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© 2001 The American Physical Society. M. V. was partially supported by a research grant of the Fundación Universidad Carlos III de Madrid. Financial support from CICYT Grant No. MAT99-1706E is also acknowledged. M. V. is thankful for the hospitality received during her stay at the LBNL/NCEM. W. G. acknowledges support from the Max Kade Foundation for his stay at Berkeley. Work at LBNL/NCEM was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098., Charge neutrality and stoichiometry impose severe restrictions on the mechanisms of epitaxial growth of complex oxides. The fundamental question arises of what is the minimum growth unit when sample thickness is reduced beyond the size of the unit cell. We have investigated the growth mechanism of YBa_(2)Cu_(3)O_(7) cuprate superconductor, using a consistent approach based on the growth of noninteger numbers of YBa_(2)Cu_(3)O_(7) layers in YBa_(2)Cu_(3)O_(7)/PrBa_(2)Cu_(3)O_(7) superlattices. Ex situ chemical and structural analysis evidence a 2D block-by-block mechanism in which the minimum growth units are complete unit cell blocks, growing coherently over large lateral distances., Fundación Universidad Carlos III de Madrid, CICYT, Max Kade Foundation, Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Department of Energy, Depto. de Estructura de la Materia, Física Térmica y Electrónica, Fac. de Ciencias Físicas, TRUE, pub

© 2002 The American Physical Society. We thank Professor Ivan Schuller for helpful conversations and interesting suggestions. This work was supported by Spanish CICYT MAT 2000 1468. Work at LBNL/NCEM was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098., We report on the growth mechanism of YBa_(2)Cu_(3)O_(7-x). Our study is based on an analysis of ultrathin YBa_(2)Cu_(3)O_(7-x) (YBCO) layers in c-axis-oriented YBa_(2)Cu_(3)O_(7-x)/PrBa_(2)Cu_(3)O_(7)superlattices. We have found that the release of epitaxial strain in very thin YBCO layers triggers a change in the dimensionality of the growth mode. Ultrathin epitaxially strained YBCO layers with thickness below 3 unit cells grow in a blockby-block two -dimensional mode that is coherent over large lateral distances. Meanwhile, when the thickness increases and the strain relaxes, layer growth turns three-dimensional, resulting in rougher layers and interfaces., Spanish CICYT, Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Department of Energy, Depto. de Estructura de la Materia, Física Térmica y Electrónica, Fac. de Ciencias Físicas, TRUE, pub

Low-frequency noise properties of single CuO nanowire devices were investigated under gas sensor operation conditions in dry and humid synthetic air at 350¿°C. A 1/f noise spectrum was found with the normalized power spectral density of current fluctuations typically a factor of 2 higher for humid compared to dry atmosphere. A core-shell nanowire model is proposed to treat the noise as parallel combination of gas-independent bulk and gas-dependent surface noise components. The observed increase in 1/f noise in the presence of water vapor is explained in terms of Hooge's mobility fluctuation model, where the increased surface noise component is attributed to carrier scattering at potential fluctuations due to hydroxyl groups at the nanowire surface.

The synthesis, local structure, and magnetism of lamellar iron(III) oxyhydroxide - surfactant composites prepared by two different methods have been investigated in detail. In the first method, Fe(II) solutions are oxidized by H2O2 in the presence of CnH2n+1OSO3- Na+ surfactants (n = 10, 12, 14, 16, 18), leading to lamellar composites with an inorganic wall thickness of around 28 Å. When a second method is used, namely, aging an Fe(III) solution for selected times after slightly increasing the pH with NH3 and subsequent addition of the surfactant, the inorganic wall thickness can be tuned between 19 and 26 Å, employing the same surfactants. EXAFS analysis of the Fe K edge X-ray absorption spectra reveals that the local structure of the inorganic part is a reminder of those found for the bulk iron oxyhydroxides goethite and akaganéite; that is, [Fe(O,OH)6] octahedra are predominantly connected by common edges and corners, the ratio of edge to corner sharing being similar to the mentioned bulk oxyhydroxides. Whereas coordination numbers for the first oxygen coordination shell are around 6, confirming an octahedral (or distorted octahedral) coordination around the Fe ions, coordination numbers found for the second and third Fe···Fe neighbors are low (around 2), indicating the presence of a considerable amount of vacancies around the central absorber ion or, as an alternative description, a low degree of condensation of the oxyhydroxide. Complementary to the local structural picture given by EXAFS, Mössbauer spectra elucidate the inorganic iron oxyhydroxide walls to be built up by domains of different crystallinity. The crystallinity is sensitive to the synthesis conditions used in the preparation. For example, under aging in the presence of NH3, longer aging times and higher temperatures result in a larger overall crystallinity of the inorganic part. By care