Phenomenology of the 0.7 conductance feature

We describe a phenomenological model for the conductance feature near 0.7×2e²/h that occurs in quantum point contacts. We focus on the transconductance at finite source–drain bias and contrast our model with the results expected from a single-particle picture. Good agreement is seen in comparing the model with experimental data, taken on ultra-low-disorder GaAs induced electron systems. Although simple, our phenomenology suggests important boundary conditions for an underlying microscopic theory.     

Operators on the Lattice of Pseudovarieties of Finite Semigroups

L (F) of pseudovarieties of finite semigroups that attempts to take full advantage of the underlying lattice structure, Auinger, Hall and the present authors recently introduced fourteen complete congruences on L (F). Such congruences provide a framework from which to study L (F) both locally and globally. For each such congruence ρ and each U∈L (F) the ρ-class of U is an interval [U ^ρ, U _ρ]. This provides a family of operators of the form U⇒Uρ on L (F) that reveal important relationships between elements of L (F). Various aspects of these operators are considered including characterizations of U ^ρ, bases of pseudoidentities for U ^ρ, instances of commutativity (U ^ρ)^σ = U ^σ)^ρ, as well as the semigroups generated by certain pairs of such operators.     

Influence of N cluster states on band dispersion in GaInNAs quantum wells

We use a modified band-anticrossing (BAC) model to investigate the band dispersion in a GaN_xAs_1-x/AlGaAs quantum well (QW) as a function of hydrostatic pressure. The band edge mass increases considerably more quickly with pressure than in the case of a GaAs/AlGaAs QW, and the subband separation also decreases significantly. We predict that the strong anticrossing interaction between the GaAs host conduction band and isolated N levels will inhibit tunnelling through the QW for a range of energy above the isolated N levels. The energy of N resonant states depends strongly on details of the local environment, giving a broader calculated distribution of N states in GaInNAs compared to GaNAs.     

Some properties of record values coming from the geometric distribution

The independence between spacings of record values and of individual record values is well known when the records come from a geometric distribution. Here we examine the form a function of two record values must have if we require independence from a lower record value. Also similar questions are examined in relation to conditional expectation and conditional variance.     

Metal-phenoxyalkanoic acid interactions—12: The crystal and molecular structure of μ-aqua-di-μ-[(2-chlorophenoxy)ethanoato-(O,O′)]-bis[(2-chlorophenoxy)-ethanoato-bis(pyridine)nickel(II)]-water (1/1)

The crystal structure of the pyridine adduct of nickel (2-chlorophenoxy)-ethanote, {[Ni₂(2-CPA)₄(py)₄(H₂O)] · H₂O}, has been determined from three dimensional X-ray data at 293 K. Crystals of the complex are triclinic, space group P$\text{1}$, with two dimers in a unit cell of dimensions a = 21.740(5), b = 14.312(7), c = 9.044(2)Å, α = 96.69(4), β = 84.99(4), γ = 101.20(3)°. The structure was solved by direct methods and refined to 0.059 for 2682 ‘observed’ reflections. The dimer consists of two face-to-face NiO₄N₂ octahedra joined at their apical positions by a bridging water molecule [Ni(1)-Ow; 2.14(1)Å; Ni(2)-Ow, 2.10(1)Å; Ni(1)-Ôw-Ni(2), 116.9(4)°]. Two bidentate carboxyl groups of the 2-CPA molecules bridge the other two corners of the triangular face while the other two 2-CPA ligands are unidentate [Ni-O, 2.05(1)Å (mean)]. The coordination about each Ni is completed by two cis-related pyridine nitrogens [Ni-N; 2.11(1)Å (mean)]     

Finite-amplitude effects on ultrasound beam patterns in attenuating media

Some problems relevant to medical ultrasonics are addressed through experimental measurements of focused, pure-tone beam patterns under quasilinear conditions where significant nonlinearities are manifested. First, measurements in water provide a comparison of the beam patterns of the fundamental and nonlinearly generated harmonics against recent theoretical predictions of others. The radial beamwidths, presence and spacing of sidelobes, axial distances to peak pressures, focal shock parameter, time-domain waveform asymmetry, and post-focal falloff of the fundamental through fifth harmonics are discussed relative to various models under preshock conditions (sigma less than 1). Second, the focused sources are placed in a more attenuating fluid to mimic the behavior of these fields in tissue. The changes in beam characteristics are examined relative to measurements at the same intensities in water, and relative to theoretical predictions. The results suggest that, given a known linear (low-intensity) focused beam pattern in water, guidelines can be followed to predict the beam pattern of the fundamental and higher harmonics at higher intensities in water, and then in attenuating media such as tissue.     

Completely regular semigroups as semigroups of partial transformations

Communicated by G. Lallement     

Green chemistry approaches for the purification of pharmaceuticals

To enable R&D of new drug products with a more green process emphasis, a full understanding of the impact of chromatography in the discovery phase of drug development is crucial. With large quantities of small scale (<1000 mg) separations typically evaluated and purified, a chromatographic technique needs to be easily integrated into workflows and readily robust. At Novartis, integration of supercritical fluid chromatography (SFC) has allowed for increases in productivity, decreases in toxic/hazardous solvent consumption and a more green approach to chromatography. The purpose of this review is to introduce the reader to new techniques available to organizations for the analysis and purification of mg-kg scale purifications that are routinely handled within discovery. Evaluation of a variety of techniques available to the industry has led to the implementation of SFC, with some of the pros of each technique noted below: An overview of enantiomeric purification methodologies is highlighted below in Table 1.