The diffusion (${\mathit{D}}_{\mathit{s}}$) of entangled and unentangled symmetric block copolymers is compared, above and below the ordering transition. The lamellar samples were not macroscopically oriented, and thus ${\mathit{D}}_{\mathit{s}}$ reflects a combination of mobilities parallel and perpendicular to the lamellar planes. For unentangled (Rouse) chains, ${\mathit{D}}_{\mathit{s}}$ is unaffected by the microstructure, whereas entangled (reptating) chains are retarded significantly; this difference reflects an interesting combination of thermodynamic and entanglement constraints. Furthermore, this retardation persists into the nominally disordered state.

Magnetic resonance and calorimetric studies of 4’-n-pentyl-4-cyanobiphenyl confined to Vycor glass, a random network of pores with average diameter ∼70 Å, are reported. Under such severely constrained ocnditions, the nematic-to-isotropic transition is replaced by a continuous evolution of orientational order in the pores. Our findings are explained by a Landau type of model where the liquid crystal is confined to independent pore segments, suggesting that the glassylike orientational order is to a large extend locally determined.

Seven structurally distinct condensed phases in the phase diagram of water-supported Langmuir monolayers are explained in the framework of the Landau theory of phase transitions. The present treatment, which covers two-dimensional hexatic and solid phases and describes the transitions between them, involves the coupling of three order parameters. One of the parameters governs the collective tilt of the molecules, while the other two describe one-dimensional ‘‘weak crystallization’’ along the bond direction and normal to it with the herringbone ordering of the molecular backbone planes.

The dynamics of a very fragile glass-forming polymer is studied by time of flight neutron scattering in a time scale from ${10}^{\mathrm{-}13}$ to ${10}^{\mathrm{-}11}$ s. We observe a crossover from a Debye-like to a non-Debye behavior at a characteristic time which is effectively independent of temperature and momentum transfer Q. We also confirm the results previously obtained about the non-Debye α relaxation in a different time scale (${10}^{\mathrm{-}11}$ to ${10}^{\mathrm{-}8}$ s). The results here reported open a new outlook for understanding the fast dynamics of glass-forming systems.

We present evidence for the existence of a prewetting transition in the mixed region of the phase diagram of the binary liquid mixture methanol-cyclohexane. Our measurements reveal the existence of a first order thin to thick film transition as the temperature is decreased towards the separation temperature ${\mathit{T}}_{\mathit{S}}$ at a temperature ${\mathit{T}}_{\mathrm{pw}}$ above ${\mathit{T}}_{\mathit{S}}$ and distinct from it. This transition is strongly hysteretic. The determined prewetting line approaches the coexistence curve tangentially. Also, the prewetting critical point is found to display 2D Ising-like criticality.

A surface structure analysis technique that provides chemical state discriminated structural information has been developed using x-ray standing wave measurements with photons in the x-ray absorption near edge structure. This near edge x-ray standing wave technique was applied to the surface structure analysis of the partially oxidized surface of a well-defined sulfur-passivated GaAs(111)B system. Oxidized sulfur atoms were about 22% of the surface sulfur atoms and were randomly distributed, while the unoxidized sulfur atoms in a Ga-S bonding state remain at the first layer As sites though they also disordered.

We have determined the diffusion constant for H on Ni(100) using the path-centroid formula for the transition rate, the embedded atom method for the potential, and the numerical quantum Monte Carlo technique. We find a distinct transition from semiclassical motion over the barrier to quantum tunneling through the barrier, in accordance with recent experiments by Lin and Gomer. The diffusion constant at high temperatures agrees with the experimental results; the transition, however, is located at too low a temperature (40 K) compared with experiment (100 K).

An effective medium theory for studying the electronic structure of the orientationally disordered ${\mathit{M}}_3$${\mathrm{C}}_{60}$ fullerides is developed and applied to study various normal properties. The theory is based on a cluster-Bethe-lattice method in which the disordered medium is modeled by a three-band Bethe lattice, into which we embed a molecular cluster whose scattering properties are treated exactly. Various single-particle properties and the freqeuncy-dependent conductivity are calculated in this model, and comparison is made with numerical calculations for disordered lattices, and with experiment.

Commuting time translations and electric translations are used for the specification of time dependent solutions of the Schrödinger equation for a Bloch electron in a periodic in time electric field. Explicit one-band expressions are found for the quasienergies and the corresponding time dependent solutions.

For the first time the tight-binding model and the recursion method are used to study the electron energy spectrum of hexagonal ice (${\mathit{I}}_{\mathit{h}}$), which is characterized by disorder in its protonic subsystem. It is shown that, if the ice rules are fulfilled, the local density of states (LDOS) does not depend upon the proton configuration, but the violation of ice rules results in the strong disturbances of LDOS. Electron energy spectra of ${\mathrm{H}}_3$${\mathrm{O}}^{+}$ and ${\mathrm{OH}}^{\mathrm{-}}$ ions are also calculated.

The weak order parameter observed below the 17.5 K transition in ${\mathrm{UrU}}_2$${\mathrm{Si}}_2$ has been variously attributed to magnetic dipoles, quadrupoles, or octupoles. By polarized neutron scattering we show that the order parameter breaks time reversal invariance and so cannot be quadrupolar or a spin nematic. Of the five dipolar and octupolar order parameters that are then allowed by symmetry, polarization analysis reveals that only the spin dipole, ${\mathit{M}}_2$, orders at the transition.

The electrical conductivity σ (extrapolated to T=0) of uncompensated Si:P indicates a crossover as a function of P concentration N at ${\mathit{N}}_{\mathrm{cr}}$ slightly above the metal-insulator transition at ${\mathit{N}}_{\mathit{c}}$. For N>${\mathit{N}}_{\mathrm{cr}}$ the exponent of σ∼(N-${\mathit{N}}_{\mathit{c}}$${)}^{\mathrm{\mu }}$ is μ≊0.64, while μ≊1.3 for ${\mathit{N}}_{\mathit{c}}$<N<${\mathit{N}}_{\mathrm{cr}}$. At ${\mathit{N}}_{\mathrm{cr}}$ dσ/dT changes sign from negative for N>${\mathit{N}}_{\mathrm{cr}}$ to positive for N<${\mathit{N}}_{\mathrm{cr}}$. $sigma— in a magnetic field also yields μ≊1. The apparent discrepancy between uncompensated and compensated semiconductors is traced back to a difference in the (nonuniversal) width of the critical region.

We present ac measurements of the diagonal conductivity ${\mathrm{\sigma }}_{\mathit{x}\mathit{x}}$, in the integer quantum Hall regime for frequency f between 0.2 and 14 GHz and temperature T≥50 mK. Re(${\mathrm{\sigma }}_{\mathit{x}\mathit{x}}$) was obtained from the measured attenuation of a coplanar transmission line on the sample surface. For f≳1 GHz, ${\mathrm{\sigma }}_{\mathit{x}\mathit{x}}$ peaks between IQHE minima broaden as f increases, roughly as (ΔB)∝${\mathit{f}}^{\gamma }$, where ΔB is the peak width. γ=0.41±0.04 for spin-split peaks, and for spin-degenerate peaks γ=0.20±0.05. We interpret our data in terms of dynamic scaling. Our γ, when compared with existing estimates of the localization length exponent ν, is consistent with assigning a dynamic exponent z=1.

Swept-frequency measurements (1–600 MHz) of the vortex resistivity in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ were performed in fields from 0.5 to 8 T. At all fields studied, both the real and imaginary parts of the resistivity display a power-law frequency dependence with exponents ${\mathrm{\alpha }}_1$ and ${\mathrm{\alpha }}_2$ that depend only on the reduced field h=H/${\mathit{H}}^{\mathrm{*}}$(T). The scaling field ${\mathit{H}}^{\mathrm{*}}$(T) is identified with the critical field for the solid-to-liquid transition. The power-law dependence and the frequency variation of the phase angle are consistent with the model of Fisher, Fisher, and Huse. In particular, we confirm that the frequency dependence of the phase angle is qualitatively distinct on boths sides of the transition. The field dependence of ${\mathrm{\alpha }}_1$ and ${\mathrm{\alpha }}_2$ provides an improved determination of the critical phase angle (65.7°) and the dynamic exponent z (3.7).

Using a high speed magneto-optic technique we have investigated the dynamics of a new flux instability in thin superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ films exposed to an external magnetic field of several ${10}^{\mathrm{-}2}$ T. The instability was nucleated by a 5 ns laser pulse, which heated a small spot of the sample in a region of high shielding currents. Two subsequent regimes in the development of the instability are discovered, which give rise to strikingly different flux distributions. The formation of flux branches, which are characteristic for the second stage, occurs on a time scale of a hundred nanoseconds.

We report a detailed comparison of the ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ and the planar ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ NMR shifts K and spin-lattice relaxation rates 1/${\mathit{T}}_1$ in oriented ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$ powder with y≃0 and y=0.37. For y=0.37, the ratio R of the O relaxation rate to the Y relaxation rate increases with decreasing temperature above ${\mathit{T}}_{\mathit{c}}$ and ${\mathit{T}}_1$TK at Y sites is not constant, which suggests that the antiferromagnetic spin correlations between adjacent ${\mathrm{CuO}}_2$ layers grow with decreasing temperature. The magnitude of R in both y=0.37 and y≃0 is smaller than evaluated from an ionic model of the hyperfine Hamiltonian.

The absorption coefficient of liquid He droplets is reported in the energy range from the first electronic excitation up to the ionization limit. It is obtained from total fluorescence yield curves. The size of He droplets is varied between approximately ${10}^6$ atoms/particle to small clusters containing on the order of 50 atoms. The character of the excited states is discussed in view of different theoretical approaches, i.e., molecular excitations, Frenkel and Wannier type excitons. It turned out that the Frenkel and Wannier model being very successful for the description of excited states in insulators has severe shortcomings in the case of liquid He. Furthermore, the implications for the structural peculiarities of liquid He are addressed.

I argue that the separation of the optical response into nonreciprocal (magnetic) and reciprocal parts can be unequivocally done in a microscope designed to image magnetic structure, only when the source and the detector satisfy the condition of the exact backscattering. Usually the condition is not under control, and the magneto-optic setup may display a magnetic mirage—a spurious picture of a random magnet. The contrast of the speckle-type mirage created by imperfections in the object increases together with the spatial resolution. The pseudomagnetic rotation angle is estimated.

The changes of the nonlinear optical response by single atoms adsorbed on free-electron-like metal surfaces are calculated in the low-frequency limit. Density-functional theory and an extension of the atom+jellium chemisorption model by Lang and Williams allowing for the presence of an external static field is used. The results enable a detailed discussion of the mechanisms responsible for the characteristic effects of adsorbates on second-harmonic generation as evident from recent experiments on metal surfaces.

We report experimental results from a high power X-band traveling wave tube amplifier desgined to eliminate sidebands due to reflections from its output. The amplifier has a very low energy velocity, such that the time it takes a wave to be reflected from the output to the input is of the order of, or greater than, the electron beam pulse duration. The bandwidth of the output spectrum is limited by the very narrow passband of the periodic structure. The amplifier has been operated at power levels of up to 160 MW at 9 GHz for pulse durations of 50 ns.