Computer-enhanced polarized video microscopy has been used to study the discrete layer-by-layer surface ordering transitions of freely suspended films of 4-($n$-nonyloxy) benzylidene-4-($n$-butyl) aniline (9O.4). Stable tilted hexatic smectic-$I$ surface layers form on both free surfaces of the 2D-liquid smectic-$A$ interior in a monolayer, bilayer, trilayer,...sequence as $T\to T_c$. The appearance of the first ten surface layers is described very well by the simple power-law form, $L=L_0{t}^{-\frac{1}{3}}$, predicted for the surface freezing transition in a system with van der Waals forces.

A multicomponent order parameter for surface melting is introduced which depends both on the structure of the bulk crystal and on the orientation of the crystal surface. Each order-parameter component, $\tilde{M}\left({\mathrm{Q}}^{\parallel }\right)$, corresponds to a reciprocal-lattice vector, ${\mathrm{Q}}^{\parallel }$, of the two-dimensional lattice parallel to the surface. For complete surface melting, all order-parameter components with $\left|{\mathrm{Q}}^{\parallel }\right|$ vanish simultaneously but the associated critical behavior depends on the length of ${\mathrm{Q}}^{\parallel }:\tilde{M}\left({\mathrm{Q}}_i^{\parallel }\right)$ vanishes faster than $\tilde{M}\left({\mathrm{Q}}_j^{\parallel }\right)$ if $\left|{\mathrm{Q}}_i^{\parallel }\mathrm{}\right|>\mathrm{}\left|\mathrm{}{\mathrm{Q}}_j^{\parallel }\right|$. For a surface of low symmetry, this implies a marked anisotropy of the melting process.

We present examples of two-dimensional crystals with thirteen to eighteen strong electron scatterers per unit cell which have almost perfect twelvefold, tenfold, and eightfold diffraction patterns in the regions in $k$ space where current experiments are performed. These examples show that it is at present premature to identify the recently discovered octagonal and dodecagonal phases as quasicrystals.

Strong intensity oscillations have been found in RHEED during epitaxial growth at 77 K. This temperature is too low for thermally activated diffusion and establishes that the deposited atom uses its latent heat of condensation to skip across the surface, preferentially coming to rest at growing island edges, to achieve quasi—layer-by-layer growth. This growth mechanism implies that RHEED oscillations should be observable at 0 K. The data also provide insight into the basic principles governing RHEED oscillations.

Diffraction peaks corresponding to diffraction lines in photographic observations were found in high-angular-resolution diffractometry of the 220 thermal diffuse spot of a germanium single crystal in the Laue arrangement. The experiment was performed by means of monochromatized synchrotron radiation and a triple-crystal diffractometer with a collimator and an analyzer of grooved silicon crystals. Agreement between experimental peak profiles and calculations based on dynamical diffraction theory gives decisive evidence of anomalous transmission of x rays scattered by phonons for the first time.

We report in situ scanning tunneling microscopy at atomic-layer resolution of the topographic changes accompanying electrochemical reactions. In dilute perchloric acid, Au(111) terraces roughen during the electrochemical formation and reduction of more than a monolayer of gold oxide, but anneal in minutes at moderate potential. Trace chloride enhances step motion, prevents the observation of roughening, and promotes the dissolution of gold during oxidation and rereduction. These observations illustrate the role adsorbates have in determining the surface mobility of substrate atoms.

We combine chemical lattice imaging with digital pattern recognition to map, at near-atomic resolution, the compositional change across GaAs/AlGaAs interfaces of the highest optical quality. These maps quantify the information content of each unit cell of the lattice image. Our results show that state-of-the-art GaAs/AlGaAs interfaces contain substantial atomic roughness on scales finer than suggested by optical measurements.

We propose a new metastable diatomic hydrogen complex in crystalline Si. Our ab initio calculations show that the complex is about 1.2 eV more stable than two separated neutral hydrogen atoms in their most energetically favorable configurations. A novel diffusion mechanism, based on metastable and molecular hydrogen-complex formation and dissociation, is presented.

We consider sums over directed paths, interfering due to quenched random elements controlling either the phase or the sign of each term. The former enter the determination of electronic transport in the localized regime, and the latter that of correlations in the high-temperature phase of the Ising spin-glass. Numerical computations in two dimensions indicate anomalous fluctuations dominated by untypical paths, with scaling behavior similar to that of random but positive impurities. This is argued to follow from a bound state, due to a subtle attraction between pairs, in replica space.

The local carbon-carbon bonding in evaporated ($e$-C) and ion-sputtered ($i$-C) amorphous carbon has been investigated by transmission electron-energy-loss spectroscopy (EELS) and by ($e, 2e$) spectroscopy. The EELS data indicate that the $e$-C is graphiclike and the $i$-C is diamondlike. The ($e, 2e$) data demonstrate that the concentration of $s{p}^2$ bonding is ∼100% in both materials ($0.85\le c_{s{p}^2}\le 1.0$) for $e$-C and $0.75\le c_{s{p}^2}\le 1.0$ for $i$-C). There is significant rehybridization of the $\pi$ orbital in $i$-C.

The rates at which hot electrons scatter from the $\Gamma$ valley to the $L$ and $X$ valleys in GaAs have been measured as a function of electron energy. Scattering times are determined from the relative efficiency of recombination of hot electrons with neutral acceptors at low injected-carrier densities. Representative scattering times are ${\tau }_{\Gamma L}=540\mathrm{}\pm 120$ fsec for 0.48-eV electrons and ${\tau }_{\Gamma X}=180\mathrm{}\pm 40$ fsec for 0.58-eV electrons. Our results enable us to reconcile the large range of scattering rates reported in other experiments and demonstrate the power of this cw probe to study subpicosecond electron dynamics.

When the magnetic field is not aligned along one of the crystallographic axes of a quasi-1D conductor, the electronic motion is periodic only for special values of the field orientation. For these values, a cascade of spin-density-wave subphases is expected as the field increases. In each subphase the Hall resistance per layer is ${\rho }_{\mathrm{xy}}=\frac{\mathrm{qh}}{p{e}^2}$. $q$ depends on the orientation of the field and $p$ on its amplitude. The electron motion involves large orbits which spread over $q$ unit cells. The integer quantization of these orbits implies a fractional quantization of the density of carriers.

The powder magnetic susceptibility $\chi \mathrm{}\left(T\right)\mathrm{}$ of ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_{4-y}$ is found to scale with doped hole concentration $p=x-2y$ according to a law of corresponding states for $0\le p\le 0.20$, thereby allowing ${\chi }^{\mathrm{Pauli}}\mathrm{}\left(p\right)\mathrm{}$ of the holes and ${\chi }^{2\mathrm{D}}(p, T)$ of the ${\mathrm{Cu}}^{+2\mathrm{}}$ spin sublattice to be separated and precisely evaluated. ${\chi }^{\mathrm{Pauli}}$ increases with $p$. The shape of ${\chi }^{2\mathrm{D}}\mathrm{}\left(T\right)\mathrm{}$ is that of the spin-½ square-lattice Heisenberg antiferromagnet; however, by $p=0.20\mathrm{}$, the in-plane Cu-Cu superexchange coupling constant and effective magnetic moment per Cu ion are both largely suppressed.

In this Letter we describe a new technique for investigating phase transitions and dynamics in interacting electron systems. This technique is based on the derivation and self-consistent solution of infinite-order conserving approximations. It provides a new approach to the study of two-particle correlations with strong frequency and momentum dependence. We use this technique to derive a low-temperature phase diagram and dynamic correlation functions for the two-dimensional Hubbard model.

The Zeeman effect on the magnetoresistance in high-temperature superconductors is investigated above the transition temperature. It is found that this effect is dominant for the positive magnetoresistance in a parallel magnetic field and the phase relaxation time ${\tau }_{\varphi }$ can be estimated by a comparison of this term with experimental data for single-crystal films. This value is consistent with an estimate from the orbital effect of the Maki-Thompson term in a perpendicular magnetic field, and a temperature dependence of ${\tau }_{\varphi }$ proportional to $\frac{1}{T}$ is obtained from the experimental data.

Phase locking of relativistic magnetrons has been achieved at power levels of ∼3 GW at 2.8 GHz, exceeding previous phase-locking power levels by 3 orders of magnitude. Two relativistic magnetrons interact directly through a short waveguide of length $l\sim \frac{n\lambda }{2}$ to allow locking. Power-density enhancement due to source coherence is directly measured in the radiation field. Phase locking occurs in ∼5 ns and is reproducible. Extension to 10-100 GW appears feasible with arrays of oscillators.

We give an approximate formula for the Lyapunov exponent of the product of $N$ independent random matrices $M_k$ all distributed according to the same probability distribution. We consider the case of a Bernoullian distribution, $M_k=A$ with probability $p$ and $M_k=B$ otherwise. The method holds fixed the number of matrices of a given type, thus reducing fluctuations. We compare our analytic result to numerical calculations of the Lyapunov exponent for matrices $A$ and $B$ related to localization in the one-dimensional Anderson model where the approximation appears very precise for a wide range of energies. We give the fluctuation spectrum for the Lyapunov exponent also in terms of our approximation.

The behavior of solitary waves has been studied both experimentally and numerically in a strongly dissipative Toda lattice. In the absence of dissipation the system is an integrable soliton system, but due to dissipation the solitary wave decreases, and develops a tail. Numerical simulation of the head-on collision of two solitons with different amplitudes indicates that the scattering is nevertheless purely elastic. This suggests that certain parts of soliton theory should be applicable in systems where the wave itself is not conserved.

The effective interaction between interfaces or membranes is renormalized by thermally excited shape fluctuations of these surfaces. For a large class of interactions, this renormalization leads to a complex phase diagram which is governed by an unusual renormalization-group flow. This flow exhibits a line of renormalization-group fixed points and leads to essential singularities and nonuniversal critical exponents; it must, however, be distinguished from the well-known Kosterlitz-Thouless flow since it has a parabolic character.

The proper classical limit $\hslash \to 0$ of the quantum Bethe-Ansatz thermodynamics gives a new set of integral equations for the thermodynamics of the classical massive-Thirring-sine-Gordon model. The theory contains the thermodynamic information about breathers and solitons, and reproduces the transfer-matrix-method result for arbitrary coupling constant.