P. G. Medaglia

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.

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.

Energy and symmetry of dd excitations in undoped layered cuprates measured by Cu L 3 resonant inelastic x-ray scattering

We measured the high-resolution Cu L-3 edge resonant inelastic x-ray scattering (RIXS) of undoped cuprates La2CuO4, Sr2CuO2Cl2, CaCuO2 and NdBa2Cu3O6. The dominant spectral features were assigned to dd excitations and we extensively studied their polarization and scattering geometry dependence. In a pure ionic picture, we calculated the theoretical cross sections for those excitations and used these to fit the experimental data with excellent agreement. By doing so, we were able to determine the energy and symmetry of Cu-3d states for the four systems with unprecedented accuracy and confidence. The values of the effective parameters could be obtained for the single-ion crystal field model but not for a simple two-dimensional cluster model. The firm experimental assessment of dd excitation energies carries important consequences for the physics of high-T-c superconductors. On the one hand, we found that the minimum energy of orbital excitation is always >= 1.4 eV, i.e. well above the mid-infrared spectral range, which leaves to magnetic excitations (up to 300 meV) a major role in Cooper pairing in cuprates. On the other hand, it has become possible to study quantitatively the effective influence of dd excitations on the superconducting gap in cuprates.

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.

Electrical transport properties of artificially layered films of [BaCuO2](2)/[(Sr,Ca)CuO2](n)

In this letter we show that SrCuO2/BaCuO2 and CaCuO2/BaCuO2 based artificial structures of very good crystallographic quality can be grown by pulsed laser deposition. We demonstrate that the most important parameters which determine the superconductive properties are the chemical composition, the oxygen growth pressure, and the CO2 contamination. A major difference is found in the behavior of resistivity between films containing Sr and Ca, respectively. Namely [BaCuO2]2/[SrCuO2]n films never show any trace of superconductivity, while [BaCuO2]2/[CaCuO2]n films result in having, for growth oxygen pressures PO2⩾0.2 mbar, a full transition with Tc (zero resistance) up to 46 K. Furthermore we demonstrate that even a tiny amount of CO2 in the oxygen growth atmosphere can suppress completely the superconducting properties of these artificial structures.

Epitaxial LiNbO3 thin films grown by pulsed laser deposition for optical waveguides

We have grown good-quality epitaxial LiNbO3 thin films on (0001) sapphire substrates using a pulsed laser deposition technique. The growth conditions permitted us to deposit c-axis oriented films avoiding the problem of the LiNb3O8 Li-deficient phase. The chemical composition of thin films was investigated by secondary ion mass spectrometry and resulted to coincide with the target composition. Guided propagation was demonstrated at 632 and 488 nm with thin films (∼1200 A).

Structure of superconducting [BaCuOx]2/[CaCuO2]n superlattices on SrTiO3(001) investigated by x-ray scattering

Structural properties of [BaCuO x ] 2 /[CaCuO 2 ] n superconducting superlattices with a number of Ca-CuO 2 layers ranging between 2 and 3 were investigated using synchrotron radiation, by coplanar (symmetrical and asymmetrical) X-ray diffraction and also in grazing incidence geometry. This extensive X-ray diffraction characterisation allowed to confirm the pseudomorphic growth of [Ba-CuO x ] 2 /[CaCuO 2 ] n superlattices on the SrTiO 3 (001)-oriented substrate. Reciprocal space maps in symmetrical configuration, i.e. by specular reflection, showed the effect of a slight miscut of the substrate on the inclination of the rods. Finally, the interface and surface morphology were investigated by diffuse scattering obtaining the in-plane and out-of-plane correlation properties of the interface roughness and these results were compared with the surface roughness measured by atomic force microscopy.

Superconducting properties of [ BaCuO x ] 2 / [ CaCuO 2 ] n artificial structures with ultrathick CaCuO 2 blocks

The electrical transport properties of

Growth, structural and electrical characterization of [(Ca,Sr)CuO2]m/[BaCuO2]n superlattices

[{\mathrm{BaCuO}}_{x}{]}_{2}/[{\mathrm{CaCuO}}_{2}{]}_{n}(\mathrm{C}\mathrm{B}\mathrm{C}\mathrm{C}\mathrm{O}\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}n)