A series of new stable operating states of a homogeneously broadened laser have been observed. As the pump power is varied, thresholds are reached where the power and spectrum of the laser change discontinuously. In these states the laser simultaneously oscillates at two frequencies whose separation is given by the Rabi frequency or one of its subharmonics. A theoretical explanation is given which includes field-induced structure within the homogeneous line.

This paper reports the first conclusive observation of ion Bernstein waves near the second-harmonic cyclotron layer in a single-ion-species tokamak plasma. The interpretation that the ion Bernstein waves are generated through fast-wave mode conversion at the second-harmonic cyclotron layer is supported by wave dispersion measurements as well as the observation of the long-wavelength fast wave by laser scattering techniques.

The angle and energy distribution of a 800-keV, 6-kA, 100-ns electron beam have been measured in an experiment in which this beam interacts with an initially cold hydrogen plasma of density 7×${10}^{19}$ ${\mathrm{m}}^{-3\mathrm{}}$. Towards the end of the beam pulse both distributions become Gaussian. They are explained by the assumption of collisions of beam electrons with three-dimensional "blobs" containing fields with amplitudes of ${10}^8$ V/m. Spectroscopic measurements give average fields of ${10}^6$ V/cm. These observations are interpreted as evidence for the occurrence of Langmuir collapse.

The linear stability of resistive ballooning modes in a high-temperature tokamak plasma is examined, with account taken of the effect of temperature gradients and finite parallel electron thermal conductivity. It is found that for moderate values of $\beta$, where $\beta$ is the ratio of the thermal to magnetic pressure, resistive ballooning modes may be completely stabilized.

Remarkable parallels in the behavior of diffusion-limited aggregation and two-fluid displacements in porous media exist; hence, the former can be used to simulate the latter. Both processes can be described by the application of Laplace's equation with similar boundary conditions. Displacements can be stabilized by reversing the flow direction and interfacial tension can be incorporated to broaden dendrites or fingers. Furthermore, diffusion-limited aggregation can be used to simulate flow in anisotropic or inhomogeneous porous media.

A Hall-type voltage induced by the fast diffusion of hydrogen in a metal (palladium) in a strong magnetic field has been detected. The sign and the magnitude of this voltage imply that hydrogen in palladium acts as a particle of charge $Z_{\mathrm{H}}=1\mathrm{}$. The presence of this effect is strong experimental evidence for the existence of the controversial direct-force term (i.e., the action of an electric field on the unscreened charge of an ion in a metal) in the theory of electromigration.

Implantation of As ions into (111) surfaces of V single crystals at room temperature results in the formation of supersaturated solid solution. Channeling studies revealed that at 14 at.% As the implanted region starts to rotate with respect to the bulk. The angle of rotation is perpendicular to a (110) plane, the slip plane, and increases with increasing As concentration. At 20 at.% As polygonization of the implanted region occurs. Results from (110) surfaces support the assumption that the implantation process causes a slip mechanism which gives rise to the rotation and the polygonization of the implanted layer.

We present a theoretical study of electronic properties of the Pd-Si(111) interface in the early stages of interaction. Three models are considered: a chemisorbed Pd layer on Si, an epitaxial ${\mathrm{Pd}}_2$Si layer on Si, and a near-surface Si layer containing interstitial Pd atoms. By comparing the results of our calculations with the spectroscopic data, we find that the model of mixing interstitial Pd atoms in Si is the most appropriate one. This suggests an interpretation of the interfacial reaction in terms of a precursor state of formation of Pd silicide.

Spin-polarized tunneling is used to measure the energy difference $\delta$ between spin-up and spin-down electrons in a magnetic field in thin films of superconducting A1. $\delta$ decreases from $2{\mu }_{\mathrm{B}}H$ at low temperature and field to $0.8\left(2{\mu }_{\mathrm{B}}H\right)$ as the phase boundary $T_c\mathrm{}\left(H\right)\mathrm{}$ is approached. This change in $\delta$ provides the first measurement of the $l=0\mathrm{}$ antisymmetric Fermi-liquid parameter $G^0$ in a real metal and the first direct evidence for the renormalization of the Pauli field in high-field superconductivity.

We present a theory of the temperature-dependent conductance $G\mathrm{}\left(T\right)\mathrm{}$ of one-dimensional systems which depends only on the general principles of quantum resonance. Resonant and nonresonant conductances and temperature parameters $T_0$ are found to be related in a way which is in qualitative agreement with recent experiments. A new low-temperature regime is predicted in which $G\mathrm{}\left(T\right)\mathrm{}$ should change abruptly from Mott type to $T$ independent or metallic.

A unique class of electronic-vibronic photoluminescence spectra in silicon is observed exclusively after noble-gas ion implantation and partial annealing. Characteristic features are mass-dependent systematic line shifts, systematic behavior in the defect formation and annealing, and the common trigonal defect symmetry. The spectra are closely related with the previously studied $I_1$ (1.018-eV) spectrum emitted by an intrinsic lattice defect. It is concluded that the noble gases are incorporated into the intrinsic defect in an analogous way, constituting a class of related deep optical defects.

Direct confirmation of the one-dimensional $S=\frac{1}{2}$ $\mathrm{XY}$ character of the interactions between the pseudospins of the ${\mathrm{Pr}}^{3+}$ ions in Pr${\mathrm{Cl}}_3$ is obtained through a study of the pseudospin dynamics by means of electronic Raman scattering, with and without a magnetic field. This is concluded from an analytic calculation of the spectral moments and from a numerical finite-chain calculation of the line shapes, both of which are in excellent agreement with the experimental data.

We report the dependence of the percolation threshold of the three-dimensional sticks systems on aspect ratio and on macroscopic anisotropy. This Monte Carlo study is the first determination of percolation thresholds for randomly oriented objects in three-space. The results show that the above dependence is determined by the excluded volume of the sticks. However, the total excluded volume of randomly oriented objects is lower than that of the same objects in parallel alignment.

A rigorous theory for low-energy scattering in quantum systems in $n$ dimensions, $n>~2$, with general, possibly long-range, interactions is presented. It gives new insights into lowenergy phenomena; e.g., for $n=2\mathrm{}$ the controversy around the existence of scattering parameters is resolved. Furthermore, the important role of a parametrization of scattering observables in the study of spin-polarized quantum systems is illustrated. For $n=3\mathrm{}$, an analytic expansion of the phase shift and scattering length for charged, finite-size particle scattering, around their neutral counterparts, is derived.

The problem of breaking the gauge symmetry for lattice gauge theories with the $\mathrm{U}\mathrm{}\left(N\right)\mathrm{}$ gauge group in the limit $N\to \infty$ is discussed. It is shown that the breakdown of the gauge symmetry proved recently to occur in two dimensions occurs in higher dimensions. Some aspects of this phenomenon are clarified.

The parity-nonconservation asymmetry $A_{\gamma }$ in the decay of polarized ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}_{}^{*}{}_{}{}^{}$ (110-keV) nuclei has been measured to high precision and is found to be $A_{\gamma }=-(6.8\mathrm{}\pm 1.8)\times {10}^{-5\mathrm{}}$. Absolute uncertainties have been largely removed. This new value for $A_{\gamma }$ together with recent nuclear structure calculations provides severe constraints on the weak meson-nucleon coupling constants, in particular on $f_{\pi }$ and $h_{\rho }^0$.

A basic difficulty in the measurement of the radiation characteristics of autoionizing states in strong coherent laser fields is removed by considering the interaction with a beam of many atoms, so that a properly defined steady-state spectrum can be calculated. Such a spectrum has both coherent and incoherent components. These can be used for studying the confluence of coherences, Stark splitting, Fano asymmetry parameters, etc.

A novel interference effect that is unique to the recently introduced technique of Raman heterodyne detection of NMR is illustrated for the impurity-ion crystal ${\mathrm{Pr}}^{3+}$: YAl${\mathrm{O}}_3$. Experiments confirm theoretical arguments that the two inequivalent Pr nuclear sites in the host lattice generate coherent Raman signals of opposite sign that interfere. A necessary requirement, which can now be examined in detail, is that the principal nuclear quadrupole $Z$ axis of Pr be incongruent in the ground and optically excited electronic states.

The topological properties of patterns which arise in free motion of matter under random but smooth initial velocity distributions are considered. Potential and solenoidal motions are discussed. The cosmological role of this phenomenon is also briefly discussed.

Initial operation of the tokamak fusion test reactor has concentrated upon confinement studies of Ohmically heated hydrogen and deuterium plasmas. Total energy confinement times (${\tau }_E$) are 0.1-0.2 s for a line-average density range (${\overline{n}}_e$) of (1-2.5)×${10}^{19}$ ${\mathrm{m}}^{-3\mathrm{}}$ with electron temperatures of $T_e\mathrm{}\left(0\right)\mathrm{}\sim 1.2-2.2$ keV, ion temperatures of $T_i\mathrm{}\left(0\right)\mathrm{}\sim 0.9-1.5$ keV, and $Z_{\mathrm{eff}}\sim 3$. A comparison of Princeton large torus, poloidal divertor experiment, and tokamak fusion test reactor plasma confinement supports a dimension-cubed scaling law.