bluuebunny/crossref_metadata_2025 huggingface dataset:
STRIPE PHASE OF THE EXTENDED HUBBARD MODEL
Authors: CITRO ROBERTA
Published: April 2003

Within a strong-coupling perturbative approach, based on a Cumulant Expansion of the extended single-band Hubbard model, we analyze the low-energy behavior of the chargevertex function. It is shown that quantum critical charge fluctuations drive the system towards a stripe-phase characterized by a finite wave-vector that yields information on the stripe order. Some relevant features observed in ARPES experiments on Bi2212 and LSCO are qualitatively reproduced.
QUASIPARTICLES IN HIGH-TEMPERATURE SUPERCONDUCTORS
Authors: CITRO ROBERTA , MARINARO MARIA , NAKAGAWA K.
Published: April 2003

We study the quantum criticality effects induced by a singular charge vertex on the quasiparticle spectral function of an extended single-band Hubbard model. It is shown that the spectral intensity computed in a strong-coupling approach, reproduces the Momentum Distribution Curve (MDC) and the Energy Distribution Curve (EDC) of ARPES experiments on Bi 2 Sr 2 CaCu 2 O 8+δ.
PINNING ENERGY AND VORTEX-PHASE DIAGRAM OF MgB2 BULK MATERIALS
Authors: GHIGO G. , BOTTA D. , CHIODONI A. , GERBALDO R. , GOZZELINO L. , LAVIANO F. , MEZZETTI E. , MINETTI B. , GIUNCHI G. , CERESARA S.
Published: April 2003

By using ac susceptibility measurements at different frequencies and dc fields we investigated the creep properties of MgB 2 bulk samples. Two independent analysis have been used in order to determine the behavior of the pinning energy in the whole investigated temperature and field ranges. The main result of the first analysis (χ′′-peak analysis) is that the attempt frequency v 0 in the Arrhenius formula is about 104 Hz , a value significantly lower than that usually reported for HTS's. By means of the second analysis a field dependent maximum of the pinning energy as a function of temperature has been found. It turns out that the phase points corresponding to the peak of the pinning energy are coincident with the irreversibility line, as determined by magnetization cycles. The results coming from different and independent analysis lead to the determination of a vortexphase diagram for bulk MgB 2, which seems to be largely dominated by creep phenomena.
MAGNETIC BEHAVIOR OF MAGNESIUM DIBORIDE SUPERCONDUCTING THIN FILMS
Authors: GRIMALDI G. , GAMBARDELLA U. , PACE S. , FERRANDOM V. , FERDEGHINI C. , SIRI S.
Published: April 2003

We report the experimental results of the field H and temperature T dependent magnetization M measurements of MgB 2 superconducting thin films deposited by pulsed laser deposition. MgB 2 thin films were obtained from a precursor layer deposited in UHV at room temperaure from stoichiometric target, then an annealing treatment in Mg atmosphere allowed to crystallize the superconducting phase. From the hysteresis loops at each temperature of measurement the critical current density Jc(H) has been extracted according to Brandt's model for a transverse geometry. At low temperatures an anomalous behavior has been observed in the hysteresis loops: as the exernal magnetic field approaches the zero field region on the upper and lower branches, a double peak of the magnetization occurs. This magnetic flux jumping can be explained within an adiabatic critical state model, and can be ascribed to thermomagnetic instabilities as in bulk samples.
WEAK FERROMAGNETIC FLUCTUATIONS IN GdSr2RuCu2O8 COMPOUND
Authors: CIMBERLE M. R. , CUOCO M. , NOCE C. , VECCHIONE A. , GOMBOS M. , ARTINI C. , PACE S.
Published: April 2003

It has been investigated the static magnetic response of the polycrystalline rutheno-cuprate oxide GdSr 2 RuCu 2 O 8. The susceptibility χ shows an upturn at T C ~ 134 K which indicates the onset of a magnetic instability. Above that transition we have analyzed within a spin mode coupling theory the dependence of χ-1 in the form of ~ (T - TC)γ. The main finding is that the value of the critical exponent γ can be understood as mainly due ferromagnetic fluctuations, assuming that the system is close to its quantum paramagnetic instability and that the magnetization comes primarily from itinerant moments, rather than moments localized to the atomic sites.
TIME REVERSAL BREAKING SUPERCONDUCTING STATE IN THE PHASE DIAGRAM OF THE CUPRATES
Authors: SANGIOVANNI G. , CAPONE M. , CAPRARA S.
Published: April 2003

We review and extend a previous study1 on the symmetry of the superconducting state, stimulated by recent tunneling and Andreev reflection measurements giving robust evidences for the existence of a dx2-y2 + idxy order parameter in the overdoped regime of two different cuprates. Looking for a possible second-order phase transition from a standard dx2-y2 to a mixed and time reversal breaking state, we confirm the results of our previous analysis on La 2-x Sr x CuO 4. In the case of Y 1-y Ca y Ba 2 Cu 3 O 7-x as well, among all the allowed symmetries, the dx2 - y2 + idxy is the most favored one and the unconventional state is likely to occur in a small dome of the phase diagram located in the optimal-overdoped region and at very low temperatures.
SUPERCONDUCTIVITY IN A SYSTEM OF ITINERANT ELECTRONS COUPLED TO LOCALIZED BIPOLARONS
Authors: ROMANO ALFONSO
Published: April 2003

We analyze the superconducting phase which can develop in a system of itinerant electrons interacting with self-trapped lattice bipolarons, in turn coupled to purely local vibrational modes. Starting from a standard mean-field decoupling of the boson-fermion exchange term describing processes in which a bipolaron decays into a pair of free electrons and vice-versa, we analyze some features of the superconducting phase stemming from the simultaneous condensation of electrons and bipolarons, with a specific attention to the dependence of the amplitude of the order parameters on the carrier densities and on the boson-phonon coupling.
EFFECTS OF GRANULAR STRUCTURE ON THE DC TRANSPORT IN UNDERDOPED SUPERCONDUCTORS
Authors: DALLACASA V. , ROMANÓ L.
Published: April 2003

A percolation model where carriers tunnel between metallic islands distributed in an insulating background is used to examine the electrical conductivity of underdoped cuprates above Tc. The process is thermally activated with an activation energy of the order of the Coulomb charging energy. The tunneling probability is described in terms of the distributions of the size r and separation s of the islands whose average values are increasing function of the doping. An insulating - like behaviour of the form ρ ≃ exp (To/T)1/4 is obtained at very low temperatures followed by a more metallic regime (ρ ≃ exp (Too/T)1/2) at higher temperatures. A comparison with experimental data in LASCO systems confirms the expected behaviour indicating a size of the regions ranging from 10A to 50A.
ELECTRON-PHONON SPECTRAL FUNCTION AND TWO-BAND MODEL IN TUNNELING MEASUREMENTS ON MgB2
Authors: UMMARION G. A. , GONNELLI R. S. , DOLGOV O. V. , SHULGA S. V.
Published: April 2003

In the present work we calculate for the first time the density of states of MgB 2 for different tunneling directions by directly solving the two-band Eliashberg equations in the real-axis formulation starting from the first-principle calculation of the interband and intraband electron-phonon spectral functions. This complicated numeric procedure allows preserving the fine structures of the DOS in the phonon energy range. We show that the numeric inversion of the standard single-band Eliashberg equations when applied to the densities of states obtained by the two-band model may lead to artifacts in the extracted electron-phonon spectral function α2 F(ω). We also suggest that the small DOS structures produced by the two-band inter-action at energies between 20 and 100 meV can be observed only at very low temperature in junctions in perfect clean limit.
STUDY OF THE VORTEX DYNAMIC IN MGB2 BY HARMONIC SUSCEPTIBILITY MEASUREMENTS
Authors: SENATORE C. , POLICHETTI M. , ZOLA D. , DI MATTEO T. , PACE S. , GIUNCHI G. , CERESARA S. , RIPAMONTI G.
Published: April 2003

In order to investigate the flux line dynamic in the recently discovered MgB 2 superconductor, we have measured the ac magnetic susceptibility as a function of the temperature T and the ac field frequency f on a bulk sample. In particular we have analysed the fundamental and the third harmonics χ1,3(T) = χ′1,3(T) + iχ′′1,3(T) by comparison of the experimental data with both analytical and numerical results. We show that in the temperature range below Tp(χ′′I) (i.e. the peak temperature of the imaginary part for the fundamental harmonic) the measured curves for χ3(T) do not agree with the curves predicted by the Bean model for the critical state, and that the shape of the third harmonic, both in the real and the imaginary part, is strongly affected by the frequency f, as a consequence of the thermally activated vortex motion in a non linear flux diffusion regime. The evidence of this behaviour allows one to estimate the value of the pinning barrier for the analysed sample.
SUPERCONDUCTIVITY AND FERROMAGNETISM IN HYBRID SYSTEMS
Authors: CUOCO MARIO , NOCE CANIO
Published: April 2003

Interplay of superconductivity (SC) and ferromagnetism (FM) is studied in a system constituted of alternated planes with FM metallicity and spin singlet SC, respectively. The phase diagram of the SC, FM, and normal state is obtained as the amplitude of the single particle charge transfer between the planes is varied. One important finding is the appearance of a region with coexistence of SC and FM at small values of the interlayer hopping amplitude, though the FM exchange coupling is below the critical value indicated by the Stoner criterium. We consider the role of the direct spin exchange coupling with respect to the degree of hybridization between FM and SC fermions and how those mechanisms may destroy the coexisting phase. Relation to exotic superconductors recently discovered, as the ruthenate-cuprate compound of the class RuSr 2 GdCu 2 O 8, is also presented.
JOSEPHSON DEVICES FOR QUANTUM COMPUTING
Authors: CHIARELLO F. , CASTELLANO M. G. , LEONI R. , TORRIOLI G. , COSMELLI C. , CARELLI P.
Published: April 2003

Computing tools, all based on classical logic at the moment, present intrinsic limitations that can be overcome by using quantum logic. In this direction, superconducting Josephson devices have been proved to be very suitable candidates for the realization of quantum computing tools. We present some basic elements of quantum computing, possible strategies for the implementation of quantum gates by using Josephson devices, and recent experimental results in this field.
BSCCO SUPERCONDUCTING RESONATORS IN THE RADIOFREQUENCY DOMAIN
Authors: BIANCHETTI M. , MALAGOLI A. , SCATI N. , SIRI A. S. , GRASSO G.
Published: April 2003

BSCCO/Ag-based RLC resonators have been assembled and characterized with the aim of studying the behaviour of such conductors when subjected to a radiofrequency current flow. The inductive part of the circuits is constituted by several turns of a multifilamentary BSCCO/Ag tape fabricated by the Powder-In-Tube method, and treated in such a way that the outer silver sheath is completely removed. A high-Q multilayer capacitor is directly soldered between the extremities of the superconducting tape, its value being chosen in order to tune the resonance frequency around 10 MHz. The quality factor of the assembled RLC circuits has been characterized as a function of the induced current, in order to relate the observed dissipative behaviour to the current penetration mechanism occurring in the superconducting tape.
CHEMCIAL TAILORING OF ELECTRONIC DOPING IN YSr2Cu3O7±d SUPERCONDUCTOR
Authors: GILIOLI E. , LICCI F. , PRODI A. , GAUZZI A. , MAREZIO M.
Published: April 2003

With the aim to optimize the carrier concentration and T c of YSr 2 Cu 3 O 7± d (YSCO), we studied the effects of aliovalent substituents for Y and Cu1 in both as-prepared and annealed samples. Ca 2+ and Ce 4+ and Al 3+ were substituted for Y and Cu1, respectively. The samples were synthesized at 1050°C, under a pressure of 30 Kbar, in a multi-anvil apparatus. The post-preparation annealings were carried out in static oxygen, nitrogen or under dynamic vacuum. The samples were characterized by X-ray diffraction and found to be almost single-phase at doping concentrations ≤ 10%. T c was determined by resistivity and magnetic susceptibility Vs temperature measurements. It was found to decrease in Ca-doped samples, to slightly increase for Al-doping or to remain almost constant for Ce-doping. These results seem to support the hypothesis that as-prepared YSCO is overdoped. For any substitution > 10%, T c decreased or vanished and the samples had a high content of secondary phases. T c of doped compounds also decreases after post-preparation annealings.
FULLY INTEGRATED DC SQUID MAGNETOMETERS FOR ULTRA LOW FREQUENCY NOISE APPLICATIONS
Authors: DI RUSSO C. , GRANATA C. , LONGOBARDI L. , MONACO A. , RUSSO M. , FORTUNATO F.
Published: April 2003

Fully integrated dc SQUID magnetometers for ultra low frequency noise applications have been designed, fabricated and characterized. The basic design of the sensor is based on the Ketchen-type magnetometer. A moat surrounding the washer have been inserted to reduce 1/f noise due to hopping of flux vortices, trapped in the SQUID body during the cool-down process. To decrease flux motions in the pick-up coil structure a turn width as small as possible has been used. Further Josephson junctions with high areas have been employed to reduce the 1/f arising from critical currant fluctuations. An additional positive feedback (APF) circuit and a feedback coil for Flux-Locked-Loop (FLL) have been integrated on the same chip to avoid any additional noise due to an external APF circuit. Details about design criteria and experimental results concerning voltage-flux characteristics and noise performances of the devices are presented. Due to a low magnetic field noise, such devices well meet the requirements of ultra-low frequency noise applications.
FABRICATION PROCESS OF Nb/Al/Nb SNS JOSEPHSON JUNCTIONS FOR SUPERCONDUCTING ELECTRONICS APPLICATIONS
Authors: LACQUANITI V. , MAGGI S. , STENI R. , CAGLIERO C. , ROCCI R. , ANDREONE D.
Published: April 2003

Overdamped Josephson junctions are gaining interest in many fields of superconducting electronics. The characteristic voltage ICRN is the parameter limiting the highest possible speed and voltage resolution of the junctions. Nb/Al/Nb technology is a good candidate for these devices because of its wide range of characteristic voltages. In our laboratory we have so far fabricated Nb/Al/Nb SNS junctions having (a) high critical currents and smooth interfaces, (b) low critical currents and smooth interfaces, (c) low critical currents and rough interfaces. However, further improvements to this technology are still needed.
GROWTH OF MgB2 THIN FILMS BY MEANS OF IN SITU DEPOSITION TECHNIQUES
Authors: ROGAI R. , GALLUZZI V. , MANCINI A. , CELENTANO G. , PETRISOR T. , RUFOLONI A. , VARESI E. , GRASSANO G. , BOFFA V. , GAMBARDELLA U.
Published: April 2003

We report on the growth of MgB 2 thin films by means of Pulsed Laser Deposition (PLD) and Electron Beam (EB) deposition techniques. In order to develop an in-situ deposition procedure both techniques have been exploited following two approaches: the "as grown" procedure, where the superconducting phase is formed during the film growth, and no further process is performed, and the "annealing" procedure, where precursor layers are deposited and annealed in argon atmosphere. In the case of EB evaporated films, the "as grown" procedure revealed to be inadequate, because of the low reactivity of the thermally evaporated species and the high magnesium volatility. On the contrary, using PLD, the higher reactivity of the plasma species promotes the formation of the superconducting phase at deposition temperature as low as 350°C. In the "annealing" procedure, different kinds of precursor layers have been studied, in order to reduce and prevent the fast Mg evaporation at high temperature. Different annealing processes were investigated in order to promote the interdiffusion and reaction between Mg and B. The films were characterised by means of resistivity measurements and X-ray analyses. The surface morphology was observed by SEM microscopy.
DYNAMICAL BEHAVIOR OF A SQUID RING COUPLED TO A QUANTIZED ELECTROMAGNETIC FIELD
Authors: MIGLIORE R. , MESSINA A.
Published: April 2003

In this paper we investigate the dynamical behavior of a SQUID ring coupled to a quantized single-mode electromagnetic field. We have calculated the eigenstates of the combined fully quantum mechanical SQUID-field system. Interesting phenomena occur when the energy difference between the usual symmetric and anti-symmetric SQUID states equals the field energy ℏωF. We find the low-energy lying entangled stationary states of the system and demonstrate that its dynamics is dominated by coherent Rabi oscillations.
GROWTH OF SINGLE CRYSTALS IN Ru-Sr-Gd-Cu-O SYSTEM
Authors: MELE P. , ARTINI C. , COSTA G. A. , CARNASCIALI M. M. , FERRETTI M. , MASINI R.
Published: April 2003

The possibility to grow single crystals in the Ru-Sr-Gd-Cu-O system applying flux methods has been investigated. Several kinds of crystals were obtained: needle-like crystals of spurious phases (basically CuO and SrO), SrRuO 3, and ruthenates. We also observed that Gd was generally depleted from the ruthenate crystals, and it was spread in the residual matrix on the wall of crucible. Some small ruthenocuprate single crystals were indeed observed.
ANALYSIS OF ADVANTAGES OBTAINABLE USING MgB2 IN THE DESIGN OF ELECTROMAGNETS WITH IRON YOKE FOR MRI
Authors: MASULLO G. , MATRONE A. , SANTORO A. , BESIO S. , PITTALUGA S. , TREQUATTRINI A.
Published: April 2003

The discovery of superconductivity in MgB 2 gives the chance to study the application in magnet design in conjunction with the use of cryocoolers. Central field and compactness of conventional copper electromagnets with iron yoke can be improved using superconducting materials. The possibility to use MgB 2 is under investigation. A critical current density of 1000 A/mm 2 at 4.2K, at self-field, can be considered a typical value for this material. Taking into account temperature and magnetic field dependence and constructional packing factors we have considered a more realistic engineering current for our study. In the frame of a cooperation among CRIS and ESAOTE, the feasibility of a MRI (Magnetic Resonance Imaging) electromagnet by using MgB 2 will be investigated. For a preliminary magnet configuration, a typical iron profile has been considered to drive the magnetic flux and to obtain sufficient final magnetic field homogeneity. Additional considerations concern possible modification in magnetic design to take maximum advantage by the use of MgB 2 conductor.
AN IRRADIATION FACILITY FOR IN-VACUUM HEAVY-ION IRRADIATION OF LARGE AREA SUPERCONDUCTORS
Authors: ROVELLI A. , AMATO A. , BOTTA D. , CHIODONI A. , GERBALDO R. , GHIGO G. , GOZZELINO L. , LAVIANO F. , NEGRO M. , MEZZETTI E. , MARTINI L. , CURCIO F.
Published: April 2003

An irradiation facility to irradiate "long" stripe-shaped high-Tc superconductors by means of GeV heavy ions is described. This facility was designed and installed at INFN - Laboratori Nazionali del Sud (LNS), Catania, Italy. It consists into a moving apparatus automatically driven at a given velocity in order to obtain the in-vacuum exposure of contiguous zones of the moving target at a programmed ion fluence. The whole apparatus is automatically controlled by a home-made remote control program. The main parts of the apparatus are the mechanical set-up and the electronic circuit to create feedback signals to control both the beam current and the velocity of the sample holder, the high accuracy system for the measure of the integrated charge and the beam calibration set-up (beam shape and homogeneity). This apparatus is, at our best knowledge, the first one especially designed to create nanostructures across "large area" superconducting devices by means of the irradiation. Some significant results of 4.2 GeV Au-ion irradiation of a long high-Tc superconducting multifilamentary Ag/BSCCO-2223 tape are shown.
SWITCHING PHENOMENA AND PULSED OPERATION IN LONG JOSEPHSON DEVICES
Authors: ROTOLI G. , LATEMPA R. , PARLATO L. , PEPE G. P.
Published: April 2003

Recent findings by G.Pepe et al. (Appl.Phys.Lett.89, 2770, 2001) have given new insights over the dynamics of Josephson junctions subject to electronic pulse injection. These experiments can be considered important steps toward the study of non-equilibrium effects involved in junctions under very fast (e.g. laser induced) pumping signals. In the governing equations (sine-Gordon or coupled sine-Gordon depending on the system) pulse operation enters via Josephson current distribution or via bias current distribution. By means of numerical simulations here we study the dependence of the switching to resistive state on fast local depression of Josephson current. Next, using simulated electronic pulse train we show how the flip-flop operating mode found in G.Pepe et al. is a common feature of single or stacked Josephson systems without regard for the pulse waveform or the presence of the gap structure in the I-V characteristics. The effect of the length and of the magnetic field is also discussed.
ELECTRO-THERMAL RESPONSE OF A VOLTAGE-BIASED HIGH-TC BOLOMETER
Authors: MONTICONE E.
Published: April 2003

The characteristics of voltage biased HTc bolometers have been analysed by numerically solving the heat continuity equation of the system for ohmic and non-ohmic behaviour. The solutions show that the time constant and the linearity range of response depend strongly on biasing condition. For a bolometer in ohmic regime and operating in strong electrothermal feedback, the time constant can be reduced of more of one order of magnitude and linearity range increased of about 100 times with respect to the same device biased at low bias voltage. More limited improvements are obtained if the bolometer is in non-ohmic regime.
LASER DEPOSITION OF YBCO FILMS ONTO Ni–BASED SUBSTRATES
Authors: CANTORO M. , COPPEDÈ N. , CAMPOSEO A. , LABARDI M. , PARDI L. , FUSO F. , ALLEGRINI M. , ARIMONDO E. , BALDINI A. , BOTARELLI A. , LANCIA M. , MASCIARELLI G.
Published: April 2003

We have applied pulsed laser deposition (PLD) to fabricate YBCO/YSZ films (short length) onto biaxially textured NiFe substrates. Macroscopic characterization of the samples has been carried out by electrical measurements, XRD patterns and pole figures. Our best films are characterized by T cs around 89 K and transition widths around 5–6 K. Bragg–Brentano spectra and pole figures acquired on the {111} peak for YSZ and {013} for YBCO show the achievement of the correct crystalline structure and stoichiometry both in YSZ and in YBCO layers. They are largely in-plane textured (respectively (001) <110> for YSZ and (001) <100> for YBCO), and the superconducting YBCO layer exhibits a relatively low mosaicity. Atomic-force microscopy (AFM) analysis confirmed the presence of large grains (1–2 μm typical dimensions), and the occurrence of a complex morphology in the YBCO layer.
NB/AL2O3/AL/MGB2 LARGE AREA THIN FILMS HETEROSTRUCTURES: POSSIBLE OBSERVATION OF TUNNELING FROM BOTH DIRTY AND CLEAN LIMIT MGB2
Authors: CARAPELLA G. , MARTUCCIELLO N. , COSTABILE G. , FERDEGHINI C. , FERRANDO V. , GRASSANO G.
Published: April 2003

We report the demonstration of dc and ac Josephson effects as well as conductance spectra measurements on Nb/Al2O3/Al/MgB2 thin films heterostructure. The heterostructure exhibits moderately hysteretic current-voltage characteristic with a dc Josephson current branch and regular microwave-induced Shapiro steps. Conductance spectra measurements point toward a two-gap scenario and are explained with the probable presence of tunneling both from dirty limit regions, reflecting a proximized order parameter Δdirty ≈ 2 meV , and from clean limit regions, reflecting a proximized order parameter Δ3D ≈ 0.9 meV .
MAGNETIC FIELD CONTROLLED SUPERCONDUCTING - RESISTIVE AND RESISTIVE - SUPERCONDUCTING TRANSITIONS IN HIGHLY HYSTERETIC JOSEPHSON JUNCTIONS
Authors: LATEMPA R. , CARAPELLA G. , COSTABILE G. , PEPE G. P. , PARLATO L. , RUOTOLO A.
Published: April 2003

We report measurements of the switching induced by a rapidly varying magnetic field in a single, small, fully hysteretic Josephson junction. We demonstrate that the variation of the magnetic field, generated by a current step in a control line deposited on the top of the junction, can control the transitions from Josephson branch to the resistive branch and vice versa. The transition from resistive branch to Josephson branch is not trivial for an hysteretic junction. With a suitable choice of the magnetic pulse shape, a "deterministic flip-flop regime" can be achieved in this system.
RF SQUID BASED DEVICES FOR MQC
Authors: LONGOBARDI L. , CORATO V. , GRANATA C. , ROMBETTO S. , RUSSO M. , RUGGIERO B. , SILVESTRINI P.
Published: April 2003

We present different types of integrated planar SQUID devices in view of Macroscopic Quantum Coherence (MQC) experiments. These devices include an rf SQUID as a probe to test the MQC, coupled to a readout system based on a dc SQUID sensor. Both dc SQUID and flux transformer are in a gradiometric configuration to minimize the electromagnetic noise from environment. We report data on dc SQUID performances as well as preliminary results on the decay from the metastable flux state for rf SQUID-based devices above the crossover temeperature (classical limit).
HIGH QUALITY SmBa2Cu3O7-X THIN FILMS FOR MICROWAVE APPLICATIONS
Authors: BOFFA M. A. , BOBBA F. , CUCOLO A. M. , MONACO R.
Published: April 2003

In this paper we report on the analysis of the surface properties of c-axis oriented, highly epitaxial SmBa 2 Cu 3 O 7-X thin films grown by high oxygen pressure d.c. sputtering technique on (100)k LaAlO 3 substrates. Structural and morphological characterizations of the samples were carried out by means of X-Ray diffraction and AFM analysis. The AFM images show a surface morphology with characteristic screw dislocations with roughness of 2-3 nm on 1 μ m 2 area. The superconducting properties indicate a critical temperature Tc(R=0)>90K and a critical current density Jc>1*106A/cm 2 at T=77K. Preliminary microwave measurements on meanderline microstrip resonators show surface residual resistances Rs<13μΩ a 1 GHz at T=4.2K .
CARBON INFLUENCE IN THE SYNTHESIS OF MgB2 BY A MICROWAVE METHOD
Authors: AGOSTINO A. , BONOMETTI E. , VOLPE P. , TRUCCATO M. , MANFREDOTTI C. , OLIVERO P. , PAOLINI C. , RINAUDO G. , GOZZELINO L.
Published: April 2003

We describe a method to produce MgB 2 bulk via a microwave processing, which has not been reported yet for this material. We used two experimental arrangements, which were different in the form of the graphite thermal activator responsible for the microwave absorption. The reaction products have been analyzed from the morphological, structural and electronic point of view. The results show that the critical temperature is decreased by about 4 K when the graphite can diffuse inside the reaction cell and is incorporated in the samples, even if no evidence of carbon substitution in the MgB 2 lattice is given by the lattice constants. We point out that excellent conductivity features are achieved for nearly carbon-free samples.
HIGH QUALITY FULLY IN-SITU MgB2 THIN FILMS OBTAINED BY DC MAGNETRON SPUTTERING
Authors: MAGLIONE M. G. , CHIARELLA F. , DI CAPUA R. , VAGLIO R. , SALVATO M. , MARITATO L. , PRISCHEPA S. L.
Published: April 2003

MgB 2 thin films were grown in-situ at INFM- University of Naples by a magnetron sputtering technique in a UHV system (10-7 Pa) equipped with 3 focused 2′′ magnetron sources (a stoichiometric MgB 2 and metallic Mg and B targets by Superconducting Components Inc.). The substrates (sapphire or MgO) were placed "on axis" at 7 cm from the target surface on the surface of a molybdenum heater that could be operated up to 1000°C under vacuum.

Best results were obtained codepositing MgB 2 and Mg at equal sputtering power (500W) for 10 min on cold substrates, resulting in a Mg rich Mg-B precursor film. The films were then annealed inhyphen;situ at 830°C for 10 min in a In sealed Nb box in presence of saturated Mg vapor.

The process is highly reproducible and can be easily scaled to produce large area films.

The resulting films were about 1μm thick, with 100nm surface roughness as measured by AFM Resistive transition showed a maximum T c of 35 K and a transition width lower than 0.5 K.

The residual resistivity ratio was 1.6 for the best sample.

Resistivity measurements in external magnetic field up to 8 T have been performed both in parallel and perpendicular configuration. The upper critical magnetic field vs. temperature behavior has been determined from the experimental data and the superconducting anisotropy has been calculated for samples with different T c .
ANOMALOUS DOPING DEPENDENCE OF THE FLUCTUATION-INDUCED DIAMAGNETISM IN SUPERCONDUCTORS OF YBCO FAMILY
Authors: LASCIALFARI A. , RIGAMONTI A. , TEDESCO P. , ROMANO' L. , VARLAMOV A. , EMBRIACO D.
Published: April 2003

SQUID magnetization measurements in oriented powders of Y1-xCaxBa2Cu3Oy from the underdoped to the overdoped regime are discussed in the light of an anomalous precursor fluctuating diamagnetism occurring above the bulk superconducting transition temperature. It is emphasized how both in the underdoped as well as in the overdoped YBCO compounds, the fluctuating magnetisation differs strongly from the predictions based on the Ginzburg-Landau anisotropic functional, at variance of what occurs in optimally doped YBCO. A theoretical description based on the extension of a recent theory for fluctuations of the order parameter in a layered systems of vortices, is briefly outlined. It is shown that such an approach, essentially assuming droplets with a local superconducting temperature above the bulk TC, accounts for most of the experimental findings, including irreversibility effects.
89Y NMR SPECTRA AND RELAXATION IN UNDERDOPED AND OVERDOPED REGIMES OF SUPERCONDUCTING Ca-DOPED YBCO6 AND YBCO7
Authors: CARRETTA P. , LASCIALFARI A. , RIGAMONTI A. , TEDESCO P. , TEDOLDI F.
Published: April 2003

The results of a study of 89 Y NMR line shape and Knight shift, of the spin-echo dephasing time and of spin lattice relaxation time in Ca-doped YBCO compounds are presented. The underdoped and the overdoped regimes are achieved by means of Ca 2+ for Y 3+ substitutions in the parent antiferromagnet YBa 2 Cu 3 0 6.1 and in the chain-full YBa 2 Cu 3 0 7, respectively. The experimental findings are discussed in the light of models of localized Cu 2+ electrons and of delocalized Fermi-like carriers with antiferromagnetic correlation. Insights are obtained in regards of the density of states evolution with doping and of the occurrence, in the underdoped regime, of low-energy magnetic excitations causing a divergence of the relaxation rates on cooling, accompanied by a concurrent broadening of the NMR line. It is argued how a revision of the commonly accepted view for the microscopic magnetic properties in the YBCO family is required in order to attain a comprehensive justification of the variety of experimental data.
ANELASTIC SPECTROSCOPY STUDY OF THE CHARGE ORDER TRANSITION OF LiMn2O4
Authors: PAOLONE A. , CANTELLI R. , ROUSSE G. , MASQUELIER C. , FERRETTI M.
Published: April 2003

We studied the charge order transition of LiMn 2 O 4 by means of anelastic spectroscopy, measuring the Young's modulus, E, and the elastic energy loss function, Q-1. Our measurements confirm the occurrence of a phase transition at 296 K on cooling, showing a large hysteresis on heating. Around the transition temperature the Young's modulus shape reveals that there is at least one elastic constant, possibly c44, decreasing lowering the temperature. Moreover the anelastic spectroscopy results confirm that the transformation is characterised by the coexistence of two phases in a narrow temperature range. The time constant of the transformation of the cubic into the orthorhombic phase sharply decreases lowering T from 296 to 290 K and then increases on further cooling to 289 K. The strong decrease of the time constant around 290 K can be related to the reduction of the c44 elastic constant.
SUPERCONDUCTING FLUCTUATIONS AND RELATED FLUCTUATING DIAMAGNETISM IN YBCO124
Authors: LASCIALFARI A. , TEDESCO P. , ZUCCA I.
Published: April 2003

Magnetization M(H,T) measurements for magnetic field in the range H=5÷70000 Oe and T≥Tc(H) , have been performed in oriented powders of the strongly anisotropic cuprate YBCO124 in order to study the fluctuating diamagnetism and the dimensionality of the spectrum of the superconducting fluctuations. The experimental findings are discussed in the light of scaling laws and/or of anisotropic GL functional in terms of possible field-induced 2D-3D dimensional crossover. For high fields (H≥10000 Oe), it is found that the scaled magnetization -Mfl/TH 0.5 is an universal function of ε/ H 1/2v (for ε = T/T c(0)-1) and the critical exponent ν ≈ 0.72 is close to the value ν ≈ 0.67 obtained in 3D anisotropic YBCO123 systems. However, the 3D scaling law that predict a crossing of the curves Mfl(T,H = const)/H 0.5 vs. T at T c (0) is not verified. Also the 2D scaling laws for Mfl(T,H = const) are not obeyed. At intermediate fields (1000 < H < 10000 Oe), the curves Mfl(T,H = const) cross around Tc(0) thus suggesting a 2D behavior, while the critical exponent turn out ν ≈ 0.86. For H<1000 Oe, an anomalous behavior is noted and the curves Mfl(T,H=const) cross at Tc(0) + 1 K . The experimental findings suggest a dependence of the dimensionality of the superconducting fluctuations from the field. Finally the experimental data in YBCO124 are compared to the ones in underdoped YBCO123, where the occurrence of locally superconducting droplets above the bulk Tc was argued to occur.
ANISOTROPIC THERMAL EXPANSION IN DIBORIDES AS A FUNCTION OF MICRO-STRAIN
Authors: AGRESTINI S. , DI CASTRO D. , SANSONE M. , SAINI N. L. , BIANCONI A. , SACCONE A. , DE NEGRI S. , GIOVANNINI M. , COLAPIETRO M.
Published: April 2003

Thermal expansion of diborides ( AB 2), with different intercalated atoms (A) is studied as a function of temperature in the range of 100-370 K by high-resolution x-ray powder diffraction. The results indicate a well defined relationship between the anisotropy of the thermal expansion and the micro-strain of the boron layers, ε a = (a-a0)/a0 (where a0 = c/1.08 is the equilibrium a-axis for an unstrained AB2 system), determined by the atomic radius of the intercalated atoms. The thermal expansion is isotropic at εa = 0 (i.e., near c/a = 1.08) while the AB 2 system is unstrained, and it gets anisotropic away from the equilibrium. The anisotropy increases with increasing micro-strain in both directions (positive or negative, i.e. tensile or compressive) suggesting that the micro-strain is a key variable to define the state of diborides. As a matter of fact, the MgB 2 with the highest T c shows a tensile micro-strain, εa = 6% and a large thermal expansion anisotropy.
SYNTHESIS AND CHARACTERIZATION OF SUPERCONDUCTING MgCNi3
Authors: ARTINI C. , COSTA G. A. , MAGNONE E. , CIMBERLE M. R. , MASINI R.
Published: April 2003

The intermetallic perovskite MgCNi 3, superconducting with a Tc of 8 K, is particularly interesting from several points of view: in spite of the large amount of nickel present the compound is not ferromagnetic; nevertheless band structure calculations put in evidence a Van Hove singularity just above the Fermi level suggesting that MgCNi3 is on the border of magnetic instability and that may be driven to ferromagnetism by partial substitution of Mg with a monovalent metal as Li or Na. The material is also interesting because of the presence of two kinds of pinning centres, grain boundaries and nanometer-scale precipitates inside the grains, combination which is not found in other metallic superconductors. We have synthesized polycrystalline samples with composition MgCNi 3 by a solid state route. Both the resistive and the magnetic measurements show a double superconductive transition, the first one occurring at a temperature of 10 K, higher than that reported in literature (8 K) and a second one at 6 K. High temperature resistivity shows a metallic behaviour.
EFFECT OF SUBSTRATE ON THE EPITAXIAL GROWTH OF BOROCARBIDE THIN FILMS
Authors: FERDEGHINI CARLO , GRASSANO GIUSEPPE , BELLINGERI EMILIO , MARRÈ DANIELE , RAMADAN WEGDAN , FERRANDO VALERIA , BENEDUCE CONCETTA
Published: April 2003

The influence of the substrate on the epitaxial growth of borocarbide thin films was extensively studied. LuNi 2 B 2 C thin films were deposited by pulsed laser ablation from a stoichiometry target in high vacuum condition on different kind of substrates. Different cuts of MgO crystals with different surface crystallographic symmetry were employed. All the samples show a good c-axis orientation and rocking curves around LuNi 2 B 2 C (004) reflection show important differences both in width and in shape for the samples deposited on the different substrates. The growth of the film is effectively driven by the substrate even in the case of big lattice mismatch as in the case of MgO substrates. Different behaviors of the in-plane texture were observed on the different MgO cuts: an almost perfect epitaxial growth was obtained on the MgO (110) whereas multiple in-plane orientations are present on MgO (100) and (111). These results and all the observed orientations are well explained and understood in term of a simple geometrical near-coincident site lattice model (NCSL). A correlation between structural and superconducting properties is observed and the properties of best samples are similar to monocrystals ones ( T c = 16.4 K Δ T c = 0.3 K , RRR = 13).
MICROWAVE COMPLEX RESISTIVITY OF YBCO OVER A CONTINUOUS FREQUENCY SPECTRUM
Authors: SARTI S. , TOSORATTI N. , FASTAMPA R. , GIURA M. , SILVA E. , CUCOLO A. M. , BOFFA M.
Published: April 2003

We present measurements of the complex resistivity of YBCO thin filmsas a function of frequency, in the microwave range. Data arecollected through a Corbino disc geometry, allowing for a continuous frequency spectrum in the range 1-20 GHz. We focus on the behaviour of the imaginary part of the resistivity as a function of temperature. We show that it presents a broad peak justbelow Tc, whose amplitude and width depend on the frequency. We also discuss to what extent the experimental findings can be explained using the standard two fluid model, and which are the possible responsibles for the deviations from these predictions.
TEMPERATURE AND X-RAY ILLUMINATION EFFECTS IN OXYGEN DOPED La2CuO4
Authors: CAMPI G. , DI CASTRO D. , DELL'OMO C. , AGRESTINI S. , SAINI N. L. , BIANCONI A. , BIANCONI G. , BARBA L. , COLAPIETRO M.
Published: April 2003

We have investigated charge ordering in an oxygen doped La 2 CuO 4.1 crystal by high resolution x-ray diffraction using synchrotron radiation. Thanks to the high brilliance synchrotron radiation it has been possible to record a large number of weak superstructure spots due to charge ordering around the main peaks of the average structure. A study of the charge modulations with stage 3.5, and their behaviour as a function of the temperature and the intensity of the x-ray incident flux is reported. We are able to distinguish a microscopic decomposition in; 1) a stable domain, due to charges self trapped into a crystal of static ordered strings of finite length (~ 145 Å) and; 2) bubbles of 'superstripes' showing, as a function of photon dose, a decreasing coherence length at T = 220 K.
DYNAMICS OF THE LOW TEMPERATURE GLASS-LIKE PHASE IN MANGANITES
Authors: CASTELLANO C. , CORDERO F. , CANTELLI R. , FERRETTI M.
Published: April 2003

We report anelastic spectroscopy measurements of La1-xCaxMnO3performed in order to better characterize the nature of the phase transitions and microscopic lattice relaxation processes present in these materials. A peak in the imaginary part of the elastic susceptibility presents a behaviour typical of inhomogeneous and glass-like systems. We have performed a quantitative analysis calculating the temperature dependent distribution function of the energy barriers of the fluctuations characterizing this nanostructured state.
MICROSTRUCTURES OF SPUTTERED ORIENTED Si/CeO2 BILAYERs YBa2Cu3O7-δ/Si INTEGRATED MICROELECTRONICS
Authors: CHIODONI A. , MEZZETTI E. , BOTTA D. , GOZZELINO L. , MINETTI B. , PIRRI C. F. , TRESSO E. , CAMERLINGO C. , TALLARIDA G. , BARUCCA G. , FABBRI F.
Published: April 2003

In the framework of a research aimed to superconductor/semiconductor integrated electronics, we have grown a-axis oriented YBa 2 Cu 3 O 7-δ (YBCO) thin films on silicon (100) substrates with (111) oriented insulating buffer layers of cerium dioxide ( CeO 2), using magnetron sputtering deposition techniques. The properties of the cerium dioxide layer have been preliminary optimized by means of several layout and by monitoring the growing procedures through X-ray diffraction, AFM and TEM techniques. The lattice matching between CeO 2 and YBCO resulted to be worsened by an amorphous thin SiO 2 layer at the Si/CeO 2 interface, that decouples the buffer orientation from the seed orientation. However, it was possible to grow a relatively thick, optimally textured layer of CeO 2 without spurious orientations. The YBCO films deposited on top of this layer result preferentially a-axis oriented. The transition widths are very large, jet well controllable and reproducible. Some technological applications can be already envisaged.
SYNTHESIS AND PHYSICAL PROPERTIES OF RuSr2GdCu2O8
Authors: ARTINI C. , CARNASCIALI M. M. , COSTA G. A. , FERRETTI M. , CIMBERLE M. R. , MASINI R.
Published: April 2003

The synthesis and physical characterization of single phase polycrystalline samples with composition RuSr 2 GdCu 2 O 8 has been performed. We report on the electrical and magnetic measurements carried out in order to investigate and relate the properties of this compound subjected to different annealing routes. We show that while intrinsic properties as T c and structural properties as lattice parameters remain unchanged, the physical properties are strongly affected by the final annealing in flowing Oxygen. Coexistence of superconductivity and ferromagnetism is observed in all specimens studied, while Meissner effect visibility is induced by the annealing process.
ON THE COEXISTENCE OF SUPERCONDUCTIVITY AND SPIN-FREEZING IN UNDERDOPED CUPRATES
Authors: RIGAMONTI A. , EREMIN M. , CAMPANA A. , CARRETTA P. , CORTI M. , LASCIALFARI A. , TEDESCO P.
Published: April 2003

The divergences, on cooling, of NMR-NQR relaxation rates and muon spin rotation in underdoped cuprates, until now interpreted in terms of glassy spin freezing of magnetic moments resulting from charge inhomogeneities, are critically reconsidered. The relaxation data are analyzed in the light of the lack of dependence from an external magnetic field, of the strength of the effective field driving the divergences and of the stretched exponential character of the recovery processes. By resorting also to the theoretical conclusions of the extended t-J model including the intersite Coulomb interactions, it is argued that the divergences of the relaxation rates, rather than arising from magnetic moments could be related to sliding motions of vortex-antivortex orbital currents coexisting with d-wave superconducting state.
IN-PLANE AND OUT OF PLANE RESISTIVITIES IN BSCCO (2212) SINGLE CRYSTALS AT DIFFERENT DOPING LEVEL
Authors: FASTAMPA R. , GIURA M. , SARTI S. , MARCON R. , SILVA E.
Published: April 2003

Using an eight terminal geometry in standard current/voltage contact configurations we have determined both the out-of-plane and the in-plane d.c. resistivity in BSCCO(2212) samples at different doping level. Comparison of the temperature behaviors for these transport quantities highlights that use of these configurations results in incorrect determinations for the in-plane components. Exploiting the possibility of crossed comparisons when using a multi-terminal technique we are able to ascribe this effect at presence on the sample of surface layer with oxygen contents different from the bulk.
CORRELATION BETWEEN LOCAL OXYGEN DISORDER AND ELECTRONIC PROPERTIES IN SUPERCONDUCTING RESR2CU3O6+X(RE = Y, YB)
Authors: PRODI A. , GAUZZI A. , GILIOLI E. , LICCI F. , MAREZIO M. , BOLZONI F. , ALLODI G. , DE RENZI R. , BERNARDINI F. , MASSIDDA S. , PROFETA G. , CONTINENZA A. , RADAELLI P. G.
Published: April 2003

This work aims at understanding the large reduction of superconducting critical temperature Tc observed in YSr 2 Cu 3 0 6+x as compared to its YBa 2 Cu 3 O 6+x counterpart (ΔTc = -30 K ). We report on a combined study of structural and electronic properties of RESr 2 Cu 3 O 6+x(RE = Y, Yb) polycrystalline samples. Neutron diffraction data and Cu NQR spectra show that, contrary to REBa 2 Cu 3 O 6+x RESr 2 Cu 3 O 6+x is locally tetragonal and no CuO chains are formed. This arises from the random occupancy of oxygen along the a- or b- direction in the basal planes. Ab-initio calculations of the electronic structure using the full-potential linearized-augmented-plane-wave method (FLAPW) in the local density approximation (LDA) suggest that the CuO chains are not formed because of the large elastic strain associated with the orthorhombic distortion produced by the chain formation. In addition, by using a [Formula: see text] supercell simulating the absence of chains, we find that oxygen disorder greatly alters the band structure near the Fermi level. Our analysis indicates that this alteration leads to a reduction of hole transfer from the CuO chains to the CuO 2 planes, which accounts for the reduction of Tc experimentally observed.
SECOND PEAK FEATURES IN BSCCO SINGLE CRYSTALS OBSERVED BY THE MAGNETO-OPTICAL ANALYSIS
Authors: GOZZELINO L. , BOTTA D. , GERBALDO R. , GHIGO G. , LAVIANO F. , MEZZETTI E. , FASTAMPA R.
Published: April 2003

We performed magneto-optical analysis on slightly overdoped Bi 2 Sr 2 CaCu 2 O 8+x single crystals, looking for the second peak effect. On the basis of a method suitably developed to extract from the magneto-optical images the main electrodynamic quantities (magnetic induction field, B z , and current density, J), the local magnetisation, M s and the local J distribution maps were obtained, respectively. For the first time (at our best knowledge) patterns clearly due to a second peak in the M s vs. B z and J vs. B z curves were put in evidence and correlated each other by a magneto-optical analysis. The second peak slightly shifts with the temperature and with the position inside the sample, while its sharpness and depth do increase when the considered area is lowered. All the experimental evidences point towards a straightforward correlation between the second peak effect and the local disorder at micrometric scale.
INCLINED SUBSTRATE DEPOSITED CeO2 FILMS BY ELECTRON BEAM EVAPORATION ON RANDOMLY ORIENTED METALLIC SUBSTRATE
Authors: MANCINI A. , CELENTANO G. , FABBRI F. , GALLUZZI V. , PETRISOR T. , RUFOLONI A. , VARESI E. , VANNOZZI A. , ROGAI R. , BOFFA V. , GAMBARDELLA U.
Published: April 2003

A study on CeO 2 film growth on randomly oriented metallic substrate using lnclined Substrate Deposition (ISD) technique was performed in order to develop a biaxially aligned buffer layer for YBa 2 Cu 3 O 7-δ (YBCO) coated conductors. The influence of deposition parameters, as the substrate inclination angle α with respect to the CeO 2 vapor direction, deposition temperature and film thickness, on structural and morphological properties of the film was investigated. At substrate temperature between 200°C and 700°C a biaxial texture was observed for α ranging from 15° to 75°. The minimum value of the ϕ-scan full width at half maximum (FWHM) on (002) poles of about 13.5° was obtained for film 2 μm thick deposited at 200°C and α=55°. Morphological analyses on cross-sectioned samples revealed a columnar structure, typical for this deposition technique, with spaced grains and a tile like surface.