We report the first direct observation of ripplon excitations at large wave vectors. The experiments were performed on liquid${\mathrm{-}}^4$He films of atomic thickness adsorbed on a graphite substrate. Neutron-scattering techniques were used to determine the inelastic structure factor of surface (ripplon) and bulklike (phonon-roton) elementary excitations. The lowest lying mode at low wave vectors is experimentally shown to be a surface mode; its dispersion relation agrees well with the quantum-hydrodynamic theory of Edwards and Saam. The predictions of recent microscopic models are compared to the data at high energies and momenta.

A low-energy electron diffraction analysis of the clean K(110) surface has been performed at 25 K, providing a quantitative structural determination of a clean alkali-metal surface. The K(110) surface exhibits a novel surface phase consisting of a lateral shear displacement between the first two surface planes while preserving their two-dimensional periodicity. The atomistic description of this surface structure is related to that of the bulk martensitic phase transitions of the alkali metals.

We propose a model for the motion of the fluid flow, for the electric field, and for the concentration map around the branches during electrodeposition of ramified metal clusters in two-dimensional cells. This model accounts for the observed distribution of ions in the solution around the metal tips.

The behavior of singlet- and triplet-state bipolarons in liquid ammonia has been studied by quantum simulation. The singlet-state electron density is a peanut-shaped cavity structure with peaks in the distribution about 7 Å apart, whereas the triplet-state electron density consists of two distinct cavities. These findings are consistent with a large number of experimental observations and with a model proposed by Mott. In the singlet state, the dynamics of the electrons is characterized by novel hopping events.

We report the first measurement of cyclotron resonance in a metallic charge-transfer salt. Carrier pockets are observed with effective masses of 0.40${\mathit{m}}_{\mathit{e}}$ and 0.94${\mathit{m}}_{\mathit{e}}$. In contrast, masses of 2.0${\mathit{m}}_{\mathit{e}}$ and 2.4${\mathit{m}}_{\mathit{e}}$ are seen in magnetotransport. We propose that the transport masses are enhanced by electron-electron interactions, whereas, following Kohn’s theorem, the mass measured by cyclotron resonance is independent of these interactions. These measurements therefore represent the first direct gauge of the electron-electron interaction in a metallic charge-transfer salt.

We analyze the behavior of the Hall conductivity in weak magnetic fields, near the disorder-induced metal-insulator transition. Using the mapping onto the nonlinear σ model we identify a new operator which is relevant to the scaling of the Hall conductivity in weak magnetic fields. Using the Wilson-Polyakov renormalization-group procedure we find that the perturbative behavior of the Hall conductivity, which is observable in two dimensions in the weakly localized regime, is not simply related to the critical behavior of the Hall conductivity. Our theory predicts that the Hall number vanishes at the mobility edge with an exponent g=1 in agreement with several experimental observations.

A high-resolution Laplace-transform deep-level-transient spectroscopy has been used to study electon emission from the DX centers related to group-IV (silicon) and group-VI (tellurium) donor elements in ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As (0.25<x<0.76). This provides experimental evidence that substitutional-interstitial atom motion is responsible for DX behavior and for the associated metastability effects. The atom which is subjected to this transition is, for DX(Si), silicon itself, and so only one group of peaks is observed in the spectra; while for DX(Te), it can be either gallium or aluminum, producing two groups of peaks.

A Landauer formula for the current through a region of interacting electrons is derived using the nonequilibrium Keldysh formalism. The case of proportionate coupling to the left and right leads, where the formula takes an especially simple form, is studied in more detail. Two particular examples where interactions give rise to novel effects in the current are discussed: In the Kondo regime, an enhanced conductance is predicted, while a suppressed conductance is predicted for tunneling through a quantum dot in the fractional quantum Hall regime.

Momentum transfer due to electron-electron interaction between two quasi-two-dimensional electron gases, whose edges are separated by a distance d, is studied at low temperatures (T). The coupling between the gases results from the direct Coulomb (screened) interaction and from the interaction mediated by virtual phonon exchange. The former has a ${\mathit{T}}^2$ dependence and varies approximately as ${\mathit{d}}^{\mathrm{-}4}$ for large d. The latter depends very weakly on d and, combined with the former, accounts extremely well for the T and d dependences of the relaxation time as measured by Gramila et al. [Phys. Rev. Lett. 66, 1216 (1991)].

The spin-orbit scattering cross section of many different impurities (Cu, Zn, Ga, Ag, Cd, In, Sn, Sb, Au, Tl, Pb, Bi) in Mg is measured by means of weak localization. We find two distinct dependences of ${\mathrm{\sigma }}_{\mathrm{so}}$. (i) For the same valence of the impurity ${\mathrm{\sigma }}_{\mathrm{so}}$ shows a powerlaw as a function of the atomic number Z as ${\mathit{Z}}^{\mathit{p}}$ with p≃5 for the noble metals, and (ii) ${\mathrm{\sigma }}_{\mathrm{so}}$ depends on the valence of the impurity; for s impurities ${\mathrm{\sigma }}_{\mathit{s}\mathit{o}}$ is considerably smaller than for p impuruties. This suggests that the atomic electronic wave functions are quite well preserved after dissolving the atom.

A novel approach to the superconducting Hall effect is developed, based on the opposing drift of the thermally excited quasiparticles. These collide quasielastically with the hydrodynamic superfluid velocity field circulating far outside the core of a vortex, thereby transferring momentum to the latter. The predicted Hall angle, by BCS theory, is of the order of ${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$ divided by the Fermi energy, has sign opposite to that in the normal state, because of the backflow, and disappears at low temperature.

We study the properties of interacting flux lines with Monte Carlo simulation for parameters appropriate for the high-temperature superconductor Ba-Sr-Ca-Cu-O. A nonlocal potential suggested by Sudbo/ and Brandt is used. The pair-correlation function and its melting behavior are discussed.

We have measured the stress-strain properties of whiskerlike samples of the high-${\mathit{T}}_{\mathit{c}}$ material ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{\mathit{x}}$. We observe an anomalous temperature dependence of Young’s modulus (Y) around 300 K. The magnitude of Y falls by a factor of 3–4 between 270 and 330 K. We also observe hysteresis in the stress-strains curves in this same temperature range. The stress and strain relax toward the middle of the hysteresis loop with a characteristic time of approximately 20 sec. All these phenomena are reproducible as either the stress or temperature is cycled.

Current-voltage characteristics of uniform Al wire networks with various values of frustration index f are found to be consistent with a transition to a fully superconducting (R=0) state. Scaling of these characteristics, usually indicative of a continuous transition, is found to occur at special irrational f values such as the inverse golden mean ${\mathrm{\tau }}^{\mathrm{-}1}$ but not at nearby low-order rationals. London-loop simulations for these cases suggest a self-similar ground state which is most likely never attained due to the presence of nearly degenerate disordered states such as those found in glasses.

Symmetries of the Hubbard model in the large-U limit are discussed. WE show that the global SU(2) symmetry of the full even-lattice Hubbard model generated by Yang’s η-pairing operators becomes a local symmetry in the large-U limit and is applicable to even-odd mixed lattices. This extends the well-known SU(2) local symmetry of Anderson, for which we construct the generators. The associated local gauge theory of the extended symmetry can be similarly formulated and has interesting physical implications: new mean-field theories with parity-changed pairings. We point out the importance of searching for the parity-dependent Aharanov-Bohm-type of effects related to these local gauge symmetries.

Bulk-sensitive, polarization-dependent oxygen K- and copper L-edge absorption spectra of single-domain ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ with ${\mathrm{CuO}}_2$ planes tilted off the sample surface were measured. A novel experimental configuration was utilized to achieve complete polarization geometry and eliminate optical path variations. Contrary to previous measurements, our data show for both intrinsic and doped holes a significant amount of O2${\mathit{p}}_{\mathit{z}}$ character, likely associated with apical O, but a very weak Cu3${\mathit{d}}_{3\mathit{z}}^2$-${\mathit{r}}^2$ character. The quantitative results impose strong contraints on superconductivity models invoking out-of-plane orbitals, and suggest that the apical O 2${\mathit{p}}_{\mathit{z}}$ orbital may play an important role in high-${\mathit{T}}_{\mathit{c}}$ phenomena.

We have observed the direct vibrational excitation (v=0→v=1) of ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ molecules colliding with a Cu(111) surface. The dependence of this process on kinetic energy is found to be similar for both molecules, resembling the function determined for dissociative chemisorption of molecules in the v=0 state. This suggests that translational-vibrational coupling is strong only when molecules closely approach the dissociation barrier.

We report the observation of toroidal and higher genus vesicles of diacetylenic phospholipids, a class of polymerizable amphiphiles. When unpolymerized, the vesicles exhibit different toroidal shapes in quantitative agreement with recent theoretical predictions. When partially polymerized, only a specific family of shapes has been observed: the Clifford torus or the branch of nonaxisymmetric shapes obtained by its conformal transformations. Assuming that partially polymerized vesicles are permeable on short time scale, we give a physical explanation of our findings. We also report the results of a variational calculation which approximates the nonaxisymmetric shape problem for finite spontaneous curvature.

The electric-field-induced structures of small glass spheres in silicone oil produce laser diffraction patterns which verify, for the first time, the body-centered-tetragonal lattice structure predicted by theory. By a mechanism totally unlike that of conventional x-ray scattering, the incident laser beam propagates through the lattice of close-packed spheres via stable optic modes along regular arrays of transparent spheres, and then produces diffraction patterns after exiting from the lattice.

The mean-square displacement 〈Δ${\mathit{r}}^2$(τ)〉 of particles in concentrated suspensions is measured at times sufficiently short to observe the transient nature of hydrodynamic interactions. For all volume fractions φ, the velocity autocorrelation function decays as a power law R(τ)∼${\mathrm{\tau }}^{\mathrm{-}3/2}$. A remarkable scaling with φ is observed for the time-dependent self-diffusion coefficient ${\mathit{D}}_{\mathit{s}}$(τ)=〈Δ${\mathit{r}}^2$(τ)〉/6τ: If ${\mathit{D}}_{\mathit{s}}$(τ) is scaled by its asymptotic value and if time is scaled by a viscous time inversely proportional to the shear viscosity of the suspension, all the data fall onto a single master curve.