We have carried out an experimental study of weak localization of photons in a rigid, nonabsorbing, disordered medium. Unlike recent results reported for particles in a fluid, we observe large-amplitude fluctuations in the scattering intensity as a function of angle. Ensemble averaging of these sample-specific fluctuations decreases their intensity and uncovers a sharp peak in the backscattering direction. These results emphasize the common origin of weak localization of electrons, as a wave phenomenon, and of photons which are inherently wavelike.

We report measurements of the nonlinear radiation intensity and phase (wave refractive index) of a free-electron laser amplifier operating in the collective (Raman) regime. The laser generates up to ∼ 100 kW of rf power at a frequency of 9.3 GHz and with an efficiency of ∼ 10%. Power saturation, efficiency, synchrotron oscillations, and the rf phase are studied as functions of electron beam energy, current, and axial distance within the helical wiggler. Excellent agreement with nonlinear theory that takes cognizance of electron trapping in the combined ponderomotive and space-charge potential well is obtained.

We examine experimentally the stability of a Stokes layer in a fluid near a boundary whose temperature is modulated as $T_{0}cos\omega t$. We define an appropriate Rayleigh number for the problem and determine its critical value. Increased stabilization is observed to accompany a reduction in the Prandtl number. We observe hysteresis effects near the critical Rayleigh number, including a double hysteresis loop, which appear qualitatively similar to recent predictions of Roppo, Davis, and Rosenblat.

The energy content of high-density, purely lower-hybrid-current-driven plasmas has been measured under a variety of operating conditions on the Alcator-C tokamak. The energy stored in the current-carrying electron tail can be a large fraction of the total kinetic energy in an rf-driven plasma, while the energy residing in the bulk plasma is comparable to that of a similar Ohmic discharge. At densities ${\overline{n}}_e<~3\times {10}^{13}$ ${\mathrm{cm}}^{-3\mathrm{}}$, the global confinement time of the rf-driven plasma is similar to that of an Ohmic plasma, but degrades relative to its Ohmic counterpart at higher densities and/or rf powers.

A simple technique is proposed for probing the single-particle and collective excitations in a Fermi liquid via a standing magnetic surface wave of arbitrary $\omega$ and fixed $k$ generated by a meanderline coil. The calculated power absorption spectrum for ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ displays singularities associated with the $l=0\mathrm{}$ spin-wave mode and a Doppler-shifted spin resonance of the single-particle excitations.

We report a high-resolution, small-angle x-ray study of aggregated gold colloids over the range 0.0003 to 0.08 ${\mathrm{\AA }}^{-1\mathrm{}}$. We are able to fit our data with a simple model that correctly accounts for nonfractal short-range order with a crossover to long-range fractal correlations. This provides new information on the structure of real aggregates, and new insight into the aggregation processes which lead to their formation.

Epitaxial growth of metastable phases is discussed. A linearized theory predicts new ordered compounds stabilized by the interaction of atomic size differences with epitaxial strain during growth.

Electrochemically synthesized polythiophene is a very promising conducting polymer able to support tiny metallic particles which could be useful in various applications (e.g., conducting leads, catalysis,...). Time-resolved in situ investigation of the process of the inclusion of metallic aggregates has been achieved by dispersive x-ray absorption spectroscopy. The unique capability of this new structural tool comes from the association of the properties of synchrotron radiation with a photodiode array used as a position-sensitive detector.

Inhomogeneous systems are modeled by use of the maximum-entropy method so that only explicit properties of their structure are taken into account, with no arbitrary assumptions. A phase diagram describes the evolution of one inhomogeneous system into another. It contains first- and second-order transitions, with lattice animals, percolation, and a lattice gas as special cases. Two order parameters are identified. A tentative mapping onto the Potts model results in continuously varying critical exponents. New methods, including Monte Carlo, are proposed for maximum-entropy problems.

ENDOR experiments on paramagnetic thermal donors are presented for the first time. They show that the ESR spectra of all thermal donors identified by ir bands are superimposed in one narrow line ("$\mathrm{NL}8\mathrm{}$"). Only ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ superhyperfine interactions were found and determined for up to seven neighbor shells of four thermal donors. From the analysis of the data with effective-mass theory it is concluded that the thermal-donor defect most probably does not contain a central Si atom assumed in most current models. It is tentatively suggested that it is an ${\mathrm{O}}_2$ molecule.

Switching and narrow-band noise, phenomena usually associated with charge- and/or spindensity waves, have been observed for the first time in a semiconductor heterojunction. These phenomena occur in the recently reported condensed phase of the two-dimensional electron gas which forms at a GaAs/${\mathrm{Ga}}_{0.77}$${\mathrm{Al}}_{0.23}$As heterojunction.

We have performed narrow-band noise and dc conductivity measurements on the charge-density-wave (CDW) conductor Nb${\mathrm{Se}}_3$ in magnetic fields up to 75 kG. In the lower CDW state, the anomalous magnetoresistance is shown to result from a direct magnetic-field-induced conversion of normal to CDW carriers. The $H$ and $T$ dependence of the new CDW carrier concentration, and hence CDW order parameter, is explicitly determined.

The magnetic susceptibility of ${\mathrm{Eu}}_x{\mathrm{Sr}}_{1-x}{\mathrm{As}}_3$ spin-glasses is highly anisotropic: For $x=0.24\mathrm{}$ and $x=0.3\mathrm{}$ the longitudinal and transverse susceptibilities (with respect to the orientation of magnetic moments in antiferromagnetic Eu${\mathrm{As}}_3$) exhibit spin-glass-like maxima at different temperatures, with the longitudinal freezing occurring at higher temperature. The magnetic specific heat displays a temperature dependence not previously found in spin-glasses, $C_M=aT \exp (-\frac{\Delta E}{k_{\mathrm{B}}T})$. This indicates a gap around zero energy in the density of states of magnetic excitations, which arises from the anisotropy.

The dynamic effective mass in heavy-fermion systems is different from the bare mass, unlike in one-component Fermi liquids. This is used to explain the appearance of a heavy mass in the London penetration depth at all temperatures. $F_0^s\sim {10}^2$ and $F_1^s\sim 1$ are found, reflecting the strong frequency dependence but weak momentum dependence of the conduction-electron self-energy.

Point-contact tunneling is used to make what we believe to be the first measurement of a superconducting energy gap ($\Delta$) in an organic superconductor. For a $\beta -{\left[bis\right(\mathrm{ethylenedithio}\left)\mathrm{tetrathiafulvalene}\right]}_2\mathrm{Au}{\mathrm{I}}_2$ single crystal, $\Delta$ shows consistent field and temperature dependences, but is more than 4 times larger than weak-coupling BCS value, implying extremely strong coupling. We speculate that very low-frequency modes may be responsible.

We report magnetoresistance studies of Al and Ag wires of width 35 to 110 nm which probe the electron phase-breaking rate. We find that this rate at low temperatures is determined by one-dimensional electron-electron scattering with small energy transfers. This confirms the importance of this mechanism for electron energy loss in one-dimensional systems, as suggested by Al'tshuler et al., and defines clearly the relevant dimensional length scales.

The microstructure of sedimentary rocks is studied by small-angle neutron scattering for length scales between 5 and 500 Å. In limestones and dolomites, we find that the pore surfaces are effectively smooth above 50 Å, but there is evidence for roughening on shorter length scales. In sandstones and shales, the pore surfaces show fractal character due to the presence of clay. The fractal dimension is nonuniversal. We attribute these observations to impurity effects, which can lower the surface tension and maximize the surface area.

Experimental evidence for spontaneous excitation of ultralow-frequency (ULF) waves in the ionosphere through the use of high-frequency (HF) pumping is reported. The ULF waves were excited through beating of two HF pumps. The observed ULF power indicates conversion efficiency exceeding the one predicted by the Manley-Rowe relations consistent with theoretical expectations for stimulated excitation in the semicollisional regime. ULF wave excitation was also observed through the use of one amplitude-modulated HF pump, consistent with generation of ULF magnetic fields caused by the coupling of the heater-induced temperature gradient to ionospheric gradient.

The influence of the boundary conditions on the finite-size dependence of the mass gap in the 1D Ising model with a transverse field is studied by means of combined exact results and large-size numerical calculations. The well-known exponential form (for transverse fields smaller than the critical one) is only recovered in a limited range of parameters. A power-law behavior for the mass gap is even found in the case of antiperiodic boundary conditions.

It is shown that no continuous process in which the energy tensor of accreted matter remains bounded and satisfies the weak energy condition in a neighborhood of the apparent horizon can reduce the surface gravity of a black hole to zero within a finite advanced time. This gives a precise expression to the third law of black-hole mechanics.