Nonlinear evolution of surface morphology in InAs/AlAs superlattices via surface diffusion

Continuum simulations of self-organized lateral compositional modulation growth in InAs/AlAs short-period superlattices on InP substrate are presented. Results of the simulations correspond quantitatively to the results of synchrotron x-ray diffraction experiments. The time evolution of the compositional modulation during epitaxial growth can be explained only including a nonlinear dependence of the elastic energy of the growing epitaxial layer on its thickness. From the fit of the experimental data to the growth simulations we have determined the parameters of this nonlinear dependence. It was found that the modulation amplitude don't depend on the values of the surface diffusion constants of particular elements.Comment: 4 pages, 3 figures, published in Phys. Rev. Lett. http://link.aps.org/abstract/PRL/v96/e13610

The speciation and genotyping of Cronobacter isolates from hospitalised patients

The World Health Organization (WHO) has recognised all Cronobacter species as human pathogens. Among premature neonates and immunocompromised infants, these infections can be life-threatening, with clinical presentations of septicaemia, meningitis and necrotising enterocolitis. The neurological sequelae can be permanent and the mortality rate as high as 40 – 80 %. Despite the highlighted issues of neonatal infections, the majority of Cronobacter infections are in the elderly population suffering from serious underlying disease or malignancy and include wound and urinary tract infections, osteomyelitis, bacteraemia and septicaemia. However, no age profiling studies have speciated or genotyped the Cronobacter isolates. A clinical collection of 51 Cronobacter strains from two hospitals were speciated and genotyped using 7-loci multilocus sequence typing (MLST), rpoB gene sequence analysis, O-antigen typing and pulsed- field gel electrophoresis (PFGE). The isolates were predominated by C. sakazakii sequence type 4 (63 %, 32/51) and C. malonaticus sequence type 7 (33 %, 17/51). These had been isolated from throat and sputum samples of all age groups, as well as recal and faecal swabs. There was no apparent relatedness between the age of the patient and the Cronobacter species isolated. Despite the high clonality of Cronobacter , PFGE profiles differentiated strains across the sequence types into 15 pulsotypes. There was almost complete agreement between O-antigen typing and rpoB gene sequence analysis and MLST profiling. This study shows the value of applying MLST to bacterial population studies with strains from two patient cohorts, combined with PFGE for further discrimination of strains

Surface morphology and magnetic anisotropy in (Ga,Mn)As

Atomic Force Microscopy and Grazing incidence X-ray diffraction measurements have revealed the presence of ripples aligned along the $[1\bar{1}0]$ direction on the surface of (Ga,Mn)As layers grown on GaAs(001) substrates and buffer layers, with periodicity of about 50 nm in all samples that have been studied. These samples show the strong symmetry breaking uniaxial magnetic anisotropy normally observed in such materials. We observe a clear correlation between the amplitude of the surface ripples and the strength of the uniaxial magnetic anisotropy component suggesting that these ripples might be the source of such anisotropy.Comment: 3 pages, 4 figures, 1 table. Replaced with published versio

Mn incorporation in as-grown and annealed (Ga,Mn)As layers studied by x-ray diffraction and standing-wave uorescence

A combination of high-resolution x-ray diffraction and a new technique of x-ray standing wave uorescence at grazing incidence is employed to study the structure of (Ga,Mn)As diluted magnetic semiconductor and its changes during post-growth annealing steps. We find that the film is formed by a uniform, single crystallographic phase epilayer covered by a thin surface layer with enhanced Mn concentration due to Mn atoms at random non-crystallographic positions. In the epilayer, Mn incorporated at interstitial position has a dominant effect on lattice expansion as compared to substitutional Mn. The expansion coeffcient of interstitial Mn estimated from our data is consistent with theory predictions. The concentration of interstitial Mn and the corresponding lattice expansion of the epilayer are reduced by annealing, accompanied by an increase of the density of randomly distributed Mn atoms in the disordered surface layer. Substitutional Mn atoms remain stable during the low-temperature annealing.Comment: 9 pages, 9 figure

Nickel(II), Copper(II) and Zinc(II) Complexes of 9-[2- (Phosphonomethoxy)ethyl]-8-azaadenine (9,8aPMEA), the 8-Aza Derivative of the Antiviral Nucleotide Analogue 9-[2-(Phosphonomethoxy)ethyl] adenine (PMEA). Quantification of Four Isomeric Species in Aqueous Solution

The acidity constants of the twofold protonated acyclic nucleotide analogue 9-[2-(phosphonomethoxy)- ethyl]-8-azaadenine, H2(9,8aPMEA)±, as well as the stability constants of the M(H;9,8aPMEA)+ and M(9,8aPMEA) complexes with the metal ions M2+ =Ni2+, Cu2+ or Zn2+, have been determined by potentiometric pH titrations in aqueous solution at I=0.1 M (NaNO3) and 25℃. The result for the release of the first proton from H2(9,8aPMEA)+ (pKa= 2.73), which originates from the (N1)H+ site, was confirmed by UV-spectrophotometric measurements. Application of previously determined straight-line plots of log KMM(R-PO3) versus PKH3(R-HPO3)' for simple phosph(on)ate ligands, R- PO-, where R represents a residue without an affinity for metal ions, proves that the primary binding site of 9,8aPMEA2- is the phosphonate group for all three metal ions studied. By stability constant comparisons with related ligands it is shown, in agreement with conclusions reached earlier for the Cu(PMEA) system [PMEA2-=dianion of 9-[2- (phosphonomethoxy)ethyl]adenine], that in total four different isomers are in equilibrium with each other, i.e. (i) an open isomer with a sole phosphonate coordination, M(PA)op, where PA2-=PMEA2-or 9,8aPMEA2-, (ii) an isomer with a 5-membered chelate involving the ether oxygen, M(PA)cl/o, (iii) an isomer which contains 5- and 7-membered chelates formed by coordination of the phosphonate group, the ether oxygen and the N3 site of the adenine residue, M(PA)cl/O/N3, and finally (iv) a macrochelated isomer involving N7, M(PA)cl/]N7. The Cu2+ systems of PMEA2- and 9,8aPMEA2- behave quite alike; the formation degrees for Cu(PA)op, CuM(PA)cl/O, Cu(PA)cl/O/N3 and Cu(PA)cl/N3 are approximately 16, 32, 45 and 7%, respectively, which shows that Cu(PA)cl/N7 is a minority species. In the Ni2+ and Zn2+ systems the open isomer is the dominating one followed by M(PA)cl/O, but there are indications that the other two isomers also occur to some extent

Effect of inter-wall surface roughness correlations on optical spectra of quantum well excitons

We show that the correlation between morphological fluctuations of two interfaces confining a quantum well strongly suppresses a contribution of interface disorder to inhomogeneous line width of excitons. We also demonstrate that only taking into account these correlations one can explain all the variety of experimental data on the dependence of the line width upon thickness of the quantum well.Comment: 13 pages, 8 figures, Revtex4, submitted to PR

Magnetic Proximity Effect in Perovskite Superconductor/Ferromagnet Multilayers

$\mathrm{YBa_2Cu_3O_7/La_{2/3}Ca_{1/3}MnO_3}$ superconducting/ferromagnetic (SC/FM) multilayers have been studied by neutron reflectometry. Evidence for a characteristic difference between the structural and magnetic depth profiles is obtained from the occurrence of a structurally forbidden Bragg peak in the FM state. The comparison with simulated reflectivity curves allows us to identify two possible magnetization profiles: a sizable magnetic moment within the SC layer antiparallel to the one in the FM layer (inverse proximity effect), or a ``dead'' region in the FM layer with zero net magnetic moment. The former scenario is supported by an anomalous SC-induced enhancement of the off-specular reflection, which testifies to a strong mutual interaction of SC and FM order parameters.Comment: 4 pages, 2 figures, submitted to PR