High-T(c) Superconductivity at the Interface between the CaCuO2 and SrTiO3 Insulating Oxides.

At interfaces between complex oxides it is possible to generate electronic systems with unusual electronic properties, which are not present in the isolated oxides. One important example is the appearance of superconductivity at the interface between insulating oxides, although, until now, with very low T(c). We report the occurrence of high T(c) superconductivity in the bilayer CaCuO(2)/SrTiO(3), where both the constituent oxides are insulating. In order to obtain a superconducting state, the CaCuO(2)/SrTiO(3) interface must be realized between the Ca plane of CaCuO(2) and the TiO(2) plane of SrTiO(3). Only in this case can oxygen ions be incorporated in the interface Ca plane, acting as apical oxygen for Cu and providing holes to the CuO(2) planes. A detailed hole doping spatial profile can be obtained by scanning transmission electron microscopy and electron-energy-loss spectroscopy at the O K edge, clearly showing that the (super)conductivity is confined to about 1-2 CaCuO(2) unit cells close to the interface with SrTiO(3). The results obtained for the CaCuO(2)/SrTiO(3) interface can be extended to multilayered high T(c) cuprates, contributing to explaining the dependence of T(c) on the number of CuO(2) planes in these systems.

Magnetic behavior of Ni nanoparticle films produced by two laser irradiations in vacuum.

Ni Nanoparticle assembled thin films were prepared using an unconventional approach based on the use of a secondary nanosecond (ns) ultraviolet (UV) laser irradiation interacting with the plume of ablated nanoparticles (NPs) during the femtosecond pulsed laser deposition (fs-PLD). The secondary laser beam determines the reduction of the NPs size and their dispersion by partial vaporization of the NPs during their flight from the target to the substrate. The proper selection of the time delay between fs and ns laser pulses allows the latter to interact selectively with different parts of the NPs plume, controlling, to a certain degree, the reduction of the NPs size and dispersion. Another original effect of the UV laser irradiation is the change in the deposited films topology, due to a reduction of the NP-aggregates density and size, fostering non-uniform dense assemblies of NPs with concentration well above the percolation threshold, with the consequent reduction of the influence of the exchange interactions on the macroscopic magnetic properties. The magnetic behavior of the films prepared using two laser beams with respect to that obtained in the case of fs-PLD only is characterized by higher H(c) values (up to approximately 70%) and a good compromise between the hysteresis loops squareness and moderate exchange interactions, strongly correlated with the NPs topology.

Dispersion of magnetic excitations in the cuprate La2CuO4 and CaCuO2 compounds measured using resonant x-ray scattering.

By resonant inelastic x-ray scattering in the soft x-ray regime we probe the dynamical multiple-spin correlations in the antiferromagnetic cuprates La2CuO4 and CaCuO2. High resolution measurements at the copper L3 edge allow the clear observation of dispersing bimagnon excitations. Theory based on the ultrashort core-hole lifetime expansion fits the data on these coherent spin excitations without free parameters.

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.

Direct measurement of sheet resistance Rsquare in cuprate systems: evidence of a fermionic scenario in a metal-insulator transition.

The metal-insulator transition (MIT) has been studied in Ba(0.9)Nd(0.1)CuO(2+x)/CaCuO2 ultrathin cuprate structures. Such structures allow for the direct measurement of the 2D sheet resistance R( square), eliminating ambiguity in the definition of the effective thickness of the conducting layer in high temperature superconductors. The MIT occurs at room temperature for experimental values of R(square) close to the 25.8 kOmega universal quantum resistance. All data confirm the assumption that each CaCuO2 layer forms a 2D superconducting sheet within the superconducting block, which can be described as weak-coupled equivalent sheets in parallel.

Very large purely intralayer critical current density in ultrathin cuprate artificial structures.

Ultrathin artificial high temperature superconducting structures, consisting of (Ba(0.9)Nd0.1)CuO2+x and CaCuO2 layers, were grown by pulsed laser deposition. Intralayer superconductivity at 60 K was obtained for a structure consisting of a single (CaCuO2) block sandwiched between two (Ba(0.9)Nd0.1)CuO2+x charge reservoir blocks. The purely intralayer critical current density was measured at 4.2 K and resulted to be larger than 10(8) A/cm(2). These findings clearly show that interaction between nearest neighbor (CaCuO2) layers is not essential for high T(c) superconductivity and strongly supports the physical model based on the idea that intralayer interaction alone is responsible for high temperature superconductivity.