The effect of the sweeping velocity on the hysteresis loop of a bistable system is examined in the case where the velocity is so large that the system is prevented from undergoing any transition during the forward sweep. The experiments, made on optical and electronic devices, evidence a dramatic instability (‘‘hesitation’’) of the return path of the hysteresis loop for a critical value of the sweeping velocity. The main features of this generic phenomenon are well covered by a one-dimensional analytic theory which provides scaling laws in good agreement with the observations.

Langmuir monolayers and freely suspended smectic films can exhibit a spontaneous breaking of chiral symmetry. The order parameter that characterizes this symmetry breaking is coupled to variations in the direction of molecular tilt. As a result, chiral symmetry breaking leads to the spontaneous formation of complex equilibrium patterns with either 1D or 2D modifications in the direction of molecular tilt. A Landau theory for this pattern formation gives a general phase diagram, which includes a uniform nonchiral phase, a striped pattern, a square lattice, and a uniform chiral phase.

The equilibrium shape of a large crystal is uniquely determined by the anisotropic surface energy. In sharp contrast, we show here that the equilibrium shape of a small crystal is not unique, for temperatures below the roughening temperature. Instead, the degree of faceting is an oscillatory function of crystal size. Using low-energy electron microscopy, we observe oscillations in the shape of silver crystallites as they grow; we demonstrate theoretically that such oscillations with size occur even in thermodynamic equilibrium.

${\mathrm{O}}_2$ adsorbed on Pd(111) is photodesorbed and photodissociated when irradiated with 5.0 eV light. Polar and azimuthal angle-resolved time-of-flight spectra of the directly photodesorbed neutral molecules were measured. Both desorption flux and the translational energy were found to depend on the desorption angle. In particular a marked maximum in the translational energy at a polar angle of 60° with ${\mathit{C}}_{3\mathit{v}}$ symmetry is found for the superoxo species. This behavior is interpreted to be correlated with the binding geometry of the molecules before desorption.

Inelastic light scattering spectra of the one-dimensional electron gas in GaAs quantum wires embedded in a strong perpendicular magnetic field show long-wavelength collective excitations and display multiple structures that indicate the magnetoroton density of states. The observed shift of the q∼0 intersubband magnetoplasmons from the cyclotron frequency is the signature of 1D behavior. At low temperatures spin polarization of the 1D system is revealed by the exchange enhancement of spin-flip excitations.

A random-matrix formula is derived for the variance of an arbitrary linear statistic on the transmission eigenvalues. The variance is independent of the eigenvalue density and has a universal dependence on the symmetry of the matrix ensemble. The formula generalizes the Dyson-Mehta theorem in the statistical theory of energy levels. It demonstrates that the universality of the conductance fluctuations is generic for a whole class of transport properties in mesoscopic systems.

We present a theory of electron transport in novel disordered waveguides that support different numbers N and M of modes propagating in opposite directions, i.e., ‘‘quantum railroads’’ (QRR). Anderson localization and the integer quantum Hall effect are special cases of our theory. More generally, our analytic results based on scattering matrix theory and our numerical simulations show that disorder results in directed localization in QRR’s. The limiting transmittance of a macroscopic QRR is T=‖N-M‖ in the majority direction but T’=0 in the minority direction.

The diagonalization of a Hamiltonian in the subspace spanned by many quasiparticle states has been performed. The result, compared to the full Hilbert space diagonalization, shows that the anyonic picture of the hierarchy theory provides a very accurate description of the low-energy physics of the fractional quantum Hall system. The implication of this study on the equivalence between the hierarchy theory and the composite fermion theory is discussed.

We calculate the magnetic field and temperature dependence of the critical current density ${\mathit{j}}_{\mathit{c}}$ along the c axis in Josephson-coupled layered systems when the applied magnetic field is parallel to the c axis. ${\mathit{j}}_{\mathit{c}}$ decreases with field and temperature because of the thermal fluctuations of the vortices which induce a phase difference across the junctions. The anisotropy ratio γ∝1/ √${\mathit{j}}_{\mathit{c}}$ acquires a field and temperature dependence. The decoupling phase transition line above which the superconducting current along the c axis vanishes is determined.

We report ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ NMR shift (K) and spin-lattice relaxation (${\mathit{T}}_1$) data on oriented ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ powders, with x=1 and 0.64. The anisotropy of K is shown to be dominated by that of the spin susceptibility on the oxygen sites ${\mathrm{\chi }}_{\mathit{c}}^{\mathrm{}\left(\mathrm{O}\right)\mathrm{}}$/${\mathrm{\chi }}_{\mathit{a}\mathit{b}}^{\mathrm{}\left(\mathrm{O}\right)\mathrm{}}$=1.2±0.05, which agrees with that expected on the Cu sites, and with a single-spin fluid picture for the ${\mathrm{CuO}}_2$ band. The anisotropy of ${\mathit{T}}_1$ is compatible with a nearly isotropic spin lifetime in the q∼0 dynamic susceptibility. Deviations from ${\mathit{T}}_1$TK∼const are found at high T for both the underdoped (x=0.64) and the overdoped sample (x=1).

To investigate the band structure of nonmetallic ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Y}}_{\mathit{x}}$${\mathrm{Cu}}_2$${\mathrm{O}}_{\mathit{y}}$ (x=0.4–1.0), photon-induced hopping has been employed as a new method, in which transmission spectroscopy has been carried out from 10 to 12 000 ${\mathrm{cm}}^{\mathrm{-}1}$. The photon-induced-hopping conduction edge has been found at 5200 ${\mathrm{cm}}^{\mathrm{-}1}$. This edge does not shift with (x=0.4–1.0), whereas the absorption is proportional to 1-x. The charge-transfer absorption edge at 12 000 ${\mathrm{cm}}^{\mathrm{-}1}$ also does not shift with x. Using this method, the bandwidth has been determined to be 5200 ${\mathrm{cm}}^{\mathrm{-}1}$. Obvious midgap states have not been observed.

We report ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ NQR and NMR studies of low frequency spin dynamics in the paramagnetic state of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ for 0≤x≤0.15 in the temperature range up to 900 K. For x=0, the results were in good agreement with the theoretical predictions for an S=1/2 two-dimensional quantum Heisenberg antiferromagnet. For Sr-doped samples, we found that d spins exhibit, above 300 K, the characteristic behavior expected for a local moment system even for the highest-${\mathit{T}}_{\mathit{c}}$ phase ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_4$, and cross over to an itinerant regime before superconductivity sets in.

Fe(100) films and wedge-shaped overlayers were grown epitaxially onto Ag(100) and investigated in situ by means of the surface magneto-optic Kerr effect. The reversible spin reorientation transition between perpendicular and in-plane alignment at ${\mathit{T}}_{\mathit{R}}$, which is below the Curie temperature ${\mathit{T}}_{\mathit{C}}$, is characterized for critical thicknesses ${\mathit{d}}_{\mathit{R}}$ along the length of a wedge. A similar asymmetry is identified within pseudogap regions in the vicinity of both ${\mathit{T}}_{\mathit{R}}$ and ${\mathit{d}}_{\mathit{R}}$ in which the remanent magnetization is small but nonvanishing.

The spontaneous macroscopic polarization of perovskite ${\mathrm{KNbO}}_3$ is calculated as a Berry’s phase using the Bloch functions of the tetragonal crystal. The result P=0.35 C/${\mathrm{m}}^2$ supports the measured value 0.37 C/${\mathrm{m}}^2$ and implies that earlier data from hysteresis loops are too low. The polarization is linear in the ferroelectric distortion; the Born effective charges show strong variations from nominal ionic values and a large inequivalence of the O ions. Linearity posteriori demonstrates that the polarization of perovskites at finite temperature can be safely predicted assuming the ions frozen in their time-averaged positions.

We report the first direct measurements of femtosecond electronic dephasing in CdSe nanocrystals using three-pulse photon echoes and a novel mode-suppression technique. We are able to separate the dynamics of the coherently excited LO phonons from the underlying electron-hole dephasing by suppressing the quantum beats. The homogeneous linewidth of these materials at 15 K results from electronic dephasing in ∼85 fs, approximately half of which is due to acoustic phonon modes. Contributions from acoustic phonons dominate the homogeneous linewidth at room temperature.

We report a new mechanism of photoluminescence up-conversion in undoped GaAs-AlGaAs asymmetric coupled quantum wells. Under band-to-band excitation, resonant excitation of the ${\mathit{E}}_1$→${\mathit{E}}_2$ intersubband transition is shown to result in a finite nonthermal occupation of the ${\mathit{E}}_2$ subband, giving rise to an ${\mathit{E}}_2$→${\mathit{HH}}_1$ photoluminescence. The induced luminescence closely follows the polarization selection rules and the spectral dependence of the intersubband absorption. We show that this up-conversion scheme can be used to estimate the intersubband scattering time, which is found to be 2.5±0.7 ps.

Possible enhancement of (d,d) nuclear fusion rate under clusters impact has been investigated. (${\mathrm{CD}}_4$${)}_{\mathit{n}}^{+}$ cluster were sent onto deuterated polyethylene targets and protons resulting from fusion were detected. Using light clusters (n from 1 to 5) of constant velocity we measured essentially constant proton yields per incident deuteron, i.e., observed no deviation from binary (d,d) fusion cross sections. With heavier clusters (n=110,145) we could only determine upper limits of proton yields which seem to be entirely accounted for by beam contamination.

We have measured the directions and velocities of Ne, Ar, and Xe atoms scattering from perfluorinated ether and hydrocarbon liquids to probe the relationship between the microscopic roughness of liquid surfaces and gas-liquid collision dynamics. Impulsive energy transfer is governed by the angle of deflection: head-on encounters deposit more energy than grazing collisions. Many atoms scatter in the forward direction, particularly at glancing incidence. These results imply that the incoming atoms recoil locally from protruding C-H and C-F groups in hard spherelike collisions.

We present a new type of cluster algorithm that strongly reduces critical slowing down in simulations of vertex models. Since the clusters are closed paths of bonds, we call it the loop algorithm. The basic steps in constructing a cluster are the breakup and the freezing of vertices. We concentrate on the case of the F model, which is a subset of the six-vertex model exhibiting a Kosterlitz-Thouless transition. The loop algorithm is also applicable to simulations of other vertex models and of one- and two-dimensional quantum spin systems.

We bring out the connection between Berry’s geometric phase and a gauge invariant expression introduced by Bargmann. We exploit this relationship to show that the Güoy effect, which referes to the phase jump experienced by a focused beam as it crosses the caustic, is the geometric phase associated with the underlying Lobachevskian hyperbolic geometry.