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Two-dimensional electron gases (2DEGs) formed at oxide interfaces show a large variety of functional properties of major physical interest. Here, the peculiar electric transport behavior of the 2DEG formed at the LGO/STO oxide interface is studied under the application of light pulses of different amplitude, duration, and repetition rate, and by varying the sample temperature from 8 to 300 K. The experimental results evidence a persistent photoconductivity, intimately related to the complex physics of this system. These findings suggest the possibility of using the oxide interfaces for advanced applications as, for example, energy conversion or information storage.

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.

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.

Weak ferromagnet/superconductor (F/S) proximity bilayers have been fabricated and characterized for basic physics studies concerning the ultrafast carrier dynamics in layered materials. The normalized reflectivity change (Δ R / R ) as a function of the time delay between the pump and the probe laser beams has been measured in F/S heterostructures formed by a low critical temperature superconductor (Nb) with a NiCu overlayer, and a high T c superconductor (YBCO) covered by Au/NiCu and manganite (LSMO) overlayers. The attention is mainly focused to the investigation of nonequilibrium excitation dynamics inside different bilayers in the low temperature region. The presence of the weak ferromagnetic overlayer produces faster optical relaxation times with respect to sole superconducting films. The results are promising in view of potential applications as ultrafast kinetic inductance superconducting photodetectors as confirmed by preliminary time-resolved photoimpedance experiments on both Nb and NiCu(21 nm)/Nb(70 nm) samples.

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 present the results of collaboration between the groups of Naples and Genoa Universities/INFM, focused on thin films superconducting borocarbides. In particular a careful study of the deposition of non-magnetic LuNi 2 B 2 C thin films by Pulsed Laser Deposition (PLD) on MgO and other substrates will be presented. While samples with good c-orientation and superconducting properties are obtained in wide range of deposition parameters, in-plane micrometric oriented structures are obtained only in a narrow range of conditions, as evidenced by X-rays measurements and AFM (atomic force microscopy) analyses. A special set-up for the in situ interchanging of shadow masks has been developed in order to deposit the diferent patterned layers and different buffer and/or barrier layers. New results on the temperature dependence of the LuNi 2 B 2 C gap using S/N contact junctions are presented and discussed. The obtained value of the superconducting gap and its temperature dependence agrees very well with the BCS prediction. We present also microwave measurements of the surface impedance performed on large area films obtained by sputtering technique on low loss sapphire substrates. The data confirm the overall BCS-like behavior of these compounds.

(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.

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.

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 present novel low-field magnetoresistive devices based on the ferromagnetic manganite La0.7Sr0.3MnO3 operating in the current in plane (CIP) configuration. In these planar spin-valve devices, a focused Ga+ beam is used to create pinning centers for magnetic domain walls. The spin-dependent scattering of polarized electrons at the domain walls (DW) is responsible for the magnetoresistance observed in the patterned tracks. The magneto-transport properties of these devices are interpreted within a model for DW magnetoresistance. Applications such as magnetic data storage can be envisaged for the structures we investigated. (C) 2006 Elsevier B.V. All rights reserved.

Superconducting thin films of the Tl 2 Ba 2 CaCu 2 O x (2212) and Tl 2 Ba 2 Ca 2 Cu 3 O x (2223) phases have been prepared through a combined approach of MOCVD and thallium vapour diffusion. The effects due to the nature of the BaCaCuO(F) precursor matrix on the formation of the TlBaCaCuO phase film have been investigated. Fluorinated [BaCaCuO(F)] matrices have been deposited from Ba(hfa) 2 ·tetraglyme, Ca(hfa) 2 ·tetraglyme and Cu(acac) 2 via metalorganic chemical vapour deposition (MOCVD) on yttria-stabilized zirconia (YSZ) substrates. Both fluorinated [BaCaCuO(F)] and unfluorinated (BaCaCuO) matrices proved suitable for vapour diffusion thallium incorporation using selected Tl 2 O 3 :BaO:CaO:CuO mixtures. The formation of the superconducting 2212 and 2223 films is discussed depending upon the fluorinated and unfluorinated nature of the precursor matrices.

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

We present femtosecond pump-probe spectroscopy studies of time-resolved optical reflectivity of all-oxide YBa2Cu3O7/La0.7Sr0.3MnO3 (YBCO/LSMO) superconductor/ferromagnet (S/F) bilayers consisting of a 100-nm-thick YBCO base layer and either 10 or 35 nm LSMO cap thickness. At temperatures far below the YBCO superconducting transition T-C, samples with a 10 nm F overlayer show a photoresponse that is similar to, but faster than, pure-YBCO, 100-nm-thick reference samples, while close to T-C and above (up to similar to 160 K) we observe a signature of both the electronic and spin response that cannot be interpreted as an incoherent sum of contributions from the two layers. The photoresponse of the S/F structures with the 35-nm LSMO layer always qualitatively follows that of the pure LSMO, but with a shorter relaxation time. In all cases, the YBCO/LSMO nonequilibrium dynamics can be modeled using a generalized multitemperature model, which is a superposition of the dynamics of the three-temperature models that are used to describe the superconductor and ferromagnet subsystems, respectively. The long term of the photoresponse signal of the S/F bilayer can be well fitted with the two characteristic relaxation times. DOI: 10.1103/PhysRevB.87.134514

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 present various concepts and experimental procedures to fabricate biepitaxial YBa 2Cu 3O 7-x grain boundary Josephson junctions. Different types of structures can be realized including "π-loops" in which one of the junctions is intrinsically π-phase shifted. Due to its versatility the biepitaxial technique offers the possibility to investigate basic aspects of the physics of grain boundary Josephson junctions, and also the influence of intrinsic and extrinsic d-wave induced effects in transport properties. Further, we discuss the possibility to produce circuits in which "0-" and "π-loops" are controllably located on the same chip, and intrinsic d-wave effects can be exploited.

YBa 2 C 3 O 7− δ artificial grain boundary Josephson junctions have been fabricated by a recently implemented biepitaxial technique, employing MgO and CeO 2 as seed layers. The Josephson properties of asymmetric grain boundaries (GBs) characterised by a misalignment of the c -axis (45° c -axis tilt or twist) are presented. A comparison with GBs characterised by 45° c -axis tilt or twist and by an additional rotation of 45° of the a – b planes of one electrode has been also performed. The differences observed in the transport properties have been interpreted in terms of d x 2 − y 2 wave symmetry of the order parameter and faceting of the grain boundary.

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.

Phase-sensitive, spatially resolved optical second-harmonic-generation experiments were performed on LaAlO${}_{3}$/SrTiO${}_{3}$ heterostructures. Lateral inhomogeneities on a length scale of $\ensuremath{\approx}30$ $\ensuremath{\mu}$m are found when a one-unit-cell-thick epitaxial monolayer of LaAlO${}_{3}$ is grown on TiO${}_{2}$-terminated SrTiO${}_{3}$ single crystals. The inhomogeneity is absent in samples with LaAlO${}_{3}$ layers of more than one unit cell. The results are discussed in the framework of electronic, oxidic, and chemical inhomogeneities.

An extensive surface characterization of hydrofluoric acid (HF) etched and annealed SrTiO3 single crystals, vacuum-annealed below 300 degrees C, reveals the formation of a two-dimensional electron gas (2DEG). A joint scanning tunneling spectroscopy and low-energy electron diffraction analysis allows us to associate the surface metallic state (characterized by the presence of a nonzero density of states close to the Fermi level) with the low-temperature-annealed highly ordered 1 x 1 reconstructed SrTiO3 surface hosting two-dimensional carriers. Meanwhile, a gap opens in the tunneling spectrum of 2 x 1 reconstructed, high-temperature-annealed surfaces. X-ray photoemission spectroscopy shows that the metallic state is associated with the surface formation of Ti3+. Recently published photoemission data demonstrated the formation of a 2DEG on the surface of cleaved SrTiO3, while scanning tunneling spectroscopy on crystals heated at high temperature revealed gaplike features: Our results can help reconcile this seemingly contradicting phenomenology observed so far by scanning tunneling spectroscopy and photoemission spectroscopy.

Using high-resolution transmission electron microscopy and image simulation techniques in combination with ab initio calculations, we show the existence of two different superlattices of crystallographic shear planes, analogous to the Magnéli phases of rutile, in oxygen-deficient films of anatase TiO2 epitaxially grown on LaAlO3 substrates. (103)- and (101)-oriented shear plane structures are detected in the outer film region and in proximity of the film/substrate interface, respectively. We show that these shear planes are characterized by TiO-like cubic local structures, which can deviate from the TinO2n-1 stoichiometry of the classical rutile-derived Magnéli phases, particularly in the outer part of the film. Computed formation energies provide insights into the thermodynamic stability of the observed structures and their relations to the growth dynamics.

The polarization induced at the LaAlO${}_{3}$/SrTiO${}_{3}$ interface has been explored by optical second harmonic generation (SHG) spectroscopy. Our data reveal that an orbital reconstruction of the SrTiO${}_{3}$ surface occurs when three epitaxial monolayers of LaAlO${}_{3}$ are deposited on top of it. The reconstruction manifests as a spectral modification of a Ti${}^{4+}(3d)$ state at the LaAlO${}_{3}$/SrTiO${}_{3}$ interface which is accompanied by a displacement of the Ti${}^{4+}$ ions in adjacent SrTiO${}_{3}$ 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 LaAlO${}_{3}$ coverage.

In order to analyze the effect of strain on the magnetic properties of narrow-band manganites, temperature- and field-dependent susceptibilities of about 8.5-nm-thick epitaxial ${\text{Pr}}_{0.7}{\text{Ca}}_{0.3}{\text{MnO}}_{3}$ films, respectively, grown on (001) and (110) ${\text{SrTiO}}_{3}$ substrates, have been compared. For ultrathin samples grown on (001) ${\text{SrTiO}}_{3}$, a bulklike cluster-glass magnetic behavior is found, indicative of the possible coexistence of antiferromagnetic and ferromagnetic phases. On the contrary, ultrathin films grown on (110) substrates show a robust ferromagnetism with a strong spontaneous magnetization of about $3.4{\ensuremath{\mu}}_{\text{B}}/\text{Mn}$ atom along the easy axis. On the base of high-resolution reciprocal space mapping analyses performed by x-ray diffraction, the different behaviors are discussed in terms of the crystallographic constraints imposed by the epitaxy of ${\text{Pr}}_{0.7}{\text{Ca}}_{0.3}{\text{MnO}}_{3}$ on ${\text{SrTiO}}_{3}$. We suggest that for growth on (110) ${\text{SrTiO}}_{3}$, the tensile strain on the film $c$ axis, lying within the substrate plane, favors the ferromagnetic phase, possibly by allowing a mixed occupancy and hybridization of both in-plane and out-of-plane ${e}_{\text{g}}$ orbitals. Our data allow to shed some physics of inhomogeneous states in manganites and on the nature of their ferromagnetic insulating state.

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 report on the observation in current–voltage characteristics of YBa2Cu3O7−δ (YBCO) step-edge Josephson junctions of resistive branches generated by a current injected close to the grain-boundary region through two symmetrical Au/YBCO contacts. The current amplitude and voltage tunability of the branches is measured as a function of injected current and sample temperature and the observed effects are explained in terms of flux-flow dynamics in long Josephson junctions. As the voltage of the branches can be tuned up to 2.5 mV, the investigated phenomenon is also appealing for practical applications in the submillimeter-wave range of the electromagnetic spectrum. We estimate the available power and the linewidth of the electromagnetic radiation associated with typical bias points.

YBa2Cu3O7-x 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 degrees a-axis tilt or 45 degrees a-axis twist). These types of grain boundaries are still mostly unexplored. We carried out a complete characterization of their transport properties and microstructure. Junctions and de SQUIDs associated with these grain boundaries exhibit an excellent Josephson phenomenology and high values of the ICRN product and of the magnetic flux-to-voltage transfer parameter respectively. Remarkable differences in the transport parameters of tilt and twist junctions have been observed, which can be of interest for several applications. A maximum speed of Josephson vortices as calculated from the voltage step values of the order of 2 x 10(6) m s(-1) is obtained. These devices could also have some impact on experiments designed to study the symmetry of the order parameter, exploiting their microstructure and anisotropic properties. High-resolution electron microscopy showed the presence of perfect basal plane faced boundaries in the cross sections of tilt boundaries.

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 ${\text{LaAlO}}_{3}$ and ${\text{SrTiO}}_{3}$. 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 ${\text{LaAlO}}_{3}$ 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 ${\text{LaAlO}}_{3}$ 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.

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.

The thermal stability of the superconducting phase of nominal composition YBa2Cu3O7−x-sintered pellets has been studied with respect to different temperatures (ranging from 300 to 950° C), time (ranging from 1 to 72 h), oxygen partial pressure (from 4 Pa to 1 atm) and carbon dioxide partial pressure (from 10−4 Pa to 1 atm). Annealed samples were characterized by X-ray diffraction analysis, optical microscopy, and resistive measurements of the superconductive transition temperature. A stability field of the orthorhombic and tetragonal phases was obtained, showing a region of coexistance. The decomposition of the 1 2 3 phase is found to be strongly influenced by the presence of a small amount of CO2 (1 p.p.m.) in the sintering atmosphere. A sintering process is proposed to avoid the formation of by-products.

We investigated the nanosecond-scale time decay of the blue-green light emitted by nominally pure $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ following the absorption of an intense picosecond laser pulse generating a high density of electron-hole pairs. Two independent components are identified in the fluorescence signal that show a different dynamics with varying excitation intensity and which can be modeled as a bimolecular and unimolecolar process. An interpretation of the observed recombination kinetics in terms of interacting electron and hole polarons is proposed.

The authors have performed experiments on current-induced domain wall (DW) displacement in La0.7Sr0.3MnO3 nanostructures patterned by gallium (Ga) focused-ion-beam milling. A dc current is found to assist or hinder, according to polarity, an external magnetic field in the depinning of a DW trapped in a nanoconstriction. For large enough currents, the DW depinning occurs in the absence of external magnetic field. The depinning current depends on the transverse anisotropy constant of the region toward which the DW is displaced.

The resistivity of thin La0.7Sr0.3MnO3 films was first investigated in a wide temperature (T) range (10-750 K). Films grown by different techniques and on several substrates enabled to analyze samples with different amounts of disorder. The aim of this work was to elucidate the nature of the metal-insulator (M-I) transition occur-ring at T= T-p in these films and its relation with the different kinds of inhomogeneities they could present like intrinsic electric disorder and co-existence of two different electrical and/or magnetic phases. The low-temperature resistivity state was described mostly by a law which scales as T-alpha with alpha approximate to 2.5. This supports the theoretical proposal of single magnon scattering in presence of minority spin states localized by the disorder. In the whole range of temperatures the experimental data are found to be consistent with a phase separation (PS) scenario. In order to go through the origin of the characteristic length scale of inhomogeneity found, preliminarily low frequency noise measurements as a function of T in a range of temperature around the M-I transition were made. The samples used were patterned using photolithography into bridges with various widths and lengths. No clear sign of separation phase dynamic has been observed in our noise measurements. Unexpectedly the normalized Hooge parameter alpha(H)/n was found not to be volume (Q) independent. The LSMO electrical properties may strongly be driven by disorder and new design for magnetoresistance sensors may have to take into account their intrinsic PS. (C) 2007 Published by Elsevier Ltd.

The structure of SrTiO3 (100) (STO) single crystal surfaces has been investigated by grazing incidence x-ray diffraction. The authors found that chemically etched STO is composed similar to 75% of an ideal TiO2 surface layer and similar to 25% of SrO and it is very stable when annealed in ultrahigh vacuum or in O-2 at high temperatures. A monolayer of SrO, epitaxially grown by pulsed laser deposition, is found to cover only a fraction of the TiO2 layers and exhibits a 2x2 reconstruction with strong structural changes after annealing in conditions typically used for the deposition of oxides. (c) 2007 American Institute of Physics.

We investigate the structural, chemical, and magnetic properties on BiFe0.5Cr0.5O3 (BFCO) thin films grown on (001) (110) and (111) oriented SrTiO3 (STO) substrates by x-ray magnetic circular dichroism and x-ray diffraction. We show how highly pure BFCO films, differently from the theoretically expected ferrimagnetic behavior, present a very weak dichroic signal at Cr and Fe edges, with both moments aligned with the external field. Chemically sensitive hysteresis loops show no hysteretic behavior and no saturation up to 6.8 T. The linear responses are induced by the tilting of the Cr and Fe moments along the applied magnetic field. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

(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 magnetic and magnetotransport properties of La0.7Sr0.3MnO3/Permalloy heterostructures are investigated. La0.7Sr0.3MnO3 films were grown on (110) SrTiO3 in order to induce an in-plane anisotropy with the easy axis along the [001] direction. An easy axis of magnetization was induced in the Permalloy in the same direction through the application of a magnetic field during the growth. The magnetic characterization of the bilayer is presented and discussed. The transport properties of devices operated in the current perpendicular to plane mode are shown. The possibility of fabricating magnetoresistive devices by simply depositing a ferromagnetic counterelectrode on a manganite film is demonstrated. (c) 2006 American Institute of Physics.

We have studied the effect of thickness on the structure and superconductivity of ${\mathrm{Nd}}_{1.2}{\mathrm{Ba}}_{1.8}{\mathrm{Cu}}_{3}{\mathrm{O}}_{z}$ (NdBCO) epitaxial films deposited on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ (100) substrates. High-quality NdBCO films were deposited by high oxygen pressure sputtering. The samples have been carefully characterized by transport measurements and x-ray diffraction using conventional and synchrotron radiation sources. Two structural transformations are observed. The first one is a crossover from the orthorhombic and twinned structure, typical of thick samples, to a pseudotetragonal phase at thickness lower than 60 unit cells. The pseudotetragonal phase exhibits peculiar x-ray diffuse scattering around the Bragg reflections, associated with nanostructured twinned domains. The second one appears at a thickness lower than 20 unit cells and is assigned to a pseudotetragonal-tetragonal transition. The transport properties are not modified at the first crossover, while a substantial decrease in ${T}_{c}$ and in the number of holes is observed at the pseudotetragonal-tetragonal transformation. Our results suggest that the structural rearrangement and the transport properties are explained by the ordering of Cu(1)O chains. A simple model, able to explain the experimental results and capturing the essential features of the Cu(1)O chains nucleation process in the ${\mathrm{R}}_{1+x}{\mathrm{Ba}}_{2\ensuremath{-}x}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ ($\mathrm{R}=\mathrm{Y}$ and rare-earth elements) compounds, is presented.

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.

The resistivity of thin $La_{0.7}A_{0.3}MnO_{3}$ films ($A=Ca, Sr$) is investigated in a wide temperature range. The comparison of the resistivities is made among films grown by different techniques and on several substrates allowing to analyze samples with different amounts of disorder. In the low-temperature nearly half-metallic ferromagnetic state the prominent contribution to the resistivity scales as $T^{\alpha}$ with $\alpha \simeq 2.5$ for intermediate strengths of disorder supporting the theoretical proposal of single magnon scattering in presence of minority spin states localized by the disorder. For large values of disorder the low-temperature behavior of the resistivity is well described by the law $T^{3}$ characteristic of anomalous single magnon scattering processes, while in the regime of low disorder the $\alpha$ exponent tends to a value near 2. 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 also in films doped with strontium ($A=Sr$)., Comment: 5 figures

High quality, very flat ${\mathrm{Nd}}_{1+x}{\mathrm{Ba}}_{2\ensuremath{-}x}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ films have been grown by sputtering and analyzed by low energy electron diffraction, scanning tunneling microscopy (STM) and grazing incidence x-ray diffraction (GIXD) employing synchrotron radiation, in order to investigate the surface structure and morphology. The refinement of the GIXD data has been performed on the basis of structural models sensitive to the nature of the terminating layer. The interpretation of the results provides a picture of the surface structure that is in full agreement with STM results. The surface is composed by two terraces with different termination, one of which is an ordered and complete Cu(1)-O layer and the other an incomplete BaO layer that partially covers a disordered Cu(1)-O layer. Atomic vacancies and steps bounding terraces with a height of about $0.4\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, are present on the surface.

Clear evidence about the influence of growth orientation on the stoichiometry of sputtered oxide thin films is reported and discussed. The growth of stoichiometric and of non-stoichiometric films is respectively obtained, from the same CaRuO3 target, on the surface of (110) and (100) perovskite substrates. Such phenomenon has been systematically investigated by deposition of pairs of samples in the same deposition run. Our data show that in samples deposited on (100) perovskites, Ca excess leads to the formation of a single Ca-rich phase with stoichiometry Ca1 + x Ru1-x O3, resulting in a dramatic effect on transport properties. Structural characterization based on X-ray diffraction proves that the orthorhombic structure is preserved in a wide stoichiometry range. The striking evidence that substrate orientation influences film stoichiometry is discussed in terms of growth kinetics and chemical properties of sputtered species.

A detailed study of the correlation between the superconducting critical temperature and the strain in as-grown, high-quality, c-axis epitaxial, Nd-doped YBa 2 Cu 3 O 7 - Φ (YBCO) films is reported. Samples with thicknessesranging from 8 to 250 unit cells have been deposited by high-pressure oxygen sputtering on LaAlO 3 (100) single-crystal substrates. The a, b, and c axis of the films have been accurately determined by x-ray diffraction from the measurements of the (005) and the (038)-(308) reflections. Below a certain thickness, a crossover from orthorhombic to tetragonal structure is observed, together with a decrease of the critical temperature. These results cannot be explained by the elastic deformation of the film, but point to inelastic strain induced by the large mismatch with the substrate related to an oxygen reorganization in the Cu(1)-O plane.

CaRuO3 thin films have been grown by on axis sputtering on various (110) SrTiO 3 , (110) LaAlO 3 , and (100) NdGaO 3 substrates. The growth of high-quality twin free samples, having the a axis aligned with the substrate normal, and the b and c axis aligned with the principal in-plane substrate directions, is demonstrated. The mechanism of domain selection is addressed in terms of free energy minimization during the tetragonal to orthorhombic transition, taking into account both strain and surface energy contributions. On the base of experimental data, it is shown that minimization of surface energy plays a major role in determining film orientation. Obtained samples are especially suited for the investigation of anisotropy in the macroscopic properties of Ca/Sr ruthenates.