Search

The coupling between lattice vibration quanta and valence electrons can induce charge-density modulations and decisively influence the transport properties of materials, e.g., leading to conventional superconductivity. In high-critical-temperature superconductors, where electronic correlation is the main actor, the actual role of electron-phonon coupling (EPC) is being intensely debated theoretically and investigated experimentally. We present an in-depth study of how the EPC strength can be obtained directly from resonant inelastic x-ray scattering (RIXS) data through the theoretical approach derived by Ament et al. [Europhys. Lett. 95, 27008 (2011)]. The role of the model parameters (e.g., phonon energy ω0, intermediate state lifetime 1/Γ, EPC matrix element M, and detuning energy Ω) is thoroughly analyzed, providing general relations among them that can be used to make quantitative estimates of the dimensionless EPC g=(M/ω0)2 without detailed microscopic modeling. We then apply these methods to very high-resolution Cu L3-edge RIXS spectra of three Nd1+xBa2−xCu3O7−δ films. For the insulating antiferromagnetic parent compound, the value of M as a function of the in-plane momentum transfer is obtained for Cu-O bond-stretching (breathing) and bond-bending (buckling) phonon branches. For the underdoped and the nearly optimally doped samples, the effects of Coulomb screening and of charge-density-wave correlations on M are assessed. In light of the anticipated further improvements of the RIXS experimental resolution, this work provides a solid framework for an exhaustive investigation of the EPC in cuprates and other quantum materials.

A study of the interface structure of superconducting and non-superconducting CaCuO2/SrTiO3 heterostructures grown on NdGaO3(110) substrates is reported. Using the combination of high resolution x-ray reflectivity and surface diffraction, the crystallographic structure of superconducting and non-superconducting samples has been investigated. The analysis has demonstrated the excellent sharpness of the CaCuO2/SrTiO3 interface (roughness smaller than one perovskite unit cell). Furthermore, we were able to discriminate between the superconducting and the non-superconducting phase. In the former case, we found an increase of the spacing between the topmost Ca plane of CaCuO2 block and the first TiO2 plane of the overlaying STO block, relative to the non-superconducting case. These results are in agreement with the model that foresees a strong oxygen incorporation in the interface Ca plane in the superconducting heterostructures.

This paper reports on the room temperature ambipolar diffusion of oxygen vacancies and electrons in epitaxial films of pure CeO2 and 20% Sm-doped CeO2 a few tens of nanometers thick. To this aim, we fabricated heteroepitaxial structures consisting of an epitaxial ceria film sandwiched between a Pt upper electrode and a conducting substrate of Nb-doped SrTiO3 (lower electrode). The observed decay of the electrical conductivity was investigated. In particular, following this approach, we could measure room temperature diffusion coefficient in thin monocrystalline dielectric layers. While pure ceria is interesting for memory device applications, in doped ceria the resistive switching involves diffusion mechanisms that occur on much longer time scales. Under this respect, doped ceria is particularly suitable to be used in artificial neuronal devices.

This article reports on the interplay between structural properties of the interface region and emerging proton conductivity in thin films of insulating $\mathrm{BaZr}{\mathrm{O}}_{3}$ perovskite deposited by pulsed laser ablation onto $\mathrm{NdGa}{\mathrm{O}}_{3}$ wide-band-gap insulators. High-resolution transmission electron microscopy and surface x-ray diffraction reveal the presence of a large number of misfit dislocations at the interface, allowing the full relaxation of the epitaxial strain. An analysis of the x-ray diffraction patterns reveals strain relaxation that occurs over a thickness of about 3 nm, equally divided between the film and the substrate. Electrical impedance spectroscopy measurements show a sizeable proton conductance that can mainly be attributed to the defective interface. Assuming that proton conduction occurs in the interface layer, values of interface conductivity of about 0.5 S/cm at 650 \ifmmode^\circ\else\textdegree\fi{}C are estimated with an activation energy of about 0.86 eV. Experimental findings are explained assuming that the defective interface layer can accommodate a very large concentration of carriers whose mobility is somewhat hindered by the high concentration of structural defects at the interface.

Doped perovskite oxides have been widely studied in recent years as proton conducting solid electrolytes for different electrochemical devices, e.g. Solid Oxide Fuel Cells (SOFCs). By using the proton conductor as the electrolyte in a SOFC, the fuel dilution issue can be avoided due to the water formation at the cathode side instead of the anode. Among all the alkaline-earth cerates and zirconates, Y doped BaZrO3 (BZY) has been considered one of the most promising electrolyte for the SOFCs applications for its significant proton conductivity as well as chemical stability.[1] However, most reported fuel cell performance values for this material is much lower than oxide ion conducting ceramics such as YSZ owing to an unavoidable microstructure change, i.e. high grain boundary density and low degree of the crystallinity. In order to correlate the microstructure with the conductivity, we prepared BZY thin films by Pulsed Laser Deposition and discussed the effect of the microstructure on the proton conductivity. In this presentation I will show the conductivity behavior as a function of temperature and thickness by Electrochemical Impedance Spectroscopy (EIS).[2] I will focus on the interface between BZY and substrate NdGaO3 and show the cross-sectional electrochemical activity change with nanoscale resolution by Electrochemical Strain Microscopy (ESM). [3] I will highlight the interplay between the interface defects and the electrochemical performance by comparing the EIS, ESM results with Scanning Transmission Electron microscopy (STEM). I will further show the complementary study by bulk sensitive hard x-ray photoemission spectroscopy measurements which, together with Density Functional Theory calculations, demonstrate that Y and, to some extent, also Zr ions, substitute for Ba near the interface with the substrate. [1] A. D’Epifanio, E. Fabbri, E. Di Bartolomeo, S. Licoccia, E. Traversa, Fuel Cells, 1,69-762008 (2008). [2] V. Foglietti, Nan Yang, A. Tebano, C. Aruta, E. Di Bartolomeo, S. Licoccia, C. Cantoni, G. Balestrino, Applied Physics Letters 104 , 081612 (2014). [3] Nan Yang, C. Cantoni, V. Foglietti, A. Tebano, A. Belianinov, E. Strelcov, S. Jesse, D. Di Castro, E. Di Bartolomeo, S. Licoccia, S. V. Kalinin, G. Balestrino and C. Aruta, Nano Letters, 10.1021/acs.nanolett.5b00698, (2015).

Fundamental understanding of interface mechanisms that generate unexpected physical properties in the cuprate/manganite heterostructures is essential for possible applications as spintronic devices. Here, CaCuO2/La1-xSrxMnO3(CCO/LSMO) superlattices are investigated, where the infinite-layer cuprate CCO does not have apical oxygen for Cu in the Ca-plane so that the Mn-O-Cu superexchange coupling can be only present at the MnO2-(La,Sr)O-CuO(2)interface. Two different doping states for the manganites are studied, namelyx= 0.1 andx= 0.3, corresponding to the ferromagnetic insulating and ferromagnetic metallic states, respectively, in the manganite phase diagram. Linear and circular dichroism in X-ray absorption by synchrotron radiation at Cu and Mn L-edges clearly demonstrate that in the absence of apical oxygen in the cuprate block, the magnetic coupling between the LSMO and CCO is weakly ferromagnetic when LSMO is metallic, while it is antiferromagnetic when LSMO is insulating, along with an increased Cu 3d(3z(2)-r(2)) and Mn 3d(3z(2)-r(2)) orbital occupation. It is proposed that the Mn 3d valence band upward shift driven by the enhancement of Mn(3+)content in the underdoped sample enhances the orbital hybridization. The stronger hybridization at the interface MnO2-(La,Sr)O-CuO(2)gives rise to the antiferromagnetic coupling between cuprate and underdoped manganite.

Physical review materials 3(9), 094406 (2019). doi:10.1103/PhysRevMaterials.3.094406, The electronic properties of CaCuO$_2$/La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO) superlattices are determined by the electronic structure of the structural units and in particular their interfaces. The electronic structure of LSMO is governed by a metal-insulator transition, which is controlled by the thickness of the units and the sample temperature, resulting in a systematic downward band shift for metallic samples (i.e., thick LSMO units, low temperature). We present a systematic study of the changes in the valence-band structure and screening features in Mn 2p and Cu 2p core-level spectra. The results show that hybridization of Cu 3d orbitals with out-of-plane O 2p orbitals can be systematically tuned by controlling the band alignment at the interface via the metal-to-insulator transition of the LSMO units. This opens a new route to rational design of functional interfaces and control of orbital reconstructions., Published by APS, College Park, MD

Samaria-doped ceria (SDC) thin films are particularly important for energy and electronic applications such as microsolid oxide fuel cells, electrolyzers, sensors, and memristors. In this paper, we report a comparative study investigating ionic conductivity and surface reactions for well-grown epitaxial SDC films varying the samaria doping concentration. With increasing doping above 20 mol % of samaria, an enhancement in the defect association is observed by Raman spectroscopy. The role of such associated defects on the films̀ oxygen ion transport and exchange is investigated by electrochemical impedance spectroscopy and electrochemical strain microscopy (ESM). The measurements reveal that the ionic transport has a sharp maximum in ionic conductivity and drops in its activation energy down to 0.6 eV for 20 mol % doping. Increasing the doping concentration further up to 40 mol %, it raises the activation energy substantially by a factor of 2. We ascribe the sluggish transport kinetics to the "bulk" ionic-near ordering in case of the heavily doped epitaxial films. Analysis of the ESM first-order reversal curve measurements indicates that these associated defects may have a beneficial role by lowering the activation of the oxygen exchange "surface" reaction for heavily doped 40 mol % of samaria. In a model experiment, through a solid solution series of samaria doped ceria epitaxial films, we reveal that the occurrence of associated defects in the bulk affects the surface charging state of the SDC films to increase the exchange rates. The implication of these findings is the design of coatings with tuned oxygen surface exchange by controlling the bulk associated clusters for future electrocatalytic applications.

The growing demand for miniaturized systems for energy conversion and storage has prompted extensive research aimed at fabricating electrochemical devices, such as solid oxide fuel cells for portable applications (μ-SOFCs). This goal requires reducing the operating temperature by producing components as thin films. Nowadays, there is a huge gap between macroscopic device-level electrochemical techniques and microscopic electrochemical techniques to study atomic-level diffusion and transport process. Recently, a new scanning probe microscopy technique, named electrochemical strain microscopy (ESM) has been developed in order to study the structure and physical properties on the microscopic level. After applying the dc bias to the tip, the oxygen vacancies and protons injection or annihilation can occur, depending on the working conditions and the material properties. The subsequent oxygen ion or proton movement results in localized strain under the tip, which can be detected through dynamic surface displacement.[1] This technique can be very useful to investigate the electrochemical activity at the nano-scale in ionic or proton conductors. In this presentation, I will discuss several examples of high-resolution studies of pure and Sm-doped CeO2 epitaxial thin films surfaces which allowed to understand water interaction and conductivity mechanisms on the nanoscale. In particular I will show: (1) The effect of the different doping concentrations on Sm1-xCexO2-a in terms of different transport mechanism and different type of charge carriers [2]. (2) The role of granularity and porosity on the oxygen ion or proton conduction 20% Sm doped CeO2 nanocrystalline films [3]. (3) The bias dependent mechanisms of irreversible cathodic and anodic processes on a pure CeO2 film.[4] [1] A. Kumar et al. Nature Chemistry, 2011, 3,707. [2] Nan Yang et al. ACS Nano DOI: 10.1021/nn505345c (2014) [3] S. Doria, Nan Yang, et al. Appl. Phys. Lett. 103, 171605 (2013) [4] Nan Yang et al. Nanotechnology 25, 075701 (2014).

We demonstrate that combining standing-wave (SW) excitation with resonant inelastic x-ray scattering (RIXS) can lead to depth resolution and interface sensitivity for studying orbital and magnetic excitations in correlated oxide heterostructures. SW-RIXS has been applied to multilayer heterostructures consisting of a superconductor La$_{1.85}$Sr$_{0.15}$CuO$_{4}$(LSCO) and a half-metallic ferromagnet La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO). Easily observable SW effects on the RIXS excitations were found in these LSCO/LSMO multilayers. In addition, we observe different depth distribution of the RIXS excitations. The magnetic excitations are found to arise from the LSCO/LSMO interfaces, and there is also a suggestion that one of the dd excitations comes from the interfaces. SW-RIXS measurements of correlated-oxide and other multilayer heterostructures should provide unique layer-resolved insights concerning their orbital and magnetic excitations, as well as a challenge for RIXS theory to specifically deal with interface effects., Comment: 18 pages, 3 figures, plus supplemental materials (14 pages)

The asymmetry between electron and hole doping in high critical-temperature superconducting (HTS) cuprates is key information for the understanding of Cooper pair formation mechanisms. Despite intensive studies on different cuprates, a comprehensive description of related magnetic and charge excitations is still fragmentary. In the present work, artificial cuprates were used to cover the entire phase diagram within the same HTS family. In particular, Cu ${L}_{3}$-edge resonant inelastic x-ray scattering (RIXS) measurements were performed on artificial $n$- and $p$-type infinite layer (IL) epitaxial films. Beside several similarities, RIXS spectra show noticeable differences in the evolution, with doping level, of magnetic and charge intensity and damping. Compatible trends can be found in spectra measured on bulk cuprates, as well as in theoretical calculations of the spin dynamical structure factor $S(\mathbf{q},\ensuremath{\omega})$. The findings give a deeper insight into the evolution of collective excitations across the cuprate phase diagram, and on underlying general features, only connected to the doping type. Moreover, they pave the way to the exploration of general properties of HTS physics over a broad range of conditions, by means of artificial compounds not constrained by the thermodynamic limitations governing the chemical stability of bulk materials.

A systematic study by reversible and hysteretic electrochemical strain microscopy (ESM) in samples of cerium oxide with different Sm content and in several working conditions allows disclosing the microscopic mechanism underlying the difference in electrical conduction mechanism and related surface activity, such as water adsorption and dissociation with subsequent proton liberation. We have measured the behavior of the reversible hysteresis loops by changing temperature and humidity, both in standard ESM configuration and using the first-order reversal curve method. The measurements have been performed in much smaller temperature ranges with respect to alternative measuring techniques. Complementing our study with hard X-ray photoemission spectroscopy and irreversible scanning probe measurements, we find that water incorporation is favored until the doping with Sm is too high to allow the presence of Ce3+. The influence of doping on the surface reactivity clearly emerges from all of our experimental results. We find that at lower Sm concentration, proton conduction is prevalent, featured by lower activation energy and higher electrical conductivity. Defect concentrations determine the type of the prevalent charge carrier in a doping dependent manner.

The role of trivalent rare-earth dopants on the cerium oxidation state has been systematically studied by in situ photoemission spectroscopy with synchrotron radiation for 10 mol % rare-earth doped epitaxial ceria films. It was found that dopant rare-earths with smaller ionic radius foster the formation of Ce3+ by releasing the stress strength induced by the cation substitution. With a decrease of the dopant ionic radius from La3+ to Yb3+, the out-of-plane axis parameter of the crystal lattice decreases without introducing macroscopic defects. The high crystal quality of our films allowed us to comparatively study both the ionic conductivity and surface reactivity ruling out the influence of structural defects. The measured increase in the activation energy of films and their enhanced surface reactivity can be explained in terms of the dopant ionic radius effects on the Ce4+ -> Ce3+ reduction as a result of lattice relaxation. Such findings open new perspectives in designing ceria-based materials with tailored properties by choosing suitable cation substitution.

In high Tc superconductors the magnetic and electronic properties are determined by the probability that valence electrons virtually jump from site to site in the CuO2 planes, a mechanism opposed by on-site Coulomb repulsion and favored by hopping integrals. The spatial extent of the latter is related to transport properties, including superconductivity, and to the dispersion relation of spin excitations (magnons). Here, for three antiferromagnetic parent compounds (single-layer Bi2Sr0.99La1.1CuO6+delta, double-layer Nd1.2Ba1.8Cu3O6 and infinite-layer CaCuO2) differing by the number of apical atoms, we compare the magnetic spectra measured by resonant inelastic x-ray scattering over a significant portion of the reciprocal space and with unprecedented accuracy. We observe that the absence of apical oxygens increases the in-plane hopping range and, in CaCuO2, it leads to a genuine 3D exchange-bond network. These results establish a corresponding relation between the exchange interactions and the crystal structure, and provide fresh insight into the materials dependence of the superconducting transition temperature., 9 pages, 4 figures, 1 Table, 42 references

We report on the characterization of resistive switching devices based on epitaxial CeO2 thin films as a functional material. CeO2 epitaxial thin films were grown by the pulsed laser deposition technique on conductive substrates. Platinum and titanium nitride top electrodes (TE) were successively deposited. Very good performances, in terms of resistivity switching and multilevel operation capability, were obtained using the Pt TE. The dependence of the low resistance and high resistance state on the TE material and on the CeO2 film thickness were explained. The electrical characteristics of these heterostructures make them promising as synapse for neuromorphic computation, but suggest also their use with multi-valued digital systems or multibit memory cells.

Rapidly growing attention is being recently directed towards the investigation of the ionic conducting properties of oxide film hetero-structures. Experimental evidence has been reported showing that interfacial phenomena at hetero-phase interfaces give rise to faster ion conduction pathways than the bulk or homo-phase interfaces. Nonetheless, a deeper understanding of the interface transport properties is still needed to exploit these effects. In this work we have investigated the growth mechanism of different superlattices fabricated by pulsed laser deposition (PLD) coupling doped and undoped cerium and zirconium oxides. Single crystalline MgO wafers were selected as deposition substrates. The superlattice structures were obtained by means of a thin buffer layer of SrTiO3 (STO). The growth mechanism was investigated by reflection high energy electron diffraction (RHEED) and X-ray diffraction (XRD) analyses.

We measured high resolution Cu $L_3$ edge resonant inelastic x-ray scattering (RIXS) of the undoped cuprates La$_2$CuO$_4$, Sr$_2$CuO$_2$Cl$_2$, CaCuO$_2$ and NdBa$_2$Cu$_3$O$_6$. 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 them to fit the experimental data with excellent agreement. By doing so, we were able to determine the energy and symmetry of Cu-3$d$ 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 one hand, having found that the minimum energy of orbital excitation is always $\geq 1.4$ eV, i.e., well above the mid-infrared spectral range, 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., 22 pages, 11 figures, 1 table

The interface between mismatched perovskite oxides presents novel physical and chemical features of interest for the development of engineered materials with tailored properties. In this article, we report on the presence and properties of a regular network of misfit dislocations (MDs), which self-assembles at the interface between a thin film of the proton conductor BaZr 0.8 Y 0.2 O 2.9 and the substrate, the (110) oriented NdGaO 3 wide-gap insulator. The conductivity properties of this system strongly depend on the atmosphere at which a thermal annealing is carried out. Namely, a conductivity increase is observed after annealing at 450 °C in wet atmosphere. An annealing in dry environment at the same temperature brings back the conductivity to its pristine value. X-ray diffraction measurements show a strong increase of the strain field associated with the dislocation network, after annealing in wet atmosphere, suggesting that hydroxyl groups mostly accumulate in the cores of the interface MDs rather than in the bulk of the BaZr 0.8 Y 0.2 O 2.9 thin film. A further annealing in dry environment allows to recover the initial value of the strain field. The reported data indicate a strong involvement of the interface MDs in the transport properties of perovskite oxide interfaces.

Low dimensional superconducting systems have been the subject of numerous studies in the recent past, with the aim of achieving a higher and higher critical temperature (T c ). The recent improvement in film deposition techniques has allowed the realization of artificial heterostructures, with atomically flat surfaces and interfaces, where novel properties appear that are not present in the single constituent. For instance, quasi-2D superconductivity was found at the interface between different oxides. In this review we analyze, in particular, the quasi-2D superconductivity occurring at the interface between two non-superconducting oxides, mostly cuprates. Throughout a comparison of the superconducting properties of different oxide heterostructures and superlattices, we propose a phenomenological explanation of the behavior of the T c as a function of the number of conducting CuO2 planes. This is achieved by introducing two different interactions between the superconducting 2D sheets. This interpretation is finally extended also to standard high T c cuprates, contributing to the solution of the long-standing question of the dependence of T c on the number of CuO2 planes in these systems.

Thin films of samarium-oxide-doped (20 mol%) ceria (SDC) are grown by pulsed-laser deposition (PLD) on (001) MgO single-crystal substrates. SrTiO3 (STO) prepared by PLD is used as a buffer layer on the MgO substrates to enable epitaxial growth of the fluorite-structured SDC film; the STO layer provides a proper crystalline match between SDC and MgO, resulting in highly crystalline, epitaxial SDC films grown in the (001) orientation. Film conductivity is evaluated by electrochemical impedance spectroscopy measurements, which are performed at various temperatures (400–775 °C) in a wide range of oxygen partial pressure ( pO2) values (10exp-25 - 1 atm) in order to separate ionic and electronic conductivity contributions. At 700 °C, SDC/ STO films on (100) MgO exhibit a dominant ionic conductivity of about 7x10-2S cm-1, down to pO2 values of about 10-15 atm. The absence of grain boundaries make the SDC/STO/MgO heterostructures stable to oxidation-reduction cycles at high temperatures, in contrast to that observed for the more disordered SDC/STO films, which degraded after hydrogen exposure.

Substitution of Ca with Ba in CaCuO 2 increases the lattice parameter which in turn modifies all interactions between the constituent ions. Here we study the effect on the bimagnon propagating in the Cu-O planes as seen with Resonant Inelastic X-ray Scattering at the Cu-L 3 peak. The bimagnon energy is reduced by a factor of 0.67 ±0.06 while its spectral weight decreases at least by a factor of 0.5. This scaling is understood from the dependence on superexchange, which is linear in the former case and quadratic in the second. Moreover the scaling of the bimagnon energy can be understood from the distance dependence of the transfer integrals.

Electrolyte thin films of Ce0.8Gd0.2O1.9-δ (GDC20) ionic conductor were deposited by pulsed laser deposition (PLD) for the fabrication of micro solid oxide fuel cells (SOFCs). The crystal structure and morphology of the films deposited on different substrates were investigated as a function of the growth parameters, using X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) analysis. The film conductivity was measured in parallel and perpendicular directions of the film substrate using electrochemical impedance spectroscopy (EIS) measurements. Pt or La0.8Sr0.2Co0.8Fe0.2O3-δ (LSCF) films were used as electrodes in symmetrical cells. For parallel conductivity measurements, two electrodes were deposited by PLD on the surface of the film, while for perpendicular conductivity, thin film hetero- structure were grown sandwiching the electrolyte film between the two electrodes.

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 Tc. We report the occurrence of high Tc superconductivity in the bilayer CaCuO2/SrTiO3, where both the constituent oxides are insulating. In order to obtain a superconducting state, the CaCuO2/SrTiO3 interface must be realized between the Ca plane of CaCuO2 and the TiO2 plane of SrTiO3. 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 CuO2 planes. In addition, 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 CaCuO2 unit cells close to the interface with SrTiO3. The results obtained for the CaCuO2/SrTiO3 interface can be extended to multilayered high Tc cuprates, contributing to explaining the dependence of Tc on the number of CuO2 planes in these systems.

Scanning probe bias techniques have been used as a method to locally dope thin epitaxial films of La(2)CuO(4) (LCO) fabricated by pulsed laser deposition. The local electrochemical oxidation of LCO very efficiently introduces interstitial oxygen defects in the thin film. Details on the influence of the tip voltage bias and environmental conditions on the surface morphology have been investigated. The results show that a local uptake of oxygen occurs in the oxidized films.

Yttrium-doped barium zirconate (BZY) thin films recently showed surprising electric transport properties. Experimental investigations conducted mainly by electrochemical impedance spectroscopy suggested that a consistent part of this BZY conductivity is of protonic nature. These results have stimulated further investigations by local unconventional techniques. Here, we use electrochemical strain microscopy (ESM) to detect electrochemical activity in BZY films with nanoscale resolution. ESM in a novel cross-sectional measuring setup allows the direct visualization of the interfacial activity. The local electrochemical investigation is compared with the structural studies performed by state of art scanning transmission electron microscopy (STEM). The ESM and STEM results show a clear correlation between the conductivity and the interface structural defects. We propose a physical model based on a misfit dislocation network that introduces a novel 2D transport phenomenon, whose fingerprint is the low activation energy measured.

We describe the main ideas and some experimental outcomes of two experiments, both aimed to study quantum tunneling effects in high-T-C structures. In the first experiment, macroscopic quantum tunneling is demonstrated in grain boundary YBaCuO Josephson junctions revealing dissipation levels lower than expected and opening novel perspectives for quantum circuitry. In the second, the low temperature dissipation is dominated by quantum tunneling of individual Pearl vortices in ultra-thin CaBaCuO systems characterized by extremely long Pearl lengths. (c) 2006 Elsevier B.V. All rights reserved.

Pulsed laser deposition in a molecular beam epitaxy environment has been used to deposit high quality thin films of BaCuO2+x, CaCuO2, and superconducting (BaCuO2+x)(2)/(CaCuO2)(2) artificial superlattices. In situ reflection high energy electron diffraction (RHEED) has shown that the situ x-ray diffraction spectra growth mechanism is two dimensional, and ex confirmed the growth rate deduced from RHEED oscillations. The BaCuO2+xCaCuO2 films alone are not superconducting; however, in infinite layer based heterostructures the occurrence of charge transfer amongst layers containing different alkaline earth ions can give rise to superconductivity. Structural features of these heterostructures can be engineered over a wide range and, consequently, their superconducting properties studied.

Superlattices based on standard high temperature superconductors (HTS) such as YBa2Cu3O7?x and Bi2Sr2CunCan?1Ox have been extensively used to investigate many fundamental issues in HTS physics such as dimensionality, vortex dynamics, proximity effects, epitaxial strain effects, interplay between superconductivity and magnetism. Recently, a different approach based on thin layers of the (Ca, Sr, Ba)CuO2 compounds (infinite layers compounds) has been applied to the synthesis of superconducting cuprate heterostructures. Ideal infinite layer (IL) compounds alone are not superconducting; however, in IL-based heterostructures, the occurrence of charge transfer among layers containing different alkaline earth ions can give rise to superconductivity. Structural features of these heterostructures can be engineered in a wide range and, consequently, their superconducting properties are studied. The highlights of these investigations are reviewed here.

We propose a Ginzburg-Landau phenomenological model for the dependence of the critical temperature on microscopic strain in tetragonal high-[Formula: See Text] cuprates. Such a model is in agreement with the experimental results for LSCO under epitaxial strain, as well as with the hydrostatic pressure dependence of [Formula: See Text] in most cuprates. In particular, a nonmonotonic dependence of [Formula: See Text] on hydrostatic pressure, as well as on in-plane or apical microstrain, is derived. From a microscopic point of view, such results can be understood as due to the proximity to an electronic topological transition (ETT). In the case of LSCO, we argue that such an ETT can be driven by a strain-induced modification of the band structure, at constant hole content, at variance with a doping-induced ETT, as is usually assumed.

The optical phonon spectrum of the La0.7Sr0.3MnO3 pseudocubic manganite has been investigated by means of far infrared reflectance measurements on high-quality films of different thickness grown by Pulsed Laser Deposition on LaAlO3 and SrTiO3 substrates. The manganite phonon spectrum, as obtained by properly modelling the measured reflectances, does not strongly depend on substrate and film thickness d. Measurements on thick-films (100 < d < 200 nm) on SrTiO3 allow to precisely determine line profile and peak-frequency of bending (\(\nu_{B} = 336\pm 3\) cm-1) and stretching (\(\nu_{S} = 588\pm \)3 cm-1) phonons of the bulk manganite. The bending phonon is broadened by an extra component around 365 cm-1, which strongly decreases in intensity on decreasing d below 100 nm. Although this finding is not yet fully understood, the unambiguous conclusion is that thick-films on SrTiO3 are ideal samples to get detailed information on the phonon spectrum of a bulk manganite.

Spatial variability of conductivity in ceria is explored using scanning probe microscopy (SPM) with galvanostatic control. Ionically blocking electrodes are used to probe the conductivity under opposite polarities to reveal possible differences in the defect structure across a thin film of CeO2. Data suggests the existence of a large spatial inhomogeneity that could give rise to constant phase elements during standard electrochemical characterization, potentially affecting the overall conductivity of films on the macroscale. The approach discussed here can also be utilized for other mixed ionic electronic conductor (MIEC) systems including memristors and electroresistors, as well as physical systems such as ferroelectric tunneling barriers.

Given the layered structure of cuprate high-transition-temperature superconductors (HTS), essentially constituted by infinite layer (IL) blocks, containing the CuO2 planes, separated by charge reservoir (CR) blocks, the use of heterostructures and superlattices, made by stacking HTS with different magnetic and conductivity properties (metallic, superconducting, insulating), such as YBa2Cu3O7−x, PrBa2Cu3O7−x, and Bi2Sr2CunCan−1Ox, have been extensively exploited to investigate many fundamental issues in HTS physics. Successively, superconducting heterostructures made by a metallic (but non-superconducting) cuprate and an insulating cuprate have been successfully synthesized, such as CaCuO2/BaCuO2+x and La2CuO4/La2−xSrxCuO4, with the idea to mimic the intrinsic IL/CR structure of HTS, revealing the circumstance that interface effects are important for the occurrence of high T c superconductivity. The discovery of 2D electron gas at the interface between two insulating perovskites induced us to develop a new kind of heterostructures based on an insulating cuprate with IL structure, CaCuO2 (CCO), and an insulating perovskite, SrTiO3 (STO), with the idea that the 2D electron gas eventually formed at the interface could dope the CuO2 planes in CaCuO2 block, giving rise to high T c superconductivity. Indeed, CCO/STO superlattices are superconducting at T c = 50 K. The doping of CCO occurs thanks to extra oxygen ions, which are incorporated at the CCO/STO interface during the growth in strongly oxidizing conditions. The CR role is thus here played by the interface layer, and the superconductivity is confined at the interface within few CCO unit cells. Structural features of these heterostructures can be engineered in a wide range and, consequently, their superconducting properties are studied. The highlights of these investigations are reviewed here.

We propose a Ginzburg-Landau phenomenological model for the dependence of the critical temperature on microscopic strain in tetragonal high-Tc cuprates. Such a model is in agreement with the experimental results for LSCO under epitaxial strain, as well as with the hydrostatic pressure dependence of Tc in most cuprates. In particular, a nonmonotonic dependence of Tc on hydrostatic pressure, as well as on in-plane or apical microstrain, is derived. From a microscopic point of view, such results can be understood as due to the proximity to an electronic topological transition (ETT). In the case of LSCO, we argue that such an ETT can be driven by a strain-induced modification of the band structure, at constant hole content, at variance with a doping-induced ETT, as is usually assumed.

Epitaxial heterostructures consisting of an yttrium doped barium cerate, BaCe0.8Y0.2O3,(BCY) layer sandwiched between two yttrium doped barium zirconate, BaZr0.8Y0.2O3,(BZY) thin layers have been deposited on insulating (001) MgO substrates by pulsed laser deposition. The first BZY layer was aimed at improving the lattice match with the MgO substrate, the second at protecting the BCY layer. Ionic conductivity has been studied in the 300 °C–600 °C temperature range as a function of the BCY thickness in dry air and wet 5% H2 in Ar atmosphere. In both atmospheres, heterostructures showed a conductivity slightly larger than that of BCY pellets sintered under optimized conditions. Such a result has been attributed to the absence of blocking grain boundaries in the epitaxial heterostructures due to their good crystallographic quality. The BCY sandwich heterostructures showed an improved chemical stability relative to standard BCY pellets.

International audience; Here we report a structural and electronic investigation of the recently discovered CaCuO2/SrTiO3 superconducting artificial superlattices based on two insulating oxides. We show that Tc = 50 K is reached in the superlattices grown on NdGaO3 substrate under optimized growth conditions. If the superlattices are grown on LaAlO3 substrate the Tc is drastically reduced due to the much higher lattice mismatch between CaCuO2 and LaAlO3, which has lattice parameters substantially smaller than NdGaO3. The consequent in-plane lattice contraction also makes the incorporation of extra oxygen atoms more difficult, leading to a decrease of hole doping and Tc. Finally, x-ray absorption spectroscopy study as a function of the thickness of the CaCuO2 block in CaCuO2/SrTiO3 superconducting superlattices has revealed the important role played by the interfaces as the location of hole doping and superconductivity in these systems.

The in-plane (δσab(T,H)) and out-of-plane (δσc(T,H)) components of the magnetoconductivity tensor of Bi2Sr2CaCu2O8+x films have been simultaneously measured in magnetic fields parallel to the film conducting planes (B⩽1 T ) . A negative c-axis magnetoresistance is measured for the first time in this geometry. Both δσab and δσc are well described by the fluctuation theory in the temperature range Tc

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.

The structural disorder in ( Ba 2 Cu 2 O 4+x )/( CaCuO 2) n superlattices with ultrathin individual layers was investigated. Numerical simulations of x-ray spectra were carried out following a kinematical approach in a simplified model structure. The model assumes a two-dimensional layer by layer growth in which mixed composition layers can be corrugated to adjust the internal stresses due to the large mismatch between the constituent oxides. An additional random disorder is added to take into account the experimental dispersion in the amount of material deposited in each iteration. The model was successfully used to simulate x-ray spectra for samples with non integer n ranging from 2 to 16, grown in optimal conditions to obtain a superconducting behavior. The same model was used to simulate spectra of samples grown at lower pressures monitored with in-situ RHEED diagnostic. In this case, random disorder results more than one order of magnitude smaller.

EXAFS (Extended X-ray Absorption Fine Structure) and XANES (X-Ray Absorption Near Edge Structure) spectroscopies have been used to probe the local bonding of the artificially layered superconducting (BaCuO2)2/(CaCuO2)n (n = 2, 3, 4) superlattices. Fluorescence-yield measurements have been performed above the Cu and Ba K-edges. This study shows that the charge reservoir (CR) block of the Ba/Ca superlattice contains oxygen vacancies randomly distributed in the CuO2 planes and that the two apical oxygen atoms are displaced out of the Ba plane giving rise to buckled BaO planes. These oxygen atoms have a distorted pyramidal and octahedral coordination around Cu at Ba/Ca interface and in the (BaCuO2) block, respectively.

The electrical transport properties of $[{\mathrm{BaCuO}}_{x}{]}_{2}/[{\mathrm{CaCuO}}_{2}{]}_{n}(\mathrm{C}\mathrm{B}\mathrm{C}\mathrm{C}\mathrm{O}\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}n)$ underdoped high-temperature superconducting superlattices grown by pulsed laser deposition have been investigated. Starting from the optimally doped $\mathrm{C}\mathrm{B}\mathrm{C}\mathrm{C}\mathrm{O}\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}2$ superlattice, having three ${\mathrm{CuO}}_{2}$ planes and ${T}_{c}$ around 80 K, we have systematically increased the number n up to 15 moving toward the underdoped region and hence decreasing ${T}_{c}.$ For $ng11$ the artificial structures are no longer superconducting, as expected, for a uniformly distributed charge carrier density inside the conducting block layer. The sheet resistance of such artificial structures $(n\ensuremath{\approx}11)$ turns out to be quite temperature independent and close to the two-dimensional quantum resistance 26 k\ensuremath{\Omega}. A further increase of the number of ${\mathrm{CuO}}_{2}$ planes results in an insulator-type dependence of $R(T)$ in the wide range of temperatures from room temperature to 1 K. The value of the sheet resistance separating the superconducting and the insulating regimes supports the fermionic scenario of the superconductor-insulator transition in these systems.

The structural disorder in $({\mathrm{Ba}}_{2}{\mathrm{Cu}}_{2}{\mathrm{O}}_{4+x})/({\mathrm{CaCuO}}_{2}{)}_{n}$ superlattices with ultrathin individual layers consisting of a noninteger number of unit cells is investigated. X-ray diffration and reflection high energy electron diffraction show that a two-dimensional layer by layer growth occurs and that layers of mixed composition are corrugated to adjust the internal stresses. On the basis of these conclusions we can outline an explanation for the behavior of ${T}_{c}$ vs n in $({\mathrm{Ba}}_{2}{\mathrm{Cu}}_{2}{\mathrm{O}}_{4+x})/({\mathrm{CaCuO}}_{2}{)}_{n}$ superlattices.

We investigate the epitaxial properties of LiNbO 3 films grown by pulsed laser deposition on different single-crystal substrates such as SrTiO 3 (1 0 0), NdGaO 3 (1 1 0) and MgO(1 1 1) by using several X-ray diffraction methods: conventional X-ray diffraction, X-ray reciprocal space mapping and azimuthal X-ray scans. The different domain structures and mosaic spreads observed for films deposited on different substrates are explained by different crystallographic symmetry of substrate surfaces and by different lattice mismatch values. In particular, the LiNbO 3 films grown on NdGaO 3 (1 1 0) exhibit two twinned crystalline domains which are tilted in a well-defined fashion in order to minimize the strain energy and to accommodate the lattice mismatch between the film and the substrate. The films grown on SrTiO 3 (1 0 0) show four domains and a larger mosaic spread, while the films grown on MgO(1 1 1) are single oriented due to the good lattice accommodation.

Polarised extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies have been used to probe the local bonding of the infinite layer compounds CaCuO2 and SrCuO2 and of artificially layered superconducting (BaCuO2)2/(CaCuO2)n and nonsuperconducting (BaCuO2)2/(SrCuO2)2 superlattices. Fluorescence-yield measurements have been performed at T=77 K above the Cu and Ba K-edges. The XANES spectrum of CaCuO2 is described in terms of mixing of electronic configurations in the final state. EXAFS results show that the charge reservoir (CR) block of the Ba/Ca superlattice contains 0.6 oxygen vacancies per unit cell randomly distributed in the CuO2 plane sandwiched between the BaO planes and that the two apical oxygens are not in the Ba plane but are shifted toward Cu by 0.21 A. These oxygens have a pyramidal and distorted octahedral coordination with Cu at the Ba/Ca interface and in the (BaCuO2)2 block, respectively. The results clarify the origin of the oxygen doping mechanism in these artificially layered compounds.

The structure of (BaCuO 2 ) 2 /(CaCuO 2 ) 2 superconducting superlattices (SLs), obtained by the pulsed-laser deposition technique, was investigated by X-ray diffraction (XRD) and X-ray specular reflectivity (XSR). Simulations of XRD and XSR data obtained by varying the structure factor and, consequently, the chemical composition of the hypothesized structure lead to an accurate structural determination of the investigated SLs. For the (BaCuO 2 ) 2 layers of the investigated superconducting SLs we found that the copper-to-barium ratio obtained by the simulations is less than one. Moreover, the copper vacancies are localized only in the CuO 2 planes between barium atoms. Non-stoichiometric compounds with an excess of oxygen are compatible with the experimental data, indicating the Ba 2 Cu 2-x O 4-y layers to be the reservoir charge blocks' in the superconducting SLs.

During the last decade a lot of efforts have been undertaken to explain the unusual normal state properties of high temperature superconductors (HTS) in the framework of unconventional theories based on strongly interacting electrons, pre-formed Cooper pairs, polaron mechanism of super conductivity etc. A different approach to this problem would be to develop the perturbation theory for interacting electrons in the normal phase of strongly anisotropic superconductors without specifying the origin of this interaction. The Cooper channel of interelectron interaction is equivalent to the superconducting fluctuations which are unusually strong in HTS. We show that the peculiarities of such systems not only lead to the increase of the magnitude but are also frequently responsible for the change of the hierarchy of different fluctuation effects and even of the sign of the total corrections. As a result the fluctuation contributions can manifest themselves in very unusual forms. The first and well known result i...