Thickness-dependent thermal properties of amorphous insulating thin films measured by photoreflectance microscopy

In this work, we report on the measurement of the thermal conductivity of thin insulating films of SiO2 obtained by thermal oxidation, and Al2O3 grown by atomic layer deposition (ALD), both on Si wafers. We used photoreflectance microscopy to determine the thermal properties of the films as a function of thickness in the 2 nm to 1000 nm range. The effective thermal conductivity of the Al2O3 layer is shown to decrease with thickness down to 70% for the thinnest layers. The data were analyzed upon considering that the change in the effective thermal conductivity corresponds to an intrinsic thermal conductivity associated to an additional interfacial thermal resistance. The intrinsic conductivity and interfacial thermal resistance of SiO2 were found to be equal to 0.95 W/m·K and 5.1 × 10− 9 m2K/W respectively; those of Al2O3 were found to be 1.56 W/m·K and 4.3 × 10− 9 m2K/W

Oxygen non-stoichiometry in Ru-1212 and Ru-1222 magnetosuperconductors

Here we report the results of thermogravimetric (TG) analysis on the oxygen non-stoichiometry of RuSr2GdCu2O8 (Ru-1212) and RuSr2(Gd0.75Ce0.25)2Cu2O10(Ru-1222) samples. With TG annealings carried out in O2 and Ar atmospheres it was found that the oxygen content in Ru-1212 remains less affected upon various annealings, while for Ru-1222 wider-range oxygen-content tuning is possible. When heated in H2/Ar atmosphere the both phases release oxygen upon breaking down to mixtures of metals (Ru and Cu) and binary oxides (CeO2, Gd2O3, and SrO) in two distinct steps around 300 and 450 oC. This reductive decomposition reaction carried out in a thermobalance was utilized in precise oxygen content determination for these phases. It was found that the 100-atm O2-annealed Ru-1212 sample was nearly stoichiometric, while the similarly treated Ru-1222 sample was clearly oxygen deficient. X-ray absorption near-edge (XANES) spectroscopy was applied to estimate the valence of Ru in the samples. In spite of the fact that the Ru-1212 phase was shown to possess less oxygen-deficient RuO2 layer, the valence of Ru as probed with XANES was found to be lower in Ru-1212 than that in Ru-1222.Comment: 11 pages text, 4 pages Figs. To ISS 2002 YOKOHAMA for PHYSICA

Dimensionality-driven insulator–metal transition in A-site excess non-stoichiometric perovskites

Coaxing correlated materials to the proximity of the insulator–metal transition region, where electronic wavefunctions transform from localized to itinerant, is currently the subject of intensive research because of the hopes it raises for technological applications and also for its fundamental scientific significance. In general, this tuning is achieved by either chemical doping to introduce charge carriers, or external stimuli to lower the ratio of Coulomb repulsion to bandwidth. In this study, we combine experiment and theory to show that the transition from well-localized insulating states to metallicity in a Ruddlesden-Popper series, La0.5Srn+1−0.5TinO3n+1, is driven by intercalating an intrinsically insulating SrTiO3 unit, in structural terms, by dimensionality n. This unconventional strategy, which can be understood upon a complex interplay between electron–phonon coupling and electron correlations, opens up a new avenue to obtain metallicity or even superconductivity in oxide superlattices that are normally expected to be insulators

Superconducting coupling across a spin-filtering manganite tunnel barrier with magnetic disorder

Ferromagnetic insulator Josephson junctions consisting of Nb/Au/Pr0.8Ca0.2MnO3/La1.85Sr0.15CuO4 layers were fabricated. Non-linear current-voltage characteristics suggest the presence of superconducting coupling between the Nb/Au and La1.85Sr0.15CuO4 layers across a ferromagnetic Pr0.8Ca0.2MnO3 tunnel barrier. Tunneling spectra showed clear conductance peaks due to quasi-particle tunneling. Superconducting gap energies determined from the tunneling spectra were consistent with the temperature dependence of the critical current of the junctions. We argue that magnetic inhomogeneity in the ultrathin tunnel barrier plays a role in linking the superconducting states in Nb/Au and La1.85Sr0.15CuO4