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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

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 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.

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 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.

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.

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; 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.

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.

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 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.

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.

Sr3Ru2O7/Sr2RuO4 eutectic system is investigated by electron backscattering diffraction (EBSD) and X-ray diffraction (XRD). The eutectic growth enables the solidification of the two phases in an ordered lamellar pattern extending along the growth direction, namely the b-axis direction. The eutectic material thus provides in the a-c plane two distinct interfaces having different microstructures with respect to the growth direction. Our analysis shows that, across the inplane c-axis direction (characterized by a poor lattice matching), the b-axis orientation is not constant at the individual interfaces, showing an orientation spread of about 5 degrees. However, across the in-plane a-axis direction (characterized by a good lattice matching), the b-axis orientation does not change within a few tenths of degree (about 0.25 degrees). Such information at nanoscale is also verified on a macroscopic level by standard XRD investigation.

We present here measurements carried out by using a dielectrically loaded copper cavity operating at 7 GHz on MgB/sub 2/ thin films synthesized by hybrid physical-chemical vapor deposition. Microwave data on samples having critical temperatures above 41 K, very low resistivity values, and residual resistivity ratio between 15 and 17, are presented. The dependence of the nonlinear surface losses and of the third order intermodulation products on the power feeding the cavity is analyzed, with the aim of shedding a light on the primary dissipation mechanisms. At low power, data from both power dependence and third order products seem to indicate that we are observing a nonlinear response which is inherent to the conventional s-wave nature of this novel compound. At intermediate-to-high circulating power values, vortex penetration appears to be the limiting mechanism for the power handling capability of MgB/sub 2/ films.

We report structural and transport proprieties of MgB/sub 2//MgO and MgB/sub 2//AlN multilayers for MgB/sub 2/ Josephson junctions. The MgB/sub 2/ layers were grown by hybrid physical chemical vapor deposition (HPCVD). The epitaxial MgB/sub 2//MgO/MgB/sub 2/ trilayers were grown in situ in the HPCVD system. The AlN layers were grown at room temperature by pulsed laser deposition, and the MgB/sub 2//AlN/MgB/sub 2/ trilayers were deposited ex situ with the deposition of the AlN layer between the depositions of the top and bottom MgB/sub 2/ layers. Although slightly less perfect than in films grown directly on sapphire and SiC substrates, excellent superconducting and transport properties were obtained in the MgB/sub 2/ layers in both heterostructures. The result addressed only the first of many issues about the adequacy of using MgO and AlN as the barrier materials for all-MgB/sub 2/ planar Josephson junctions, i.e. the ability to obtain good structural and superconducting properties in both electrode layers using the trilayer deposition process. It allows us to further investigate the conditions for better wetting and coverage of the insulator layers, as well as other critical issues in the fabrication of all-MgB/sub 2/ planar Josephson junctions.

We have studied the degradation of MgB/sub 2/ thin films, grown by hybrid physical-chemical vapor deposition (HPCVD), by submerging MgB/sub 2/ films into de-ionized water at different temperatures. The thickness, room temperature resistance, and superconducting transition were measured as a function of time in water and the temperature of the water. Results show that the films degrade faster at room temperature than at 0/spl deg/C. The room temperature resistance increases with time. The thickness of the MgB/sub 2/ films decreases rapidly initially, but then saturates. The resultant material is amorphous and insulating. Both T/sub c/(onset) and T/sub c/(0) of the MgB/sub 2/ films decrease with time in water, while the superconducting transition become broader. The effects of the exposure to other solvents and photoresist were also studied, and it was found that isopropanol does not cause MgB/sub 2/ films to degrade.

Lanthanum manganites with a massive concentration of La defects can be stabilized in the form of thin films, by exploiting the structural stress produced by a substrate such as SrTiO3. They undergo an insulator-to-metal transition (IMT) like those doped by divalent ions, which is here studied by measuring the optical conductivity of LaxMnO films with x = 0.66, 0.88, 0.98 and 1.10, and with , from the far infrared to the near ultraviolet, and between 20 and 300 K. The IMT is here a slow process which continues down to 100 K at least, more than 250 K below its onset at the Curie temperature Tc and at the TIMT measured from the dc resistivity. The metallization is here monitored through the increase of the Drude term and a transfer of spectral weight from a 'hard' midinfrared band MIR-2 peaked between 3000 and 5000 cm−1 at room temperature, to a 'soft' midinfrared band MIR-1 at ~ 1000 cm−1 and to the Drude term. This evidence is in good agreement with a model of phase separation below Tc, where insulating and paramagnetic regions of small polarons coexist with conducting and ferromagnetic regions populated by large polarons and free carriers.

Superconducting Sr1–xLaxCuO2 thin films have been grown on GdScO3 substrates by reflection high-energy electron diffraction calibrated layer-by-layer molecular-beam epitaxy. X-ray diffraction analysis has confirmed the infinite layer structure after an in situ vacuum annealing step. In situ photoemission spectroscopy indicates that the vacuum annealing step employed immediately after film growth to achieve superconducting films results in oxygen loss from the films. The superconducting critical temperature depends on the La content x, with the highest value obtained for x∼0.10. Resistivity as a function of temperature ρ(T) curves of optimally doped samples show a T2 temperature dependence characteristic of a scattering process where electron-electron interactions dominate.

International audience; SrRuO3 is a strong itinerant ferromagnet showing many features of 'bad metals' together with Fermi liquid behavior at very low temperature. The intriguing magnetic and transport properties of SrRuO3 are tightly related to structure, as commonly observed in transition metal oxides. Here we report on the correlation of structural parameters (lattice constant and surface roughness) with the critical behavior of resistivity at the Curie point in SrRuO3 thin films deposited on (001) SrTiO3 by PLD. By varying the deposition conditions we were able to obtain a wide variety of structural properties in our samples, thus allowing us to perform a systematic study. Our analysis demonstrates the direct correlation between the critical temperature TP and the lattice out-of-plane parameter. Furthermore, the value of the critical exponent is proved to be a good physical parameter to quantify the microscopic order of SrRuO3 samples.

Double-exchange (DE) mechanisms in $R{E}_{1\ensuremath{-}x}A{E}_{x}{\text{MnO}}_{3}$ manganites (where $RE$ is a trivalent rare-earth ion and $AE$ is a divalent alkali-earth ion) relies on the strong exchange interaction between two ${\text{Mn}}^{3+}$ and ${\text{Mn}}^{4+}$ ions through interfiling oxygen $2p$ states. Nevertheless, the role of $RE$ and $AE$ ions has ever been considered ``silent'' with respect to the DE conducting mechanisms. Here we show that a new path for DE mechanism is indeed possible by partially replacing the $RE\text{\ensuremath{-}}AE$ elements by ${\text{Mn}}^{2+}$ ions, in La-deficient ${\text{La}}_{x}{\text{MnO}}_{3\ensuremath{-}\ensuremath{\delta}}$ thin films. X-ray absorption spectroscopy demonstrated the relevant presence of ${\text{Mn}}^{2+}$ ions, which is unambiguously proved to be substituted at La site by resonant inelastic x-ray scattering. ${\text{Mn}}^{2+}$ is proved to be directly correlated with the enhanced magnetotransport properties because of an additional hopping mechanism trough interfiling ${\text{Mn}}^{2+}$ ions. Such a scenario has been theoretically confirmed by calculations within the effective single-band model. The use of ${\text{Mn}}^{2+}$ both as a doping element and an ion electronically involved in the conduction mechanism reveals an unconventional phenomenon in transport properties of manganites. More important, such a strategy might be also pursed in other strongly correlated materials.

We measure the optical conductivity of (SrMnO3)n/(LaMnO3)2n superlattices (SL) for n=1,3,5, and 8 and 10 < T < 400 K. Data show a T-dependent insulator to metal transition (IMT) for n \leq 3, driven by the softening of a polaronic mid-infrared band. At n = 5 that softening is incomplete, while at the largest-period n=8 compound the MIR band is independent of T and the SL remains insulating. One can thus first observe the IMT in a manganite system in the absence of the disorder due to chemical doping. Unsuccessful reconstruction of the SL optical properties from those of the original bulk materials suggests that (SrMnO3)n/(LaMnO3)2n heterostructures give rise to a novel electronic state., Published Online in Nano Letters, November 8, 2010; http://pubs.acs.org/doi/abs/10.1021/nl1022628; 5 pages, 3 figures

We report on superconducting magnesium diboride MgB(2) thin films grown on both YSZ and MgO substrates, with two different orientations, namely [110] and [211]. MgB(2) off-axis growth mode (namely, with the c-axis tilted with respect to the film surface's normal) is achievable on these substrates. Depending on the type and orientation of the substrate, tilting angle can be varied. As a consequence of tilted growth, anisotropic transport properties are observed. In very thin films, resistance measurements provide an estimate of the resistivity anisotropic ratio rho(c)/rho(ab), where rho(c) is the resistivity along the c axis and rho(ab) is the in-plane resistivity. All these findings clearly demonstrate that tilted MgB(2) films offer new exciting possibilities to both investigate intrinsic fundamental properties of MgB(2) and to explore possible applications in planar superconducting devices

The magnetic and electronic modifications induced at the interfaces in ${({\text{SrMnO}}_{3})}_{n}/{({\text{LaMnO}}_{3})}_{2n}$ superlattices have been investigated by linear and circular magnetic dichroism in the $\text{Mn}\text{ }{L}_{2,3}$ x-ray absorption spectra. Together with theoretical calculations, our data demonstrate that the charge redistribution across interfaces favors in-plane ferromagnetic (FM) order and ${e}_{g}({x}^{2}\ensuremath{-}{y}^{2})$ orbital occupation, in agreement with the average strain. Far from interfaces, inside ${\text{LaMnO}}_{3}$, electron localization and local strain favor antiferromagnetism (AFM) and ${e}_{g}(3{z}^{2}\ensuremath{-}{r}^{2})$ orbital occupation. For $n=1$ the high density of interfacial planes ultimately leads to dominant FM order forcing the residual AFM phase to be in-plane too, while for $n\ensuremath{\ge}5$ the FM layers are separated by AFM regions having out-of-plane spin orientation.

In a metallic manganite, the substitution of ${\mathrm{Mn}}^{+3}$ by ${\mathrm{Ga}}^{+3}$ locally destroys both the ferromagnetic order and the Jahn--Teller distortion, without heavily affecting the crystal structure. One can thus observe an unusual metal-to-insulator transition, which is induced neither by the temperature nor by changes in the divalent-ion doping. This transition is studied here by comparing the infrared reflectivity of five samples of ${\mathrm{La}}_{2∕3}{\mathrm{Sr}}_{1∕3}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}{\mathrm{O}}_{3}$, with $x$ increasing from 0 to 0.30. The dramatic effect of Ga on the transport properties is monitored through the decrease in the number of free carriers and the increase in their effective mass, until a good metal such as ${\mathrm{La}}_{2∕3}{\mathrm{Sr}}_{1∕3}\mathrm{Mn}{\mathrm{O}}_{3}$ is turned into an insulator at all temperatures. A simple model which links the measured optical parameters to the magnetization $M(x,T)$ well describes the behavior of the plasma frequency, the scattering rate, and the midinfrared absorption, throughout the Ga-induced metal-to-insulator transition.

A series of $\mathrm{Mg}{\mathrm{B}}_{2}$ thin films systematically disordered by neutron irradiation have been studied by scanning tunneling spectroscopy. The $c$-axis orientation of the films allowed a reliable determination of the local density of states of the $\ensuremath{\pi}$ band. With increasing disorder, the conductance peak moves towards higher voltages and becomes lower and broader, indicating a monotonic increase of the $\ensuremath{\pi}$ gap and of the broadening parameter. These results are discussed in the framework of two-band superconductivity.

We have used scanning SQUID magnetometry to image vortices in ultrathin $({\mathrm{Ba}}_{0.9}{\mathrm{Nd}}_{0.1}{\mathrm{CuO}}_{2+x}{)}_{m}/\phantom{\rule{0ex}{0ex}}({\mathrm{CaCuO}}_{2}{)}_{n}$ high temperature superconductor samples, with as few as three superconducting ${\mathrm{CuO}}_{2}$ planes. The Pearl lengths ($\ensuremath{\Lambda}=2{\ensuremath{\lambda}}_{L}^{2}/d$, ${\ensuremath{\lambda}}_{L}$ the London penetration depth, $d$ the superconducting film thickness) in these samples, as determined by fits to the vortex images, agree with those by local susceptibility measurements, and can be as long as 1 mm. The in-plane penetration depths ${\ensuremath{\lambda}}_{ab}$ inferred from the Pearl lengths are longer than many bulk cuprates with comparable critical temperatures. We speculate on the causes of the long penetration depths, and on the possibility of exploiting the unique properties of these superconductors for basic experiments.

We have investigated by means of noise spectroscopy the transport properties of half-metal La0.7Ba0.3MnO3 (LBMO) thin films deposited on MgO substrates. A reduced metal–insulator transition temperature and a peculiar noise behaviour are observed in the films grown on MgO substrates, when compared with similar films grown on SrTiO3 substrates. In particular, a large increase in noise is observed below the metal–insulator transition temperature, associated with a current induced reduction in the excess noise level. This finding is explained in terms of the spin torque effect between regions with depressed Curie temperatures among the ferromagnetic metallic domains. The proposed theoretical model, taking into account the half-metal character of manganites, describes well the experimental data.

The two tone intermodulation distortion arising in MgB2 thin films synthesized by hybrid physical-chemical vapour deposition is studied in order to probe the influence of the two bands on the nonlinear response of this superconductor. The measurements are carried out by using a dielectrically loaded copper cavity operating at 7 GHz. Microwave data on samples having critical temperatures above 41 K, very low resistivity values, and residual resistivity ratio larger than 10, are shown. The dependence of the nonlinear surface losses and of the third order intermodulation products on the power feeding the cavity and on the temperature is analyzed. At low power, the signal arising from distortion versus temperature shows the intrinsic s-wave behavior expected for this compound. Data are compared with measurements performed on Nb and YBCO thin films using the same technique., Revised version. Submitted to APL on October 7th 2005

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.

We review results on two novel types of grain-boundary Josephson junctions in the high-critical-temperature cuprate superconductors. The quality of YBa(2)Cu(3)O(7-delta) biepitaxial grain-boundary Josephson junctions has been improved using a new growth geometry to the point that "tunnel-like" characteristics are reproducibly obtained. The consequent low barrier transmission probabilities are apparently favorable for isolating intrinsic d-wave induced effects. We also focus on peculiar aspects of spontaneous currents in HTS Josephson junctions, which are unique and reflect the intimate relation between vortex matter and the Josephson effect. Bicrystal grain-boundary Josephson junctions have also been fabricated using artificial superlattice films with as few as six superconducting copper-oxygen planes. Extrinsic and intrinsic d-wave effects in these junctions are discussed within the framework of novel designs for pi-circuitry and qubits. (C) 2004 American Institute of Physics.