Because of the recently raised suspicion about the correctness of the conventional (axial) identification of ${}_{}{}^3{}_{}{}^{}\mathit{A}$, we have studied the flow properties of the axiplanar state, the only other possible candidate for the A phase. We show that for small ‘‘planar’’ mixture, both phases have very similar flow properties. However, the superflow of the axiplanar phase is always more stable than that of the axial phase, in qualitative (though not yet quantitative) agreement with current experiments. We also propose a method to distinguish these two states clearly using flow experiments.

A surface-tension-gradient-induced growth phenomenon is reported in lipid monolayer films. The surface tension gradient is introduced to the monolayer film by local, subtle heating using the microscope light. This surface tension gradient is responsible for compressing the illuminated area continuously and induces the growth of domains of liquid condensed phase. The evolution of morphology from fractal to faceted and then to dendritic patterns is investigated by in situ observations. The mechanism of pattern formation in this system is discussed.

We have observed macroscopic island (macroisland) formation in Stranski-Krastanov growth of Ge on Si(001) surfaces under various growth conditions using transmission electron microscopy. The interplay between surface morphological evolution and defect formation during growth was revealed. We show that the nucleation of macroislands is predominantly heterogeneous. Furthermore, evidence is presented that macroisland formation is mediated by a particular defect which results from the coalescence of small faceted islands.

We have performed resistivity and near-ir absorption measurements on ${\mathrm{NiI}}_2$ under pressure that strongly suggests that closure of the charge-transfer energy gap is the fundamental cause of the insulator-to-metal transition in ${\mathrm{NiI}}_2$. Our findings support the framework proposed by Zaanen, Sawatsky, and Allen outlining the electronic structure of transition-metal compounds.

Quasi-one-dimensional Peierls systems with quantum and thermal lattice fluctuations can be modeled by a Dirac-type equation with a Gaussian-correlated off-diagonal disorder. A powerful new method gives the exact disorder-averaged Green function used to compute the optical conductivity. The strong subgap tail of the conductivity has a universal scaling form. The frequency and temperature dependence of the calculated spectrum agrees with experiments on KCP(Br) and trans-polyacetylene.

Coulomb blockade effects are investigated in lateral transport through a quantum dot defined in a two-dimensional electron gas. Tunneling through excited states of the quantum dot is observed for various tunneling barriers. It is shown that transport occurring via transitions between ground states with different numbers of electrons can be suppressed by the occupation of excited states. Measurements in a magnetic field parallel to the current give evidence for tunneling processes involving states with different spin.

Using a new giant magnetoresistance structure we measure the spin-up mean free paths of ferromagnetic metals. At room temperature we find for Co ${\lambda }^{+}$=55±4 Å, for Fe ${\lambda }^{+}$-15±2 Å, and for ${\mathrm{Ni}}_{80}$${\mathrm{Fe}}_{20}$ alloy ${\lambda }^{+}$=46±3 Å. Combining these results with the Mott picture of transport we deduce the spin-down mean free paths, and find for Co ${\lambda }^{\mathrm{-}}$≲6 Å, for Fe ${\lambda }^{\mathrm{-}}$=21±5 Å, and for ${\mathrm{Ni}}_{80}$${\mathrm{Fe}}_{20}$ ${\lambda }^{\mathrm{-}}$≲6 Å.

Electronic states of Ni(7 9 11) surfaces were studied by angle-resolved ultraviolet photoelectron spectroscopy using synchrotron radiation the existence of a surface electronic state characteristic of the step and one associated with the terrace is shown from studies of the effect of alkali metal adsorption and the polarization dependence of photoelectron spectra.

We investigate dielectric and transport properties of periodic metallic systems, when electron motion within a primitive cell is chaotic. For a large ‘‘conductance’’ of a single primitive cell, the integration over the Fermi surface provides an effective averaging which leads to the universal two-point correlation function. by identifying the quasimomenta with Aharonov-Bohm fluxes, the supermatrix approach enables us to determine the dynamic dielectric function and optical conductivity, and to describe exactly the transition from ballistic to diffusive motion. The latter manifests itself even at length scales much larger than the period of the structure.

The low lying excited states in a system of coupled layers of electrons in a gallium arsenide heterostructure are strongly affected by many-body correlations between the electrons for densities as high as ${\mathit{r}}_{\mathit{s}}$=5. For a layer spacing of less than 100 nm the function Imχ(q,ω) near q=1.9${\mathit{k}}_{\mathit{F}}$ has a large peak at small ω which is apparently a soft-mode precursor to a charge density wave instability. This new peak should be observable with present experimental techniques in realistic samples. The linear gradient of the ‘‘acoustic’’ plasmon is depressed so much by many-body effects that for ${\mathit{r}}_{\mathit{s}}$=5 it can become degenerate with the single-particle excitation spectrum for all q.

We report on observations of the magnetic flux lattice in 2H-${\mathrm{NbSe}}_2$ using the magnetic decoration technique. For low applied magnetic fields and with the field applied parallel to the c axis of the crystal, we find a disordered flux lattice with short-ranged positional and orientational order. As the tilt angle of the applied magnetic field increases, we find that the orientational order in the flux lattice grows dramatically. At high tilt angles, we find a hexatic solid with long-ranged orientational order but only short-ranged positional order. This is the first observation of a flux lattice in which the orientational order is stabilized by changing the tilt angle of the applied magnetic field in an anisotropic superconductor.

Evidence of a vortex-glass transition in amorphous ${\mathrm{Mo}}_3$Si films on sapphire is manifested by the universal critical exponents (ν≊2/3,z≊3) obtained from dc electrical transport measurements. The exponents are consistent with those found in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ single crystals with random point defects, indicating that the vortex-galss transition in both systems belongs to the same universality class. In contrast, ac transport measurements from 100 Hz to 3 MHz suggest that the dominant vortex dynamics in amorphous ${\mathrm{Mo}}_3$Si thin films is diffusion, and that in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ single crystals it is the critical relaxation of thermally induced dislocations.

We have observed, by 2D positron annihilation, the ridge-shaped Fermi surface related to the chains in the compound series (R)${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ for R=Y, Dy, Ho, and Pr. In the case of Y we have sighted the clear signature of the ridge in the fourth Brillouin zone and have observed the effect of substituting Cu with Ni, Zn, and Al. The presence of the ridge is also seen in ${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ which has an insulating character. This shows that chains have a Fermi surface but contribute little to conductivity. This observation is discussed in view of the above alloying experiments, and in view of the recent model of Fehrenbacher and Rice.

We analyzed the symmetry of the superconducting-phase condensate of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{\mathit{x}}$ with photoemission at high energy and angular resolution. The result unambiguously demonstrates that the photohole must include non-s-wave components. Under the hypothesis of a pure d-wave symmetry, this would be identified as ${\mathit{d}}_{\mathit{x}\mathit{z}}$+${\mathit{d}}_{\mathit{y}\mathit{z}}$. Our superconducting state data are, specifically, incompatible with pure ${\mathit{d}}_{\mathit{x}}^2$-${\mathit{y}}^2$ symmetry. Mixed symmetries, however, cannot be excluded as long as they include a non-s-wave component. We specifically observe a change in the symmetry of electronic states between the normal state and the superconducting state.

We calculate the exact dynamical magnetic structure factor S(Q,E) in the ground state of a one-dimensional S=1/2 antiferromagnet with gapless free S=1/2 spinon excitations, the Haldane-Shastry model with inverse-square exchange, which is in the same low-energy universality class as Bethe’s nearest-neighbor exchange model. Only two-spinon excited states contribute, and S(Q,E) is found to be a very simple integral over these states.

The magnetic susceptibilities of polycrystalline ${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$${\mathrm{GeO}}_3$ were measured to study the effects of nonmagnetic impurities on the spin-Peierls transition. The transition temperature drastically reduces upon Zn doping and the spin-Peierls state collapses around x=0.03. The Zn doping causes another phase transition in the samples with 0.02≤x≤0.08, which proved to be spin-glass-like. The present Letter provides the first study of the spin-Peierls system with impurities.

The imaginary part of the magnetic susceptibility of an insulating glass doped with dysprosium ions (Kramers ions), measured at high frequency (680 MHz) and low temperature (14 mK), is strongly dependent on the amplitude of the electromagnetic field. This nonlinear effect, which is not observed in a similar glass doped with holmium ions (non-Kramers ions), is attributed to magnetic tunneling states.

Magnetic moments μ(N) of iron clusters in a molecular beam, with temperatures ranging from 100 to 1000 K are investigated from their Stern-Gerlach deflections. We find that at a temperature of 120 K, μ (25≤N≤130) is 3${\mathrm{\mu }}_{\mathit{B}}$ per atom, decreasing to about the bulk value (2.2${\mathrm{\mu }}_{\mathit{B}}$ per atom) near N=500. For all sizes, μ decreases with increasing temperature, and is approximately constant above a temperature ${\mathit{T}}_{\mathit{C}}$(N). For example, ${\mathit{T}}_{\mathit{C}}$(130) is about 700 K, and ${\mathit{T}}_{\mathit{C}}$(550) is about 550 K (${\mathit{T}}_{\mathit{C}}$ bulk =1043 K). Limitations of the superparamagnetic model due to rotational effects are discussed.

Stimulated optical emission from the lowest exciton state in atomically smooth semiconductor quantum wires is observed for the first time. The wires are formed by the ssT intersection of two 7 nm GaAs quantum wells. The optical emission wavelength is nearly independent of pump levels. This absence of band-gap renormalization in the laser emission indicates a marked increase in the stability of the exciton in one dimension.

The influence of a laterally inhomogeneous electrostatic potential of an adsorbate-covered surface on the resonant charge transfer in ion-surface scattering is investigated by means of the time-dependent Anderson-Newns Hamiltonian. The adsorbate-induced random modulation of the projectile orbital energy level is treated as a stochastic process, and its role in the charge transfer is evaluated analytically, in the Gaussian-process approximation, for low projectile speeds and low surface coverage by alkali atoms.