Localization and the integer-quantized Hall effect: Diffusion constant for high Landau levels and white noise potential

The diffusion constant and the diagonal conductivity for non-interacting electrons in a two-dimensional, disordered system are studied. A homogeneous magnetic field perpendicular to the electron system is assumed. For weak short-range random potentials and high fields the Landau quantum numbern can be used as expansion parameter. In the limit of high Landau levels the system shows metallic behaviour. Corrections for finiten decrease the conductivity and indicate localized states in the whole energy band. A breakdown of the expansion and stronger localization are observed only for the lowest Landau levels if the typical experimental length scale of the quantized Hall effect is used.     

A PULSED LASER AND PULSE RADIOLYSIS STUDY OF AMPHIPHILIC CHLOROPHYLL DERIVATIVES WITH PDT ACTIVITY TOWARD MALIGNANT MELANOMA

Two amphiphilic derivatives of chlorophyll, which have high potential as photodynamic therapy sensitizers for malignant melanoma have been investigated by a combination of laser flash photolysis and pulse radiolysis. It is shown that direct excitation of monomeric forms of these molecules in both hydrophilic and hydrophobic environments produces significant yields of the corresponding triplet states, which have been characterized in terms of spectral and kinetic parameters. In both environments, scavenging of the triplets by oxygen produces singlet oxygen, O₂(^lΔ₈), with essentially unit efficiency as evidenced by time-resolved IR luminescence measurements.     

Long range one-dimensional ordering of lead phthalocyanine monolayer on InSb(1 0 0) (4 × 2)/c(8 × 2)

Sub-monolayer and monolayer of lead phthalocyanine deposited on InSb(1 0 0) (4 × 2)/c(8 × 2) surface have been investigated by scanning tunneling microscopy and low energy electron diffraction. Molecules first adsorb on the indium rows of the (4 × 2)/c(8 × 2) structure in the [1 1 0] direction and diffuse at the surface in order to form two-dimensional islands. The molecule-substrate interaction stabilizes the PbPc molecules on the In rows. It weakens the interaction between molecules located in adjacent rows resulting in numerous gliding planes between the molecular chains, in the direction parallel to the rows. At monolayer completion, a long-range one-dimensional order is adopted by the molecules in the [1 1 0] direction.     

Design of the cold atom PHARAO space clock and initial test results

In this paper we describe the cold atom clock PHARAO, designed for microgravity operation. All elements of the PHARAO engineering model have been manufactured and delivered to CNES, the French space agency. We present the clock design, its main characteristics, and initial science operation. PHARAO is one of the main components of the Atomic Clock Ensemble in Space payload that is scheduled to fly on board the International Space Station in 2010. PACS 07.87.+v; 06.30.Ft; 95.55.Sh; 32.80.Pj     

Sub-Ramsey numbers for arithmetic progressions

Letm 3 andk 1 be two given integers. Asub-k-coloring of [n] = {1, 2,...,n} is an assignment of colors to the numbers of [n] in which each color is used at mostk times. Call an arainbow set if no two of its elements have the same color. Thesub-k-Ramsey number sr(m, k) is defined as the minimumn such that every sub-k-coloring of [n] contains a rainbow arithmetic progression ofm terms. We prove that((k – 1)m ²/logmk) sr(m, k) O((k – 1)m ² logmk) asm , and apply the same method to improve a previously known upper bound for a problem concerning mappings from [n] to [n] without fixed points.Research supported in part by Allon Fellowship and by a Bat Sheva de-Rothschild grant.Research supported in part by the AKA Research Fund of the Hungarian Academy of Sciences, grant No. 1-3-86-264.     

Conductometric study of cationic and anionic complexes in propylene carbonate

Conductivity data are used to determine thermodynamic complex formation constants for cases in which both the initial electrolyte and the complexed electrolyte form ion pairs. Using the method described in the text, the complex formation constants of Li⁺, Na⁺ and K⁺ with the crown ether 18-crown-6 and of Li⁺ with the ligand triphenylphosphine oxide in propylene carbonate have been evaluated from conductance data. The complexation of AgBr in propylene carbonate solutions of n-etrabutylammonium bromide has also been studied by the measurement of molar conductivities. The results of these studies indicate that ion pairing should not be neglected, even in high permittivity solvents such as propylene carbonate, and that the ion pair association constants correlate well with structural studies on cation-crown ether molecular conformations.     

Gamma-hydroxyalkenals are oxidatively cleaved through Michael addition of acylperoxy radicals and fragmentation of intermediate beta-hydroxyperesters

Oxidative cleavage of arachidonate (C(20)) and linoleate (C(18)) phospholipids generates truncated C(8) or C(12) gamma-hydroxyalkenal phospholipids as well as C(5) or C(9) carboxyalkanoate phospholipids, which are abundant in atherosclerotic plaques. The gamma-hydroxyalkenals promote foam cell formation by scavenger receptor CD36-mediated endocytosis. The carboxyalkanoates are potent regulators of endothelial cell functions that may promote atherogenesis. We now report an unexpected biosynthetic interconnection; the carboxyalkanoates can be generated through oxidative cleavage of the gamma-hydroxyalkenals with the loss of three carbons. This unprecedented transformation is shown to involve Michael addition of an acylperoxy radical and fragmentation of the resulting beta-hydroxyperester.     

Two combinatorial problems on posets

Bialostocki proposed the following problem: Let nk2 be integers such that k|n. Let p(n, k) denote the least positive integer having the property that for every poset P, |P|p(n, k) and every Z _k -coloring f: P Z _k there exists either a chain or an antichain A, |A|=n and _aA f(a) 0 (modk). Estimate p(n, k). We prove that there exists a constant c(k), depends only on k, such that (n+k–2)²–c(k) p(n, k) (n+k–2)²+1. Another problem considered here is a 2-dimensional form of the monotone sequence theorem of Erdös and Szekeres. We prove that there exists a least positive integer f(n) such that every integral square matrix A of order f(n) contains a square submatrix B of order n, with all rows monotone sequences in the same direction and all columns monotone sequences in the same direction (direction means increasing or decreasing).     

Mössbauer conversion electron studies of tantalum metal surface layers

Conversion electrons following the Mössbauer absorption of the 6.2 keV -rays of¹⁸¹Ta were observed in backscattering geometry. Mössbauer spectra for tantalum single crystal and foil surfaces (mean-depth 330 Å) are compared with normal transmission spectra for tantalum foils (bulk). While no broadening of lines is observed for carefully polished single crystal furfaces, foil surfaces show considerably broader lines than bulk spectra. The linewidth and isomer shift indicate an increase of the concentration of absorbed residual gases at the foil surfaces. The observed dispersion term arising from the interference between photo- and conversion electrons for this E1-transition, depends only on the absorber thickness.