A numerical study of the internal kink mode is performed for a family of tokamak equilibria with circular cross section and parabolic pressure profile. If the ${\beta }_p$ values are sufficiently small, the internal kink mode becomes stable.

It is shown that the concepts developed for understanding superfluid turbulence in thermal counterflow can be successfully applied when the He II is also in uniform rotation. In particular, the association of a critical length scale with the turbulent transition, when extended naturally to the rotating superfluid, yields a simple result in good agreement with the data of Yarmchuck and Glaberson.

It is shown that electroabsorption measurements provide an elegant test to decide between the two most favored theories of Urbach's rule at excitonic absorption edges, namely the model of the field-ionized exciton by Dow and Redfield and the model of the momentarily trapped exciton by Sumi and Toyozawa. Experiments performed at the intrinsic Urbach tails of the ionic insulators CuCl and TlCl give support to the field-ionization model.

Some new correlation inequalities are described which bound large-distance behavior of correlations in ferromagnets from above by correlations at intermediate distances. Among applications are (1) an inequality, $\eta <1\mathrm{}$, on the decay of correlations at the critical point; (2) an inequality $\chi \geqq coth\left(\frac{1}{2}m\right)$ relating the zero-field susceptibility and the mass gap in a nearest-neighbor ferromagnet; (3) a finite algorithm for rigorously computing a sequence of upper bounds guaranteed to converge to the true transition temperature in Ising ferromagnets.

High-resolution angle-resolved photoemission spectroscopy is employed to study surface states on the Cu(111) surface, with use of synchrotron radiation in the energy range $18<~\hslash \omega <~120$ eV. The results reveal a novel periodic oscillatory behavior in surface-state emission intensity which leads to identification of additional new surface states. A spectral decomposition theory is proposed to explain the physical origin of the oscillations. It describes the measured intensity profiles and predicts that the oscillations are universal for all surface states.

It is argued that the recent prediction of the $a$ dependence of the coupling, $g\left(a\right)\mathrm{}$, of an effective lattice gauge theory for quarkless quantum chromodynamics is confirmed by a recent numerical evaluation of $g\left(a\right)\mathrm{}$ for a latticized SU(2) gauge theory, thus providing strong evidence for the dynamical role of instantons. Additional predictions, testing the validity of the quantum chromodynamics bag model, are presented.

On the basis of quantum chromodynamics (QCD) and the recombination model the quark decay function is calculated. The inclusive distribution for a quark and an antiquark in a quark jet can be determined by the perturbative method in QCD. The hadronization of quarks and antiquarks is described by a recombination function that is completely fixed in a recent study of the structure function. No free parameters are involved and the agreement with the data is excellent.

The second Born cross section for electron capture from a hydrogenlike atom of atomic number $Z_T$ by a bare ion of atomic number $Z_P$ and speed $v$ is evaluated for $(\frac{Z_P{e}^2}{\hslash v}\ll 1$ and $Z_T$ arbitrary. The ratio of the second to first Born cross sections increases rapidly as $Z_T$ increases; for $\left(\frac{Z_T{e}^2}{\hslash v}\right)\gtrsim 1$ this ratio is very large. These results indicate that second- and higher-order Born terms must be considered in calculating the cross section for inner-shell electron capture (or the time-reversed process).

New long-term (days) measurements of the evolution of turbulence for a Rayleigh-Bénard system with aspect ratio $\Gamma =4.72\mathrm{}$ reveal a turbulent state, with a threshold near the critical Rayleigh number $R_c$, which consists of a random background time dependence and rare, randomly spaced, major events. These events are discussed in terms of the analog of a particle under the influence of a stochastic driving force and in a potential with two minima.

It is possible to design a storage-ring free-electron laser so that the electrons remain trapped in the optical potential wells formed by the laser fields. Such a design eliminates the heating of the electrons caused by the interaction and could yield both high power output and efficiency.

A simple model for the nonlinear saturation of an instability is studied via numerical solution of the resonant three-wave coupling equations in the case where the high-frequency wave is linearly unstable and the two lower-frequency waves are linearly damped. The solutions are shown to undergo interesting qualitative cahnges as the damping rate of the stable waves is increased. These changes include bifurcations to increasingly complicated periodic motions and the appearance of apparently chaotic motions, possibly indicative of the presence of a strange attractor.

We have studied samples of 98.5% $o$-${\mathrm{D}}_2$ in a diamond anvil cell to pressures of 150 kbar at 5 K by means of Raman scattering. At 50-60 kbar the $E_{2\mathrm{g}}$ phonon and roton cross and hybridize. The coupling has been determined. The phase transition from the symmetric ground state at low pressure to an orientationally ordered ground state, predicted to occur between 31 and 73 kbar, does not take place. A predicted hcp-fcc structural phase transition is not observed.

Measurements of the first- and second-order quadrupole energy-level shifts are performed in a ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$ NMR experiment in a single crystal of NaCN. Twelve orthorhombic domains are found below the phase transition at 15°C. They consist of six pairs of domains with each one oriented by an angle $2\beta$ relative to the other member of the pair. The angle is around the $c$ orthorhombic axis which coincides with the cubic [100] direction of the room-temperature pseudocubic phase. The angle $\beta$ is found to be ±(8.5°±1.5°) at 247°K.

Isothermal-isobaric Monte Carlo computer experiments on melting in a two-dimensional Lennard-Jones system indicate that the transition is first order, in contrast to the two-stage, second-order melting behavior suggested as a possibility by Halperin and Nelson.

The results of a heat-capacity study of the region between one and two layers in the Kr/graphite (Grafoil) system are presented. Heat-capacity anomalies corresponding to the commensurable-incommensurable transition, the commensurable-fluid transition, and a previously unobserved incommensurable-fluid transition delimit the high-coverage extent of the registered phase. A phase diagram proposed on the basis of the heat-capacity data suggests the possibility of a new type of multicritical point.

The scaling theory of Anderson localization by Abrahams et al. is used for dirty conductors just above the Anderson transition. When the inelastic mean free path, $l_{\mathrm{ph}}$, is smaller than the coherence length, $\xi$, in the extended phase, the conductivity increases with temperature like ${l_{\mathrm{ph}}}^{-1\mathrm{}}$. This may be related to the quite general correlation between large resistances and their negative temperature derivatives found by Mooij. $l_{\mathrm{ph}}>\xi$ is required to distinguish between the extended and localized regimes.

Two new features, finite-size effects and a new hydrodynamic eigenvalue problem, are essential for a consistent discussion of the scattering of light by a fluid in a non-equilibrium steady state. The light-scattering structure factor is computed for the case of a steady thermal gradient and a steady shear flow in a fluid. The height differences found in the Brillouin peaks differ from those obtained by Ronis, Procaccia, and Oppenheim and Machta, Oppenheim, and Procaccia.

Sharp intensity peaks are observed in the fluorescence spectra from dye-impregnated single polystyrene microspheres which are not seen in the spectra from bulk material. The resonant peaks are shown to correspond to the natural modes of oscillation of a dielectric sphere, and excellent correlation is found between experimental and theoretically predicted spectra. The results can be applied to the interpretation of inelastic emission (fluorescence and Raman) spectra obtained for the chemical speciation of aerosols and particulates.

Infrared measurements of the cyclotron resonance in the two-dimensional electron gas reveal a remarkable line narrowing and shift if and only if the lowest Landau level is partially occupied. This behavior cannot be explained in terms of single-electron models, and strongly suggests a highly correlated or crystallized ground state, whose properties are compared with existing theories.

A valence change from $f^{13}$ to $f^{14}$ occurs in ytterbium atoms in a mixed-valent ytterbium-aluminum alloy during autoionization and Auger-electron emission. In both of these emission processes the $4f$-shell occupancy increases when the shell becomes more tightly bound under the influence of the core hole. The quenching of the core hole, which terminates the process, occurs after the valence change, so that the $f^{13}$ initial state is not manifest in the spectra.