A. Tebano

High proton conduction in grain-boundary-free yttrium-doped barium zirconate films grown by pulsed laser deposition

Reducing the operating temperature in the 500-750 °C range is needed for widespread use of solid oxide fuel cells (SOFCs). Proton-conducting oxides are gaining wide interest as electrolyte materials for this aim. We report the fabrication of BaZr(0.8)Y(0.2)O(3-δ) (BZY) proton-conducting electrolyte thin films by pulsed laser deposition on different single-crystalline substrates. Highly textured, epitaxially oriented BZY films were obtained on (100)-oriented MgO substrates, showing the largest proton conductivity ever reported for BZY samples, being 0.11 S cm(-1) at 500 °C. The excellent crystalline quality of BZY films allowed for the first time the experimental measurement of the large BZY bulk conductivity above 300 °C, expected in the absence of blocking grain boundaries. The measured proton conductivity is also significantly larger than the conductivity values of oxygen-ion conductors in the same temperature range, opening new potential for the development of miniaturized SOFCs for portable power supply.

Evidence of orbital reconstruction at interfaces in ultrathin La0.67Sr0.33MnO3 films.

Linear dichroism (LD) in x-ray absorption, diffraction, transport, and magnetization measurements on thin La(0.7)Sr(0.3)MnO(3) films grown on different substrates, allow identification of a peculiar interface effect, related just to the presence of the interface. We report the LD signature of preferential 3d-e(g)(3z(2)-r(2)) occupation at the interface, suppressing the double exchange mechanism. This surface orbital reconstruction is opposite to that favored by residual strain and is independent of dipolar fields, the chemical nature of the substrate and the presence of capping layers.

Room-Temperature Giant Persistent Photoconductivity in SrTiO3/LaAlO3 Heterostructures

SrTiO(3)/LaAlO(3) interfaces show an unprecedented photoconductivity effect that is persistent even at room temperature and giant as it gives rise to a conductivity increase of about 5 orders of magnitude at room temperature. The persistent photoconductivity effects play a paramount role in the still controversial intrinsic behavior of the SrTiO(3)/LaAlO(3) interfaces, as even a limited exposure to visible light is able to strongly modify the electrical transport properties of the interface even above room temperature, while only an appropriate thermal treatment in a dark environment can completely suppress the persistent photoconductivity effect unveiling the intrinsic conduction mechanism of the interface. Moreover, our study demonstrates that the origin of the high conductivity, revealed at the STO/LAO interface at room temperature, is purely electronic.

Growth of diamond films: General correlation between film morphology and plasma emission spectra

We have studied the emission spectra from plasmas excited in several compositions of three different gas mixtures useful for the growth of diamond films, namely CH4‐H2 (the classical one), CH4‐CO2 and C2H2‐CO2 (not previously reported by other authors). In all three cases we find the same correlation between the quality of the obtained diamond films and some spectral features: in particular, the yield of the best diamond films corresponds to the presence of the emission line at wavelength 431 nm ascribed to CH, in the absence of the emission band at 505–517 nm attributed to C2. The appearance and the progressive increase of the latter corresponds to a gradual worsening of the diamond film quality. We propose such spectral features of the plasma as a general and practical gauge of the diamond film growth conditions, for any gas mixture used.

Suppression of the metal-insulator transition temperature in thin La0.7Sr0.3MnO3 films

In this paper, we illustrate an approach to discriminate between epitaxial strain and other factors responsible for the decrease of the metal–insulator transition temperature (TP) in thin La0.7Sr0.3MnO3 films grown by pulsed laser deposition. Using this approach, we have estimated the effect of the biaxial strain on TP. Ultrathin films, independent of epitaxial strain, do not show any metal–insulator transition over the full temperature range. This finding confirms the existence of an interface dead layer. The strain-independent decrease in TP, relative to its bulk value, observed for a much wider thickness range (up to about 1000A) can most likely be attributed to oxygen deficiency.

Occurrence of a high-temperature superconducting phase in (CaCuO2)n/(SrTiO3)m superlattices

We report the occurrence of superconductivity, with maximum Tc = 40 K, in superlattices (SLs) based on two insulating oxides, namely CaCuO2 and SrTiO3. In these (CaCuO2)n/(SrTiO3)m SLs, the CuO2 planes belong only to CaCuO2 block, which is an antiferromagnetic insulator. Superconductivity, confined within few unit cells at the CaCuO2/SrTiO3 interface, shows up only when the SLs are grown in a highly oxidizing atmosphere, because of extra oxygen ions entering at the interfaces. Evidence is reported that the hole doping of the CuO2 planes is obtained by charge transfer from the interface layers, which act as charge reservoir.

Preparation and characterization of LaMnO3 thin films grown by pulsed laser deposition

We have grown LaMnO3 thin films on (001) LaAlO3 substrates by pulsed laser deposition. X-ray diffraction confirms that the films are only slightly relaxed and are oriented “square on square” relative to the substrate. The measured Raman spectra closely resemble that observed in bulk LaMnO3, which indicates no relevant distortions of the MnO6 octahedra induced by the epitaxial strain. Therefore, no detectable changes in the lattice dynamics occurred in our LaMnO3 strained films relative to the bulk case. Mn55 nuclear magnetic resonance identifies the presence of localized Mn4+ states. Superconducting quantum interference device magnetization measures TN=131(3)K and a saturation moment μ=1.09μB∕Mn, revealing a small concentration of Mn4+ and placing our films within the antiferromagnetic insulating phase.

Compositional and spectroscopic study of the growth of diamond films from several gaseous mixtures

A critical experimental test of the empirical unifying scheme recently proposed by Bachmann, Leers, and Lydtyn [Diamond and Related Mater. 1, 1 (1991)] about the gas compositions useful to grow diamond films by plasma‐assisted chemical‐vapor deposition has been performed. The data confirm the main concept of the Bachmann scheme, namely, the existence of a single compositional ‘‘diamond domain’’ in a C‐O‐H triangular diagram, in which the overall gas compositions are plotted; however, quantitatively, the shape and the borders of such a diamond domain are rather different from those assumed in the work of Bachmann and co‐workers. For all the gas mixtures investigated, the changes in the plasma optical emission spectra consequent to crossing the border from the ‘‘no‐growth zone’’ into the ‘‘diamond domain’’ have been studied. These changes show universal features, which point to the presence of the same growth mechanisms over all the diamond domain, independent of the identity of the initial chemical species.

Growth of CaCuO2 and SrxCa1 –xCuO2 epitaxial films on NdGaO3 substrates by pulsed laser deposition

High quality epitaxial films of the ‘infinite layer’(IL) compound SrxCa1 –xCuO2(x= 0–1) have been grown by pulsed laser deposition on NdGaO3(110) substrates, thus including the last member of this series CaCuO2, for the first time, to our knowledge. The quality of films varied with the composition: higher quality films were obtained for x= 0 and 0.1. The experimental results have been explained by taking into account the lattice match between film and substrate: a better match results in a better film quality. A very good match is obtained on NdGaO3 substrates for IL films with small or zero Sr content. Attempts to grow IL films were also carried out on SrTiO3 and LaAlO3 substrates. On SrTiO3 substrates it was not possible to grow IL films in the absence of Sr, while on LaAlO3 substrates no IL films could be grown at all. In the case of SrTiO3, the lattice match is good only for Sr-rich films, while in the case of LaAlO3, no good match can be obtained for any composition. Such substrate effects are not unexpected due to the importance of the substrate for the pseudomorphic stabilization of the otherwise unstable IL structure.

Fast growth of Bi2Sr2Ca2Cu3O10+x and Bi2Sr2CaCu2O8+x thin crystals at the surface of KCl fluxes

We report a novel technique to grow crystals of the high‐temperature superconductor Bi2Sr2Ca2Cu3O10+x and Bi2Sr2CaCu2O8+x (2223 and 2212 BSCCO phases), both pure and Pb substituted. The technique, based on chemical transport in a thermal gradient in molten KCl, is simple and inexpensive, and yields a continuous output of crystals, with growth times of a few minutes, i.e., orders of magnitude shorter than conventional flux methods. Depending on melt aging, crystals of either the pure 2223 or the pure 2212 BSCCO phases are obtained, having zero‐resistance temperature of 105 and 90 K, respectively, and a transport critical current density of 104 A cm−2 at liquid nitrogen temperature.