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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.

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 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.

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.

The proportional regime of a double superconducting tunnel junction detector is described in terms of the diffusion equation of the nonequilibrium quasiparticles excited in the absorber by the impinging radiation. The expression of the output current signals through the junctions is obtained. Some numerical results are shown and discussed also in connection with the operation of the detector in the charge collection mode.

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.

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 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.

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.

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.

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.

An innovative biepitaxial technique for the fabrication of YBa2Cu3O72d artificial grain-boundary Josephson junctions is proposed. The structure was specifically designed to analyze the effects of anisotropy, both in the order parameter and in the crystallographic structure of YBa2Cu3O72d , on the Josephson current crossing the artificial grain boundary. Samples are grown on vicinal ~110! oriented SrTiO3 substrates, and a CeO2 film is employed as a seed layer. Crystallography and morphology of the samples are analyzed, thus allowing us to infer the nature of the grain boundary. Our measurements show that the obtained grain boundary, besides possessing a unique and complex structure, exhibits very peculiar Josephson and tunnel-like properties. The intrinsic flexibility of the biepitaxial technique is employed to modify the in-plane angle of the grain-boundary interface, allowing for the realization of symmetrical, asymmetrical, and intermediate configurations on the same substrate. Accordingly, junctions exhibiting different kinds of magnetic field dependence of the Josephson current are obtained on the same chip. Evidence of d-wave-induced effects and of a dielectric character of the barrier is obtained by the transport measurements.

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 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 YBa2Cu3O7-delta 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 "pi-loops" and "0" loops along the same grain boundary line.

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.

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

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.

High quality STJs of type Nb/Al/AlxOy/Al/Nb and Nb/Al/AlxOy/Nb/NbN, in which Al and Nb act as quasiparticle traps , were realized and tested under 6 KeV X ray radiation. The experimental results show an enhancement of the collected charge with respect to basic Nb/Nb configuration. An energy resolution of 300 eV was achieved for a 50x50 μm2 NbN-based STJ.

The plume expansion dynamics strongly affects the growth and the chemistry of pulsed laser deposited thin films. The interaction with the background gas determines the kinetic energy of the species impinging on the substrate, their angular broadening, the plasma chemistry, and eventually the cations stoichiometric ratio in oxide films. Here, we exploit two-dimensional, spectrally resolved plume imaging to characterize the diverse effects of the oxygen background pressure on the expansion dynamics of La, Ga, and LaO species during pulsed laser deposition of LaGaO3. The propagation of the ablated species towards the substrate is studied for background oxygen pressures ranging from high vacuum up to ≈10−1 mbar. Our experimental results show specie-dependent effects of the background gas on the angular distribution of the precursors within the plume. These findings suggest that even in the presence of a stoichiometric ablation and of a globally stoichiometric plume, cations off-stoichiometry can take place in the forefront portion of the plume impinging on the substrate. We show that such effect can be compensated by a proper choice of process parameters.

The effects of the order parameter symmetry and the interfere grain boundary orientation on the transport properties of YBa2Cu3O7-x biepitaxial grain boundary junctions have been investigated. 45 degrees [100] tilt and twist junctions on SrTiO3 [110] and MgO [110] substrates by employing respectively MgO and SrTiO3 as seed layers have been fabricated. More complex junctions by using CeO as seed layers have also been fabricated and analyzed. The analysis of experimental data points towards a predominant d-wave symmetry of the superconductive order parameter.

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.

(110) and (103) YBa2Cu3O7 films have been grown onto exact and vicinal (110) SrTiO3 substrates, and on vicinal (110) MgO substrates with a SrTiO3 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) YBa2Cu3O7 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 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.

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.

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.

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.. Y2O3 precipitates are found in 103. YBCO films grown on 110. SrTiO3 STO. substrates. On the same biepitaxial samples, Y2BaCuO5 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 Y2O3 precipitates

YBa(2)Cu(3)O(7-delta) (YBCO) grain boundaries characterized by a misalignment of the c-axes (45 degrees c-axis tilt or 45 degrees c-axis twist) have been obtained by employing a recently implemented biepitaxial technique. Junctions based on these grain boundaries exhibit good Josephson properties useful for applications. High values of the I(C)R(N) product and a Fraunhofer-like dependence of the critical current on the magnetic field, differently from traditional biepitaxial junctions, have been obtained. The correlation between transport properties and microstructure has been investigated by Transmission Electron Microscopy (TEM), which was also performed on previously measured junctions. The presence of atomically clean basal plane (BP) faced tilt boundaries, among other types of interfaces, has been shown. The possibility of selecting these kinds of boundaries by controlling film growth, and their possible advantages in terms of reproducibility and uniformity of the junction properties an discussed. The possibility of employing these junctions to explore the symmetry of the order parameter is also discussed.

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 transport and noise properties of Pr_{0.7}Ca_{0.3}MnO_{3} epitaxial thin films in the temperature range from room temperature to 160 K are reported. It is shown that both the broadband 1/f noise properties and the dependence of resistance on electric field are consistent with the idea of a collective electrical transport, as in the classical model of sliding charge density waves. On the other hand, the observations cannot be reconciled with standard models of charge ordering and charge melting. Methodologically, it is proposed to consider noise-spectra analysis as a unique tool for the identification of the transport mechanism in such highly correlated systems. On the basis of the results, the electrical transport is envisaged as one of the most effective ways to understand the nature of the insulating, charge-modulated ground states in manganites., Comment: 6 two-column pages, 5 figures