We present a new, rotationally invariant formalism for the theory of stimulated Raman scattering. The formalism is applied to Raman transitions of well-defined rotational symmetry, e.g., rotational Raman (Stokes) transitions, yielding the explicit dependence of gain on light polarization, phase mismatch, and frequency offset.

The scale dependence of transmission in wedged random media containing titania microstructures is measured. The diffusion coefficient and absorption length are determined from the total transmission versus thickness. These parameters are observed to determine the scale dependence of the correlation frequency of fluctuations in intensity within the transmitted speckle pattern as the laser frequency is scanned. Mean free paths as short as 1.4 and 0.35 μm are observed.

When neutrons are exposed to a large number of electromagnetic quanta of arbitrary frequency their spin rotation about a static magnetic field can be suppressed completely. Further, natural-linewidth level-crossing signals in the neutron polarization can be produced. These observed effects are interesting also in connection with neutron electric-dipole-moment and neutron oscillation experiments. They are described in the dressed-particle formalism developed in atomic physics. We were able to reconstruct the dressed-neutron energy-level diagram experimentally.

We formulate the problem of self-propulsion at low Reynolds number in terms of a gauge field over the space of shapes. The computation of this field is discussed, and carried out in some examples. We apply our results to determine maximally efficient infinitesimal swimming motions of spheres and circular cylinders.

It has been proposed that the oscillatory traveling-wave states recently observed at the onset of convection in ethanol-water mixtures can be explained in terms of the linear properties of the waves, taking into account their reflection at lateral boundaries. We present experimental results which support this picture and explore a number of properties of this physical situation, including the existence of states which are modulated by a second frequency at onset.

Nonlinear electron plasma oscillations in a warm plasma are studied by use of Lagrangean coordinates. Without recourse to amplitude expansion, the electron density is obtained as an explicit function of space and time. In the zero-temperature limit, a restriction on the previously found cold-plasma solutions is found.

The amplification of magnetic (Weibel) modes in laser-plasma interaction is investigated by use of unperturbed distribution functions given by Fokker-Planck simulations and a dispersion relation valid for all collisionality regimes. In the five cases studied, a strongly growing mode is found in the underdense plasma, where ${\overline{v}}_x^2<\frac{1}{2}{\overline{v}}_{\perp }^2$, and the usual slowly growing one in the overdense plasma. The first mode grows convectively outwards by more than ${10}^4$. The convection velocities are found to be very different from Nernst values.

Stochastic electron acceleration in intense electromagnetic waves, propagating obliquely to an ambient magnetic field, is considered. It is shown that when the waves' parallel phase velocities are supraluminous, the Hamiltonian surfaces are topologically open and, as a consequence, electrons can gain large energies. The results indicate that state-of-the-art, ground-based rf transmitters can accelerate ionospheric electrons to multimegaelectronvolt energies.

A factorization Ansatz for the three-particle direct correlation function $c^{\mathrm{}\left(3\right)\mathrm{}}$ is combined with the exact relation between $c^{\mathrm{}\left(3\right)\mathrm{}}$ and the pair function $c^{\mathrm{}\left(2\right)\mathrm{}}$ to derive a simple and tractable approximation for $c^{\mathrm{}\left(3\right)\mathrm{}}$ in dense, classical fluids. The predictions compare favorably with "exact" molecular-dynamics results for the "soft-sphere" fluid near freezing.

We have studied the dynamics of a nematic-nematic phase separation in mixtures of a mesomorphic polymer with a low-molecular-weight liquid crystal by performing temperature quenches at a number of compositions. The appearance of a new phase and its subsequent growth were observed with an optical microscope. A striking result is the formation of a regular striated pattern whose period increases with time according to a $\frac{1}{3}$-power law.

The fractal nature of collisional cascades in solids is investigated by means of a model of "idealized" collisional cascade and the Winterbon-Sigmund-Sanders theory of atomic collisions. It is shown that the fractal dimensionality increases as the cascade evolves because of the change of the effective interaction potential. The condition of a spike is shown to arise naturally from the concept of "space-filling" fractals. The characteristic kinetic energy per particle in a spike is estimated to be on the order of a few electronvolts.

Directed polymers subject to quenched external impurities (as in a polyelectrolyte in a gel matrix) are examined analytically, and numerically. Transverse fluctuations $\left|\mathrm{x}\right|$ scale with the length $t$ of the polymer as $\left|\mathrm{x}\right|\sim t^{\nu }$. In all dimensions, for sufficiently strong disorder, $\nu$ can be different from the random-walk value of ½. Extensive numerical simulations in two, three, and four dimensions in fact suggest a superuniversal exponent of $\nu =\frac{2}{3}$. The importance of the tree structure of the polymer ensemble is emphasized.

The mathematical structure of the dynamical theory for the soft-spin version of the $p$-spin-interaction ($p>\mathrm{}2\mathrm{}$) spin-glass model is related to that for the dynamical theories of the structural glass transition. The phase transitions predicted by both theories are discussed. The spin-glass transition predicted by the dynamical theory is related to a broken-replica-symmetry equilibrium calculation.

Acoustic activity in NaBr${\mathrm{O}}_3$ has been studied by measurement of the degeneracy splitting of transverse acoustic waves with neutron scattering. The splitting $\Delta \nu$ is proportional to wave vector q, instead of ${\mathbf{q}}^2$. It is the first sample which cannot be explained by phenomenological theory on acoustic activity. An additional study of Na${\mathrm{Br}}_{0.7}$${\mathrm{Cl}}_{0.3}$${\mathrm{O}}_3$ showed that NaBr${\mathrm{O}}_3$ and NaCl${\mathrm{O}}_3$ have opposite senses of acoustic activity. Further study of $\mathrm{Na}{\mathrm{Br}}_x{\mathrm{Cl}}_{1-x}{\mathrm{O}}_3$ may lead to a new subject which is concerned with the phenomenon in acoustics similar to that in ferromagnetism.

The problem of classifying two-dimensional lattices with $N$-fold rotational symmetry for arbitrary (noncrystallographic) even $N$ is shown to be equivalent to a much-studied problem in algebraic number theory. When translated into crystallographic language, the number-theoretic results establish that except for 29 even numbers $N$ there are two or more distinct lattices. The smallest $N$ for which there is more than a single lattice, however, is $N=46\mathrm{}$. We list every $N$ for which there is a unique lattice, and give the numbers of distinct lattices for all $N<\mathrm{}100\mathrm{}$.

The concentration and molecular-weight dependence of the self-diffusion coefficient for polymer solutions as well as of the cooperative-diffusion coefficient is given in the dilute and semidilute regimes. The method takes into account the gradual screening of both hydrodynamic and excluded-volume interactions as well as the entanglement constraint. The results fit well to the experimental data over the entire regime.

Magnetoresistance measurements of an electron gas formed on a solid-hydrogen substrate are reported as a function of surface disorder. High-mobility surfaces display an essentially classical positive $B^2$ field dependence. Lower-mobility surfaces, however, are characterized by the appearance of a negative magnetoresistance for fields less than ∼0.2 T, in good agreement with present weak-localization theories. On extremely disordered surfaces we find large increases in resistivity but a finite residual electron diffusivity $D\sim \frac{\hslash }{m_{\mathrm{el}}}$.

We consider the quantum transport in networks. Arguments similar to those for the quantum Hall effect show that the averaged transport coefficients are quantized. Numerical calculations for a three-loop network yield the values 0, 1, and - 1, depending on the fluxes threading the loops and the quantum state of the net. We characterize the conductance properties of such networks. We also discuss general properties of the transport coefficients in general multiloop networks.

Results of self-consistent energy-minimization calculations provide strong evidence that the ordered phases in epitaxially grown ${\mathrm{Ga}}_{1-x}{\mathrm{Al}}_x\mathrm{As}$ and strained ${\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x$ alloys are metastable, in the sense that segregation into constituents is favored. We show that the long-range order in intermediate metastable structures leads to significant changes in the electronic properties of semiconductor superlattices. Segregation gives rise to micro-quantum-wells with staggered band lineup and multiple confined states in the potential barrier.