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We present a combined resonant soft X-ray reflectivity and electric transport study of $$\hbox {LaAlO}_3$$ LaAlO 3 / $$\hbox {SrTiO}_3$$ SrTiO 3 field effect devices. The depth profiles with atomic layer resolution that are obtained from the resonant reflectivity reveal a pronounced temperature dependence of the two-dimensional electron liquid at the $$\hbox {LaAlO}_3$$ LaAlO 3 / $$\hbox {SrTiO}_3$$ SrTiO 3 interface. At room temperature the corresponding electrons are located close to the interface, extending down to 4 unit cells into the $$\hbox {SrTiO}_3$$ SrTiO 3 substrate. Upon cooling, however, these interface electrons assume a bimodal depth distribution: They spread out deeper into the $$\hbox {SrTiO}_3$$ SrTiO 3 and split into two distinct parts, namely one close to the interface with a thickness of about 4 unit cells and another centered around 9 unit cells from the interface. The results are consistent with theoretical predictions based on oxygen vacancies at the surface of the $$\hbox {LaAlO}_3$$ LaAlO 3 film and support the notion of a complex interplay between structural and electronic degrees of freedom.

We present a combined resonant soft X-ray reflectivity and electric transport study of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {LaAlO}_3$$\end{document}LaAlO3/\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {SrTiO}_3$$\end{document}SrTiO3 field effect devices. The depth profiles with atomic layer resolution that are obtained from the resonant reflectivity reveal a pronounced temperature dependence of the two-dimensional electron liquid at the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {LaAlO}_3$$\end{document}LaAlO3/\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {SrTiO}_3$$\end{document}SrTiO3 interface. At room temperature the corresponding electrons are located close to the interface, extending down to 4 unit cells into the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {SrTiO}_3$$\end{document}SrTiO3 substrate. Upon cooling, however, these interface electrons assume a bimodal depth distribution: They spread out deeper into the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {SrTiO}_3$$\end{document}SrTiO3 and split into two distinct parts, namely one close to the interface with a thickness of about 4 unit cells and another centered around 9 unit cells from the interface. The results are consistent with theoretical predictions based on oxygen vacancies at the surface of the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {LaAlO}_3$$\end{document}LaAlO3 film and support the notion of a complex interplay between structural and electronic degrees of freedom.

Electronic phase separation is crucial for the fascinating macroscopic properties of the ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ (LAO/STO) paradigm oxide interface, including the coexistence of superconductivity and ferromagnetism. We investigate this phenomenon using angle-resolved photoelectron spectroscopy (ARPES) in the soft-x-ray energy range, where the enhanced probing depth combined with resonant photoexcitation allow us access to fundamental electronic structure characteristics -- momentum-resolved spectral function, dispersions and ordering of energy bands, Fermi surface -- of buried interfaces. Our experiment uses x-ray irradiation of the LAO/STO interface to tune its oxygen deficiency, building up a dichotomic system where mobile weakly correlated $\mathrm{Ti}\phantom{\rule{0.28em}{0ex}}{t}_{2\mathrm{g}}$ electrons coexist with localized strongly correlated $\mathrm{Ti}\phantom{\rule{0.28em}{0ex}}{e}_{\mathrm{g}}$ ones. The ARPES spectra dynamics under x-ray irradiation shows a gradual intensity increase under constant Luttinger count of the Fermi surface. This fact identifies electronic phase separation (EPS) where the mobile electrons accumulate in conducting puddles with fixed electronic structure embedded in an insulating host phase, and allows us to estimate the lateral fraction of these puddles. We discuss the physics of EPS invoking a theoretical picture of oxygen-vacancy clustering, promoted by the magnetism of the localized $\mathrm{Ti}\phantom{\rule{0.28em}{0ex}}{e}_{\mathrm{g}}$ electrons, and repelling of the mobile ${t}_{2\mathrm{g}}$ electrons from these clusters. Our results on the irradiation-tuned EPS elucidate the intrinsic one taking place at the stoichiometric LAO/STO interfaces.

We report on the magnetotransport properties of a spin-polarized two-dimensional electron system (2DES) formed in LaAlO3(LAO)/EuTiO3/SrTiO3 (STO) heterostructures. We show that, at low temperature, the 2DES magnetoconductance exhibits weak antilocalization corrections related to Rashba spin-orbit scattering, in analogy with the LAO/STO 2DES. However, the characteristic spin-orbit scattering field decreases substantially for carrier density higher than 1.9×1013cm-2. We attribute this behavior to the masking effect of ferromagnetism, which sets in at the same carrier density and at a temperature below 10 K. Our work shows that, while weak antilocalization corrections to the magnetoconductance are strongly reduced by the emergence of ferromagnetism, they persist in a large part of the phase diagram of a spin-polarized oxide 2DES.

We have used grazing-angle infrared spectroscopy to detect the Berreman effect (BE) in the quasi-two-dimensional electron system (q-2DES) which forms spontaneously at the interface between SrTiO 3 (STO) and a thin film of LaAlO 3 (LAO). From the BE, which allows one to study longitudinal optical excitations in ultrathin films like the q-2DES, we have extracted at different temperatures its thickness, the charge density and mobility of the carriers under crystalline LAO (sample A), and the charge density under amorphous LAO (sample B). The latter quantity turns out to be higher than in sample A, but a comparison with Hall measurements shows that under amorphous LAO the charges are partly localized at low T with a low activation energy (about 190 K in units of kB) and are thermally activated according to a model for large polarons. The thickness of the q-2DES extracted from our spectra turns out to be 4±1 nm for crystalline LAO and 7±2 nm for amorphous LAO.

Concepts and experimental procedures to obtain different types of YBa 2 Cu 3 O 7 grain boundaries Josephson junction through the biepitaxial technique have been developed. Various seed layers (MgO, CeO 2 and SrTiO 3) and substrates (MgO, SrTiO 3) have been used providing, for the biepitaxial technique, additional flexibility in circuit design. A detailed investigation on transport properties evidenced good performances for both junctions and superconducting quantum interference devices. Experimental evidence is given of a conventional magnetic field behavior in 45° tilt Artificial Grain Boundary (AGB) for which no π-loops are predicted along the interface.

Oxide growth with semiconductorlike accuracy has led to atomically precise thin films and interfaces that exhibit a plethora of phases and functionalities not found in the oxide bulk material. This has yielded spectacular discoveries such as the conducting, magnetic, and even superconducting ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ interfaces separating two prototypical insulating perovskite materials. All these investigations, however, consider the static state at the interface, although studies on fast oxide interface dynamics would introduce a powerful degree of freedom to understanding the nature of the ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ interface state. Here, we show that the polarization state at the ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ interface can be optically enhanced or attenuated within picoseconds. Our observations are explained by a model based on charge propagation effects in the interfacial vicinity and transient polarization buildup at the interface.

Using x-ray magnetic circular dichroism and ab-initio calculations, we explore the La1-xSrxMnO3/LaAlO3/SrTiO3 (001) heterostructure as a mean to induce transfer of spin polarized carriers from ferromagnetic La1-xSrxMnO3 layer into the 2DEG (two-dimensional electron gas) at the LaAlO3/SrTiO3 interface. By out-of-plane transport measurements, the tunneling across the LaAlO3 barrier is also analyzed. Our results suggest small or vanishing spin-polarization for the 2DEG: magnetic dichroism does not reveal a neat signal on Ti atoms, while calculations predict, for the pristine stoichiometric interface, a small spin-resolved mobile charge of 2.5 x 10(13) cm(-2) corresponding to a magnetic moment of 0.038 mu(B) per Ti atom, tightly confined within the single SrTiO3 layer adjacent to LaAlO3. Such a small magnetization is hard to be detected experimentally and perhaps not robust enough to survive to structural disorder, native doping, or La1-xSrxMnO3 dead-layer effects. Our analysis suggests that, while some spin-diffusion cannot be completely ruled out, the use of ferromagnetic La1-xSrxMnO3 epilayers grown on-top of LaAlO3/SrTiO3 is not effective enough to induce robust spin-transport properties in the 2DEG. The examined heterostructure is nevertheless an excellent test-case to understand some fundamental aspects of the spin-polarized charge transfer in 2D wells.

We used x-ray absorption spectroscopy to study the orbital symmetry and the energy band splitting of (111) LaAlO${_3}$/SrTiO${_3}$ and LaAlO${_3}$/EuTiO${_3}$/SrTiO${_3}$ heterostructures, hosting a quasi two-dimensional electron system (q2DES), and of a Ti-terminated (111) SrTiO${_3}$ single crystal, also known to form a q2DES at its surface. We demonstrate that the bulk tetragonal Ti-3d D${_4}$${_h}$ crystal field is turned into trigonal D${_3}$${_d}$ crystal field in all cases. The symmetry adapted a${_1}$${_g}$ and e${^\pi_g}$ orbitals are non-degenerate in energy and their splitting, \Delta, is positive at the bare STO surface but negative in the heterostructures, where the a${_1}$${_g}$ orbital is lowest in energy. These results demonstrate that the interfacial symmetry breaking induced by epitaxial engineering of oxide interfaces has a dramatic effect on their electronic properties, and it can be used to manipulate the ground state of the q2DES.

Oxide growth with semiconductorlike accuracy has led to atomically precise thin films and interfaces that exhibit a plethora of phases and functionalities not found in the oxide bulk material. This has yielded spectacular discoveries such as the conducting, magnetic, and even superconducting LaAlO 3 / SrTiO 3 interfaces separating two prototypical insulating perovskite materials. All these investigations, however, consider the static state at the interface, although studies on fast oxide interface dynamics would introduce a powerful degree of freedom to understanding the nature of the LaAlO 3 / SrTiO 3 interface state. Here, we show that the polarization state at the LaAlO 3 / SrTiO 3 interface can be optically enhanced or attenuated within picoseconds. Our observations are explained by a model based on charge propagation effects in the interfacial vicinity and transient polarization buildup at the interface.

The recombination dynamics of electron–hole pairs of pure and n-doped strontium titanate (STO) bulk samples after an intense UV laser-pulse excitation has been investigated by collecting and analyzing the frequency- and time-resolved photoluminescence (PL) signal, as a function of the sample temperature. The observed PL decay is in the nanosecond time-scale and it shows a peculiar mixed kinetics. In particular, two decay channels are clearly singled-out: the faster one has a decay-time that is markedly dependent on the photo-excited carrier density and it is well described as a bimolecular recombination process, while the second slower channel shows an excitation-independent rate and follows an unimolecular law. The PL spectra show a considerable variation with temperature, with a previously unreported, pronounced difference between pure and doped samples emerging at high temperatures. A first discussion of the possible underlying mechanisms is attempted.

The low-temperature softening of the TO1 phonon of SrTiO3 (STO), which determines its incipient ferroelectricity, is known to be partially hindered either in the bulk under strong electric fields, or in thin STO films. Here we show, by terahertz (THz) reflectivity measurements, that a similar effect is produced in bulk STO and at zero static field by ultrathin metallic films on its surface, like a 10-nm-thick film of SrRuO3 (SRO), or the two-dimensional electron system (2DES) present at the interface with LaAlO3. In SRO/STO, the observed hardening is well explained by the depolarizing action of the SRO free electrons which follow adiabatically the ion motion. In LAO/STO, a weaker TO1 hardening could be detected by patterning the 2DES in the form of microstripes and using a polarized THz field parallel (E-) or orthogonal (E-) to the stripes. At 10 K, when TO1 is excited together with the free electrons by E-, its absorbance is harder by about 7 cm-1 than that measured when TO1 is coupled to the plasmon-polariton confined within the stripes, being excited by E-.

Advances in growth technology of oxide materials allow single atomic layer control of heterostructures. In particular delta doping, a key materials' engineering tool in today's semiconductor technology, is now also available for oxides. Here we show that a fully electric-field-tunable spin-polarized and superconducting quasi-2D electron system (q2DES) can be artificially created by inserting a few unit cells of delta doping EuTiO3 at the interface between LaAlO3 and SrTiO3 oxides(1,2). Spin polarization emerges below the ferromagnetic transition temperature of the EuTiO3 layer (T-FM = 6-8 K) and is due to the exchange interaction between the magnetic moments of Eu-4f and of Ti-3d electrons. Moreover, in a large region of the phase diagram, superconductivity sets in from a ferromagnetic normal state. The occurrence of magnetic interactions, superconductivity and spin-orbit coupling in the same q2DES makes the LaAlO3/EuTiO3/SrTiO3 system an intriguing platform for the emergence of novel quantum phases in low-dimensional materials.

(110) and (103) YBa 2 Cu 3 O 7 films have been grown onto exact and vicinal (110) SrTiO 3 substrates, and on vicinal (110) MgO substrates with a SrTiO 3 buffer layer. The samples are carefully characterised by reciprocal space mapping with x-ray diffraction, in order to investigate the features of the typical double domain of (110) and (103) YBa 2 Cu 3 O 7 structure. It is demonstrated that vicinal cut substrates allow to select one film/substrate epitaxial relation. The growth properties of these thin films deposited on vicinal surfaces are discussed.

We measure the transmittance of LaMnO3 (LMO) thin films in the spectral range from 10 000 to 38 000 cm-1 as a function of temperature. The optical conductivity obtained from the transmittance data is decomposed into five Drude-Lorentz contributions: three of them show evident changes below the Neel temperature T N $\simeq$ 140 K. We discuss the band assignment in the framework of the tight-binding and of the orbitally degenerate Hubbard models. Our results ascribe the overall low-frequency spectrum of LMO to charge-transfer mechanisms involving the t 2g and e g electrons of the Mn ions, without invoking the presence of excitonic contributions. This finding definitely promotes a Mott-Hubbard scenario for LMO and provides an estimate of the Hamiltonian parameters.

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

Segregation and competition of Y-based impurities in Y-rich biepitaxial YBCO samples is investigated. The analysis of distribution and microstructure of the precipitates is performed by high resolution electron microscopy (HREM). Y 2 O 3 precipitates are found in (103) YBCO films grown on (110) SrTiO 3 (STO) substrates. On the same biepitaxial samples, Y 2 BaCuO 5 (Y211) precipitates are found in (001) YBCO films deposited on an intermediate (110) MgO seed layer. Agglomeration of Y211 precipitates is observed near the edge in the MgO seed layer. The experimental data are discussed in the context of phase competition, nucleation, epitaxy and surface migration in the Y–Ba–Cu–O system. It is deduced that the change of supersaturation and surface energy, also governed by the favorable epitaxy of the secondary phase with the substrate or the YBCO matrix, plays a major role in determining the segregation of either Y211 or Y 2 O 3 precipitates.

We report the relationship between epitaxial strain and the crystallographic orientation of the in-phase rotation axis and A-site displacements in Pbnm-type perovskite films. Synchrotron diffraction measurements of EuFeO3 films under strain states ranging from 2% compressive to 0.9% tensile on cubic or rhombohedral substrates exhibit a combination of a-a+c- and a+a-c- rotational patterns. We compare the EuFeO3 behavior with previously reported experimental and theoretical work on strained Pbnm-type films on non-orthorhombic substrates, as well as additional measurements from LaGaO3, LaFeO3, and Eu0.7Sr0.3MnO3 films on SrTiO3. Compiling the results from various material systems reveals a general strain dependence in which compressive strain strongly favors a-a+c- and a+a-c- rotation patterns and tensile strain weakly favors a-a-c+ structures. In contrast, EuFeO3 films grown on Pbnm-type GdScO3 under 2.3% tensile strain take on a uniform a-a+c- rotation pattern imprinted from the substrate, despite strain energy considerations that favor the a-a-c+ pattern. These results point to the use of substrate imprinting as a more robust route than strain for tuning the crystallographic orientations of the octahedral rotations and A-site displacements needed to realize rotation-induced hybrid improper ferroelectricity in oxide heterostructures., Comment: submitted to PRB, comments welcomed

The indirect effect of oxygen background gas on the La/Al ratio during the growth of LaAlO3 (LAO) films by pulsed laser deposition (PLD) is analyzed, in a pressure range between 10^-3 and 10^-1 mbar. We resort to two complementary investigation methods: Rutherford backscattering spectroscopy and spectrally resolved, time-gated imaging of the laser plume. The first technique allows us to analyze the stoichiometry of the deposited films, and the latter allows us to analyze the plume expansion phase of the PLD process by collecting chemically resolved two-dimensional images of the relevant atomic/molecular species. The comparison between the results obtained by the two techniques allows us to highlight the role of the plume-gas interaction in affecting cations stoichiometry. Our results indicate that, in the considered pressure range, the background oxygen pressure affects the cations stoichiometry of the LAO films, besides determining their oxygen content.

The quality of YBa2Cu3O7-delta blepitaxial grain boundary Josephson junctions has been improved and 'tunnel-like' characteristics are reproducibly obtained. The consequent low barrier transmission probabilities are apparently favourable to isolate intrinsic d-wave induced effects. Extrinsic and intrinsic d-wave effects are discussed within the framework of novel designs for pi-circultry and qubits.

The voltage-spectral density SV(f) of the 2-dimensional electron gas formed at the interface of LaAlO3 /SrTiO3 has been thoroughly investigated. The low-frequency component has a clear 1/f behavior with a quadratic bias current dependence, attributed to resistance fluctuations. However, its temperature dependence is inconsistent with the classical Hooge model, based on carrier-mobility fluctuations. The experimental results are, instead, explained in terms of carrier-number fluctuations, due to an excitation-trapping mechanism of the 2-dimensional electron gas., 4 pages, 4 figures

Results concerning the study of the morphology and the surface chemical properties of epitaxial YBCO and highly oriented TBCCO films before and after wet etching processes are reported. X-ray photoemission spectroscopy, Auger electron spectroscopy and scanning electron microscopy are employed to get information on the surface contamination and on the defects of the samples. The chemical modifications of surfaces and the shape of etch pits are discussed.

We investigated the role of strain on the magnetic properties of La0.7Sr0.3MnO3 epitaxial films grown on substrates with different crystallographic orientation. The magnetic characterization confirms the large magnetostriction due to the tensile stress applied by the SrTiO3 substrates. The data demonstrate that the easy direction of magnetization is aligned to the elongated, in-plane axis both in (110)- and (001)-oriented films. The in-plane anisotropy of (110) films is interpreted in the frame of a simple structural model. We provide evidence of a spatial separation between a conducting ferromagnetic and an insulating paramagnetic phase at the Curie temperature. The possible effect of such phase separation on the coercitive field and on the magnetization versus temperature is discussed. (c) 2006 Elsevier B.V. All rights reserved.

Optical second harmonic generation (SHG) spectra of LaAlO${}_{3}$/SrTiO${}_{3}$, LaGaO${}_{3}$/SrTiO${}_{3}$, and NdGaO${}_{3}$/SrTiO${}_{3}$ interfaces with SrTiO${}_{3}$ substrates have been measured up to 4.2 eV as a function of film thickness and temperature. This spectral range is characterized by two-photon transitions from the valence to conduction bands of SrTiO${}_{3}$ which are classified with a model based on symmetry, selection rules, and atomic orbital overlaps. This model is further confirmed in spectral measurements as a function of film thickness, material overlayer, and temperature. SHG enhancement at low temperature indicates an increase of the interfacial polarity with decreasing temperature. This confirms the relation between SHG and the spatial translation of Ti ions which are more prone to move at lower temperatures because of the quantum-paraelectric nature of SrTiO${}_{3}$. We finally show that SHG spectroscopy captures structural details of the interfaces. In particular, we find evidence for proximity effects such as LaAlO${}_{3}$-induced distortions of the TiO${}_{6}$ octahedra or bond buckling. We also observe a correlation between SHG and the in-plane lattice mismatch between the SrTiO${}_{3}$ substrate and the LaAlO${}_{3}$, LaGaO${}_{3}$, and NdGaO${}_{3}$ overlayers.

We investigate pulsed laser ablation of LaAlO 3 and LaGaO 3 with a focus on the influence of oxygen background gas pressure on the plume expansion dynamics and deposition rate. The ablation plume is characterized by exploiting fast photography and time- and space-resolved optical emission spectroscopy. The variation of the deposition rate with the oxygen background pressure was obtained at 800 °C by reflection high-energy electron diffraction, and compared to that measured at room temperature by means of a quartz crystal microbalance. The experimental findings allow one to address the various stages of plume expansion as a function of the background oxygen pressure as well as the changes induced on the plume species kinetic energy and composition. On the base of our experimental results, the possible influence of various mechanisms, such as subplantation and oxygen vacancy formation, on the growth of oxides interfaces is addressed.

We report on a systematic study of the growth of epitaxial TiO2 films deposited by pulsed laser deposition on Ti-terminated (001) SrTiO3 single crystals. By using in-situ reflection high energy electron diffraction, low energy electron diffraction, x-ray photoemission spectroscopy and scanning probe microscopy, we show that the stabilization of the anatase (001) phase is preceded by the growth of a pseudomorphic Sr-Ti-O intermediate layer, with a thickness between 2 and 4 nm. The data demonstrate that the formation of this phase is related to the activation of long range Sr migration from the substrate to the film. The role of interface Gibbs energy minimization, as a driving force for Sr diffusion, is discussed. Our results enrich the phase diagram of the Sr-Ti-O system under epitaxial strain opening the roudeficient SrTiO phase., Comment: 8 pages, 7 figures

The polarization induced at the LaAlO3/SrTiO3 interface has been explored by optical second harmonic generation (SHG) spectroscopy. Our data reveal that an orbital reconstruction of the SrTiO3 surface occurs when three epitaxial monolayers of LaAlO3 are deposited on top of it. The reconstruction manifests as a spectral modification of a Ti4+(3d) state at the LaAlO3/SrTiO3 interface which is accompanied by a displacement of the Ti4+ ions in adjacent SrTiO3 monolayers in order to compensate for the polarization discontinuity induced at the interface. According to the SHG experiment, the carriers transferred to the interface are trapped because in spite of the orbital reconstruction, the interface remains insulating. The onset of conductivity with the formation of a two-dimensional electron liquid appears to be a subordinate effect related to a minority of carriers and occurs upon further increase of the LaAlO3 coverage.

International audience; We describe some general features in the transient behaviour of strongly correlated transition metal oxides, following ultrafast excitation by a femtosecond laser pulse. Our analysis is based on time-resolved reflectivity measurements on (V1-xCrx)2O3, a prototype Mott-Hubbard material, and the manganite compound (La0.67Sr0.33)MnO3, performed over a time window of several tenths of ps. We point out the contribution of coherent lattice oscillations-both optical and acoustic-to the overall signal, and show how they can depend on the crystallographic orientation of the material with respect to the laser beam wavevector and polarisation. In particular, an acoustic wave is always found to be present in our measurements, and we show that its oscillating behaviour is superposed to the average time evolution of the material, which can be instead related in different ways to its electronic properties and thermodynamic phase. All these effects are to be taken into account in any further analysis leading to the estimation of relevant physical parameters for correlated materials, and they appear to play a significant role not only for reflectivity techniques, but also in other kinds of ultrafast pump-probe experiments.

The pulsed laser deposition of SrTiO(3)/LaGaO(3) and SrTiO(3)/LaAlO(3) interfaces is analyzed with a focus on the kinetic energy of the ablated species. LaGaO(3) and LaAlO(3) plasma plumes were studied by fast photography and space-resolved optical emission spectroscopy. Reflection high energy electron diffraction was performed proving a layer-by-layer growth up to 10(-1) mbar oxygen pressure. The role of the energetic plasma plume on the two-dimensional growth and on the presence of interfacial defects at different oxygen growth pressures is discussed in connection with the conducting properties of the polar/nonpolar interfaces. (C) 2010 American Institute of Physics. [doi:10.1063/1.3529487]

The properties of thin films of complex oxides, such as La1−xDxMnO3− D=Ba, Ca, Sr, etc., produced by pulsed laser deposition depend critically on the experimental parameters in which laser ablation is carried out. Here, we report a comparative analysis of the pulsed laser ablation process of La0.7Ba0.3MnO3−, in oxygen background, in the ambient pressure range from 10−2 to 1 mbar, typically employed in pulsed laser deposition of manganites. The laser ablation plume was studied by using time-gated imaging and optical emission spectroscopy techniques. It was found that at a pressure of 10−2 mbar, the plume species arriving at the substrate are characterized by hyperthermal kinetic energy 10 eV, and high degree of excitation. On the contrary, at larger oxygen pressure 0.1–1 mbar, the velocity of plume species reaching the substrate, and their degree of excitation are much reduced by the confining effects of the background gas. These features explain why an appropriate choice of the experimental conditions in which the deposition process is carried out leads to better quality films, providing helpful indications to improve control over the growth process of both La1−xDxMnO3− and other perovskitic oxides.

We have designed an experiment that could allow to investigate, on the same sample, the interplay between the morphological anisotropy of high critical temperature superconductors and the effects related to the order parameter symmetry. To this aim we have employed c-axis tilt biepitaxial grain boundary YBa2Cu3O 7 - δ Josephson junctions and measured the junction properties for different junction interface orientations. The results give evidence of the possibility to tune normal state resistance and critical current density of a Josephson junction almost independently of each other. We interpret the results of this experiment as an indication that Cooper pairs and quasi-particles transport properties may be ruled by different mechanisms. Measurements apparently give evidence of the possibility to determine to some extent the presence of “π-loops” and “0” loops along the same grain boundary line.

The so-called "polar catastrophe", a sudden electronic reconstruction taking place to compensate for the interfacial ionic polar discontinuity, is currently considered as a likely factor to explain the surprising conductivity of the interface between the insulators LaAlO3 and SrTiO3. We applied optical second harmonic generation, a technique that a priori can detect both mobile and localized interfacial electrons, to investigating the electronic polar reconstructions taking place at the interface. As the LaAlO3 film thickness is increased, we identify two abrupt electronic rearrangements: the first takes place at a thickness of 3 unit cells, in the insulating state; the second occurs at a thickness of 4-6 unit cells, i.e., just above the threshold for which the samples become conducting. Two possible physical scenarios behind these observations are proposed. The first is based on an electronic transfer into localized electronic states at the interface that acts as a precursor of the conductivity onset. In the second scenario, the signal variations are attributed to the strong ionic relaxations taking place in the LaAlO3 layer.

The blue-green photoluminescence emitted by pure and electron-doped strontium titanate under intense pulsed near-ultraviolet excitation is studied experimentally, as a function of excitation intensity and temperature. Both emission spectra and time-resolved decays of the emission are measured and analyzed in the framework of simple phenomenological models. We find an interesting blue-to-green transition occurring for increasing temperatures in pure samples, which is instead absent in doped materials. The luminescence yield and decay rate measured as a function of temperature can be modeled well as standard activated behaviors. The leading electron-hole recombination process taking place in the initial decay is established to be second-order, or bimolecular, in contrast to recent reports favoring a third-order interpretation as an Auger process. The temporal decay of the luminescence can be described well by a model based on two interacting populations of excitations, respectively identified with interacting defect-trapped (possibly forming excitons) and mobile charges. Finally, from the measured doping and sample dependence of the luminescence yield, we conclude that the radiative centers responsible for the luminescence are probably intrinsic structural defects other than bulk oxygen vacancies.

Superconductive thin films of Y-Ba-Cu-O have been grown in situ by r.f. magnetron sputtering from a single stoichiometric target on Al2O3 substrates. Zero-resistivity temperatures as high as 73 K have been achieved. X-ray and EDS analyses have been performed on the samples. The dependence of film features on deposition conditions is also discussed.

We study the structural, magnetic, and transport properties of LaAlO3/EuTiO3/SrTiO3 heterostructures grown by pulsed laser deposition. The samples are characterized in situ by electron diffraction and scanning probe microscopy and ex situ by transport measurements and x-ray absorption spectroscopy. LaAlO3/EuTiO3/SrTiO3 films show a ferromagnetic transition at T

Following the recent surge of interest in n-doped strontium titanate as a possible blue light emitter, a time-resolved photoluminescence analysis was performed on nominally pure, Nb-doped and oxygen-deficient single-crystal SrTiO3 samples. The doping effects on both the electronic states involved in the transition and the decay mechanism are respectively analyzed by comparing the spectral and dynamic features and the yields of the emission. Our time-resolved analysis, besides shedding some light on the basic recombination mechanisms acting in these materials, sets the intrinsic bandwidth limit of the proposed blue light emitting optoelectronic devices made of Ti-based perovskites heterostructures in the gigahertz range.

We report preliminary results on the optical driving and spectroscopic detection of Raman-active features in thin films of the high-temperature superconductor YBa2Cu3O7 in its normal phase by means of coherent Stokes and anti-Stokes Raman scattering, in a reflection geometry. Reference measurements on germanium are also reported. We observe phonon resonances which interfere coherently with a broad electronic resonance centered at zero frequency, giving rise to characteristic asymmetric spectral features. Our measurements provide a first step towards applying nonlinear optical wave-mixing spectroscopy to correlated electron systems. (c) 2008 Optical Society of America.

We present various concepts and experimental procedures to produce biepitaxial ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ grain-boundary Josephson junctions. The device properties have an interesting phenomenology, related in part to the possible influence of $\mathrm{``}\ensuremath{\pi}$ loops.'' The performance of our junctions and superconducting quantum interference devices indicates significant improvement in the biepitaxial technique. Further, we propose methods for fabricating circuits in which 0 and $\ensuremath{\pi}$ loops are controllably located on the same chip.

The competition between a-axis and c-axis growth in $R{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ $(R=\mathrm{r}\mathrm{a}\mathrm{r}\mathrm{e}\ensuremath{-}\mathrm{e}\mathrm{a}\mathrm{r}\mathrm{t}\mathrm{h}$ or Y) superconducting thin films has been analyzed. Stoichiometric and Nd-rich NdBaCuO thin films have been deposited by dc sputtering. The relative $a/c$-axis content has been estimated by x-ray diffraction and plotted as a function of deposition temperature. Comparison of obtained results with the experimental data available for other compounds yields clear evidence that a universal behavior, valid for $R{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ oxides, is obtained as a function of a normalized deposition temperature. A model for the nucleation of supercritical clusters from a multielemental adsorbed phase, allowing us to face the problem of phase competition in complex compounds, is presented. As a particular case, the competition between different phases of a single compound is analyzed as a function of supersaturation. A general behavior for the orientation dependence as a function of a normalized deposition temperature is thus obtained. It is shown that our experimental results about the relative $a/c$-axis content fall on the theoretical curve. The same procedure is also successfully applied to all the data derived from the literature regarding the relative $a/c$-axis content of other $R{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ oxides.

We report on the expansion dynamics of laser-produced plasma plumes of complex oxides in an oxygen atmosphere. In particular, we have studied the combined effects of background gas pressure and substrate heating on the plume propagation in typical pressure and temperature regimes of oxides thin film deposition by pulsed laser deposition. Our results evidence a reduced resistance of the background gas to the plume propagation as the substrate temperature increases. The experimental data are analyzed in the frame of a model describing the plume propagation into the background gas. Our experimental findings clearly indicate that the deposition temperature might influence film growth, not only through its direct thermal effect on the surface kinetics of adatoms, but also by affecting the energetic properties of the precursors in the gas phase. (c) 2007 Elsevier B.V All rights reserved.

A generic NbN Superconducting Tunnel Junctions (STJ) technology has been developed using conventional substrates (Si and SOI-SIMOX) for making THz spectrometers including SIS receivers and RSFQ logic gates. NbN/MgO/NbN junctions with area of 1 /spl mu/m/sup 2/, Jc of 10 kA/cm/sup 2/ and low sub-gap leakage current (Vm>25 mV) are currently obtained from room temperature sputtered multilayers followed by a post-annealing at 250/spl deg/C. Using a thin MgO buffer layer deposited underneath the NbN electrodes, ensures lower NbN surface resistance values (Rs=7 /spl mu//spl Omega/) at 10 GHz and 4 K. Epitaxial NbN [100] films on MgO [100] with high gap frequency (1.4 THz) have also been achieved under the same deposition conditions at room temperature. The NbN SIS has shown good I-V photon induced steps when LO pumped at 300 GHz. We have developed an 8 levels Al/NbN multilayer process for making 1.5 THz SIS mixers (including Al antennas) on Si membranes patterned in SOI-SIMOX. Using the planarization techniques developed at the Si-MOS CEA-LETI Facility, we have also demonstrated on the possibility of extending our NbN technology to high level RSFQ circuit integration with 0.5 /spl mu/m/sup 2/ junction area, made on large area substrates (up to 8 inches).

${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (YBCO) artificial grain-boundary Josephson junctions have been fabricated, employing a recently implemented biepitaxial technique. The grain boundaries can be obtained by controlling the orientation of the MgO seed layer and are characterized by a misalignment of the c axes (45\ifmmode^\circ\else\textdegree\fi{} c-axis tilt or 45\ifmmode^\circ\else\textdegree\fi{} c-axis twist). A detailed characterization of the Josephson properties has been carried out, showing remarkable differences in the transport parameters of tilt and twist junctions. High-resolution electron microscopy showed the presence of perfect basal plane faced boundaries in the cross sections of tilt boundaries; however interfaces meandering along the sample thickness are also sometimes observed. The correlation between transport properties and microstructure is investigated and the possibility to employ these junctions to explore the symmetry of the order parameter is presented. On the basis of the analysis of microstructural data and of a qualitative growth model for (103) YBCO grains, it is suggested that clean basal plane boundaries exhibiting uniform Josephson properties could be obtained by this technique.

(110) MgO thin films have been deposited by RF sputtering on (110) SrTiO3 and used as buffer layers for YBCO deposition. The MgO films show high morphological quality, as confirmed by X-ray specular reflectivity, and narrow (approximate to 1 degrees) X-ray diffraction peaks in the rocking curves measurements. These results are discussed in the framework of an ionic oxide growth model. XRD analyses performed on the bilayer YBCO/MgO (110) confirm the epitaxial growth of the films. with cool) YBCO//(110) MgO. XRD, AFM, SEM measurements are compared with data relative to bilayers deposited on (100) SrTiO3.

The colossal magnetoresistance (CMR) manganese oxides, have been the subject of intense studies. To elucidate the mechanisms at work in the ferromagnetic metallic state is essential for the comprehension of the M–I transition and the associated CMR effect. We present here comparison among resistivities behavior in Ca and Sr-doped manganite films (x = 0.3) grown by different techniques and on several substrates allowing to analyze samples with different amounts of disorder. At low temperatures, a dominant T 2 term in the resistivity has generally been observed. Our analysis shows that for residual resistivity values larger than a critical one, there is a substantial deviation from the T 2-like behavior and that an unusual T 2.5 one is robust. This behavior supports the theoretical proposal of single magnon scattering in the presence of minority spin states localized by the disorder. In the high temperature insulating paramagnetic phase the resistivity shows the activated behavior characteristic of polaronic carriers. Finally in the whole range of temperatures the experimental data are found to be consistent with a phase separation scenario.