found for U${\mathrm{Si}}_3$, whereas the unoccupied states of U${\mathrm{Ir}}_3$ exhibit a satellite which cannot be explained by the one-particle calculation. The satellite in BIS of U${\mathrm{Ir}}_3$ is attributed to a quasiparticle bound state, and is qualitatively reproduced by a Hubbard model including Coulomb correlation within the narrow $5f$ band. The importance of our findings with reference to the magnetic properties of U intermetallics as well as pure U metal is discussed.

The growth of a strong peak at $E_{\mathrm{F}}$ is observed in photoelectron spectra of Ce${\mathrm{H}}_{2.7}$ below 70 K in addition to the hydrogen-induced band at 5 eV and the $4f$ emission at 2 eV. From the photonenergy dependence and from the absence of $4f-4d$ resonance enhancement we conclude that the peak is neither $4f$ derived nor related to screening or Kondo resonance, but is due to a surface semiconductor-metal transition. It can be explained by the removal of hydrogen from surface sites which pushes $d$ states from the hydrogen band back to $E_{\mathrm{F}}$ and results in a metallic surface dihydride.

The question of whether scattering by the almost-periodic lattice potentials of recently discovered metallic quasicrystals is able to explain the recently observed short mean free paths (about 1 Å) in these materials is discussed. It is shown that scattering by a three-dimensional, almost-periodic potential cannot account for such resistivities, but a model which includes structural defects in the three-dimensional Penrose tiling and large scattering of electrons due to $s-d$ resonant scattering can account for the experimental observations.

We present experimental results which suggest a new mechanism for the production of excited atoms by electron bombardment of alkali halides. Doppler-shift measurements show that the electronically excited atoms have a thermal velocity distribution in equilibrium with the surface temperature. Measurements of the yield of excited atoms, the distribution of population among the excited states, and the dependence on incident-electron current support a model in which the excited atoms are produced by gas-phase collisions between desorbed ground-state atoms and secondary electrons.

A resonant fast dynamo is found in chaotic shear flows. The dynamo effect is produced by resonant perturbations of the velocity field, similar to resonant diffusion in plasma physics. The dynamo is called fast because the flow produces an electric field independent of the fluid resistivity.

Phase transitions occurring after photon decoupling are shown to result in significant density perturbations.

The promise of the inflationary-Universe scenario is to free the present state of the Universe from extreme dependence upon initial data. Paradoxically, inflation is usually analyzed in the context of the homogeneous and isotropic Robertson-Walker cosmological models. We show that all but a small subset of the homogeneous models undergo inflation. Any initial anisotropy is so strongly damped that if sufficient inflation occurs to solve the flatness and horizon problems the Universe today would still be very isotropic.

We solve exactly the problem of diffusion in an arbitrary hierarchical space. We prove that for a given "tree silhouette" $0<s<\mathrm{}1\mathrm{}$ the dynamic critical exponent $\nu$ ranges from $\frac{s}{\mathrm{}(1-s)\mathrm{}}$, for either uniformly or randomly multifurcating trees, to $s$ for the most diverse ones, in qualitative agreement with a static measure of the tree's complexity. We conclude that uniform trees are optimal for information diffusion, that in thermally activated processes the temperature dependence of $\nu$ varies with the underlying tree structure, and that thin elongated trees are the only ones capable of producing a $\frac{1}{f}$ spectrum.

Beginning with rules for a large class of continuum analogs of cellular automata, we derive Langevin equations for the slow variables of the system by projection-operator methods. For purely dissipative rules, we obtain an exact result: The slow variable is linearly unstable due to a "fluctuation-enhancement" relation. This means that this slow variable for the cellular automata grows exponentially for early times. We discuss how this may be related to, for example, kinetic models of the growth of ordered structures.

Finite-temperature behavior of lattice QCD is studied with the Wilson fermion action and use of the Langevin technique for treating quarks dynamically. It is found that the transition zone from low- to high-temperature behavior does not cross the line of critical hopping parameter, but rather continues down to the strong-coupling limit. Practical implications for spectroscopic simulations at small quark masses are discussed.

The present time variation of coupling constants in superstring theories with currently favorable internal backgrounds critically depends on the shape of the potential for the size of the internal space. If the potential is almost flat, as in perturbation theory to all orders, the value of $\frac{\dot{G}}{G}$ for Newton's gravitational constant is calculable and estimated to be - 1 × ${10}^{-11\mathrm{}\pm 1}$ ${\mathrm{yr}}^{-1\mathrm{}}$. If the potential has a minimum with finite curvature due to unknown nonperturbative effects, $\frac{\dot{G}}{G}$ will become unobservably small. Improvement of the measurement of $\frac{\dot{G}}{G}$ would discriminate between the two situations. Problems with the time variation of other coupling constants are also discussed.

The three-generation phase-invariant measure of $\mathrm{CP}$ nonconservation is shown to satisfy a simple and solvable renormalization-group equation. Its value falls by four to eight orders of magnitude between the weak and grand unification scales in the standard model, as well as in its two Higgs and supersymmetric extensions. Such a small value of $\mathrm{CP}$ nonconservation at the grand unification scale can pose a problem for baryogenesis; this is avoided if there are heavy quarks with masses close to their fixed points.

We have developed an extensive model of atmospheric $\nu$ interactions which provide the backgrounds to nucleon-decay experiments. We report results from a 417-live-day exposure of the Irvine-Michigan-Brookhaven detector. During this time 401 contained events were observed at a rate and with characteristics consistent with atmospheric $\nu$ interactions. We have calculated the expected backgrounds to a variety of two- and three-body decay modes and have set lower limits on many nucleon partial lifetimes.

The production of the tensor mesons $f^0\mathrm{}\left(1270\right)\mathrm{}$ and $K^{*0\mathrm{}}\mathrm{}\left(1430\right)\mathrm{}$ and the scalar meson $S\left(975\right)\mathrm{}$ has been observed in $e^{+}{e}^{-}$ annihilation at 29 GeV center-of-mass energy by use of data obtained with the high-resolution spectrometer at the SLAC $e^{+}{e}^{-}$ storage ring PEP. The mean multiplicities for meson momenta greater than 1450 MeV/c are $〈n_{f^0}〉=0.11\mathrm{}\pm 0.04$, $〈n_{K^{*0\mathrm{}}\mathrm{}\left(1430\right)\mathrm{}}〉=0.10\mathrm{}\pm 0.06$, and $〈n_S〉=0.05\mathrm{}\pm 0.02$ per hadronic event. The fragmentation functions of the tensor mesons are in good agreement with the predictions of the Webber cluster model. The data are consistent with a predominant strange-quark content on the $S$ meson.

An approximate analytic calculation of $O\left(Z{\alpha }^2\right)$ corrections to Fermi decays is presented. When the analysis of Koslowsky et al. is modified to take into account the new results, it is found that each of the eight accurately studied $\mathcal{F}t$ values differs from the average by $\lesssim 1\sigma$, thus significantly improving the comparison of experiments with conserved-vector-current predictions. The new $\mathcal{F}t$ values are lower than before, which also brings experiments into very good agreement with the three-generation standard model, at the level of its quantum corrections.

A theoretical framework for the description of the decay of giant multipole resonances is developed. It is shown that the statistical decay of the giant resonances is not necessarily described by the Hauser-Feshbach theory because of the existence of a mixing parameter. The contribution of preequilibrium emission to the giant-resonance decay is also discussed. Application is made to the decay of the giant monopole resonance of ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$.

Fusion cross sections and second-rank-tensor analyzing powers for fusion have been measured at energies around the fusion barrier for ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$ + ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ti}$ and ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$ + ${}_{}{}^{206}{}_{}{}^{}\mathrm{Pb}$ with polarized (aligned) projectiles. The data are compared with results from coupled-channels calculations. The energy dependence of the second-rank-tensor analyzing power for fusion is well described for both systems if we take into account coupling to excited states of projectile and target. The same calculations still underpredict the cross section for fusion at subbarrier energies.

The transition to chaos in "the hydrogen atom in a magnetic field" is numerically studied and shown to lead to well-defined signature on the energy-level fluctuations. Upon an increase in the energy, the calculated statistics evolve from Poisson to Gaussian orthogonal ensemble according to the regular or chaotic character of the classical motion. Several methods are employed to test the generic nature of these distributions.

We present a mechanism for a bistability and multistability induced by a periodic modulation of a nonlinear medium. In contrast to usual ones it is nonperturbative with respect to the nonlinearity. The transmitted energy exhibits plateaus as a function of the incident intensity. The frequency versus intensity propagation diagram is fractal. The possible relevance to noise in the conductivity of systems with polarons and to optical, acoustical, or electromagnetic devices is discussed.