The phase transitions at the $R$ point in KMn${\mathrm{F}}_3$ doped with 1% and 10% of KMg${\mathrm{F}}_3$, with 1% of NaMn${\mathrm{F}}_3$, and with 1% of KCa${\mathrm{F}}_3$, have been studied with x-ray scattering techniques. The Mg impurities lower $T_c$ while the others raise $T_c$, by amounts which are roughly consistent with the differences in the atomic radii. In both cases it is found that the fluctuation-driven first-order transition becomes continuous when the concentration of impurity exceeds a critical amount.

Molecular-beam techniques have been used to probe the dynamical origin of the surface-temperature ($T_s$) dependence of the precursor-mediated dissociative chemisorption of ${\mathrm{N}}_2$ on W(100). Measurements of the angular and velocity distributions of scattered molecules have revealed that increasing $T_s$ primarily serves to reduce the fraction of precursor molecules that go on to dissociate, by biasing the kinetics in favor of desorption. In contrast, the trapping probability into the precursor state is found to be relatively insensitive to this parameter, accounting for ≲20% of the observed effects.

We present a calculation of the shifts and lifetimes of hydrogen-atom excited states in the vicinity of a jellium surface. The calculations combine accurate density-functional potentials with a complex scaling theory for obtaining resonances by means of bound-state procedures. The lifetimes of the hydrogen levels are found to be 1 to 3 orders of magnitude longer than previous estimates, a fact that has important experimental consequences. Extensions of the approach developed here should have applications to a variety of phenomena involving quasibond states in condensed-phase systems.

The temperature dependence of the dc conductivity has been measured on sputtered amorphous carbon films. The conductivity is proportional to $T^n$ with $n=15\mathrm{}-17$ over a wide temperature range, suggesting for the first time that the multiphonon tunneling transition with weak electron-lattice coupling of localized $\pi$ electrons is a probable mechanism for charge transport. This may be attributed to the delocalized nature of the $\pi$ defect in amorphous carbon.

The newly discovered even-denominator fractional quantum Hall effect at filling factor $\nu =\frac{5}{2}$ is found to collapse rapidly as the magnetic field is tilted away from the normal to the two-dimensional-electron-gas plane. No similar effect has been reported in tilted-field studies of the spin-polarized, odd-denominator effect at $\nu =\frac{2}{3} \mathrm{and} \frac{1}{3}$. Since a primary result of such tilting is increased spin splittings, the collapse of the $\nu =\frac{5}{2}$ effect strongly suggests that the underlying state is not spin polarized. Condensation into such a quantum liquid must involve substantial spin reversal.

General, separate formulas are derived for the quantum and the statistical fluctuations of any operator. The formalism is applied to the study of these fluctuations for the particle-number as well as the angular-momentum operators in a realistic case.

We present the first experimental results of quenching (ionization) of $\mathrm{H}(n_0=43\mathrm{}-49)$ atoms by a linearly polarized microwave electric field consisting of two frequencies (7.58 and 11.89 GHz). Adding a second frequency increases the quenching probability over that for only one frequency, especially where there is local stability at scaled frequencies near certain rational fractions, e.g., $n_0^3\omega =\frac{1}{6} \mathrm{and} \frac{1}{5}$. Some, but not all, two-frequency quench curves display a rich structure that appears to be related to that observed in some single-frequency quench curves.

A hyperinflation rule is found for one-dimensional systems with different periodicity or quasiperiodicity determined by a parameter $\alpha$. Physical systems described by transfer matrices are analyzed and the hyperinflation rule is shown to give rise to self-similarity in the space spanned by parameters of the system and $\alpha$.

The one-dimensional structure of high-Mach-number, transverse magnetosonic, relativistic shocks is studied computationally by kinetic, particle-in-cell methods. For amplitudes of astrophysical interest, shocks are mediated by magnetic reflection of the incoming plasma stream. The resulting coherent Larmor loops at the shock front are unstable, and rapidly thermalize downstream to form a relativistic, quasi-Maxwellian plasma in the momenta normal to the background magnetic field.

Cosmic global strings dissipate their energy into Nambu-Goldstone bosons with a $\frac{1}{k}$ energy spectrum. These Nambu-Goldstone bosons can convert to photons in the magnetic fields of various astrophysical objects, in particular galactic halos, clusters of galaxies, and extended radio sources. Evidence that this does in fact occur is found in cosmic-ray and radio-astronomy data.

We impose the requirement of one-loop finiteness on a class of recently proposed, lower-dimensional, open-string theories. It is shown that, unlike the apparent situation for closed-string theories, the finiteness requirement yields a unique lower-dimensional string.

The chiral Schwinger model is made gauge invariant by the introduction of a Wess-Zumino field, and then studied with a mass term present for the fermion. It is found that the massive gauge boson becomes unstable and light unconfined fermions exist. The model tends to the correct massless limit only if the Jackiw-Rajaraman parameter $a$ exceeds 2.

Renormalization of the current masses of light quarks by instantons is considered. It is shown that the entire low-energy current mass of the $u$ quark can be generated by this nonperturbative mass renormalization. In this case, the absence of $\mathrm{CP}$ nonconservation in the strong interactions is an automatic consequence.

We numerically study the regularization dependence of the upper bound on the Higgs-particle mass $M_{\mathrm{H}}\left(\mathrm{phys}\right)$ by varying the lattice type and interaction. Computing $f_{\pi }$ and ${\lambda }_r$ where the scalar mass is ∼ 1, we find the bound $M_{\mathrm{H}}\left(\mathrm{phys}\right)<\mathrm{}620\mathrm{}$ GeV. However, there is a small but systematic increase in the bound as the coordination number of the lattice increases. Hence, it is not meaningful to quote error estimates on such bounds.

The $K_S^0{K}_S^0{\pi }^0$ system has been studied in the exclusive reaction ${\pi }^{-}p\to K_S^0{K}_S^0{\pi }^{0}n$ at 21.4 GeV/c. Evidence for the production of the $f_1\mathrm{}\left(1285\right)\mathrm{}$ and the $\eta \mathrm{}\left(1460\right)\mathrm{}$ is presented. The $\eta \mathrm{}\left(1460\right)\mathrm{}$ is produced away from minimum momentum transfer in the presence of nonresonant $K^{*}K$ ($S$-wave) production and phase-space background. The observed mass, width, and decay properties of the $\eta \mathrm{}\left(1460\right)\mathrm{}$ are consistent with those attributed to the $\iota \mathrm{}\left(1460\right)\mathrm{}$ observed in radiative $\frac{J}{\psi }$ decay.

Cross sections for 180° inelastic electron scattering from deuterium were measured from breakup threshold to beyond the quasielastic peak for incident-beam energies of 0.843, 1.020, 1.189, and 1.281 GeV, corresponding to $0.75\le Q^2\le 2.57$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$. The data are in reasonable agreement with nonrelativistic models that include final-state interactions and meson-exchange currents. The scaling function $F\left(y\right)\mathrm{}$ for these data is generally in agreement with $F\left(y\right)\mathrm{}$ for forward-angle data at the same $Q^2$. Values of $G_M^n$ determined from the data are in good agreement with results from previous experiments.

Differential cross sections for the photodisintegration of deuterium between 6 and 9 MeV are reported. Highly monochromatic photon sources from the ($n,\gamma$) reaction and a high-resolution neutron spectrometer were employed to measure the neutron yield between 30° and 155°. The results were found to be in good agreement with the standard theory.

Calculations have been performed for the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}(\gamma ,p)$ and ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}(\gamma ,p)$ reactions at intermediate energies. Both the direct (single-particle) and pion-exchange-current mechanisms have been included using relativistic forms of the nucleon current operator and four-component nucleon wave functions.

We analyze the process of fragmentation in heavy-ion interactions from the viewpoint of growth of instabilities in hot, expanding nuclear matter. The growth rates of modes in a medium unstable to small density perturbations of finite wavelengths are calculated from a kinetic equation. This enables us to treat the case where the nucleon mean free path is comparable to or greater than the wavelength of the fluctuation, and the temperature is arbitrary. We estimate for hot nuclear matter the initial conditions which, after expansion, will result in fragmentation, evaporation, or vaporization.

Cross sections and a complete set of seven independent spin observables have been measured for inclusive quasielastic scattering of 290-MeV protons from ${}_{}{}^{54}{}_{}{}^{}\mathrm{Fe}$ at a quasifree momentum transfer of 1.36 ${\mathrm{fm}}^{-1\mathrm{}}$ and energy transfers varying from 0-90 MeV. Comparison of the results with relativistic and nonrelativistic impulse-approximation models shows that Fermi-motion averaging, a realistic nuclear response function, and medium modifications of the $\mathrm{NN}$ interaction are essential ingredients of a successful theory.