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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.

A model to describe the linear regime in a Superconducting Tunnel Junction detector has been developed. In the limit of low temperatures, the only effective processes which involve quasi-particles and phonons are the phonon scattering mechanisms and the phonon escape from the area of the detector. The response of the STJ is linear if τ esc −1 >; τ ph −1 for low energy phonons and τ esc −1 τ ph −1 for high energy phonons. The estimate of the mean energy loss ϵ by radiation per excess quasi-particle in the STJ is in agreement with previous determinations. A comparison with some experimental results taken from literature has been produced.

A theoretical analysis of the linear regime in a superconducting tunnel junction (STJ) has been carried out on the basis of a kinetic approach to the non-equilibrium superconductivity induced in the detector by an X-ray. A solution of the diffusive equation of the quasi-particles has been obtained assuming very slow recombination processes and an equilibrium 2-DELTA-phonon distribution. Under these conditions, the response of the detector is linear, that is the charge collected by the detector itself is proportional to the energy released. A relation between this released energy and the collected charge has also been obtained, thus achieving an expression of the mean energy-epsilon per excess quasi-particle. A comparison with some experimental results taken from current literature has been also reported.

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.

A YBa2Cu3O7-x artificial grain boundary Josephson junction was fabricated on to a (110) SrTiO3 substrate employing a MgO seed layer, YBa2Cu3O7-x grows mainly oriented (103) on SrTiO3 and (001) on the seed layer. An artificial 45 degrees a or b axis tilt or twist boundary can occur according to the orientation of the MgO edge with respect to the (001) in plane direction of the substrate. Josephson junctions of good quality and excellent reproducibility were obtained in microbridges patterned across the grain boundary, Typical features of the phenomenology of high critical temperature superconductor (HTCS) Josephson junction were observed. The maximum value of the ICRN product is of the order of 2-3 millivolts at T=4.2 K and of the order of 50 mu V at T=77 K respectively, The modulation of the critical current with the external applies magnetic field confirms the Josephson nature of the current in the weak link

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.

(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.

A novel fabrication procedure of high critical temperature superconductor Josephson junctions (HTCSJJ) has been developed by a 90 degrees rotation of YBa2Cu3O7-x c-axis around an in-plane direction, on the basis of concepts of the biepitaxial technique. YBa2Cu3O7-x grows oriented along (001) direction on a MgO seed layer deposited on a (110) SrTiO3 substrate and along (110) direction on the bare substrate. Josephson junctions of good quality were obtained exhibiting RSJ behavior and features characteristic of HTCSJJ phenomenology. Even if not uniform, in some samples, the nature of the critical current I-c is completely Josephson, as resulting from the I-c dependence on the applied magnetic field. The maximum measured I(c)R(N) value at T=4.2 K is 2 mV.

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.

High quality, very flat Nd1.2Ba1.8Cu3Oy thin films have been grown by sputtering on TiO2 terminated SrTiO3 (100) single crystals and their surface structure have been determined by using Grazing Incidence X-ray Diffraction (GIXD) technique, employing synchrotron radiation. A 52 unit cells film presents a double terminated surface composed by a complete and ordered Cu(1)–O plane and a disordered Cu(1)–O plane partially covered by a BaO layer, in full agreement with scanning tunneling microscopy data. A preliminary comparison between the structure of 8 and 52 u.c. NdBCO films will be 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.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

The effects of the order parameter symmetry and the interface grain boundary orientation on the transport properties of YBa/sub 2/Cu/sub 3/O/sub 7-x/ biepitaxial grain boundary junctions have been investigated. 45/spl deg/ [100] tilt and twist junctions on SrTiO/sub 3/ [110] and MgO [110] substrates by employing respectively MgO and SrTiO/sub 3/ 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.

Some of the main anomalies in conductance characteristics of high-Tc tunnel junctions are considered. Mainly, the extreme depression of gap structures, the presence of conductance peaks in corrispondence to the counterelectrode gap, and ,,zero-bias anomalies~, will be examined. The possibility of application of the proximity Mc Millan model to describe the behaviour in conductance of high-T c junctions is considered. Discussion and preliminary comparison with experimental data on YBCO junctions are also reported

We report on the characterisation of a new Josephson device, based on a YBa2Cu3O7 (YBCO) step edge junction and a control line. The control current is injected into the region close to the GB region through two Au/YBCO contacts. Feeding the device with a suitable control current, linear branches are observed in the current-voltage characteristic of the step edge junction, in the range 0.5–2.5 mV. No similar effects appear when an external magnetic field is applied. The stability of the linear branches and their wide-range voltage tunability are such that applications can potentially be developed in the field off submillimiter-wave generation and detection.

A study of the 2D vs 3D growth in c-axis Nd 1+x Ba 2x Cu 3 O 7−δ films and a study of the crossover between a-axis to c-axis growth in the Re 1+x Ba 2−x Cu 3 O 7−δ (RBCO) family is reported. We found that c-axis stoichiometric Nd 1 Ba 2 Cu 3 O 7 films exhibit a 3D layered island-growth, possibly originated by screw and half loop dislocations. On the contrary, a continuous 2D growth is found in off-stoichiometry films, characterized by Nd excess. The amount of a-axis in Nd 1+x Ba 2−x Cu 3 O 7 films deposited at different temperatures has been carefully determined by XRD. The experimental results are interpreted in the framework of a theoretical model of the nucleation of RBCO films. The model provides a quantitative description and a satisfactory fit of experimental data from our experiment and from the current litterature.

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. (C) 1999 Elsevier Science B.V. All rights reserved.

A four terminal Josephson device, based on quasiparticle injection into an YBCO step edge Josephson junction was analysed. Two injector junctions are defined on both sides of the step edge junction using native Au/YBCO barrier. The Josephson critical current is reduced by increasing the injection current. The results are explained in terms of a non equilibrium state of the YBCO electrode under quasiparticle injection.

(103) YBCO films have been grown onto exact and vicinal (110) STO substrates, and on vicinal (110) MgO substrates with a STO buffer layer. The samples are carefully characterised by reciprocal space mapping (RSM) with X-ray diffraction, in order to investigate the features of the typical double domain of (103) YBCO structure. It is demonstrated that the STO buffer grown on vicinal MgO is (110) oriented, and that it preserves the vicinal cut of the substrate. YBCO films deposited on such STO buffer layers present a single (103) domain.

We have studied the effect of thickness on the structure and superconductivity of Nd1.2Ba1.8Cu3O7+x (NdBCO) epitaxial films deposited on SrTiO3 (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 to nanostructured twinned domains. The second one appears at 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 of T-c and of 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(I)O chains. (c) 2007 Elsevier B.V. All rights reserved.

TlBaCaCuO superconducting films have been fabricated on LaAlO3 (100) buffer layers, grown by metal-organic chemical vapour deposition (MOCVD) on SrTiO3 (100) substrates. The usual two step procedure, involving the MOCVD deposition of BaCaCuO(F) matrices and a thallium vapour diffusion process, has been adopted for the preparation of the TlBaCaCuO films. The films grown on the LaAlO3 (100)/SrTiO3 (100) substrates area-axis oriented as indicated by both X-ray diffraction measurements and scanning electron micrographs. The morphological nature of the LaAlO3 buffer layer seems to be responsible for the formation ofa-axis-oriented domains, since simultaneous fabrications of TlBaCaCuO films on commercial LaAlO3 (100) and LaAlO3 (100)/SrTiO3 (100) substrates givec-axis anda-axis-oriented films, respectively. The MOCVD grown LaAlO3 buffer layers are highly (100) oriented, the full width at half maximum (FWHM) of the rocking curve of the (100) peak being as small as 0.16°.

A simple model of nucleation of YBCO thin films deposited on different substrates by vapour deposition is reported. The role of the surface energy of the smallest crystallites formed at the very early stage of film growth is discussed. Information on the surface Gibbs energy of YBCO is obtained from experimental results reported in the literature, and in particular from the analysis of single crystal growth, etch figures and indentation measurements. An estimate from ab initio calculations of the Gibbs energy of (100), (010) and (001) surfaces is presented. The obtained data allow us to estimate the surface energy of the nuclei and to achieve a qualitative description of the transition from a -axis to c -axis YBCO growth at decreasing supersaturation.

A thermodynamical model, set up to describe the nucleation of vapour-deposited YBCO thin films, is reported. The role of the surface energy of YBCO nuclei is discussed. Information on the surface Gibbs energy of YBCO (1 0 0), (0 1 0) and (0 0 1) surfaces is obtained both from experimental results reported in the literature and from ab initio calculations. These data allow us to estimate the surface energy of the nuclei and to achieve a qualitative description of the crossover from (0 0 1) to (1 0 0) orientation at increasing supersaturation.

We have designed and tested 5 × 5 mm 2 chips containing island type STJs with different geometries, fabricated on Si substrates in the configuration Nb/AlAlO x /Nb and using a NbN overlayer acting both as wiring and radiation absorber. The aim is to investigate the advantages of the trapping effect due to the Nb layer trap. In fact, for the NbN Nb bilayer the scattering time is much faster than the values calculated for other Nb based bilayers, leading to more efficient trapping processes. The junctions have been characterized by tunneling and Josephson current measurements down to 0.5 K. Experimental results concerning the response of the devices under X-ray irradiation down to 60 mK are presented and discussed.

Superconducting thin-films containing randomly disordered layers of and have been synthesized through a combined approach of metal-organic chemical vapour deposition (MOCVD) and thallium vapour diffusion. The BaCaCu matrices have been deposited onto yttria stabilized zirconia (YSZ) substrates using the second generation tetraglyme and tetraglyme (hfa denotes hexafluoroacetylacetonate), and the (acac denotes acetylacetonate) as volatile metal-organic precursors. BaCaCu precursor films of composition 2:2:3 were annealed in the presence of a mixture of thallium (III), barium, calcium and copper oxides at temperatures of 840 - and oxygen partial pressure varying from 0.04 - 0.5 atm. The formation of an oriented () intergrowth structure has been observed in narrow temperature and oxygen partial pressure ranges. The electrical resistivity data show which, in accordance with the scanning electron microscopy image, appears indicative of a material with weakly linked grains.

Superconductivity of Y 1 Ba 2 Cu 3 O 7− d c -axis epitaxial films is always depressed when the films thickness is reduced to few unit cells. Here we report a detailed study of the structure, morphology and transport properties of Y(Nd 0.05 Ba 1.95 )Cu 3 O 7− d films deposited by high pressure diode oxygen sputtering. Films having thickness between 9 and 250 unit cells deposited on LaAlO 3 substrates have been investigated. It is shown that both T c and J c are reduced by decreasing the film thickness. The effect is quite evident even in relatively thick Y(Nd 0.05 Ba 1.95 )Cu 3 O 7− d films made of 32 unit cells. Moreover it is shown that the a and b axes of the films approach one to each other with decreasing thickness. A nine unit cell film is almost tetragonal. Application of a simple elasticity theory to account for the structural changes fails to explain the experimental results. Inelastic strain induced by an oxygen disordering in the Cu(1)–O chains may explain the experimental results.

A theroretical analysis of the behaviour of an STJ detector in the non equilibrium regime induced by some external radiation has been carried, based on the kinetic equations for both junction electrodes. The analytical expression of the tunnel times and of the non equilibrium current has been obtained. The effect of the backtunneling process on the amount of collected charge is shown.

X-ray STJ detector response is analyzed in the framework of the kinetic transport equation for the non-equilibrium quasi-particles. Numerical solutions of the resulting equation which describes the diffusion of excitations in the plane of the junction are presented and discussed. A particular attention is paid to the limits of “small” and “large” junctions, with respect to the effective diffusion length of quasi-particles. An almost linear behaviour is expected, under suitable conditions, for “large” Nb-based devices, in agreement with experiments reported in the current literature. Deviations from linearity are however possible in “small” devices or in case of short electronic mean free path.

Nb/Al–AlOx/Nb–NbN Josephson junctions for use as x‐ray detectors have been fabricated. The NbN overlayer has been used to realize the trapping of quasiparticles in the Nb top layer with an increase of the charge collection efficiency. The temperature dependence of quasiparticle and Josephson critical current have been investigated and compared with that without a NbN overlayer, showing low leakage currents, about 200 pA (V=0.5 mV, T=0.67 K, A=20×20 μm2), and high dynamical resistances in the subgap region. Preliminary measurements under 6 keV x‐ray irradiation have shown an increased value of the maximum collected charge.

We investigate pulsed laser deposition of LaGaO3/SrTiO3 at 10-1 mbar oxygen background pressure, demonstrating the critical effect of the target-to-substrate distance, dTS, on the interface sheet resistance, Rs. The interface turns from insulating to metallic by progressively decreasing dTS. The analysis of the LaGaO3 plume evidences the important role of the plume propagation dynamics on the interface properties. These results demonstrate the growth of conducting interfaces at an oxygen pressure of 10-1 mbar, an experimental condition where a well-oxygenated heterostructures with a reduced content of oxygen defects is expected., Comment: 16 pages, 3 figures

Epitaxial (001)-oriented ultrathin YBCO films of different thicknesses are deposited by Inverted Cylindrical Magnetron Sputtering (ICMS) on (100) MgO single-crystal substrates. The mean films stoichiometry is determined by Rutherford Backscattering (RBS). Auger Electron Spectroscopy (AES), X-ray Photoemission Spectroscopy (XPS) and Scanning Electron Microscopy (SEM) are employed in order to analyse film growth, and identify spurious phases present in the samples.

Superconducting thin films of the TlBaCaCuO family have been prepared through a combined approach of MOCVD and thallium vapour diffusion. A three-step process has been adopted involving i) the synthesis of Ba−Ca−Cu−O films by MOCVD using «second-generation» barium and calcium sources, ii) the hydrolysis of the fluoride phases by annealing in water vapour-saturated oxygen and iii) the annealing in the presence of a thallium source. Reproducible results, in terms of phase formation, are discussed depending on the different conditions of the preannealing process. The superconducting films consist predominantly of single-phase Tl2Ba2Ca2Cu3Ox(2223). Preferential orientation of the crystallitec-axes perpendicular to the substrate surface has been observed. The films exhibit superconducting onset temperatures of 115–120 K with zero resistance higher than 100 K.

The behavior of a superconducting tunnel junction detector (STJ) in the regime of low excitation is described, and the characteristic times of the dynamical response are discussed. The effect of a proximity layer in the STJ is analyzed in terms of the McMillan model. A set of phenomenological equations for the proximized STJ driven out of equilibrium is obtained, showing the importance of the role of the proximity effect in the dynamics of the device.

We investigate the effects of ambient pressure and substrate temperature on the deposition rate of oxides grown by pulsed laser deposition in oxygen atmosphere. The deposition rate of LaGaO3 (LGO) and LaAlO3 (LAO) is studied at room temperature by means of a quartz crystal microbalance and at 800 °C by exploiting reflection high energy electron diffraction. We observe a clear dependence of the deposition rate on temperature for an oxygen pressure between 10−2 and 1 mbar. A simple model based on multiple-elastic-scattering processes thoroughly describes the observed dependence of the deposition rate on the pressure/density of the background gas.

We show that the growth of the heterostructure LaGaO3 /SrTiO3 yields the formation of a highly conductive interface. Our samples were carefully analyzed by high resolution electron microscopy, in order to assess their crystal perfection and to evaluate the abruptness of the interface. Their carrier density and sheet resistance are compared to the case of LaAlO3 /SrTiO3 and a superconducting transition is found. The results open the route to widening the field of polar-nonpolar interfaces, pose some phenomenological constrains to their underlying physics and highlight the chance of tailoring their properties for future applications by adopting suitable polar materials.

International audience; Two kinds of epitaxial structures were grown by standard pulsed laser deposition on (001) Si, namely La0.7Sr0.3MnO3/Bi4Ti3O12/CeO2/YSZ/Si (BTO-based), and La0.7Sr0.3MnO3/SrTiO3/ CeO2 /YSZ/Si (STO-based) multilayers. The samples were investigated by means of x-ray diffraction, transmission electron microscopy, magnetic and transport measurements. The Curie temperature TC of the BTO-based samples was found to be higher (360 K) than for the typical reference epitaxial LSMO film grown on (001) SrTiO3 single crystal (345 K), due to high compressive in-plane strain. The STO-based samples show high structural quality, low roughness and high TC (350 K), making them interesting candidates for use in innovative LSMO-based bolometers or spintronic devices operating at room temperature. (Some figures in this article are in colour only in the electronic version)

A simple technique for the growth of SrO-terminated SrTiO3 surfaces is reported. High quality SrTiO3 epitaxial films were grown by reflection high energy electron diffraction assisted pulsed laser deposition on suitably prepared NdGaO3 (110) substrates. The surface properties, analyzed within a growth/characterization multichamber ultrahigh vacuum system by photoemission spectroscopy performed on the core-level spectra of Sr and Ti, low energy electron diffraction-, scanning tunneling-, and atomic force microscopy, are fully consistent with a single Sr oxide termination. The availability of such high quality SrO-terminated SrTiO3 surfaces is of major importance for the controlled growth of oxide epilayers and interfaces.