bluuebunny/crossref_metadata_2025 huggingface dataset:
FRAME FORMALISM FOR THE N-DIMENSIONAL QUANTUM EUCLIDEAN SPACES
Authors: CERCHIAI B. L. , MADORE J. , FIORE G.
Published: May 2003

We sketch our application1of a non-commutative version of the Cartan "moving-frame" formalism to the quantum Euclidean space [Formula: see text]the space which is covariant under the action of the quantum group SOq(N). For each of the two covariant differential calculi over [Formula: see text] based on the R-matrix formalism, we summarize our construction of a frame, the dual inner derivations, a metric and two torsion-free almost metric compatible covariant derivatives with a vanishing curvature. To obtain these results we have developed a technique which fully exploits the quantum group covariance of [Formula: see text]. We first find a frame in the larger algebra [Formula: see text]. Then we define homomorphisms from [Formula: see text] to [Formula: see text] which we use to project this frame in [Formula: see text].
QUANTIZING FIELD THEORIES IN NONCOMMUTATIVE GEOMETRY AND THE AdS/CFT CORRESPONDENCE
Authors: KAVIANI KAMRAN
Published: May 2003

By using the approach of noncommutative geometry, we study spinors and scalars on the two layers AdSd+1 space. We have found that in the boundary of two layers AdSd+1 Space, by using the AdS/CFT correspondence, we have a logarithmic conformal field theory. This observation propose a way to get the quantum field theory in the context of noncommutative geometry.
SPECTRAL GEOMETRY OF SPACETIME
Authors: KOPF TOMÁŠ
Published: May 2003

Spacetime, understood as a globally hyperbolic manifold, may be characterized by spectral data using a 3+1 splitting into space and time, a description of space by spectral triples and by employing causal relationships, as proposed earlier. Here, it is proposed to use the Hadamard condition of quantum field theory as a smoothness principle.
SOME PROPERTIES OF NONLINEAR σ-MODELS IN NONCOMMUTATIVE GEOMETRY
Authors: DABROWSKI LUDWIK , KRAJEWSKI THOMAS , LANDI GIOVANNI
Published: May 2003

We introduce nonlinear σ-models in the framework of noncommutative geometry with special emphasis on models defined on the noncommutative torus. We choose as target spaces the two point space and the circle and illustrate some characteristic features of the corresponding σ-models. In particular we construct a σ-model instanton with topological charge equal to 1. We also define and investigate some properties of a noncommutative analogue of the Wess–Zumino–Witten model.
DEFORMATION QUANTIZATION OF COADJOINT ORBITS
Authors: LIEDÓ M. A.
Published: May 2003

A method for the deformation quantization of coadjoint orbits of semisimple Lie groups is proposed. It is based on the algebraic structure of the orbit. Its relation to geometric quantization and differentiable deformations is explored.
ON THE RESOLUTION OF SPACE-TIME SINGULARITIES
Authors: MADORE J.
Published: May 2003

In previous articles it has been argued that a differential calculus over a noncommutative algebra uniquely determines a gravitational field in the commutative limit and that there is a unique metric which remains as a classical "shadow". Some examples were given of metrics which resulted from a given algebra and given differential calculus. Here we aboard the inverse problem, that of constructing the algebra and the differential calculus from the classical metric.
CONCEPTUAL ISSUES FOR NONCOMMUTATIVE GRAVITY ON ALGEBRAS AND FINITE SETS
Authors: MAJID SHAHN
Published: May 2003

We discuss some of the issues to be addressed in arriving at a definitive noncommutative Riemannian geometry that generalises conventional geometry both to the quantum domain and to the discrete domain. This also provides an introduction to our 1997 formulation based on quantum group frame bundles. We outline now the local formulae with general differential calculus both on the base "quantum manifold" and on the structure group Gauge transforms with nonuniversal calculi, Dirac operator, Levi–Civita condition, Ricci tensor and other topics are also covered. As an application we outline an intrinsic or relative theory of quantum measurement and propose it as a possible framework to explore the link between gravity in quantum systems and entropy.
TOWARDS NONCOMMUTATIVE COMPUTING
Authors: MASCARI G. F.
Published: May 2003

This paper presents first steps of an approach to quantum information processing in the framework of higher category theory from a noncommutative mathematics perspective. The aim is to provide a unifying theory for the structure and dynamics of composite quantum information processing systems, such that states, evolution, entanglement, decoherence are modeled by abstract categorical constructions and vice versa new mathematical structures arising from higher dimensional algebra could be "tested" as computational schemes and possibly realized by physical experiments.
INTRODUCTION TO BRAIDED QUANTUM FIELD THEORY
Authors: OECKL ROBERT
Published: May 2003

Indications from various areas of physics point to the possibility that space-time at small scales might not have the structure of a manifold. Noncommutative geometry provides an attractive framework for a perhaps more accurate description of nature. It encompasses the generalisation of spaces to noncommutative spaces and of symmetry groups to quantum groups. This motivates efforts to extend quantum field theory to noncommutative spaces and quantum group symmetries. One also expects that divergences of conventional theories might be regularised in this way.
GEOMETRIC FORMULATION OF GAUGE THEORY IN M4×ZN
Authors: SAITO TAKESI , UEHARA KUNIHIKO
Published: May 2003

The N=2 Super Yang–Mills theory is reconstructed from a geometric point of view, without employing the entire context of noncommutative geometry in discrete space. This is a revised version of our previous work.
A MODEL OF GENETIC CODE BASED ON UQ→0(sl(2)⊕sl(2))
Authors: FRAPPAT L. , SORBA P. , SCIARRINO A.
Published: May 2003

A model of genetic code based on Uq→0(sl(2)⊕sl(2)) is presented. The ratios of the codon usage in the quartets and sextets for 7 biological species belonging to the vertebrate series, with a statistics of codons larger than 200.000, exhibit a correlated behaviour, which fits naturally in the model.
QUANTUM ANTI-DE SITTER SPACE AT ROOTS OF UNITY
Authors: STEINACKER HAROLD
Published: May 2003

An algebra of functions on q-deformed Anti-de Sitter space [Formula: see text] with star-structure is defined for roots of unity, which is covariant under Uq(so(2, D-1)). The scalar fields have an intrinsic high-energy cutoff, and arise most naturally on products of the quantum AdS space with a classical sphere. Hilbert spaces of scalar fields are constructed.
SLAVNOV–TAYLOR IDENTITY IN NONCOMMUTATIVE GEOMETRY
Authors: WULKENHAAR RAIMAR
Published: May 2003

We develop a framework to define quantum Yang–Mills theories on general differential algebras imitating the standard procedure.
DEPENDENCE OF THE GAP-RATIO ON THE FERMI LEVEL SHIFT IN A VAN HOVE SUPERCONDUCTOR
Authors: KASKAMALAS S. , KRUNAVAKARN B. , YOKSAN S.
Published: May 2003

To understand the origin of high temperature superconductivity model based on the close proximity of the Fermi level to a quasi-two-dimensional Van Hove Singularity (VHS) in the density of states has been suggested. Here we present our study of the dependence of the zero temperature gap to the critical temperature for s- and d-wave superconductors on the Fermi level shift (δ) from the VHS within the BCS theory. Exact numerical calculations for the s and d gap ratios are carried out and approximate analytic expressions for the ratio are given. We find that the maximum gap ratio occurs at δ=0, and it decreases symmetrically with increasing δ.
DYNAMICS AND LOCAL STRUCTURE OF COLOSSAL MAGNETORESISTANCE MANGANITES
Authors: CASTELLANO C. , CORDERO F. , CANTELLI R. , MENEGHINI C. , MOBILIO S. , SARMA D. D. , FERRETTI M.
Published: May 2003

We report Extended X-ray Absorption Fine Structure and anelastic spectroscopy measurements on hole doped manganese oxides La 1-x Ca x MnO 3 which present the colossal magnetoresistance effect. EXAFS measurements were realized both in the absence and presence of an applied magnetic field of 1.1 Tesla, in a wide temperature range (between 330 and 77 K) and at various dopings (x = 0.25 and x = 0.33). The magnetic field orders the magnetic moments so favouring the electron mobility and the reduction of Mn-O octahedra distortions. We observe the presence of four short and two long Mn-O distances (1.93 and 2.05Å respectively) above and also below the metal-insulator phase transition. The overall distortion decreases but does not completely disappear in the metallic phase suggesting the possible coexistence of metallic and insulating regions at low temperatures. The magnetic field reduces the lattice distortions showing evidence of a microscopic counterpart of the macroscopic colossal magnetoresistance. We also present preliminary anelastic relaxation spectra in a wide temperature range from 900 K to 1 K on a sample with x = 0.40, in order to study the structural phase transitions and the lattice dynamics. A double peak has been observed at the metal-insulator transition in the imaginary part of Young's modulus. This double peak indicates that the metal-insulator transition could be a more complex phenomenon than a simple second order phase transition. In particular the peak at lower temperatures can be connected with the possible presence of inhomogeneous phase structures. Another intense dissipation peak has been observed corresponding to the structural orthorhombic-trigonal transition around 750 K.
THEORY OF CRITICAL CHARGE FLUCTUATIONS AND PSEUDOGAP FORMATION IN THE SINGLE-BAND HUBBARD MODEL
Authors: CITRO ROBERTA , MARINARO MARIO
Published: May 2003

Within a single-band Hubbard model, treated by means of a strong-coupling approach based on a cumulant expansion, we investigate the existence of critical charge fluctuations (CCF) that could give rise to a phase separation (PS) or a charge density wave, by means of a Bethe-Salpeter equation for the vertex function in the particle-hole channel. We discuss the relevance of the precursor effects of charge ordering on the self-energy of the electrons and the non-Fermi liquid behavior arising from an effective electron-electron interaction.
INVESTIGATION OF THE CLUSTER SPIN-GLASS TRANSITION IN La2–xSrxCuO4 BY MEANS OF ANELASTIC SPECTROSCOPY
Authors: PAOLONE A. , CORDERO F. , CANTELLI R. , FERRETTI M.
Published: May 2003

A step-like increase of the acoustic absorption (Q-1) is found in La 2-x Sr x CuO 4 (x = 0.019, 0.025, 0.032 and 0.064) below the temperature (Tg) at which NQR and μSR measurements evidenced the formation of a cluster spin-glass (CSG) state. The sharp maxima appearing in the 139 La NQR and μSR relaxation rates indicate the slowing down of antiferromagnetic fluctuations below the measuring frequency (107-108 Hz in those experiments). The acoustic absorption does not appear to be due to the freezing of magnetic fluctuations, for which one expects a narrow peak; rather, it is attributed to the size change of the antiferromagnetically correlated clusters, produced through magnetoelastic coupling by the vibration stress.
SYNTHESIS AND CHARACTERIZATION OF HG-BASED SUPERCONDUCTING THIN FILMS PRODUCED STARTING FROM LASER-DEPOSITED PRECURSORS
Authors: BINDI M. , FUSO F. , PUCCINI N. , ARIMONDO E. , TAMPIERI A. , CELOTTI G.
Published: May 2003

Correctly c-axis oriented HgBa 2 CaCu 2 O 6+δ thin films have been produced on (100) MgO single crystal substrates and characterized. Pulsed laser deposition has been exploited to deposit Hg-free Re-doped precursor which then underwent synthesis in evacuated and sealed quartz tubes. X-ray diffraction pattern of the precursor target shows the expected composition of oxides. Scanning electron microscopy analysis have been performed on the surface of the precursor film. Hg-1212 films have been analyzed by θ-2θ Bragg-Brentano X-ray diffractometry. The patterns show little contributions in composition of Hg-1223 phase. The films exhibit a transition temperature >120 K with zero-resistance at around 115 K.
CANONICAL TRANSFORMATION OF THE HUBBARD MODEL AND W = 0 PAIRING: COMPARISON WITH EXACT DIAGONALIZATION RESULTS
Authors: CINI MICHELE , BALZAROTTI ADALBERTO , BRUNETTI RAFFAELLA , GIMELLI MARIA , STEFANUCCI GIANLUCA
Published: May 2003

We have recently developed a canonical transformation of the Hubbard and related models, valid for systems of arbitrary size and for the full plane; this is particularly suited to study hole pairing. In this work we show that exact diagonalization results of the one band Hubbard model for small clusters with periodic boundary conditions agree well with the analytical ones obtained by means of our canonical transformation. In the presence of a pairing instability, the analytic approach allows us to identify the Cooper pairs. They are W = 0 pairs, that is, singlet two-hole eigenstates of the Hubbard Hamiltonian with vanishing on-site repulsion. Indeed, we find that the Coulomb interaction effects on W = 0 pairs are dynamically small, and repulsive or attractive, depending on the filling.
THE ROLE OF CuO CHAIN LENGTH ON SUPERCONDUCTIVITY EMERGENCE IN YBa2Cu3O6.30
Authors: MANCA P. , SANNA S. , CALESTANI G. , MIGLIORI A. , DE RENZI R. , ALLODI G.
Published: May 2003

The authors investigate the tetragonal(T)-orthorhombic(O) transient zone of YBa 2 Cu 3 O 6+x , in a comparative way on pair samples one of which is produced by deintercalation [D] and the other by intercalation [I] of intrinsic oxygen in the basal Cu(1)O x plane and having the same oxygen content (k) and thermal history. Resistive and electron diffraction measurements, performed on [D] k - [I] k pairs within the composition range 0.28 ≤ k ≤ 0.32, are reported. Direct evidence is provided that the T-OII transient is a structural and electronic two phase region characterized by the (T+OII) [D] 0.30 non-superconducting (non-SC) and (OII+OIII*) [I] 0.30 superconducting (SC) phases. The OII superstructure represents the minority component of [D] 0.30 phase and the majority one of [I] 0.30 and OIII* is a new orthorhombic anti-OIII superstructure. The nature of the non-SC and SC behaviours has been comparatively investigated by NQR analysis at the T-OII borderline. The evaluated different values of the average chain lengths emphasize the role of the Cu-O chain length on the hole doping efficiency in YBCO at constant oxygen content.
SUPERCONDUCTIVITY: FROM PHYSICS TO ALCHEMY
Authors: SACCHETTI N.
Published: May 2003

In this paper a short historical account of the discovery of superconductivity and of its gradual development is given. The physical interpretation of its various aspects took about forty years (from 1911 to 1957) to reach a successful description of this phenomenon in terms of a microscopic theory At the very end it seemed that more or less everything could be reasonably interpreted even if modifications and refinements of the original theory were necessary. In 1986 the situation changed abruptly when a cautious but revolutionary paper appeared showing that superconductivity was found in certain ceramic oxides at temperatures above those up to then known. A rush of frantic experimental activity started world-wide and in less than one year it was shown that superconductivity is a much more widespread phenomenon than deemed before and can be found at temperatures well above the liquid air boiling point. The complexity and the number of the substances (mainly ceramic oxides) involved call for a sort of modern alchemy if compounds with the best superconducting properties are to be manufactured. We don't use the word alchemy in a deprecatory sense but just to emphasise that till now nobody can say why these compounds are what they are: superconductors.
ELECTRODEPOSITION OF BIAXIALLY ALIGNED TL-BASED SUPERCONDUCTORS ON AG TAPES
Authors: BELLINGERI E. , SUO HL. , GENOUD J.-Y. , SCHINDL M. , WALKER E. , FLÜKIGER R.
Published: May 2003

An alternative preparation method of Tl based superconducting tapes is discussed. We developed Ag textured tapes as substrates: this material can be used without any buffer layer owing to its weak chemical interaction with the HTSC. Electrodeposition proved to be a fast, cheap and suitable technique to deposit a precursor that can be subsequently transformed in the superconducting phase by an opportune thermal treatment. The solution is formed with nitrates of all the metals dissolved in DMSO. The as-electrodeposited precursor is a highly reactive mixture at atomic scale of all the metals; a rapid (~30 min) thermal treatment is performed in a high isostatic pressure furnace (1 bar O2/50 bar He) in order to prevent Tl evaporation. Epitaxial growth of Tl-based superconductor was achieved by this technique on <110> textured Ag, both on single crystals and on oriented tapes. Superconducting transitions are observed at ~60 K and ~118 K for Tl(1212) and Tl(1223), respectively.
DEPOSITION ON VICINAL SUBSTRATES FOR DOMAIN SELECTION IN YBa2Cu3O7 FILMS
Authors: RICCI F. , CARILLO F. , LOMBARDI F. , MILETTO GRANOZIO F. , SCOTTI DI UCCIO U. , TAFURI F.
Published: May 2003

(110) and (103) YBa 2 Cu 3 O 7 films have been grown onto exact and vicinal (110) SrTiO 3 substrates, and on vicinal (110) MgO substrates with a SrTiO 3 buffer layer. The samples are carefully characterised by reciprocal space mapping with x-ray diffraction, in order to investigate the features of the typical double domain of (110) and (103) YBa 2 Cu 3 O 7 structure. It is demonstrated that vicinal cut substrates allow to select one film/substrate epitaxial relation. The growth properties of these thin films deposited on vicinal surfaces are discussed.
FABRICATION AND CHARACTERIZATION OF LARGE GRAINS GROWN ON YBCO PELLETS BY SEEDED MELT TEXTURING
Authors: FERRETTI M. , GHISO B. , MAGNONE E. , MELE P. , UBALDINI A. , COSTA G. A. , MONOT-LAFFEZ I. , GIOVANNELLI F. , LEBLOND-HARNOIS C.
Published: May 2003

In this work, the top-seeding technique was applied to the fabrication of both melt-textured YBCO and YBCO/Ag pellets. Both REBCO single crystals and single grains of REBCO are used as seeds. The effects of metal additive and seed nature on texture quality of the samples are discussed.
HIGH PRESSURE SYNTHESIS AND CHARACTERIZATION OF YSr2Cu3Ow
Authors: GILIOLI E. , GAUZZI A. , BESAGNI T. , LICCI F. , MAREZIO M. , RADAELLI P. G.
Published: May 2003

We report the synthesis under pressure, the critical temperature and the structural characteristics of YSr 2 Cu 3 O w . The compound does not form at normal pressure, but, once formed, can be retained at normal pressure in a metastable state. The synthesis was carried out in a multi-anvil apparatus at 30 kBars and 1050°C. The as-prepared sample was found to become superconductor with a Tc of ≈63 K. Tc increased up to ≈70 K after annealing in oxygen at 290°C. The refinement of the neutron diffraction data indicated that the as prepared YSr 2 Cu 3 O w is tetragonal (a=3.7855 Å and c=11.386 Å) and the oxygen content, w , is ≈ 6.84.
Fabrication of Neodymium-Based Superconducting Bulk Materials
Authors: VECCHIONE A. , GOMBOS M. , TEDESCO P. , PACE S.
Published: May 2003

We report on Nd123 bulk samples directionally solidified in air prepared with Ba-rich Nd422 doping brown phase by top seeding growth (TSG). By using this technique, we have fabricated single domain pellets up to 3.5 cm of diameter and 1 cm of thickness. The crystallographic alignment and the texture of samples have been investigated by means of polarised light optical microscopy, scanning electron microscope, supplemented by energy dispersive spectroscopy and X-ray diffraction analysis. The resulting microstructures have shown Nd 1 Ba 2 Cu 3 O 6+d domains, and Nd 4 Ba 2 Cu 2 O 10-d particles' size distribution, depending on the used values of the fabrication process's parameters. In particular, we investigated the influence both of the growth temperature window and of the cooling rate on the growth dynamics of the samples and on the precipitates segregation mechanism was analysed.
THEEu1+xBa2-xCu3OySYSTEM: NON-STOICHIOMETRY AND ELECTRICAL CONDUCTIVITY AT HIGH AND LOW TEMPERATURE
Authors: MALAVASI L. , GHIGNA P. , FLOR G. , SPINOLO G.
Published: May 2003

We present here non-stoichiometry and electrical conductivity measurements in the low-T and high-T range for the Eu1+xBa2-xCu3Oysystem with x=0, 0.2 and 0.4. We found an anomalous trend of the Tcwith y for EuBa2Cu3Oy(Eu-123) relative to other HTSC of REBa2Cu3Oyfamily (RE=Y, Nd, Sm). On the contrary high-T data for the whole system show a behavior similar to that of others RE-123.
EFFECTS OF COMPOSITION AND TEMPERATURE ON REBCO SINGLE CRYSTALS GROWN BY FLUX METHOD
Authors: UBALDINI A. , COSTA G. A. , CARNASCIALI M. M. , FERRETTI M.
Published: May 2003

The flux method is the most popular technique to obtain high quality and large side single crystals of YBa 2 Cu 3 O x (YBCO) and REBa 2 Cu 3 O x (REBCO). It is known that, for the YBCO, the interval of temperature for growing single crystals is between about 1010°C and 930°C, whereas, for the REBCO phases, the maximal temperature changes as function of rare earth. In this work, we studied the effect of different thermal programs on the side the YBCO crystals. The initial temperature and the cooling rate were changed. We studied the influence of a different initial ratio between Nd and Ba and between Sm and Ba on the equilibrium composition in NdBCO and SmBCO crystals.
PHASE FORMATION AND TEXTURE DEVELOPMENT IN AG-SHEATHED Bi, Pb(2223) TAPES STUDIED BY DIFFERENT IN-SITU AND EX-SITU DIFFRACTION TECHNIQUES
Authors: GIANNINI E. , BELLINGERI E. , MARTI F. , DHALLÉ M. , HONKIMÄKI V. , FLÜKIGER R.
Published: May 2003

In-situ and ex-situ high energy (80÷88 keV) X-Ray diffraction from a synchrotron radiation source were performed on multifilamentary Bi, Pb(2223)/Ag tapes using a transmission scattering geometry. Several thermo-mechanical procedures were compared, focusing mainly on the texture development of both Bi, Pb(2212) and Bi, Pb(2223) phases. The effect of the periodic pressing on the texture and on the critical current is elucidated. The texture development of the Bi, Pb(2212) phase prior to its transformation into Bi, Pb(2223) was directly observed in-situ at high temperature by using a dedicated high-energy X-ray compatible furnace and a high resolution Image Plate detector. A sharp increase of the Bi, Pb(2212) grain orientation along the [00l] direction was found to occur only above 750°C. Normal state transport measurements are in full agreement with the formation mechanism and with the texture development observed. A comparison of the results with the ones provided by in-situ neutron diffraction and standard low-energy XRD in a reflection geometry is presented.
A MODEL OF STRUCTURAL DISORDER IN (Ba2Cu2O4+x)/(CaCuO2)n SUPERLATTICES
Authors: PASQUINI G. , BALESTRINO G. , LAVANGA S. , MEDAGLIA P. G. , PETROCELLI G. , TEBANO A.
Published: May 2003

The structural disorder in ( Ba 2 Cu 2 O 4+x )/( CaCuO 2) n superlattices with ultrathin individual layers was investigated. Numerical simulations of x-ray spectra were carried out following a kinematical approach in a simplified model structure. The model assumes a two-dimensional layer by layer growth in which mixed composition layers can be corrugated to adjust the internal stresses due to the large mismatch between the constituent oxides. An additional random disorder is added to take into account the experimental dispersion in the amount of material deposited in each iteration. The model was successfully used to simulate x-ray spectra for samples with non integer n ranging from 2 to 16, grown in optimal conditions to obtain a superconducting behavior. The same model was used to simulate spectra of samples grown at lower pressures monitored with in-situ RHEED diagnostic. In this case, random disorder results more than one order of magnitude smaller.
SUPERCONDUCTING PROPERTIES OF LuNi2B2C THIN FILMS
Authors: GRASSANO G. , CIMBERLE M. R. , MARRÈ D. , PALLECCHI I. , PUTTI M. , RAMADAN W. , SIRI A. S. , FERDEGHINI C. , ANDREONE A. , CASSINESE A. , DI CAPUA R. , GIANNI L. , IAVARONE M. , MILETTO F. , RICCI F. , SCOTTI DI UCCIO U. , VAGLIO R. , CANEPA F.
Published: May 2003

We present the results of collaboration between the groups of Naples and Genoa Universities/INFM, focused on thin films superconducting borocarbides.

In particular a careful study of the deposition of non-magnetic LuNi 2 B 2 C thin films by Pulsed Laser Deposition (PLD) on MgO and other substrates will be presented. While samples with good c-orientation and superconducting properties are obtained in wide range of deposition parameters, in-plane micrometric oriented structures are obtained only in a narrow range of conditions, as evidenced by X-rays measurements and AFM (atomic force microscopy) analyses.

A special set-up for the in situ interchanging of shadow masks has been developed in order to deposit the diferent patterned layers and different buffer and/or barrier layers. New results on the temperature dependence of the LuNi 2 B 2 C gap using S/N contact junctions are presented and discussed. The obtained value of the superconducting gap and its temperature dependence agrees very well with the BCS prediction.

We present also microwave measurements of the surface impedance performed on large area films obtained by sputtering technique on low loss sapphire substrates. The data confirm the overall BCS-like behavior of these compounds.
DOPING-INDUCED ENHANCEMENT OF THE TUNNELING-DRIVEN TILT RATE OF OXYGEN OCTAHEDRA IN La2-xSrxCuO4
Authors: CORDERO F. , PAOLONE A. , CANTELLI R. , FERRETTI M.
Published: May 2003

Recent anelastic relaxation experiments (acoustic absorption versus temperature) demonstrated that a large fraction of the O octahedra in La 2 CuO 4 can switch between the minima of a multiwell potential both slowly in a collective way and locally with a tunneling driven dynamics. New results are presented for low Sr doping (x ≤ 0.03) on the absorption peak (5 K for x=0, measured at 6 kHz), which indicate fast local tilting with a temperature dependent characteristic frequency τ-1 (T). Doping greatly increases the local tilt rate τ-1 and the fraction of tunneling atoms. These observations are discussed in terms of nearly independent tunnel systems making transitions between eigenstates at a rate τ-1 determined by coupling between the tilts of the octahedra and the hole excitations. The fast temperature dependence of the rate (τ-1 (T) ∝ Tn with n ≥ 5 at x = 0.008) is not consistent with the usual models of coupling between a tunneling system and conduction electrons.
CORRELATION BETWEEN STRUCTURE AND MAGNETIC TRANSPORT MECHANISMS IN YBCO FILMS
Authors: GHIGO G. , CHIODONI A. , GERBALDO R. , GOZZELINO L. , MINETTI B. , MEZZETTI E. , CAMERLINGO C. , GIANNINI C. , TAPFER L. , MARTELLI S.
Published: May 2003

This paper studies the striking correlation between nanosized structural patterns in YBCO films and macroscopic transport current. A nanosized network of parallel Josephson junctions laced by insulating dislocations is almost mimicking the grain boundary structural network. It contributes to the macroscopic properties and accounts for the strong intergranular pinning across the film in the intermediate temperature range. The correlation between the two networks enables to find out an outstanding scaling law in the (Jc, B) plane and to determine meaningful parameters concerning the matching between the vortex lattice and the intergranular defect lattice. Two asymptotic behaviors of the pinning force below the flux flow regime are checked: the corresponding vortex phases are clearly individuated.
POSSIBLE EVIDENCE OF CHARGE-STRIPE ORDERING IN THE ab-PLANE RESISTIVITY OF STRONGLY UNDERDOPED La2-xSrxCuO4 SINGLE CRYSTALS
Authors: DAGHERO D. , GONNELLI R. S. , MORELLO A. , UMMARINO G. A. , NATALE L. , STEPANOV V. A. , LICCI FRANCESCA , UBERTALLI G.
Published: May 2003

We measured the ab-plane resistivity of La 2-x Sr x CuO 4 underdoped single crystals (x = 0.052 and x=0.06) between 4.2 and 300 K by using an AC version of the Van der Pauw technique. We suggest that a possible scenario of charge-stripe ordering at the lowering of the temperature can be delineated by starting from the main features of the ρ ab (T) curves of these samples. In particular, the deviation from the linearity of the ab-plane resistivity, occurring at a temperature T ch , could be related to the beginning of the charge localization, and the upturn of ρ ab to the progressive pinning of the resulting charge stripes. By starting from an analysis of our ab-plane resistivity curves, we determined the temperature of charge ordering T ch for our samples, thus extending in consistent way the T ch vs. x curve reported in literature to the very low-doping region of the phase diagram.
INTENSITY OF NMR SIGNAL IN La1.90Sr0.10CuO4: CHARGE vs. SPIN FREEZING
Authors: JULIEN M.-H. , CAMPANA A.
Published: May 2003

We report on 139 La NMR and 63 Cu NQR measurements in La 1.90 Sr 0.10 CuO 4 above its superconducting transition temperature. We find that the intensity of the signal integrated on frequencies decreases significantly on cooling from 60 K to 28 K for 63 Cu nuclei (wipeout effect), while it remains constant for 139 La nuclei in this T-range. The distribution of 139 La spin-lattice relaxation time (T1) values in the "wipeout regime" reveals a spread of electronic fluctuation frequencies in CuO 2 planes. A simple calculation, for spin fluctuation induced relaxation, shows that 63 Cu nuclei at sites where electronic fluctuations are slowest should not be observable because of too short relaxation times. This means that the 63 Cu NQR wipeout effect may be explained primarily by slow magnetic fluctuations.
EFFECTS OF SUBSTITUTIONS OF COPPER AT THE CHAIN SITE ON THE TRANSPORT AND MAGNETIC PROPERTIES OF YBa2Cu2(Cu1-xMx)O6+w
Authors: GUAGLIO G. , GAUZZI A. , LICCI F. , MAREZIO M. , FERRO P. , BOLZONI F. , MASINI R.
Published: May 2003

We prepared polycrystalline samples of YBa 2 Cu 2( Cu 1-x M x) O 6+w, where M (=Sc, Cr, V, Fe, Al and Au) substitutes copper in the chain site, and measured their transport and magnetic properties. The substitution level x was 0.05 and 0.10 for each substituent. We determined the solubility of the different metals and the dependence of Tc, irreversibility lines and critical current density jc on M and x. An improvement of jc by a factor larger than 2 was obtained in the Fe-, Al-, Cr- and V- substituted materials. No improvement of the irreversibility line was obtained for any substituent.
89Y NMR RELAXATION AND FLUCTUATING DIAMAGNETISM IN THE OVERDOPED REGIME OF CALCIUM-DOPED YBCO
Authors: CARRETTA P. , LASCIALFARI A. , RIGAMONTI A. , TEDOLDI F. , BOLZONI F. , LICCI F.
Published: May 2003

The overdoped regime in Y 1-x Ca x Ba 2 Cu 3 O 7, for x=0.05 and x=0.10 (Tc(0)=82 K and Tc(0) = 73.8 K respectively) is studied by means of 89 Y NMR relaxation and SQUID magnetization measurements. It is shown that for hole concentration close to 0.19, where a critical point is suspected and where the antiferromagnetic correlations should disappear, 1/(T1T) is strictly constant from room temperature down to Tc. This behavior is also observed in the sample at Ca-content 0.10, where the hole concentration is 0.203. The magnetization as a function of temperature and the isothermal magnetization curves, reveal the occurrence of a marked enhancement of the fluctuating diamagnetism in the overdoped regime, with respect to optimally doped YBCO. The magnetization data cannot be justified by anisotropic Ginzburg-Landau functional for the free energy or by scaling arguments. These findings are tentatively related to superconducting "drops" created above Tc or to phase fluctuations in restricted dimensionality.
MICROWAVE DISSIPATION IN YBa2Cu3O7-δ THIN FILMS WITH IRRADIATION-INDUCED COLUMNAR DEFECTS
Authors: Silva E. , Ghigo G. , Gozzelino L. , Camerlingo C. , Sarti S.
Published: May 2003

We present measurements of the magnetic field dependent microwave dissipation in a heavy-ion irradiated thin YBa 2 Cu 3 O 7-δ film, in order to study the effects of columnar defects on the pinning of vortices. Columnar defects were produced at 45° with respect to the c axis. Measurements of the microwave dissipation at 48 GHz were taken as a function of the angle between the field and the c axis, in order to compare the response along and perpendicular to the columns. The field was in the 0.3 T range. Several temperatures near the liquid nitrogen temperature were investigated. We find a dip in the dissipation when the field is aligned to the tracks. The dip is absent when the field is aligned perpendicular to the tracks, and with the same angle with respect to the crystallographic directions. The dip is not very sharp, and shows up for angles as far as 30° from the track directions. From the measurements, we estimate the pinning frequency along the columns at 80 K to be νpc ≃ 25 GHz .
MORPHOLOGIC AND ELECTRIC CHARACTERIZATION OF YBa2Cu3Ox/Ag COMPOSITES
Authors: COSTA G. A. , MELE P.
Published: May 2003

In this work, we performed a three-dimensional analysis of YBa 2 Cu 3 O x / Ag samples microstructure, especially with regard to silver distribution. By this way, we study thoroughly the correlation between the morphology and the electrical behaviour of the samples varying the silver content. We were able to show that the percolation limit of silver in YBCO is around 7.5 wt.%. We pointed out that the silver drops tends to disorder the system at high concentration, suggesting the opportunity of texturing under uniaxial pressure.
MICROWAVE PULSED FIELD EFFECTS ON DYNAMICS OF FLUXONS IN THE CRITICAL STATE
Authors: AGLIOLO GALLITTO A. , GUCCIONE M. , LI VIGNI M.
Published: May 2003

We report experimental results on microwave second harmonic emission in different superconductors in the critical state, exposed to intense pulsed mw magnetic fields. We show that the second order response of some superconducting samples shows a magnetic hysteretic behavior. We suggest that this behavior is related to non-equilibrium phenomena induced by the mw pulses. In particular, it may be ascribed to the different time response of the fluxons in the reverse and direct critical state.
MAGNETO-OPTICAL IMAGES OF AS-GROWN AND HEAVY-ION IMPLANTED HIGH TEMPERATURE SUPERCONDUCTORS
Authors: GOZZELINO L. , CHIODONI A. , GERBALDO R. , GHIGO G. , LAVIANO F. , MEZZETTI E. , MINETTI B. , FASTAMPA R.
Published: May 2003

An experimental set-up for magneto-optical analysis equipped with a new low- temperature controlling system is described. Magneto-optical investigations of the magnetic flux distribution are performed on as-grown as well as Au-on ion-irradiated Bi-2122 single crystals. The comparative analysis of as-grown and irradiated specimens shows that the magneto-optical technique provides information complementary to those provided by other techniques, such as, for example, susceptometric characterization.
DOPING DEPENDENT FLUCTUATIONS NEAR Tc IN THE BI2212 SUPERCONDUCTOR
Authors: DI CASTRO D. , SAINI N. L. , BIANCONI A. , LANZARA A.
Published: May 2003

We have measured temperature dependent complex conductivity on Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) superconducting system at different doping using single coil technique to determine the superconducting fluctuations near the T c . The results show that the fluctuation region below T c evolves linearly with the doping and is mainly controlled by the superfluid density. On the other hand, the fluctuation region above T c shows an exponential increase with underdoping and seems to have an intimate relationship with the anomalous electronic structure in the underdoped regime.
EXPERIMENTAL STUDY OF THE RESISTIVITY TENSOR IN Bi2Sr2CaCu2O8+x CRYSTALS BY MEANS OF MULTI-TERMINAL MEASUREMENTS
Authors: SARTI S. , ESPOSITO M. , FASTAMPA R. , GIURA M. , SILVA E.
Published: May 2003

We present measurements of the in-plane (ρab) and out-of-plane (ρc) resistivity in BSCCO single crystals. The resistivities are measured simultaneously on the same sample by means of a multi-terminal technique. Results are presented in the normal state and in the superconducting state, both in presence and in absence of an applied magnetic field. We underline the main differences with standard four probe measurements, showing in which cases the new method is strictly needed to avoid erroneous results. The technique is also able to test the locality of the dynamic: we present an accurate experimental analysis of the temperature and magnetic field ranges where the dynamic is local or non local. We finally show that above Tc this method gives results reliable enough to allow for a detailed study of the excess conductivity. We find that the data do not follow simple Aslamazov-Larkin laws in temperature ranges wider 1K, and that an accurate description of the data over a wide temperature range must include the effects of a short-wavelength cutoff in the fluctuational spectrum.
RADIO FREQUENCY BEHAVIOUR OF PIT-PROCESSED BI(2223) TAPES
Authors: GRASSO G. , MALAGOLI A. , SCATI N. , GUASCONI P. , SIRI A. S.
Published: May 2003

We have studied the response of long superconducting (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O 10 based conductors fabricated by the Oxide Powder In Tube method when exposed to a radio-frequency magnetic field excitation. In particular, we have characterized their properties when employed as a part L-C resonators typical of pick-up coil devices in Magnetic Resonance Imaging.

In these conditions, Bi(2223) conductors cooled down to 77 K have shown superior properties compared to already optimized copper based devices: tri-dimensional superconducting pick-up coils conceived for a resonance frequency between 6 and 9 MHz present an improvement of the quality factor Q by more than an order of magnitude, and an increase of the signal-to-noise ratio by a factor larger than three. This promising result opens new perspectives for Bi-based conductors in the field of the detection of weak high frequency signals.
THERMAL CONDUCTIVITY IN SILVER ALLOYS USED AS SHEATH FOR BSCCO TAPES: THE IMPORTANCE OF THE PHONON CONTRIBUTION
Authors: PUTTI M. , FERDEGHINI C. , GRASSO G. , GOLDACKER W. , VASE P.
Published: May 2003

In this paper we present measurements of thermal and electrical conductivity performed on several alloys and composite alloys tipically used as sheaths of BSCCO tapes: AgAu, AgMg, AgAu-AgMg. The thermal conductivity of the alloys is analysed in terms of electron and phonon contribution and we emphasize that in not dilute alloys the phonons contribute largely to the heat transport. In order to reduce the overall thermal conductivity, this latter contribution has to be minimized.
TEST AND QUALIFICATION OF A VARIABLE TEMPERATURE VIBRATING SAMPLE MAGNETOMETER SYSTEM FOR THE MEASUREMENT OF MAGNETIZATION CYCLES UP TO +/-12T IN THE 4.2K-300K TEMPERATURE RANGE
Authors: CLOTTI M. , GISLON P. , MORONI M. , SPADONI M. , PETRISOR T. , POP V.
Published: May 2003

A complete characterisation of an Oxford Instruments Vibrating Sample Magnetometer (VSM) has been made in order to check the limits and the reproducibility of the system as well as the measurement accuracy. Several measurement cycles have then been carried out to check the extent of the difference between the actual sample temperature and the sensor reading, and the weight of this difference on the magnetization data as a function of the temperature. Moreover an unexpected lowering of a reference signal has been observed at low temperatures, most probably due to shielding currents in the structure; this effect influences the sample signals, and has to be taken into account during data analysis. Being the magnetization a relative measurement, a great attention must be paid to the calibration procedure. Calibrations with 99.999% pure Nickel and Palladium, which have magnetization signals two order of magnitude apart, give comparable calibration constants. We observed that in order to have high accuracy measurements, the calibration has to be performed on samples geometrically identical to the test ones, and periodically repeated. The repeatability of the measurements has been found to be ±1%; once corrected the image current effect, the accuracy of the measurements, at 4.2 K, is ±3%. This value was also confirmed by the direct comparison of our results with those obtained at National Institute of Standards and Technology (NIST) on the same Nb 3 Sn samples
TWO TECHNIQUES FOR BROADBAND MEASUREMENT OF THE SURFACE IMPEDANCE OF HIGH CRITICAL TEMPERATURE SUPERCONDUCTING THIN FILMS
Authors: TOSORATTI N. , FASTAMPA R. , GIURA M. , LENZI V. , SARTI S. , SILVA E.
Published: May 2003

Measurements of the surface impedance of high critical temperature superconductors (HTS) over a wide frequency range provide important information on the electrodynamic properties of these materials and a severe test for any related theoretical model. We present two experimental techniques for the measurement of the surface impedance of HTS thin films in the microwave range, based on the detection of the reflection coefficient of the sample, connected to a vector network analyzer through a coaxial line. In one case the film forms an electrical short across the terminal section of the coaxial line, according to the so called Corbino geometry. In the other, a small circular gap separates the film from the inner conductor of the cable. In the latter case, the absence of direct electrical contact between the sample and the coaxial core simplifies realization and avoids contact instability always present with large temperature variations. We describe the two necessary steps to extract the film impedance from measured data, namely the study of electromagnetic field propagation in the two structures and the calibration of the coaxial measurement line at cyogenic temperatures. Finally, we present measurements performed on YBa 2 Cu 3 O 7-δ thin films with the two techniques.
IMPORTANCE OF PHASE FLUCTUATIONS FOR THE MAGNETIC PENETRATION DEPTH OF CONVENTIONAL AND CUPRATE SUPERCONDUCTORS
Authors: LAMURA G. , LE COCHEC J. , BOK J. , GAUZZI A. , LICCI F. , VILLÉGIER J. C. , PLAÇAIS B. , MATHIEU P. , REVCOLEVSCHI A. , ERB A.
Published: May 2003

In order to unveil the origin of the gapless properties of cuprate superconductors, we study the low temperature dependence of the magnetic penetration depth λ in: a) conventional NbN superconducting films with granular and epitaxial microstructure; b) single crystals of La 2-x Sr x CuO 4, Bi 2 Sr 2 CaCu 2 O 8 (BSCCO) and YBa 2 Cu 3 O 6+x at various doping levels ranging from the under- to the overdoped regimes. The measurements are performed by using a novel single-coil technique achieving 10 pm resolution. Our experimental results are as follows. a) The granular NbN films exhibit a linear low-temperature dependence of λ similar to the dependence reported earlier in cuprates. In the epitaxial NbN films, we find the characteristic exponential behaviour of conventional s-wave superconductors. b) In all single crystals of cuprates, the data confirm the linear dependence of λ ab reported earlier. In BSCCO, we find that the slope d λ ab / dT is rapidly increasing as doping decreases. As to a), our results give evidence for a gapless behaviour in conventional s-wave superconductors induced by a granular microstructure. As to b), our analysis indicates that a simple d-wave model accounts for the experimental slope values only in the optimally and over-doped samples. In the underdoped samples, the d-wave model underestimates the large slope values observed experimentally. Such large values are rather consistent with a simple model of thermodynamic phase fluctuations suited for metals with reduced dimensionality and low carrier density, as in cuprates. We argue that this fluctuation model also accounts for the linear dependence of λ found in our granular NbN films. We discuss possible mechanisms associated with the existence of weak links across the grains, which would favour thermally activated phase fluctuations of the gap in this conventional superconductor. Finally, we put forward the hypothesis that a similar phenomenon may occur in cuprates because of the intrinsic granularity associated with phase-separated metallic and insulating regions in the superconducting ab-plane.
REAL-AXIS SOLUTION OF ELIASHBERG EQUATIONS IN VARIOUS ORDER-PARAMETER SYMMETRIES AND TUNNELING CONDUCTANCE OF OPTIMALLY-DOPED HTSC
Authors: Ummarino Giovanni A. , Gonnelli Renato S. , Daghero Dario
Published: May 2003

In the present work we calculate the theoretical tunneling conductance curves of SIN junctions involving high-T c superconductors, for different possible symmetries of the order parameter (s, d, s + i d, s + d, anisotropics and extendeds). To do so, we solve the real-axis Eliashberg equations in the case of an half-filled infinite band. We show that some of the peculiar characteristics of HTSC tunneling curves (dip and hump at eV > Δ, broadening of the gap peak, zero bias and so on) can be explained in the framework of the Migdal-Eliashberg theory. The theoretical d I/ d V curves calculated for the different symmetries at T=4 K are then compared to various experimental tunneling data obtained in optimally-doped BSCCO, TBCO, HBCO, LSCO and YBCO single crystals. To best fit the experimental data, the scattering by non-magnetic impurities has to be taken into account, thus limiting the sensitivity of this procedure in determining the exact gap symmetry of these materials. Finally, the effect of the temperature on the theoretical tunneling conductance is also discussed and the curves obtained at T = 2 K are compared to those given by the analytical continuation of the imaginary-axis solution.
FAILURE OF THE MIGDAL-ELIASHBERG THEORY OF SUPERCONDUCTIVITY IN Rb3C60
Authors: GRIMALDI C. , CAPPELLUTI E. , PIETRONERO L. , STRÄSSLER S.
Published: May 2003

We discuss the compatibility of the most accurate experimental data with the ordinary Migdal-Eliashberg theory of superconductivity in the fullerene compound Rb 3 C 60. By using different model phonon spectra we conclude that the experimental data can be fitted only by invoking an electron-phonon coupling of order λ ≃ 3. This exceedingly high value is unphysical and it is not consistent with the basic assumptions of the Migdal-Eliashberg theory. On the contrary, by relaxing the adiabatic hypothesis on which the Migdal-Eliashberg theory rest, the experimental data can be fitted by much more realistic values of λ. This generalized theory predicts also characteristic features absent in the Migdal-Eliashberg framework which can be experimentally tested
A NON-PERTURBATIVE TREATMENT OF THE GENERALIZED SU-SCHRIEFFER-HEEGER HAMILTONIAN ON A DIMER
Authors: COCOCCIONI M. , ACQUARONE M.
Published: May 2003

Starting from the Hamiltonian for a dimer which includes all the electronic and electron-phonon terms consistent with a non-degenerate orbital, by a sequence of displacement and squeezing transformation we obtain an effective polaronic Hamiltonian. The renormalized electronic interactions differ from the results of semiclassical or perturbative treatments. The properties of the variationally determined ground state of two particles in the orbital are discussed for variable dimer length in the adiabatic limit
EFFECT OF STRONG CORRELATION ON THE ELECTRON-PHONON INTERACTION
Authors: BOERI LILIA , CAPPELLUTI EMMANUELE , GRIMALDI CLAUDIO , PIETRONERO LUCIANO
Published: May 2003

High-T c superconductors are usually described as strongly correlated electronic systems. This feature deeply affects the one-particle and two-particle properties of the system. In particular, a large incoherent background developes on the top of a narrow quasi-particle peak in the one-electron spectral function. We schematize this structure with a simple phenomenological form. The corresponding Green's function is employed to calculate the charge response of the system taking into account in a proper way strong correlation effects. The effective charge interaction acquires a structure in the exchanged momentum space with a predominance of forward scattering, in agreement with previous numerical calculations. The consequences of the momentum dependence of the interaction are discussed in the framework of the nonadiabatic theory of superconductivity proposed for the high-T c materials.