We discuss the consequences of including both electron-phonon and electron-electron couplings in multiband models, focusing on numerical studies of a one-dimensional two-band model in the intermediate regime for both coupling strengths. Spin-Peierls as well as long-period, frustrated ground states are identified, reminiscent of those found in axial next-nearest-neighbor Ising models. We speculate on experimentally observable signatures of this rich phase diagram.

We have grown high quality superlattices of Dy with nonmagnetic Lu and find that the 2.4% epitaxial compression nearly doubles the ferromagnetic ${\mathit{T}}_{\mathit{c}}$ of Dy with little change in the Néel temperature. A helimagnetic phase exists over a narrow temperature range. Below ${\mathit{T}}_{\mathit{c}}$ in superlattices, 300 Å orthorhombic domains form despite epitaxial constraints, each with a magnetostrictive distortion comparable to that of bulk Dy. For the thinnest intervening Lu layers, individual ferromagnetic Dy blocks have parallel alignment; the remaining samples show antiparallel alignment, coherent over many bilayer periods.

Magnetic resonance usually is detected inductively, using resonant circuits. Recent experiments have detected magnetic resonance mechanically, using the magnetic force acting between a sample and a nearby force microscope cantilever. This article compares the sensitivity of inductive and mechanical methods for detecting magnetic resonance. We show that, as mechanical oscillators are made smaller, their ability to detect magnetic resonance signals improves, such that existing force microscope cantilevers provide a viable alternative to inductive methods for detecting magnetic resonance.

Site-specific neutralization is seen for low energy (1–5 keV) ${}_{}{}^7{}_{}{}^{}\mathrm{Li}_{}^{+}{}_{}{}^{}$ ions scattered at large angles from Na-covered Al(100). Energy spectra of the ion yield are collected near the surface normal as a function of Na coverage. ${\mathrm{Li}}^{+}$ singly scattered from Na is almost completely neutralized, while the neutralization of ions scattered from Al is fairly insensitive to coverage, indicating a large difference in the local electrostatic potential above Na and Al sites. This shows that the phenomena of resonant charge exchange is capable of resolving atomic scale variations in the surface potential.

Temperature echoes, which were first found in simulations of a simple Lennard-Jones glass, are demonstrated to exist in molecular dynamics simulations of a globular protein. Sequences of cooling and heating pulses make possible a study of protein vibrational modes as a function of temperature. Double cooling pulse echoes are used for estimating the density of states. Implications of the results for the glasslike behavior of proteins are discussed.

We show that kink solitons may exist on top of a rotating background as localized objects if the background dynamics is phase locked to a rapidly oscillating ac force. The result is demonstrated analytically for the sine-Gordon model, showing excellent agreement with numerical simulations. In the context of long Josephson junctions this phenomenon means that the well known Shapiro steps in the current-voltage characteristics change into a set of the zero-field steps according to the number of propagating fluxons.

We present a static solution for d=2 critical string theory including the tachyon T but not its potential V(T). This solution thus incorporates tachyon back reaction and, when T=0, reduces to the black hole solution. When T≠0 one finds that (1) the Schwarzschild horizon of the above black hole splits into two, resembling Reissner-Nordstrom horizons and (2) the curvature scalar develops new singularities at the horizons. These features, as we argue, will persist even with V(T) present. Some possible methods for removing these singularities are discussed.

Contrary to commonly held belief, we show that one can obtain a low value for ${\mathit{M}}_{\mathit{R}}$, the SU(2${)}_{\mathit{R}}$ breaking scale, in grand unification theories based on SO(10). This possibility emerges in the supersymmetric version of SO(10) with a judicious choice of Higgs content. The unification scale is found to be consistent with the constraint from proton decay. This result is first explicitly demonstrated using the one-loop renormalization group equations, and then a full two-loop analysis is carried out.

The proximity of multiparticle production thresholds means that the ${\mathit{Z}}^0$ resonance is formed, not by a single particle, but by a ‘‘multiplet’’ of distinct ${\mathit{Z}}^0$ bosons differing slightly in mass. The effects that these thresholds can have on the gauge-invariant pole expansion near the ${\mathit{Z}}^0$ resonance are studied. It is shown how this expansion naturally yields physical quantities from which the mass and partial widths of the unstable ${\mathit{Z}}^0$ bosons can be defined. The recent discussion on the definition of the ${\mathit{Z}}^0$ boson mass is reviewed and commented upon, stressing the distinction between the renormalized and physical masses.

It is argued that, accounting for the anisotropy of phase space, the local fluctuations of multiplicity in high energy multiparticle production are self-affine rather than self-similar. Evidence is given confirming self-affinity in phase-space distributions. A method is proposed to extract the characteristic parameter of self-affinity, the Hurst exponent, from the experimental data iteratively.

We have deduced compound nuclear isospin mixing in ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}_{}^{\mathrm{*}}{}_{}{}^{}$ and ${}_{}{}^{26}{}_{}{}^{}\mathrm{Al}_{}^{\mathrm{*}}{}_{}{}^{}$ at excitation energies between 33 and 65 MeV. The γ-ray yield from the decay of giant dipole resonances built on excited states is found to be suppressed, implying that isospin mixing is small. By comparison with data from the literature at lower excitation energy, we conclude that isospin is a better symmetry in these compound nuclei at high excitation energy.

We have measured the spin correlation coefficient (${\mathit{C}}_{\mathit{N}\mathit{N}}$) and the neutron and proton analyzing powers (${\mathit{A}}_{\mathit{n}}$ and ${\mathit{A}}_{\mathit{p}}$) for the n→p→→dγ reaction at a neutron beam energy of 183 MeV. Calculations of the spin correlation are particularly sensitive to the treatment of mesonic and isobaric currents. Predictions that include both explicit photon couplings to exchanged mesons and intermediate isobar states, and relativistic correction terms in the Hamiltonian, agree well with our data, thus demonstrating the quantitative accuracy of present treatments of meson-exchange currents at moderate momentum transfer (q∼1-2 ${\mathrm{fm}}^{\mathrm{-}1}$).

The energy dependence of the giant dipole resonance built on the isobaric analog state was measured in pion-induced double charge exchange on ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ at incident pion energies of 140–295 MeV. The cross section increases with energy. The excitation function has a typical non-spin-flip signature, expected from simple considerations and verified by sequential model calculations. Three narrower previously reported low-lying states in ${}_{}{}^{13}{}_{}{}^{}\mathrm{O}$ (${\mathit{E}}_{\mathit{x}}$=3.1, 4.5, and 6.1 MeV) exhibit a spin-flip signature and may correspond to the giant Gamow-Teller strength built on the isobaric analog state.

The mean lifetimes τ of the ${\mathrm{Ca}}^{+}$ 4p ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{3/2}$ and ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{1/2}$ levels have been measured to ≊0.3% precision using a variant of the collinear laser-beam–ion-beam spectroscopy technique. Like previously measured precision lifetimes of light alkali-metal-like systems, these data fall significantly higher than calculations by the most recent multiconfiguration Hartree-Fock and relativistic many-body perturbation theories. Discrepancies between experiment and theory for alkali-metal-like systems have implications for the interpretation of parity-violation research.

We observed a new type of sub-Doppler cooling that employs neither polarization gradients nor magnetic fields, and involves neither a damping force nor significant diffusive heating. Instead, light shifts combined with optical pumping (OP) to levels not coupled by the laser field gives transient cooling, and steady state is not achieved. The time scale is set by the OP rate. We observe both cooling and heating, but often with the opposite detuning from that of steady state. A semiclassical and a fully quantum mechanical calculation of transient cooling agree very well with one another and with our data.

Spatial distributions of photoelectrons produced by multiphoton ionization of xenon atoms are recorded by projecting the expanding photoelectron cloud onto a two-dimensional position sensitive detector. The projected image provides a direct view of the squared angular wave functions of the free electrons as well as their energy distribution. The results confirm recent observations that intermediate state resonances with 5f and 4f character establish the dominant ionization paths at low intensity, for short pulse excitation at 640 and 620 nm. At higher intensity more complex superpositions occur with formation of electrons with continuous distributions at low energies.

We present experimental results of two-color photoassociative ionization (TCPAI) spectroscopy in a magneto-optical trap. A probe light beam traces out the frequency (and intensity) dependence of TCPAI. By manipulating the intensities of the trap and repumper lasers we investigate the participation of different hyperfine ground-state levels and demonstrate the conditions under which molecular bound states figure in the collision intermediate.

Structure has been observed in the total ionization cross section of ${\mathrm{O}}_2$ above 8 eV positron impact energy. A comparison with available data on the excitation to the Schumann-Runge continuum indicates that coupling effects between the positronium formation, excitation, and direct ionization channels may be responsible.

Transition from direct ionization to delayed ionization for free ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ is observed below the ionization potential (IP=7.5 eV). It depends on the excitation wavelength as well as the pulse energy. The appearance threshold for direct ionization is found at ∼5.8 eV. These can hardly be explained by thermionic emission, therefore, we propose that triplet states at ∼1.7 eV (=7.5-5.8 eV) should play an important role in the multiphonon ionization of free ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ and the intramolecular interaction of many triplet states gives rise to delayed ionization. To confirm our proposition, we have carried out further experiments using two laser ionization.

Atomic coherence effects within the sodium ${\mathit{D}}_1$ line are shown to lead to the suppression of optical pumping, to the switching of light on and off when the coherence effects are turned on and off, and especially to lasing without inversion.