G. Cristiani

Orbital reconstruction and covalent bonding at an oxide interface.

Orbital reconstructions and covalent bonding must be considered as important factors in the rational design of oxide heterostructures with engineered physical properties. We have investigated the interface between high-temperature superconducting (Y,Ca)Ba2Cu3O7 and metallic La0.67Ca0.33MnO3 by resonant x-ray spectroscopy. A charge of about –0.2 electron is transferred from Mn to Cu ions across the interface and induces a major reconstruction of the orbital occupation and orbital symmetry in the interfacial CuO2 layers. In particular, the Cu d3z2–r2 orbital, which is fully occupied and electronically inactive in the bulk, is partially occupied at the interface. Supported by exact-diagonalization calculations, these data indicate the formation of a strong chemical bond between Cu and Mn atoms across the interface. Orbital reconstructions and associated covalent bonding are thus important factors in determining the physical properties of oxide heterostructures.

Quantitative Comparison of Mixed Conducting SOFC Cathode Materials by Means of Thin Film Model Electrodes

Geometrically well-defined model electrodes have been employed to unambiguously elucidate the individual resistive and capacitive processes of various solid oxide fuel cell cathodes by means of impedance spectroscopy. The measurements were performed on dense, thin film-type microelectrodes of La 1-x Sr x Co 1-y Fe y O 3-δ and related perovskite-type materials prepared by pulsed laser deposition and photolithography. It was found that the substitution of the A-site cation La in La 1-x Sr x Co 1-y Fe y O 3-δ by Sm and especially by Ba leads to a strong enhancement of the surface exchange kinetics, whereas a variation of the Co/Fe ratio between 0 and 1 has only little effect on this quantity at temperatures around 750°C. Furthermore, it has been studied how the electrochemical activation effect, i.e., the strong reduction of the surface exchange resistance after application of a large dc bias, depends on composition.

Dimensionality Control of Electronic Phase Transitions in Nickel-Oxide Superlattices

The structure of metal-oxide superlattices is used to control the electronic order of the system. The competition between collective quantum phases in materials with strongly correlated electrons depends sensitively on the dimensionality of the electron system, which is difficult to control by standard solid-state chemistry. We have fabricated superlattices of the paramagnetic metal lanthanum nickelate (LaNiO3) and the wide-gap insulator lanthanum aluminate (LaAlO3) with atomically precise layer sequences. We used optical ellipsometry and low-energy muon spin rotation to show that superlattices with LaNiO3 as thin as two unit cells undergo a sequence of collective metal-insulator and antiferromagnetic transitions as a function of decreasing temperature, whereas samples with thicker LaNiO3 layers remain metallic and paramagnetic at all temperatures. Metal-oxide superlattices thus allow control of the dimensionality and collective phase behavior of correlated-electron systems.

The geometry dependence of the polarization resistance of Sr-doped LaMnO3 microelectrodes on yttria-stabilized zirconia

Abstract Impedance spectroscopic studies and I–V measurements are performed at Sr-doped LaMnO3 (LSM) microelectrodes in order to elucidate the mechanism of the oxygen-reduction reaction on yttria-stabilized zirconia. The geometry dependence of the polarization resistance was investigated by systematic variations of the microelectrodes size and thickness. The relation between the resistance and the electrode geometry turns out to be bias-dependent: in the cathodic regime and close to equilibrium, the resistance is proportional to the inverse electrode area. Moreover, measurements without bias revealed an almost linear dependence of the resistance on the electrode thickness. This suggests that the relevant oxygen reduction path involves the transport of oxide ions through the bulk of the LSM cathode. In the anodic regime, however, the resistance becomes proportional to the inverse three-phase boundary length and, hence, a mechanism involving the LSM surface is most probable with a step close to the three-phase boundary being rate limiting. Experiments performed on LSM microelectrodes with thin alumina “discs” beneath the LSM to partly block the oxygen ion transport through the bulk of the electrode support this interpretation.

Orbital reflectometry of oxide heterostructures

The occupation of d orbitals controls the magnitude and anisotropy of the inter-atomic electron transfer in transition-metal oxides and hence exerts a key influence on their chemical bonding and physical properties. Atomic-scale modulations of the orbital occupation at surfaces and interfaces are believed to be responsible for massive variations of the magnetic and transport properties, but could not thus far be probed in a quantitative manner. Here we show that it is possible to derive quantitative, spatially resolved orbital polarization profiles from soft-X-ray reflectivity data, without resorting to model calculations. We demonstrate that the method is sensitive enough to resolve differences of ~3% in the occupation of Ni e(g) orbitals in adjacent atomic layers of a LaNiO(3)-LaAlO(3) superlattice, in good agreement with ab initio electronic-structure calculations. The possibility to quantitatively correlate theory and experiment on the atomic scale opens up many new perspectives for orbital physics in transition-metal oxides.

Cuprate/manganite superlattices. A model system for a bulk ferromagnetic superconductor

Abstract YBa 2 Cu 3 O 7 /La 0.67 Ca 0.33 MnO 3 superlattices (SLs) and heterostructures have been grown by pulsed laser deposition with individual layer thickness ranging from 4 to 200 unit cells for the YBa 2 Cu 3 O 7 and 10–500 unit cells for the La 0.67 Ca 0.33 MnO 3 . Whereas simple heterostructures (single layer La 0.67 Ca 0.33 MnO 3 and single layer YBa 2 Cu 3 O 7 50 nm thickness each) reproduce the intrinsic properties of the constituent material rather well with reduced critical temperatures for the phase transitions (Curie temperature 250 K, superconducting transition at T =70 K) the critical temperatures systematically vary with the SL composition due to coupling between the layers observed in the SLs. This systematic rules out a simple decoupling of the individual layers. Tentatively we ascribe the composition dependent changes of the critical temperatures to interaction effects at the electronic level.

Giant superconductivity-induced modulation of the ferromagnetic magnetization in a cuprate–manganite superlattice

Artificial multilayers offer unique opportunities for combining materials with antagonistic orders such as superconductivity and ferromagnetism and thus to realize novel quantum states. In particular, oxide multilayers enable the utilization of the high superconducting transition temperature of the cuprates and the versatile magnetic properties of the colossal-magnetoresistance manganites. However, apart from exploratory work, the in-depth investigation of their unusual properties has only just begun. Here we present neutron reflectometry measurements of a [Y(0.6)Pr(0.4)Ba(2)Cu(3)O(7) (10 nm)/La(2/3)Ca(1/3)MnO(3) (10 nm)](10) superlattice, which reveal a surprisingly large superconductivity-induced modulation of the vertical ferromagnetic magnetization profile. Most surprisingly, this modulation seems to involve the density rather than the orientation of the magnetization and is highly susceptible to the strain, which is transmitted from the SrTiO(3) substrate. We outline a possible explanation of this unusual superconductivity-induced phenomenon in terms of a phase separation between ferromagnetic and non-ferromagnetic nanodomains in the La(2/3)Ca(1/3)MnO(3) layers.

Orbital control of noncollinear magnetic order in nickel oxide heterostructures.

We have used resonant x-ray diffraction to develop a detailed description of antiferromagnetic ordering in epitaxial superlattices based on two-unit-cell thick layers of the strongly correlated metal LaNiO3. We also report reference experiments on thin films of PrNiO3 and NdNiO3. The resulting data indicate a spiral state whose polarization plane can be controlled by adjusting the Ni d-orbital occupation via two independent mechanisms: epitaxial strain and spatial confinement of the valence electrons. The data are discussed in light of recent theoretical predictions.

Uniaxial magnetic anisotropy and magnetic switching in La0.67Sr0.33MnO3 thin films grown on vicinal SrTiO3(100)

La0.67Sr0.33MnO3 ultrathin films grown on vicinal SrTiO3(100) surface show an in-plane uniaxial magnetic anisotropy with easy axis along the substrate atomic steps generated by a 10° miscut off the (100) plane. Over a large angular range, the angular dependence of magnetic switching field is found to obey the 1/cos φ law, indicating that the magnetic reversal is completed by a 180° domain nucleation and sweeping along the easy axis. However, when the applied field is perpendicular to the hard axis (φ=90°), the magnetization reversal is found to be well described by the Stoner–Wohlfarth model, in which the magnetization coherently rotates from the easy axis to hard axis.

Element Specific Monolayer Depth Profiling

The electronic phase behavior and functionality of interfaces and surfaces in complex materials are strongly correlated to chemical composition profiles, stoichiometry and intermixing. Here a novel analysis scheme for resonant X-ray reflectivity maps is introduced to determine such profiles, which is element specific and non-destructive, and which exhibits atomic-layer resolution and a probing depth of hundreds of nanometers.