International Journal of Modern Physics B

In order to understand the difference between the effects induced by chemical substitutions and those induced by applying mechanical pressure, we synthesised and characterised samples of Y(Ba_1-xSr_x)₂Cu₃O_7-d. The structural refinements based on powder neutron diffraction data were carried out as a function of x and, for x = 0.5, as a function of d. The electric and magnetic properties have also been measured and correlated with the structural parameters and oxygen content. It seems that the Ba cations play an important role in the high T_c of YBCO. Its substitution with a smaller cation corresponds to the application of a chemical pressure, but the release of the associated strain seems to play a negative role on T_c.

Fluorine substitution in the Bi(2223), Bi(2212) and Tl(1223) superconducting phases was studied. We obtained superconducting structures, never observed before, of the Bi-based superconductors by a low temperature (200-400 °C) fluorination process. Fluorine substitutes completely the oxygen sites in the Bi layers and additional F atoms are inserted in the structure. As a consequence, changes in the arrangements of cation and anions were induced, especially in the Bi and partially in the Sr layer. F-doped Tl(1223) has been prepared in the same way as Bi(2223) and Bi(2212) (low temperature fluorination), but also starting from precursor containing fluorides of different elements. No significant differences in the crystal structure have been observed between the Tl-based samples with F inclusions and without. The critical temperature (116 K) remains unchanged but a significant increase of the irreversibility field at low temperature was found.

The present work reports a systematic and accurate study on crystal structure and chemical homogeneity of underdoped La_2-xM_xCuO₄(M = Sr, Ba). Samples' structural characterization performed by Rietveld refinement of XRD powder diffraction profiles provided an accurate working trendline for the dopant concentration determination. Furthermore FT-IR skeletal analysis and SIMS investigations ensured chemical homogeneity even for samples with very low dopant contents.

This paper reports the crystallization and superconducting properties of Bi₄Sr₃Ca₃Cu₄O_x glass-ceramics. The glass was prepared using the conventional melt-quenching method. X-ray diffraction pattern has shown that the annealing at temperature of 800°C converted the glass into glass-ceramics with Bi₂Sr₂CaCu₂O_x superconducting phase (2212-phase). The superconducing transition exhibits the change in the slope that is characteristic for two different superconducting phases. It is observed that T_c (zero) increases from 20 K to 80 K with increasing time of annealing. The influence of magnetic field on the superconducting transition has been investigated. Our studies revealed a remarkable broadening of the resistive transition as a function of temperature with increasing field. The temperature width (T_c-T) of the transition for both phases increases with magnetic field according to Tinkham model.

High temperature superconducting multilayers have been obtained depositing Bi₂Sr₂CuO_6+δ(2201) and ACuO₂ layers, where A is Ca or Sr, by Molecular Beam Epitaxy (MBE) on MgO and SrTiO₃ substrates. The samples, formed by a sequence of 2201/ACuO₂ bilayers, have different thickness of ACuO₂ layers while the thickness of the 2201 layers is kept constant. The surface structure of each layer has been monitored by in situ Reflection High Energy Electron Diffraction (RHEED) analysis which has confirmed a 2D nucleation growth. X-ray diffraction (XRD) analysis has been used to confirm that the layered structure has been obtained. Moreover, one-dimensional X-ray kinematic diffraction model has been developed to interpret the experimental data and to estimate the period of the multilayers. Resistive measurements have shown that the electrical properties of the samples strongly depend on the thickness of the ACuO₂ layers.

The epitaxial growth of YBa₂Cu₃O_7-x (YBCO) films on biaxially textured substrates is one of the most promising technique for the fabrication of high current superconducting tapes operating at high temperature. Ni is very attractive as substrate because it easily develops a (100)[001] cubic texture. The low oxidation resistance represents the main drawback of the Ni substrate. In order to better assess the role of oxygen on the Ni substrates, a surface physics technique as Auger spectroscopy has been used. It has allowed to evaluate the amount of impurities for different Ni processing and exposure to the air. The results demonstrate that the surface contamination can be efficiently removed by RF sputtering before buffer layer deposition. This procedure allows to obtain CeO₂/Pd/Ni architecture by laser ablation with a good epitaxy both of Pd and CeO₂ films. On the contrary, when CeO₂ is directly deposited on Ni a low epitaxy is obtained. The Auger analysis confirms that the formation of (111) NiO at the Ni-CeO₂ interface hampers the epitaxial growth of the ceria film.

We report on the preparation and characterization of YBa₂Cu₃O_7-x/PrBa₂Cu₃O_7-x bilayers onto (100) SrTiO₃ substrates. The samples have been prepared by sequential dc sputtering processes in high oxygen pressure from stoichiometric targets. The structural characterization of the YBa₂Cu₃O_7-x and PrBa₂Cu₃O_7-x films and of the bilayers has been performed by means of X-ray diffraction. The Scanning Electron Microscopy analysis has showed that the film surfaces are flat and free of precipitates. A detailed study of the interfaces has been performed by Transmission Electron Microscopy analysis. The electrical resistivity measurements showed for the YBa₂Cu₃O_7-x films sharp superconducting transitions at 91.5 K and critical current density of about 10⁶A/cm² at 77 K, while for the PrBa₂Cu₃O_7-x films a semiconductor-like behavior has been observed.

The effect of silver and the so-called green phase, Y₂BaCuO₅ (211), on YBa₂Cu₃O_x (123) melt textured samples has been investigated to check whether better grain alignment is possible in shorter times. We demonstrate that the addition of silver to YBCO always led to enhanced texturing in short times, while not disturbing the typical microstructure of the composite 123 plus 211, that is believed essential for high critical currents in these high Tc superconductors.

The preparation of Hg(Pb)BCCO (1201) and (1223) phases was performed by hot-isostatic-pressing (HIP) using simple oxides as precursors canned in silver tube. In the case of Hg_0.8Pb_0.2Ba₂CuO_4+δ the preparation, carried out at 800 °C and 0.2 GPa for 5 hrs, yielded a single phase bulk sample. On the other hand the preparation of Hg_0.8Pb_0.2Ba₂Ca₂Cu₃O_8+δ at 820 °C and 0.2 Gpa for 5 hrs allowed the attainment of a high volume fraction of 1223 phase. Magnetic susceptibility of 1201 phase showed a granular transition around 90 K; in the case of 1223 the value was ≈128 K followed by a broadened intergranular transition.

These results show the feasibility of 1201 phase synthesis with high purity and of 1223 phase in large amount (>80%).

The possibility to obtain the samples directly in form of dense bulks (with density ≥ 85%) stresses the importance of this technique in relation to the intrinsic low thermal stability of the superconducting phases.

A review of the structural existing data on Hg-1201 shows evidence of non stoichiometry of the sample.

In this compound not only oxygen but also Hg seems to be released or, perhaps, substituted.

So, the production of bulk samples with optimal oxygen doping and homogeneus Hg content could be profitable for the study of the effect of the Hg solubility, allowing to confirm its existence till to room temperatures, and to test the effect of Hg impoverishment on other critical superconducting parameter different from Tc.

Biaxially aligned YBCO thick films on oxide buffered metallic substrates is a promising route toward the fabrication of superconducting tapes operating at liquid nitrogen temperature. The role of buffer layer is to reduce the lattice mismatch between the substrate and the YBCO film, to adapt the thermal expansion coefficient, to hamper the diffusion of Ni in YBCO film and to prevent the oxidation of the metallic substrate surface. This paper presents a study regarding CeO₂ buffer layer deposition on a new nonmagnetic (001)[100] textured Ni-V alloy substrates. The deposition of CeO₂ was performed by both pulsed laser ablation and e-beam evaporation techniques. The θ-2θ X-ray diffraction pattern mainly exhibits the (00l) peaks of CeO₂, indicating that the films are epitaxially grown with the c axis perpendicular to the substrate. Rocking curved through the CeO₂ (002) peak have a FWHM of about 6°. The SEM studies have shown that the surface is smooth, continuos and free of cracks. Texture analysis reveals a good in-plane orientation for the ablated CeO₂ film, whereas the electron beam evaporated CeO₂ shows two textures in the growth plane. Further efforts are focused on the deposition of YBCO thick film on the as buffered nonmagnetic metallic substrate.

The problem of developing non magnetic cube textured metallic substrates other than pure Ni, to be used for epitaxial deposition of Y-123 superconducting film (with an intermediate oxide buffer layer) is being tackled since three years by several laboratories. As reported in this paper, we have developed a new substrate, based on Ni-V 10 wt % alloy, which exhibits a cube texture comparable to Ni, but has a Curie temperature < 4.2 K; in addition, it has a hardness which is more than twice higher and a comparable oxidation resistance.

In order to fabricate high temperature superconducting tapes for power applications, we have analysed different buffer layer architectures grown on textured Ni substrates suitable for YBCO deposition. Due to its optimal lattice matching the studied structures present as top layer a CeO₂ film. The deposition of CeO₂ on Ni substrates was performed by pulsed laser ablation and by e-beam evaporation at different temperatures. The films obtained by the two deposition techniques have not optimal structural properties, having a polycrystalline component. The misorientation of CeO₂ is probably due to the formation of NiO at the interface between the film and the substrate during the deposition process even if no oxygen is introduced. In order to prevent Ni oxidation an intermediate 2000 Å Pd thick film was deposited by e-beam. Furthermore, the lattice mismatch between Pd and CeO₂ is smaller than that between Ni and CeO₂. The Pd layer inhibits nickel oxide formation and improves the CeO₂ epitaxial growth: the XRD pattern contains only the (001) peaks of Pd and CeO₂. SEM analysis on CeO₂/Pd/Ni shows a smooth surface free of cracks, contrary to that observed for CeO₂/Ni structure.

In this paper we report results on the growth of borocarbides ErNi₂B₂C and DyNi₂B₂C thin films by an ultra high vacuum laser ablation technique. The dependence of the film structure and properties on the deposition parameters has been studied. Moreover the possibility to obtain superconducting films with a tungsten buffer layer on MgO substrates has been successfully checked. Here we present details on the deposition procedure as well as on the structural, morphological, and physical characterization.

We successfully used the pulsed laser ablation and deposition to prepare superconducting thin films of SmBaCuO under different growth parameters. The films were grown on both MgO (100) and SrTiO₃ (100) substrates. The best samples showed a good degree of orientation, having the θ-scan around the 005 reflection an FWHM≈ 0.2°. We report the critical temperature as a function of the deposition parameters, comparing the results between the two substrates. We also estimate the critical current density at 60 K from the magnetization curve.

Thin films and artificial materials deposition, applied to the study of high temperature superconductors, can be a powerful method to achieve better comprehension of the conduction mechanisms in these materials and, in perspective, to obtain new superconducting compounds. We simulate the layered structure of the superconducting oxides by alternating infinite layer compounds (CaCuO₂, SrCuO₂, BaCuO₂) both pure and doped either with chemical substitutions or stoichiometry variations. By the first way, we obtained superlattices with a semiconducting electrical behaviour, probably because of the disorder induced by such high chemical substitutions. The second way led to superconducting films with transition temperatures around 80 K and low anisotropy. Furthermore, their artificial nature offers an unique possibility of analysing the influence of the structural properties on the superconducting behaviour, such as the correlation between the number of CuO₂ planes contained in the infinite layer block (CaCuO₂) and the transport properties.

High temperature neutron diffraction measurements were performed on Bi(2223)/Ag-sheathed monofilamentary tapes at the ILL high-flux reactor in Grenoble. Reactions leading to the conversion from Bi(2212) to Bi,Pb(2223), as well as other transformations involving secondary phases, were directly observed during the reaction heat treatment. The heating ramp and annealing conditions were exactly the same as those used for standard high-performance tapes processing. A quantitative analysis was carried out by means of a full-pattern profile refinement technique: up to 7 phases were simultaneously detected and successfully refined. An increase of the Bi(2212) phase during a slow cooling was found not to be related to a decomposition of Bi,Pb(2223), which remained stable during cooling. The role of secondary cuprates, in particular (Ca,Sr)₁₄Cu₂₄O₄₁, was investigated. Since neutron diffraction allows for an absolute measurement of the weight of crystalline matter inside the sample, it was possible to measure the total crystalline matter amount as a function of temperature and time during processing. Evidence of partial melting at high temperature was found strongly supporting the Bi,Pb(2223) formation via a nucleation-and-growth mechanism at the early stage of the process.

A comparative investigation, performed on Oxygen-Chain-equalised(k) Deintercalated [OCD]_k and Intercalated [OCD]_k pair-samples (k-pairs) of YBa₂Cu₃O_6+x by resistive T_c and electron diffraction (ED) at steps Δk=0.02-0.03, revealed that equivalent or non-equivalent microscopic oxygen-chain arrangements are produced, at the same oxygen content k, by intercalation and deintercalation of oxygen in the Cu(1)O_x chains. These arrangements manifest in T_c-singularities or T_c-splittings of the k-pairs, whose occurrence can be straightforwardly explained by taking into account the non equivalence of the structural phase diagrams of YBCO, pointed out by electron diffraction, when produced by intercalation of oxygen in the tetragonal T structure or by deintercalation from the orthorhombic OI structure. T_c-singularities are produced in the k ranges for which a superimposition of the phase diagrams occurs, whereas T_c-splitting are pointed out in correspondence of splitted structural regions. The maximum T_c-splitting is observed in proximity of the semiconductor-metal transition with the appearing of superconductivity related to an anti-OIII (OIII*) superstructure in intercalated samples at k=0.30, where the corresponding deintercalated samples are tetragonal and semiconductor. This result confirms that the vanishing of the antiferromagnetic ordering, the emergence of hole doped superconductivity and the symmetry breaking at the T-O transition are nearly coincident phenomena in YBCO. However the related critical oxygen stoichiometry is strictly dependent on the ordering parameter in the Cu(1) plane, i.e. on the formation of extended chains. The OIII* structure, for the first time observed in a systematic way at the T-O transition in intercalated samples, seems to exhibit in analogy with OII and OIII a characteristic T_c.

The vibrational dynamics in nearly stoichiometric La₂CuO_4+δ has been studied by means of anelastic relaxation and by ¹³⁹La NQR relaxation. Evidence of dynamical tilting of the octahedra in a double well local potential is obtained with both the experimental techniques. Anelastic relaxation shows a peak at T ~ 150 K for a measuring frequency in the kHz region, while the peak in the NQR relaxation at the quadrupole frequency ν_Q ~ 19 MHz occurs at T ~ 330 K. The data are analyzed in terms of a model of pseudo-diffusive cooperative motion. The measured effective barrier is 2800 K, higher than the one of the local potential becuase of the cooperative character of the motion. Upon Sr doping the ¹³⁹La relaxation is still dominated by the octahedra tilting waves for T>100 K, while allow temperatures the magnetic relaxation mechanism related to Cu²⁺ spin fluctuation dominates. It is argued how the freezing of the tilts (occurring below 100 K in the undoped La₂CuO₄) might correspond to the formation of static stripes.

We report on spatial distribution of structural and superconducting properties in directionally solidified YBCO samples. The recrystallization process of produced YBCO bar was determined by a motion into a horizontal furnace at very high pulling rate in a fixed thermal gradient. Texturing and stoichiometry of several slices cut from the original bar have been studied by XRD and SEM with coupled EDS analysis. The magnetisation cycles of all the slices (M(H) Loops) were obtained by means a Vibrating Sample Magnetometer at 70 K using applied magnetic fields up to 2 tesla.

The anisotropic in-plane electrical and superconducting behavior of (103)/(013) oriented YBCO films has been investigated. The 150 nm thick samples were fabricated on (110) SrTiO₃ substrates by dc sputtering from an inverted cylindrical magnetron, using a self-template growth process. The (103)/(013) lattice orientation was confirmed by the X-ray diffractometry measurements and by polarised-light Raman spectroscopy. The film was patterned in 20 μm width lines, in-oriented along different directions, in order to estimated the dependence of properties on the crystal orientation. Concerning the normal state electrical resistance, measured by a four contacts method at 100 K, a value ρ_ab = 0.64 mΩ cm and ρ_c = 3.50 mΩ cm is found for the bias current directed respectively along and across the copper planes. The current voltage (I-V) characteristics were measured at different temperatures. As expected the critical current density J_o is larger for the lines aligned with the copper planes, and decreases with decreasing of the angle between current bias direction and the normal to the copper planes. The influence on vortex pinning of lattice twins occurring at the (103) and (013) grain boundaries is considered by analyzing the J_o vs T dependence and the I-V characteristics compared with c-axis YBCO film standard features.

We discuss the excess conductivity above T_c due to renormalized order-parameter fluctuation in YBCO at microwave frequencies. We show that a phenomenological cut-off in the fluctuative spectrum can extend the results of the anisotropic theory of renormalized fluctuations up to the high temperature region. Measurements of the real part of the microwave resistivity at 24 GHz are performed on two YBCO films. The onset of the superconducting transition and the deviation from the linear temperature behavior can be fully accounted for by the extended theory. Very consistent values of the fundamental parameters (critical temperature, coherence lenghts, penetration depth) of the superconducting state are obtained.

The transport properties and the dynamics of the flux lines have been investigated on YBaCuO melt textured by measuring the AC susceptibility harmonics as function of the temperature, at different frequencies and amplitudes of the AC magnetic field. The interpretation of the behavior of the experimental curves has been done by their comparison with the results obtained by critical state pictures and by simulations of the diffusion processes of the magnetic field which determine the AC response in presence of different regimes of flux dynamics. The temperature dependencies of the third harmonics of the AC susceptibility at different frequencies have been used in particular to determine what is the flux dynamic regime governing the magnetic response of melt textured samples in the different experimental conditions. A good qualitative agreement has been found between the experimental curves and the ones simulated by assuming that dissipation processes are a sequence of independent flux creep and flux flow events within a collective pinning framework.

We present measurements of the microwave resistivity at 48 GHz in YBa₂Cu₃O_7-δ film, as a function of the temperature and the magnetic field. The magneto-dissipation from 60 K up to a few kelvins below T_c finds a quantitative description in terms of free flux flow. Approaching T_c, the data are not consistent with a dissipative mechanism based solely on flux dynamic. Fluxon viscosity is measured as a function of the temperature, and the in-plane coherence length is estimated. The analysis of the zero-field transition at 48 GHz in terms of anisotropic renormalized fluctuations yields identical estimate.

A model for the current assisted thermal generation of vortices in planar superconductors is proposed. The Lorentz term is added to the vortex-antivortex pairing potential. Following the KT renormalization procedure, one obtains a dependence of the critical current on the temperature and the distribution of the dissipation in the I-T plane.

This paper deals with the electromagentic nondestructive testing performed by SQUID magnetometry on stratified aluminum alloy plates as those commonly encountered in the aircraft industry. The anomalous magnetic fields generated by flaws with know electromagnetic characteristics have been modeled by a three-dimensional specific code based on a finite element formulation. The numerical solution has correctly predicted the shape of the complicated magnetic field response due to the defect. Once accuracy and reliability of experimental data taken by superconductive probe have been tested, a benchmark-like problem has been faced. Measurements performed by conventional probes like fluxgate and inductive coil have been compared with the ones taken by innovative device based on superconductive materials.

The effects of superconducting fluctuations on the diamagnetic susceptibility and on the dynamical spin susceptibility involved in ⁶³Cu NMR-NQR relaxation rate are investigated in oriented powders of underdoped YBCO123 and of YBCO124 and compared with the ones in optimally doped YBCO123. While in this latter compound the fluctuation diamagnetism is well described by an anisotropic Ginzburg–Landau (GL) functional, in underdoped YBCO123 an anomalous diamagnetism is observed, with a strong enhancement of the susceptibility, in a wide temperature range. The magnetization curves cannot be described by any GL anisotropic functional. Also in YBCO124 the fluctuation diamagnetism is hard to describe by GL-type approach, although the enhancement is not as marked as in underdoped YBCO123. In YBCO124, and in underdoped YBCO123, the temperature and field dependences of the ⁶³Cu relaxation rates W appear different from the ones in underdoped YBCO123. No field-induced decrease of W is observed, as it is expected in the case of a changeover from s to d of the orbital simmetry of the fluctuating pairs or when the character of the fluctuations is different from the GL one.

We report a study of the resistive transitions in Bi₂Sr₂CaCu₂O_8+x in presence of an applied magnetic field (up to 12 tesla) at various field orientations. We focus our attention on the results obtained with orientations parallel and perpendicular to the (a,b) planes. We show that scaling arguments, typical of dissipative regimes independent of oriented defects, can explain the experimental findings, provided a 3D-2D crossover is assumed as the temperature is lowered, due to the variation of the out of plane coherence length.

The possibility to enhance in a predictable way the superconducting properties behavior of a bulk material by introducing linearly correlated defects within a surface layer has been demonstrated. Ag/BSCCO-2223 tapes were irradiated by means of 0.25 GeV Au ions at different fluences, with a dose equivalent field B_ϕ ranging from 1 T to 5 T. The implantation depth was about 15 μm corresponding to about 15% of the total sample thickness. Sample independent enhancements of different a.c and d.c properties show up, due to vortex confinement in the whole sample. The sample independence exhibited by all the experimental findings as well as, on the other side, the dose dependence of the enhancements, allow to design in advance the performances of a monofilamentary tape where defect trenches of nanometric size are scratched.

The non-ohmic response of several II type planar superconductors in absence of magnetic field is examined as a function of temperature and current. The scaling of the R(T) data at constant current is obtained by plotting the resistance R vs. Q = [T-To(I)]/[Tm-To(I)], where To(I) and Tm are the temperature limits of the non-ohmic response in the R(T) dependence. Such scaling is obtained in all the considered samples and it seems to be independent on the boundary effects in granular specimens and on the interlayer coupling in multi-planar systems. Because of the variety of the examined samples this behaviour appears to be an intrisic characteristic of planar superconductors. The R(Q) trend is well approximated by R=Ro exp and an interpretation based on the current assisted depairing of thermally excited vortex-antivortex pairs is proposed.

The charge carrier density plays a crucial role in the transition between normal and superconducting state in the copper-oxide superconductors. In Nd_1.85Ce_0.15CuO_4+δ sample, for charge carrier density n = n_c≃ 0.07 the superconductivity sets in. In order to investigate the magnetic properties around n_c, a set of magnetic susceptibility measurements in the temperature range 3-260K for samples with different charge carriers concentrations were realized. Magnetic susceptibility curves of Nd_1.85Ce_0.15CuO_4+δ revealed novel behavior well above the critical temperature T_c: at fixed T≥45K, the susceptibility χ, reported as a function of the charge carrier density n, shows a peak around n=0.06 and drops to significantly lower values for n greater than 0.07, the value at which the samples become superconducting. The effective magnetic moment μ_eff shows the same behavior. The comparison of the present experimental results with recent transport ones gives evidence of a possible interaction between charge carriers above T_c. The possibility that this effect could be due to a bipolarons-like mechanism of charge carriers interaction is discussed.

We introduce a microscopic model for the compound Sr₂RuO₄, which is at the present the only known layered-perovskite superconductor without copper. The one-particle Hamiltonian is deduced by means of an approach that combines the extended Hückel theory and the tight-binding method. Adding two-body terms to this Hamiltonian, as the local Coulomb interaction and the Hund coupling, the magnetic properties are determined. It is found that the system is close to a ferromagnetic instability and this result supports a superconducting phase with p-wave symmetry.

We report a set of experimental results on microwave third-harmonic emission by Ba_0.6K_0.4BiO₃ crystals. Third-harmonic signals are investigated as a function of temperature, input power level and intensity of the static magnetic field. The experimental data are discussed in the framework of models previously reported in the literature.

The paper presents the structural and magnetic properties of a new non-magnetic biaxially textured substrate based on Ni_100-xV_x solid-solution for YBa₂Cu₃O_7-y tape fabrication. The effective atomic magnetic moment monotonously decreases with the vanadium concentration, causing a corresponding decrease of Curie temperature. The Curie temperature reaches the zero value at about 11.5. % of vanadium. The texturing studies revealed that (100)[001] cube texture can be easily developed up to x=11 at. %, by a cold rolling process followed by a recrystallization thermal treatment. The X-ray ω and ϕ scans have demonstrated that the samples have a good out-of-plane and in-plane texture for the whole solubility range, with FWHM of 7° and 11°, respectively. The correlation between the magnetic and structural anisotropy was also studied.

The dominating pinning mechanism in YBa₂Cu₃O_7-x thin film has been investigated by means of resistivity and ac susceptibility measurements and by structural characterizations. The aim of the work is to determine optimal pinning conditions in order to prevent the field-induced drop of the critical current density. The Clem and Sanchez model is used to extract critical current values from the susceptibility data. In this framework, the role of columnar-like defects is extensively studied. The formation of columnar pins was promoted by a suitable modification in the preparation process. In addition, columnar tracks of different density were induced by Au-ion irradiation. Analysis of the critical current dependence on the magnetic field reveals that linearly-correlated defects are effective in improving the vortex lattice stability in "matching regions" of field and temperature.

From the complete extended Hubbard-Holstein Hamiltonian we obtain a polaronic effective Hamiltonian by successive application of generalized displacement and squeezing transformations, with wavevector dependent characteristic parameters which make possible a phonon-induced long range interaction between charges, whose sign can change with the distance. As an application we consider the four-site chain, in which the bare electronic interactions are evaluated by model Wannier functions of gaussian shape. The ground states for different ranges of interactions are obtained by exact diagonalization of the effective Hamiltonian, followed by simultaneous and independent optimization of the displacement and squeezing parameters. We consider the behaviour of the static spin and charge correlation functions for the half-filled case, upon varying the shape of the Wannier functions for given electron-phonon coupling. Special attention is devoted to the influence of the phonon coupling on the presence of competing or coexisting charge- and spin-ordered states.

We have studied momentum distribution of spectral weight around the Fermi level of Bi₂Sr₂CaCu₂0_8+δ (Bi2212) superconductor by angle resolved photoemission to understand implication of the lattice and spin fluctuations on the excitation spectrum of cuprate superconductors. The photointensity reveals shadow bands at (0.5π, 0.5π) and equivalent locations, related to the spin density wave (SDW) and suppression of spectral weight around (π, 0) and equivalent locations, related to an anharmonic and incommensurate charge density wave (ICDW) in the diagonal direction. The work further uncovers the fact that the high T_c appears in a complex phase of coexisting lattice and spin fluctuations.

Recently, by exact diagonalisation methods, we reported superconducting pairing and flux quantization in Cu-O Hubbard clusters of up to 21 atoms. The pairing is due to a configuration interaction mechanism driven by symmetry, and arises when the least bound holes have vanishing mutual repulsion (W=0 pairs). The second-order, spin-flip interaction leads to a net attraction for the singlet and repulsion for the triplet. Here we show that electron bound pairs also exist and show similar flux quantization properties. Then, we extend the theory of pairing to the full plane and to all orders.

The effective pairing interaction and the superconducting transition temperature T_c, mediated by both spin and charge fluctuations, are investigated on the basis of the simplest p - d model by considering a Bethe-Salpeter equation for the vertex functions within a generalized cumulant expansion. The comparison with experimental results of hole-doped cuprates is discussed.

The critical current J_c of superconducting films is about two order of magnitude larger than that of single crystals. The dependence of such J_c on field depends on the presence and distribution of defects. We speculate that High Temperature Superconducting (HTS) films can be modeled as an agglomeration of islands coupled via Josephson contacts and then considered as a network of Long Josephson Junctions (LJJ) between superconducting islands. In order to check this hypothesis we further developed a model firstly introduced by Fehrenbacher based on the modulation of critical current densities in LJJ by means of suitable distribution of defects. The defects act by modulating the coupling energy of the junction. Critical current densities are calculated in the case of periodic and random defect distributions.

In this work we investigate some effects of the breakdown of Migdal's theorem in high and low-T_c superconductors. In the cuprates the electronic bandwidth has a very low value compared to the characteristic phonon energy and, in order to reproduce the experimental T_c and tunneling data in the framework of the Eliashberg theory for strong electron-boson interaction it is necessary to use large values of the coupling constant. Our purpose is to numerically quantify the correlation between the amount of violation of the Migdal's theorem and the renormalization of the coupling constant λ, of the gap Δ and of T_c by examining different kinds of superconducting materials. For this task we use the theoretical approach developed by Grimaldi, Pietronero and Strässler [PRB 52, 10516 & 10530, 1995], together with the real-axis direct solution of the Eliahsberg equations in s-wave pair symmetry.

The authors report on ⁶³Cu NMR relaxation measurements in YBa₂Cu₃O_6+x samples having similar T_c's but a different structural ordering of the Cu(1)O chains. It is found that the gap in the spin excitations estimated from ⁶³Cu(2) nuclear spin-lattice relaxation rate 1/T₁, varies from 110 K up to 190 K depending on the sample. This observation suggests that there is not a one to one relationship between the hole concentration and the amplitude of the spin-gap in the normal state of YBa₂Cu₃O_6+x. The sizeable variation in the gap, related to the different ordering of the Cu(1)O chains, is tentatively explained in the framework of a phase separation scenario where Cu(1)O chains act as pinning wells for the stripes.

We report here the experimental evidence of semiconductor-superconductor transitions in relatively thick Nb/Al structures. The temperature dependence of resistivity ρ(T) shows a sharp superconducting transition followed by either a normal metallic behavior in low-resistivity samples, or a semiconducting behavior when the sample resistivity at 10 K is above 100 μΩ cm. We discuss here the fabrication conditions and the electron localization regime associated with the measured ρ(T) dependence.

¹³⁹La and ⁶³Cu NQR relaxation rates are used to obtain insights on the effects of itinerant holes on the magnetic in-plane correlation length, for x ≤ 0.04 in La_2-xSr_xCuO₄. For these concentrations, corresponding to spin-glass behavior at low temperature, evidence for the microsegregation of holes along stripes is presented. Preliminary relaxation results for an amount of Sr doping leading to superconductivity are also presented and discussed in terms of the crossover from the spin-glass to the underdoped superconducting regime: for x = 0.06 a slowing down of spin fluctuations is evidenced and superconductivity is found to coexist with spin-freezing.

We report on measurements of normal-insulator-superconductor tunnel junctions at temperatures below 1K. The junctions have been fabricated by shadow mask technique and e-beam lithography and have submicron dimensions. The measurements on our junctions show that their characteristics are very sensitive to temperature changes making them suitable to be used as a sensitive secondary thermometer. To demonstrate this we use this tunnel structure for sensing the electron temperature drop in a copper strip of an electronic micro-refrigerator, that was made by arrays of tunnel junctions, as well.

A novel class of sensors is constituted by radiation detectors based on superconducting tunnel junctions, which could lead in the next future to an advanced instrumentation with an interesting fall out for applications. So far, this area is restricted to LTc superconductors. Among them, niobium has the highest critical temperature and is therefore less demanding in terms of operational temperature. The best performances achieved so far in pure Nb-based junction are far away from the expected theoretical limit. The reason of this discrepancy could be attributed to various problems. Quasiparticle diffusion and edge losses seem to be responsible for the broadening of the signal amplitude spectra so often observed and reported in literature. In this paper we report on efforts to investigate the response to X-Ray of Nb-based superconducting tunnel junctions in terms of a model which includes the quasiparticle diffusion and the edge losses. Our model takes into account the contribution from both the electrodes of the junction and well reproduces the experimental data.

We have numerically studied the static and dynamic behavior of a three terminal vortex-flow transistor based on a parallel array of Josephson junctions. We explicitly show advantages and disadvantages for different device geometric configurations, systematically changing the symmetry of the bias-current distribution. Using parameters typical of high-temperature superconducting junctions at 77K, we have analysed for each configuration the dependence of the critical current on the control current, and the voltage-current curves. We have calculated gain and transresistance for different bias conditions, and explained the numerical results in terms of the vortex dynamics in the array. This study allows us to establish limits for the operating regime of these devices when used as high-frequency amplifiers.

We report recent results on the development of two different kinds of SNS Josephson junctions which can be used as quantum D/A converters. We have fabricated these devices using as a barrier either a thick 50-100 aluminum layer or a not-stabilized tantalum oxide layer. For both technologies we have determined the dependence of the main electrical properties of the junctions on the fabrication process. Measurements of the rf properties for the Nb/Al/Nb junctions and of the magnetical properties for the Nb/TaO_x/Nb junctions are also reported.

We present the results of a numerical simulation of the dependence of the Josephson critical current on the parallel magnetic field in the presence of a large number of Abrikosov 'monopole' vortices for the case of a circular junction. The influence of the junction edge on the distribution of the magnetic field of each vortex was taken into account by introducing image monopole vortices. Our simulations show that the dependence of the Josephson critical current on the parallel magnetic field (the I_c vs H_par dependence) critically depends on the existence of a vortex located near the center of the junction. The simulation data are compared with experiments made on circular Nb/Al-AlO_x/Nb junctions. A good agreement has been found between calculated and I_c vs H_par dependences measured after the application of a perpendicular magnetic field.

We report experiments on energy level quantization in Nb/AlOx/Nb Josephson junctions while measurements on SQUIDs are in progress. Data on the presence of energy level quantization are shown at temperatures above the classical-quantum regime crossover temperature. This has been possible by extending the measurements of the rate of the escape from the zero-voltage state at higher sweep bias rate in order to induce non-stationary conditions in the energy potential describing the junction dynamics. Data are compatible with extremely low "effective" dissipation in our system. Theoretical predictions on quantum effects related to the presence of non-stationary conditions in the potential associated to a rf SQUID are reported in view of new possible experiments.

In order to optimize the magnetic field sensitivity we have followed two different approaches. In the former we have analyzed the influence of the quality of YBCO films on the superconducting properties and noise characteristics of bicrystal grain boundary junctions and dc-SQUIDs. For this purpose we have studied and compared three different YBCO deposition processes. In the latter we have reported the different criteria for the design of SQUID magnetometers usable for applicalions in shielded and unshielded environment.

The gap voltage shift of Nb Josephson junctions fabricated on different substrates is measured under optical illumination at several conditions of light intensity and chopping frequency. The signal dependence on the chopping frequency for junctions realized on glass substrates is bolometer-like with a strong thermal coupling between film and substrate. The responsivity of a junction of 50 μm × 20 μm is 50 V/W in vacuum and at liquid helium temperature, and its time constant is lower than 10 μs. The linearity of the response on the optical power extends over five order of magnitude. The response of junctions fabricated on silicon substrates is flat versus the chopping frequency and about 100 times lower than a junction on glass with the same incident optical power. The shift of the gap at temperatures between 1.2 K and 4.2 K has been measured to separate the nonequilibrium effects from the heating effects.

The influence of superconducting fluctuations on the conductance spectra of superconductor-insulator-normal metal (S-I-N) tunnel junctions is considered. We discuss how the tunneling conductance is affected as the strength of fluctuations is changed close to the superconducting critical transition.

We study the tunneling spectra for superconductor-insulator-normal metal (S-I-N) tunnel junctions with an s-wave or a d-wave superconductor within the weak-coupling model. We deduce the temperature behavior of tunneling conductance and their peak positions as well as of the zero-bias conductance. The results obtained allow us to discriminate among the two singlet spin states.

A new possible indirect way of testing pair symmetry in high-Tc superconductors has been set up. The degree of intrinsic gap depression at Superconductor-Insulator [S-I] interfaces required to match I_c(T)R_n(T) data in HTS Josephson junctions depends on the pair symmetry of the material itself, so that an higher fraction of d-wave symmetry for the order parameter requires less gap depression, while an higher fraction of s-wave corresponds to a larger degree of gap depression. In order to obtain a general reference value for the intrinsic amount of gap depression at S-I interfaces the de Gennes condition has been used, and resulting reduced I_c(T)R_n(T) data have been calculated in the framework of a mixed (s+id)-wave pair symmetry for the depressed order parameter ranging from pure s to pure d-wave. This model has been tentatively applied to two junctions' made of very different HTSs: YBCO and BKBO, yielding a result of almost pure d-wave for YBCO and of pure s-wave for BKBO.

Electron beam irradiation has been used to induce controllable variations in the properties of YBa₂Cu₃O_7-x biepitaxial grain boundary Josephson junctions. Electron irradiation decreased the critical current of the junctions I_C and increased the normal state specific resistivities. The junction parameters can be adjusted controllably by applying an appropriate dose. The original properties of the junctions could be partly restored by isothermal annealing A correlation between the transport properties and the microstructure was obtained by determining the ratio of a barrier thickness to the dielectric constant of the junctions with different barriers. The experiment also demonstrates frequency tunability in a resonant soliton oscillator. Preliminar results of irradiation on (103) YBCO films have been given demonstrating differences from data on c-axis films, as partly expected from the anisotropy of these materials.

In this paper we report about some of the major developments of Ag sheathed Bi(2223) tapes that are actually under study in view of a possible practical application of these conductors. Efforts on improving the tape behavior in presence of alternating currents and high magnetic fields have to be done together with a severe optimization of the Powder-In-Tube method. A clear reduction of the magnetic AC losses is observed by AC susceptibility measurements performed on twisted Bi(2223) tapes prepared at the University of Geneva, with a resistive barrier capable of reducing the coupling losses between filaments. Finally, we report on multifilamentary tapes developed with a special configuration that can be of great interest when a magnetic field is applied to the tape at a significant angle with respect to the a-b planes of the superconductor.

We report on the synthesis and structural and electrical characterization of high quality Tl₂Ba₂Ca₁Cu₂O_x superconducting thin films. The samples have been prepared ex-situ by a combined approach of Metal-Organic Chemical Vapor Deposition (MOCVD) and thallium vapor diffusion. The films have been grown on 10×10 mm² (100) LaAlO₃ substrates. X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray analyses (EDX) have investigated the morphological and compositional nature of the films. The transport properties have been measured using both a four-probes and an inductive method. The highest critical temperature and critical current density are 104 K and 1 × 10⁶A/cm² respectively. The microwave response of two samples has been studied using a microstrip resonator technique. The best surface resistance values are below 200 μΩ at 1.2 GHz and 4.2 K. Measurements of the field dependence of the surface resistance have been performed.

The influence of the geometrical parameters on the behaviour of the AC losses in multifilamentary Ag-BSCCO(2223) tapes is being more and more evident. Recently, the effects of configurational parameters like the ratio between the overall width and thickness of tapes without barriers as well as their precise arrangements has been put in evidence either theoretically or experimentally. With the goal to go deeper into the comprehension of this relation, we characterized a serie of multifilamentary tapes with gradually different filament arrangements. They were measured in a self-field AC transport system at different frequencies, for current values up to the critical state. The results so obtained are discussed taking into account of some theoretical models.

The properties of high quality single side and double side YBa₂Cu₂O_7-δ (YBCO) thin films deposited on LaA1O₃ substrates by pure oxygen d.c. sputtering are reported. The single side films are highly c-axis oriented with their a (or b) axis parallel to that of the substrate. The critical temperature is higher than 91 K with transition width less than 0.5 K, while the critical current density Jc is higher than 3 MA/cm² at 77 K. The best double side samples exhibit no appreciable differences in the superconducting properties of the films deposited on the two different sides. X-ray analyses show in both cases highly c-axis oriented thin films with comparable values of the c-axis length (1.169 nm). The microwave properties are tested by a microstrip resonator technique. The surface resistance is lower than 80 μΩ at 77 K and f=5.4 GHz while the penetration depth at low temperature is in the range 150-200 nm.

The new generation of astronomical detectors based on Superconducting Tunnel Junctions (STJs) is capable of detecting photons in a wide range of wavelengths, and, in some cases, even of counting single photons. Since these detectors operate at very low temperature (down to 100 mK), a natural candidate for the readout electronics is the dc-SQUID, a superconducting device that can work as a current preamplifier, reaching very good noise performance. In this paper we present measurements on a homemade dc-SQUID, showing the response of the device to small current pulses, which simulate the real signals from an STJ. An optimal filtering processing of the data is performed, in order to extract from the noisy data the best estimate for the pulse height, which, in a real experiment with single optical photons, would be the parameter that carries the spectroscopic information.

The feasibility of a 5 T magnet by using Bi-2223/Ag tapes at 30 K has been investigated. Preliminary computation demonstrates that the target is attainable by assembling 20 double pancakes manufactured by co-winding two Bi-2223/Ag tapes having overall cross-section of 0.25 × 3.5 mm² and 25% fill factor, being the expected operating current about 200 A. The pancakes located at the edges are the most critical because the component of the magnetic field orthogonal to the tape surface is large enough to strongly depress J_c. To overcome this problem the edge pancakes could be fabricated by using Bi-2223/Ag tapes with new concept design (e.g. with oriented filaments).

The optimal design of the high temperature superconducting coils of a micro-SMES is described. Two formulations of the problem are considered which minimize the overall dimensions of the device or the volume of the superconducting coils. Out of the device the stray field is lower than the desired value. The coils are solenoids which can be arranged in an axial symmetric or toroidal configuration. The results in correspondence to different values of the stored energy and other parameters of the design are reported and discussed.

Multilayered Bi-2223 composite conductors fabricated by means of the "Accordion folding" method appear to be suitable for applications as current leads (CL). In this work, we report on the results of electrical characterization of silver-gold alloy sheathed Bi-2223 conductors up to 400 mm long, having a DC overall critical current density J_{c eng} exceeding 3000 A/cm² at 77 K. The I_c evolution from the first to the third thermal treatment as well as the uniformity along the specimen lenght has been monitored. Lastly, calculations of the magnetic field profile across stacked multilayered Bi-2223 conductors are reported and discussed.