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Kagome materials have emerged as a setting for emergent electronic phenomena that encompass different aspects of symmetry and topology. It is debated whether the XV$_6$Sn$_6$ kagome family (where X is a rare earth element), a recently discovered family of bilayer kagome metals, hosts a topologically non-trivial ground state resulting from the opening of spin-orbit coupling gaps. These states would carry a finite spin-Berry curvature, and topological surface states. Here, we investigate the spin and electronic structure of the XV$_6$Sn$_6$ kagome family. We obtain evidence for a finite spin-Berry curvature contribution at the center of the Brillouin zone, where the nearly flat band detaches from the dispersing Dirac band because of spin-orbit coupling. In addition, the spin-Berry curvature is further investigated in the charge density wave regime of ScV$_6$Sn$_6$, and it is found to be robust against the onset of the temperature-driven ordered phase. Utilizing the sensitivity of angle resolved photoemission spectroscopy to the spin and orbital angular momentum, our work unveils the spin-Berry curvature of topological kagome metals, and helps to define its spectroscopic fingerprint., Comment: 21 pages, 4 figures

Phase transitions are key in determining and controlling the quantum properties of correlated materials. Here, by using the combination of material synthesis and photoelectron spectroscopy, we demonstrate a genuine Mott transition undressed of any symmetry breaking side effects in the thin films of V

Perovskite-based heterostructures have recently gained remarkable interest, thanks to atomic-scale precision engineering. These systems are very susceptible to small variations of control parameters, such as two-dimensionality, strain, lattice polarizability, and doping. Focusing on the rare-earth nickelate diagram, LaNiO3 (LNO) catches the eye, being the only nickelate that does not undergo a metal-to-insulator transition (MIT). Therefore, the ground state of LNO has been studied in several theoretical and experimental papers. Here, we show by means of infrared spectroscopy that an MIT can be driven by dimensionality control in ultrathin LNO films when the number of unit cells drops to 2. Such a dimensionality tuning can eventually be tailored when a physically implemented monolayer in the ultrathin films is replaced by a digital single layer embedded in the Ruddlesden-Popper Lan+1NinO3n+1 series. We provide spectroscopic evidence that the dimensionality-induced MIT in Ruddlesden-Popper nickelates strongly resembles that of ultrathin LNO films. Our results can pave the way to the employment of Ruddlesden-Popper Lan+1NinO3n+1 to tune the electronic properties of LNO through dimensional transition without the need of physically changing the number of unit cells in thin films.

Focused Ion Beam patterning has become a widely applied technique in the last few decades in the micro- and nanofabrication of quantum materials, representing an important advantage in terms of resolution and versatility. However, ion irradiation can trigger undesired effects on the target material, most of them related to the damage created by the impinging ions that can severely affect the crystallinity of the sample, compromising the application of Focused Ion Beam to the fabrication of micro- and nanosized systems. We focus here on the case of Bi2Se3, a topological material whose unique properties rely on its crystallinity. In order to study the effects of ion irradiation on the structure of Bi2Se3, we irradiated with Ga+ ions the full width of Hall-bar devices made from thin films of this material, with the purpose of inducing changes in the electrical resistance and characterizing the damage created during the process. The results indicate that a relatively high ion dose is necessary to introduce significant changes in the conduction. This ion dose creates medium-range lateral damage in the structure, manifested through the formation of an amorphous region that can extend laterally up to few hundreds of nanometers beyond the irradiated area. This amorphous material is no longer expected to behave as intrinsic Bi2Se3, indicating a spatial limitation for the devices fabricated through this technique.

Phase transitions are key in determining and controlling the quantum properties of correlated materials. Here, by using the powerful combination of precise material synthesis and angle resolved photoelectron spectroscopy, we show evidence for a genuine Mott transition undressed of any symmetry breaking side effects in the thin-films of V$_{2}$O$_{3}$. In particular, and in sharp contrast with the bulk V$_{2}$O$_{3}$ crystals, we unveil the purely electronic dynamics approaching the metal-insulator transition, disentangled from the structural transformation that is prevented by the residual substrate-induced strain. On approaching the transition, the spectral signal evolves surprisingly slowly over a wide temperature range, the Fermi wave-vector does not change, and the critical temperature appears to be much lower than the one reported for the bulk. Our findings are on one side fundamental in demonstrating the universal benchmarks of a genuine non-symmetry breaking Mott transition, extendable to a large array of correlated quantum systems and, on the other, given that the fatal structural breakdown is avoided, they hold promise of exploiting the metal-insulator transition by implementing V$_{2}$O$_{3}$ thin films in devices.

The occurrence of oxygen-driven metal-insulator-transition (MIT) in SrNbO3 (SNO) thin films epitaxially grown on (110)-oriented DyScO3 has been reported. SNO films are fabricated by the pulsed laser deposition technique at different partial O-2 pressure to vary the oxygen content and their structural, optical, and transport properties are probed. SNO unit cell has been found to shrink vertically as the oxygen content increases but keeping the epitaxial matching with the substrate. The results of Fourier-transform infra-red spectroscopy show that highly oxygenated SNO samples (i.e., grown at high oxygen pressure) show distinct optical conductivity behavior with respect to oxygen deficient films, hence demonstrating the insulating character of the formers with respect to those fabricated with lower pressure conditions. Tailoring the optical absorption and conductivity of strontium niobate epitaxial films across the MIT will favor novel applications of this material.

The structural, electronic, and magnetic properties of Sr-hole-doped epitaxial La

The properties of half-metallic manganite thin films depend on the composition and structure in the atomic scale, and consequently, their potential functional behavior can only be based on fine structure characterization. By combining advanced transmission electron microscopy, electron energy loss spectroscopy, density functional theory calculations, and multislice image simulations, we obtained evidence of a 7 nm-thick interface layer in La

The properties of half-metallic manganite thin films depend on the composition and structure in the atomic scale, and consequently, their potential functional behavior can only be based on fine structure characterization. By combining advanced transmission electron microscopy, electron energy loss spectroscopy, density functional theory calculations, and multislice image simulations, we obtained evidence of a 7 nm-thick interface layer in La0.7Sr0.3MnO3 (LSMO) thin films, compatible with the formation of well-known dead layers in manganites, with an elongated out-of-plane lattice parameter and structural and electronic properties well distinguished from the bulk of the film. We observed, for the first time, a structural shift of Mn ions coupled with oxygen vacancies and a reduced Mn valence state within such layer. Understanding the correlation between oxygen vacancies, the Mn oxidation state, and Mn-ion displacements is a prerequisite to engineer the magnetotransport properties of LSMO thin films.

Electrostatically defined perovskite oxide heterostructures, in which carriers are confined by the modulation of the A-site ion charge, offer new possibilities of tuning the magnetic properties of manganite oxides. We investigate the preferential orientation of ferromagnetic and antiferromagnetic moments in ultrathin ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ layers embedded in antiferromagnetic $\mathrm{Sr}{\mathrm{MnO}}_{3}$ as they undergo a metal-to-insulator transition with decreasing thickness. Our results evince the role of orbital occupation, metallicity, and competition of different magnetic phases, in absence of spurious effects occurring in thin films as a result of symmetry breaking at ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ interfaces and of incorporation of oxygen vacancies.

We report on the reproducible surface topological electron states in Bi2Se3 topological insulator thin films when epitaxially grown by Pulsed Laser Deposition (PLD) on (0 0 1)-oriented SrTiO3 (STO) perovskite substrates. Bi2Se3 has been reproducibly grown with single (0 0 1)-orientation and low surface roughness as controlled by ex-situ X-ray diffraction and in situ scanning tunnel microscopy and low-energy electron diffraction. Finally, in situ synchrotron radiation angle-resolved photo-emission spectroscopy measurements show a single Dirac cone and Dirac point at E B ∼ 0.38 eV located in the center of the Brillouin zone likewise found from exfoliated single-crystals. These results demonstrate that the topological surface electron properties of PLD-grown Bi2Se3 thin films grown on (0 0 1)-oriented STO substrates open new perspectives for applications of multi-layered materials based on oxide perovskites.

In this work, we investigate the effects of the ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ structural phase transition on the magnetic properties of an amorphous magnetic thin film of CoFeB in contact with it. ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ thin films are deposited epitaxially on sapphire substrates, reaching bulklike properties after few nm of growth. By means of temperature dependent Kerr effect characterizations, we prove that crossing the ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ structural phase transition induces reproducible and reversible changes to CoFeB magnetic properties, especially to its coercive field. By decreasing the oxide layer thickness, its effects on the magnetic layer decreases, while reducing the magnetic layer thickness maximizes it, with a maximum of 330% coercive field variation found between the two ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ structural phases. By simply tuning the temperature, this systematic study shows that the engineering of ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ structural transition induces large interfacial strain and thus strong magnetic property variations to an amorphous thin film, opening wide possibilities in implementing strain-driven control of the magnetic behavior without strict requirements on epitaxial coherence at the interface.

Perovskite-based heterostructures have recently gained remarkable interest, thanks to atomic-scale precision engineering. These systems are very susceptible to small variations of control parameters, such as two-dimensionality, strain, lattice polarizability, and doping. Focusing on the rare-earth nickelate diagram, LaNiO3 (LNO) catches the eye, being the only nickelate that does not undergo a metal-to-insulator transition (MIT). Therefore, the ground state of LNO has been studied in several theoretical and experimental papers. Here, we show by means of infrared spectroscopy that an MIT can be driven by dimensionality control in ultrathin LNO films when the number of unit cells drops to 2. Such a dimensionality tuning can eventually be tailored when a physically implemented monolayer in the ultrathin films is replaced by a digital single layer embedded in the Ruddlesden–Popper Lan+1NinO3n+1 series. We provide spectroscopic evidence that the dimensionality-induced MIT in Ruddlesden–Popper nickelates strongly resembles that of ultrathin LNO films. Our results can pave the way to the employment of Ruddlesden–Popper Lan+1NinO3n+1 to tune the electronic properties of LNO through dimensional transition without the need of physically changing the number of unit cells in thin films.

$V_2O_3$ has long been studied as a prototypical strongly correlated material. The difficulty in obtaining clean, well ordered surfaces, however, hindered the use of surface sensitive techniques to study its electronic structure. Here we show by mean of X-ray diffraction and electrical transport that thin films prepared by pulsed laser deposition can reproduce the functionality of bulk $V_2O_3$. The same films, when transferred in-situ, show an excellent surface quality as indicated by scanning tunnelling microscopy and low energy electron diffraction, representing a viable approach to study the metal-insulator transition (MIT) in $V_2O_3$ by means of angle-resolved photoemission spectroscopy. Combined, these two aspects pave the way for the use of $V_2O_3$ thin films in device-oriented heterostructures.

The study of ionic materials on nanometer scale is of great relevance for efficient miniaturized devices for energy applications. The epitaxial growth of thin films can be a valid route to tune the properties of the materials and thus obtain new degrees of freedom in materials design. High crystal quality Sm

The electronic properties of strontium ruthenate SrRuO3 perovskite oxide thin filmsare modified by epitaxial strain, as determined by growing on different substrates by pulsedlaser deposition. Temperature dependence of the transport properties indicates that tensilestrain deformation of the SrRuO3 unit cell reduces the metallicity of the material as well as itsmetal-insulator-transition (MIT) temperature. On the contrary, the shrinkage of the Ru–O–Rubuckling angle due to compressive strain is counterweighted by the increased overlap of theconduction Ru-4d orbitals with the O-2p ones due to the smaller interatomic distances resulting intoan increased MIT temperature, i.e., a more conducting material. In particular, in the more metallicsamples, the core level X-ray photoemission spectroscopy lineshapes show the occurrence of anextra-peak at the lower binding energies of the main Ru-3d peak that is attributed to screening,as observed in volume sensitive photoemission of the unstrained material.

The electronic properties of anatase titanium dioxide (${\mathrm{Ti}\mathrm{O}}_{2}$) thin films epitaxially grown on ${\mathrm{La}\mathrm{Al}\mathrm{O}}_{3}$ substrates are investigated by synchrotron-x-ray spectroscopy [x-ray absorption spectroscopy (XAS), x-ray photoemission spectroscopy (XPS), and angle-resolved photoemission spectroscopy (ARPES)] and infrared spectroscopy. The ${\mathrm{Ti}}^{3+}$ fraction in ${\mathrm{Ti}\mathrm{O}}_{2\ensuremath{-}x}$ is varied either by changing the oxygen pressure during deposition or by postgrowth annealing in ultrahigh vacuum (UHV). Structural investigation of the ${\mathrm{Ti}\mathrm{O}}_{2}$ thin films provides evidence of highly uniform crystallographic order in both as-grown and in situ UHV-annealed samples. The increased amount of ${\mathrm{Ti}}^{3+}$ as a consequence of UHV annealing is calibrated by in situ XPS and XAS analysis. The as-grown ${\mathrm{Ti}\mathrm{O}}_{2}$ samples, with a low $\mathrm{Ti}$${}^{3+}$ concentration, show distinct electronic properties with respect to the annealed films, namely, absorption in the midinfrared (MIR) region correlated with polaron formation, and another peak in the visible range at 1.6 eV correlated with the presence of localized defect states (DSs). With the increasing level of ${\mathrm{Ti}}^{3+}$ induced by the postannealing process, the MIR peak disappears, while the DS peak is redshifted to the near-infrared region at about 1.0 eV. These results indicate the possibility of tailoring the optical absorption of anatase ${\mathrm{Ti}\mathrm{O}}_{2}$ films from the visible to the near-infrared region.

Two-dimensional (2D) metallic states induced by oxygen vacancies (${V}_{O}\mathrm{s}$) at oxide surfaces and interfaces provide opportunities for the development of advanced applications, but the ability to control the behavior of these states is still limited. We used angle resolved photoelectron spectroscopy combined with density-functional theory (DFT) to study the reactivity of ${V}_{O}$-induced states at the (001) surface of anatase ${\mathrm{TiO}}_{2}$, where both 2D metallic and deeper lying in-gap states (IGs) are observed. The 2D and IG states exhibit remarkably different evolutions when the surface is exposed to molecular ${\mathrm{O}}_{2}$: while IGs are almost completely quenched, the metallic states are only weakly affected. DFT calculations indeed show that the IGs originate from surface ${V}_{O}\mathrm{s}$ and remain localized at the surface, where they can promptly react with ${\mathrm{O}}_{2}$. In contrast, the metallic states originate from subsurface vacancies whose migration to the surface for recombination with ${\mathrm{O}}_{2}$ is kinetically hindered on anatase ${\mathrm{TiO}}_{2}$ (001), thus making them much less sensitive to oxygen dosing.

International audience; We report the integration of high-quality epitaxial La 2/3 Sr 1/3 MnO 3 (LSMO) thin films onto SrTiO 3 buffered Silicon-on-Sapphire (SOS) substrates by combining state-of-the-art thin film growth techniques such as molecular beam epitaxy and pulsed laser deposition. Detailed structural, magnetic and electrical characterizations of the LSMO/STO/SOS heterostructures show that the LSMO film properties are competitive with those directly grown on oxide substrates. X-ray magnetic circular dichroism measurements on Mn L 2,3 edges show strong dichroic signal at room temperature, and angular-dependent in-plane magnetic properties by magneto-optical Kerr magnetometry reveal isotropic magnetic anisotropy. Suspended micro-bridges were thus finally fabricated by silicon micromachining, thus demonstrating the potential use of integrating LSMO magnetic layer on industrially compatible SOS substrates for the development of applicative MEMS devices.

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.

We report the study of anatase TiO2(001)-oriented thin films grown by pulsed laser deposition on LaAlO3(001). A combination of in situ and ex situ methods has been used to address both the origin of the Ti(3+)-localized states and their relationship with the structural and electronic properties on the surface and the subsurface. Localized in-gap states are analyzed using resonant X-ray photoelectron spectroscopy and are related to the Ti(3+) electronic configuration, homogeneously distributed over the entire film thickness. We find that an increase in the oxygen pressure corresponds to an increase in Ti(3+) only in a well-defined range of deposition pressure; outside this range, Ti(3+) and the strength of the in-gap states are reduced.

The electronic properties of hole- and electron-doped manganites were probed by a combination of x-ray absorption and photoemission spectroscopies. Hole-doped La0.7Ba0.3MnO3 and electron-doped La0.7Ce0.3MnO3 thin films were epitaxially grown on SrTiO3 substrates by means of pulsed laser deposition. Ex-situ x-ray diffraction demonstrated the substrate/film epitaxy relation and in-situ low energy electron diffraction provided evidence of high structural order of film surfaces. By combining synchrotron x-ray absorption and x-ray photoemission spectroscopy, evidence of Mn ions into a 2+ state as a result of the Ce 4 + substitution in the electron-doped manganites was provided. Angular resolved photo-emission spectroscopy (ARPES) results showed a predominance of z2-orbitals at the surface of both hole- and, unexpectedly, electron-doped manganites thus questioning the validity of the commonly accepted scenario describing the electron filling in manganites’ 3d orbitals in oxide manganites. The precise determination of the electronic and orbital properties of the terminating layers of oxide manganites paves the way for engineering multi-layered heterostructures thus leading to novel opportunities in the field of quantum electronics.

The study of ionic materials on nanometer scale is of great relevance for efficient miniaturized devices for energy applications. The epitaxial growth of thin films can be a valid route to tune the properties of the materials and thus obtain new degrees of freedom in materials design. High crystal quality SmxCe1-xO2-δ films are here reported at a high doping level up to x = 0.4, thanks to the good lattice matching with the (110) oriented NdGaO3 substrate. X-ray diffraction and transmission electron microscopy demonstrate the ordered structural quality and absence of Sm segregation at the macroscopic and atomic level, respectively. Therefore, in epitaxial thin films, the homogeneous doping can be obtained even with the high dopant content not always approachable in bulk form, getting even an improvement of the structural properties. In situ spectroscopic measurements by X-ray photoemission and X-ray absorption show the O 2p band shift toward the Fermi level, which can favor the oxygen exchange and vacancy formation on the surface when the Sm doping is increased to x = 0.4. X-ray absorption spectroscopy also confirms the absence of ordered oxygen vacancy clusters and further reveals that the 5d eg and t2g states are well separated by the crystal field in the undistorted local structure even in the case of a high doping level up to x = 0.4.

International audience; The crystal structural quality and the strain induced by the substrate strictly impose the magnetic and transport properties of La0.67Sr0.33MnO3 (LSMO) films. In particular, the magnetic anisotropy (MA) of epitaxial LSMO can be finely tuned by varying its thickness and by choosing single crystal substrates with suitable lattice mismatch with the film. Here, we have deposited LSMO films with thicknesses in the 12-50 nm range by pulsed laser deposition on different single crystal substrates inducing either compressive or tensile in-plane strain on the manganites. The epitaxial quality of films was quantified by ω-scans around (002) peak with full-width half-maximum (FWHM) values as low as 0.08° for films on the nearly matched NGO (110) substrate to 1.4° films on high mismatched MgO (001) substrate. As the epitaxial strain in thin-film increases, a significant reduction in metal-insulation transition (MIT) temperature (Tp) was observed. The magnetic properties of the films probed by Kerr magnetometry show that the symmetry of the room temperature MA varies significantly as a function of both strain and thickness. Specifically, we observed pure uniaxial MA on NGO (110) and pure biaxial MA on STO buffered MgO (001), whereas a spin reorientation from uniaxial in-plane to out-of-plane on LSAT (001) and uniaxial to nearly isotropic in-plane on STO (001) substrate as the film thickness is increased. We provide an efficient tool to tune the MA according to the specific spintronic application targeted.

We have grown untwinned YBa2Cu3O7-delta (YBCO) films on (110) MgO substrates that were preannealed at high temperature in oxygen atmosphere. The annealing results in surface reconstruction with shallow facets, which induce the suppression of the YBCO twinning domains, and the preferential alignment of the CuO chains along one of the in-plane directions of the substrate. Because of the large mismatch between the in-plane lattice parameters of film and substrate, the strain induced by the MgO into the YBCO layer is strong and very peculiar. The YBCO film is compressed, with respect to the bulk, and presents a unidirectional buckling of the atomic planes, along the chains' direction, due to a deformation of the copper-oxygen octahedra. The YBCO films, which can be grown with thicknesses down to few unit cells and oxygen doping levels spanning most of the superconducting dome, are patterned into nanowires with dimensions down to 50 nm. The anisotropies due to the untwinning state are preserved in these structures; moreover, additional anisotropies appear in ultrathin structures where strain effects become more pronounced. Such untwinned and compressively strained films can therefore be used as a platform to study the interplay between strain and the various local orders in the normal state of YBCO.

We studied the effect of naturally formed homointerfaces on the magnetic and electric transport behavior of a heavily twinned, 40 nm thick, pseudomorphic epitaxial film of La 0.7 Sr 0.3 MnO 3 deposited by molecular beam epitaxy on ferroelastic LaAlO 3 (001) substrate. As proved by high resolution X-ray diffraction analysis, the lamellar twin structure of the substrate is imprinted in La 0.7 Sr 0.3 MnO 3 . In spite of the pronounced thermomagnetic irreversibility in the DC low field magnetization, spin-glass-like character, possibly related to the structural complexity, was ruled out, on the base of AC susceptibility results. The magnetic characterization indicates anisotropic ferromagnetism, with a saturation magnetization M s = 3.2 μ B /Mn, slightly reduced with respect to the fully polarized value of 3.7 μ B /Mn. The low field DC magnetization vs temperature is non bulklike, with a two step increase in the field cooled M FC ( T ) branch and a two peak structure in the zero field cooled M ZFC ( T ) one. Correspondingly, two peaks are present in the resistivity vs temperature ρ ( T ) curve. With reference to the behavior of epitaxial manganites deposited on bicrystal substrates, results are discussed in terms of a two phase model, in which each couple of adjacent ferromagnetic twin cores, with bulklike T C = 370 K, is separated by a twin boundary with lower Curie point T C = 150 K, acting as barrier for spin polarized transport. The two phase scenario is compared with the alternative one based on a single ferromagnetic phase with the peculiar ferromagnetic domains structure inherent to twinned manganites films, reported to be split into interconnected and spatially separated regions with in-plane and out-of-plane magnetization, coinciding with twin cores and twin boundaries respectively.

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.

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 nature of the metal-insulator transition in thin films and superlattices of LaNiO3 only a few unit cells in thickness remains elusive despite tremendous effort. Quantum confinement and epitaxial strain have been evoked as the mechanisms, although other factors such as growth-induced disorder, cation non-stoichiometry, oxygen vacancies, and substrate–film interface quality may also affect the observable properties of ultrathin films. Here we report results obtained for near-ideal LaNiO3 films with different thicknesses and terminations grown by atomic layer-by-layer laser molecular beam epitaxy on LaAlO3 substrates. We find that the room-temperature metallic behavior persists until the film thickness is reduced to an unprecedentedly small 1.5 unit cells (NiO2 termination). Electronic structure measurements using X-ray absorption spectroscopy and first-principles calculation suggest that oxygen vacancies existing in the films also contribute to the metal-insulator transition., The electronic behaviour of complex oxides such as LaNiO3 depends on many intrinsic and extrinsic factors, making it challenging to identify microscopic mechanisms. Here the authors demonstrate the influence of oxygen vacancies on the thickness-dependent metal-insulator transition of LaNiO3 films.

The superconducting properties of Sr1-xLaxCuO2 thin films are strongly affected by sample preparation procedures, including the annealing step, which are not always well controlled. We have studied the evolution of Cu L-2,L-3 and O K edge x-ray absorption spectra (XAS) of Sr1-xLaxCuO2 thin films as a function of reducing annealing, both qualitatively and quantitatively. By using linearly polarized radiation, we are able to identify the signatures of the presence of apical oxygen in the as-grown sample and its gradual removal as a function of duration of 350 degrees C Ar annealing performed on the same sample. Even though the as-grown sample appears to be hole doped, we cannot identify the signature of the Zhang-Rice singlet in the O K XAS, and it is extremely unlikely that the interstitial excess oxygen can give rise to a superconducting or even a metallic ground state. XAS and x-ray linear dichroism analyses are, therefore, shown to be valuable tools to improving the control over the annealing process of electron doped superconductors. Published by AIP Publishing.

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.

We report on the growth and characterization of ultrathin YBa2Cu3O7-delta (YBCO) films on MgO (110) substrates, which exhibit superconducting properties at thicknesses down to 3 nm. YBCO nanowires, with thicknesses down to 10 nm and widths down to 65 nm, have also been successfully fabricated. The nanowires protected by a Au capping layer showsuperconducting properties close to the as-grown films and critical current densities, which are limited by only vortex dynamics. The 10-nm-thick YBCO nanowires without the Au capping present hysteretic current-voltage characteristics, characterized by a voltage switch which drives the nanowires directly from the superconducting to the normal state. We associate such bistability to the presence of localized normal domains within the superconductor. The presence of the voltage switch in ultrathin YBCO nanostructures, characterized by high sheet resistance values and high critical current values, makes our nanowires very attractive devices to engineer single-photon detectors.

We report on the growth and characterization of ultrathin YBa2Cu3O7-δ (YBCO) films on MgO (110) substrates, which exhibit superconducting properties at thicknesses down to 3 nm. YBCO nanowires, with thicknesses down to 10 nm and widths down to 65 nm, have also been successfully fabricated. The nanowires protected by a Au capping layer show superconducting properties close to the as-grown films and critical current densities, which are limited by only vortex dynamics. The 10-nm-thick YBCO nanowires without the Au capping present hysteretic current-voltage characteristics, characterized by a voltage switch which drives the nanowires directly from the superconducting to the normal state. We associate such bistability to the presence of localized normal domains within the superconductor. The presence of the voltage switch in ultrathin YBCO nanostructures, characterized by high sheet resistance values and high critical current values, makes our nanowires very attractive devices to engineer single-photon detectors.

We report the study of anatase TiO

We have studied the low temperature electrical transport properties of La-x Sr1-xCuO2 thin films grown by oxide molecular beam epitaxy on (1 1 0) GdScO3 and TbScO3 substrates. The transmission electron microscopy measurements and the x-ray diffraction analysis confirmed the epitaxy of the obtained films and the study of their normal state transport properties, removing the ambiguity regarding the truly conducting layer, allowed to highlight the presence of a robust hidden Fermi liquid charge transport in the low temperature properties of infinite layer electron doped cuprate superconductors. These results are in agreement with recent observations performed in other p and n doped cuprate materials and point toward a general description of the superconducting and normal state properties in these compounds.

SrCuO2/Sr0.9La0.1CuO2/SrCuO2 trilayers were grown by oxide-molecular beam epitaxy. The thicknesses of the top and bottom SrCuO2 layers were fixed, while the thickness of the infinite-layer electron-doped cuprate Sr0.9La0.1CuO2 central layer was systematically changed. Transmission electron microscopy, x-ray reflectivity and x-ray diffraction measurements were performed to assess the sample quality and the abruptness of the interfaces. Electrical transport measurements as a function of temperature and as a function of central layer thickness, confirm that the normal state properties of the trilayers are altered by the confinement of the charge carriers in the central layer.

In multiphase/multilayer solid electrolytes, the composition, reactivity, and structure of interfaces between materials and phases play a fundamental role for fast ion-conduction. Here, the properties of buried interfaces in prototypical fast ion-conducting LaF3/SrF2 epitaxial multilayers are investigated. Photoelectron spectroscopy--both with soft-X and high-energy photons--is applied to separate composition and reactivity of buried interfaces with respect to the outermost surface. X-ray reflectivity, high-energy electron diffraction, X-ray diffraction, atomic force and transmission electron microscopies are used to study morphology, layer crystallinity, epitaxy relations, and buried interface structure. It is found that while the alternated layers present good crystallinity and high lattice matching, with formation of almost ideal sharp interfaces, buried interfaces show a sizeable reduction of the energy barrier for F vacancy formation with respect to bare materials. A density higher by a factor of six of fluorine vacancies is observed at buried interfaces in multilayers with respect to the bare materials. This is correlated to the formation of space charge regions, favoring ion conduction. The formation of F depleted La fluoride regions at interfaces is also promoted by annealing. This is associated to the increase of ion conductivity in annealed heterostructures reported in literature.