bluuebunny/crossref_metadata_2025 huggingface dataset:
PRECISE CALCULATION OF THE CASIMIR FORCE BETWEEN GOLD SURFACES
Authors: SVETOVOY V. B. , LOKHANIN M. V.
Published: May 2003

We analyze the result of precise measurement of the Casimir force between bodies covered with gold. The values of the parameters used to extrapolate the gold dielectric function to low frequencies are very important and discussed in detail. The finite temperature effect is shown to exceed considerably the experimental errors. The upper limit on the force is found which is smaller than the measured force. Many experimental and theoretical uncertainties were excluded with gold covering and we conclude that, possibly, a new force has been detected at small separations between bodies.
GROUP-THEORETICAL STRUCTURE OF QUANTUM MEASUREMENTS AND EQUIVALENCE PRINCIPLE
Authors: CAMACHO A.
Published: May 2003

The transverse group associated to some continuous quantum measuring processes is analyzed in the presence of nonvanishing gravitational fields. This is done considering, as an example, the case of a particle whose coordinates are being monitored. Employing the so-called restricted path integral formalism, it will be shown that the measuring process could always contain information concerning the gravitational field. In other words, it seems that with the presence of a measuring process the equivalence principle may, in some cases, break down. The relation between the breakdown of the equivalence principle, at quantum level, and the fact that the gravitational field could always act as a decoherence environment, is also analyzed. The phenomena of quantum beats of quantum optics will allow us to consider the possibility that the experimental corroboration of the equivalence principle at quantum level could be taken as an indirect evidence in favor of the quantization of the gravitational field, i.e. the quantum properties of this field avoid the violation of the equivalence principle.
DECOHERENCE BY RELIC GRAVITONS
Authors: HABA Z.
Published: May 2003

A time-dependence of the decoherence resulting from an interaction with gravitons in a thermal state is investigated. The decoherence time can be small if the quantum system consists of a large number of particles. In such a case the quantum addition of amplitudes is equivalent to a classical addition of probabilities.
DECAY OF COSMOLOGICAL CONSTANT AS BOSE CONDENSATE EVAPORATION
Authors: DYMNIKOVA IRINA , KHLOPOV MAXIM
Published: May 2003

We consider the process of decay of symmetric vacuum state as evaporation of a Bose condensate of physical Higgs particles, defined over asymmetric vacuum state. Energy density of their self-interaction is identified with cosmological constant Λ in the Einstein equation. Λ decay then provides dynamical realization of spontaneous symmetry breaking. The effective mechanism is found for damping of coherent oscillations of a scalar field, leading to slow evaporation regime as the effective mechanism for Λ decay responsible for inflation without special fine-tuning of the microphysical parameters. This mechanism is able to incorporate reheating, generation of proper primordial fluctuations, and nonzero cosmological constant today.
ON DUALITY ROTATIONS IN LIGHT-LIKE NONCOMMUTATIVE ELECTROMAGNETISM
Authors: ASCHIERI PAOLO
Published: May 2003

We study electric-magnetic duality rotations for noncommutative electromagnetism (NCEM). We express NCEM as a nonlinear commutative U(1) gauge theory and show that it is self-dual when the noncommutativity parameter θ is light-like (e.g. θ0i=θ1i). This implies, in the slowly varying field approximation, self-duality of NCEM to all orders in θ.
SOME REMARKS ON UNILATERAL MATRIX EQUATIONS
Authors: CERCHIAI BIANCA L. , ZUMINO BRUNO
Published: May 2003

We briefly review the results of our paper4: We study certain perturbative solutions of left-unilateral matrix equations. These are algebraic equations where the coefficients and the unknown are square matrices of the same order, or, more abstractly, elements of an associative, but possibly noncommutative algebra, and all coefficients are on the left. Recently such equations have appeared in a discussion of generalized Born–Infeld theories. In particular, two equations, their perturbative solutions and the relation between them are studied, applying a unified approach based on the generalized Bezout theorem for matrix polynomials.
STABILITY OF LIE SUPERALGEBRAS AND BRANES
Authors: CHRYSSOMALAKOS CHRYSSOMALIS
Published: May 2003

The algebra of the generators of translations in superspace is unstable, in the sense that infinitesimal perturbations of its structure constants lead to non-isomorphic algebras. We show how superspace extensions remedy this situation (after arguing that remedy is indeed needed) and review the benefits reaped in the description of branes of all kinds in the presence of the extra dimensions.
SOME COMPUTATIONS WITH SEIBERG–WITTEN INVARIANT ACTIONS
Authors: CORNALBA LORENZO
Published: May 2003

We show, with a two-dimensional example, that the low energy effective action which describes the physics of a single D-brane is compatible with T-duality whenever the corresponding U (N) non-Abelian action is form-invariant under the noncommutative Seiberg–Witten transformations.
NONCOMMUTATIVE GEOMETRY: CALCULATION OF THE STANDARD MODEL LAGRANGIAN
Authors: ELSNER KAREN
Published: May 2003

The calculation of the standard model Lagrangian of classical field theory within the framework of noncommutative geometry is sketched using a variant with 18 parameters. Improvements compared with the traditional formulation are contrasted with remaining deviations from the requirements of physics.
SUPERGROUPS, QUANTUM SUPERGROUPS AND THEIR HOMOGENEOUS SPACES
Authors: FIORESI R.
Published: May 2003

We give a more algebraic definition of algebraic supergroup via its Hopf algebra. The Hopf algebra of the supergroup SL (m|n) is given together with its quantization. The example of supercoadjoint orbits is examined at the end.
OPEN STRINGS AND NONCOMMUTATIVE GEOMETRY OF BRANES ON GROUP MANIFOLDS
Authors: SCHOMERUS VOLKER , ALEKSEEV ANTON YU. , RECKNAGEL ANDREAS
Published: May 2003

In this letter we review some recent work on the noncommutative geometry of branes on group manifolds. In particular, we show how fuzzy spaces arise in this context from an exact worldsheet description and we sketch the construction of a low-energy effective action for massless open string modes. The latter is given by a combination of a Yang–Mills and a Chern–Simons like functional on the fuzzy world-volume. It can be used to study condensation on various brane configurations in curved backgrounds.
ENVELOPING ALGEBRA-VALUED GAUGE TRANSFORMATIONS ON NONCOMMUTATIVE SPACES
Authors: SCHRAML STEFAN
Published: May 2003

Enveloping algebra-valued gauge fields are constructed, its components depend only on a finite number of fields. To achieve this, the Seiberg–Witten map is used.
NONCOMMUTATIVE OPEN STRING THEORIES AND THEIR DUALITIES
Authors: SHEIKH-JABBARI M. M.
Published: May 2003

The recently found noncritical open string theories is reviewed. These open strings, noncommutative open string theories (NCOS), arise as consistent quantum theories describing the low energy theory of D-branes in a background electric B-field in the critical limit. Focusing on the D3-brane case, we construct the most general (3+1) NCOS, which is described by four parameters. We study S- and T-dualities of these theories and argue the existence of a U-duality group.
INHOMOGENEOUS STRING COSMOLOGY SOLUTIONS WITH REGULAR SPACE–TIME CURVATURE
Authors: PIMENTEL LUIS O.
Published: May 2003

In this work cosmological models are considered for the low energy string cosmological effective action (tree level) in the absence of dilaton potential. A two-parametric nondiagonal family of analytic solutions is found. The curvature is nonsingular, however the string coupling diverges exponentially.
EXACT SUPERSYMMETRY ON THE LATTICE
Authors: BIETENHOLZ W.
Published: May 2003

We discuss the possibility of representing supersymmetry exactly in a lattice discretized system. In particular, we construct a perfect supersymmetric action for the free Wess–Zumino model.
A NEW APPROACH FOR BOSONIZATION OF MASSIVE THIRRING MODEL IN THREE DIMENSIONS
Authors: BANERJEE R. , MARINO E. C.
Published: May 2003

We develop a new approach for bosonization in the quadratic approximation which is based on the direct comparison of current correlation functions. This is applied to the case of the massive Thirring model in three dimensions in the weak coupling regime, but with an arbitrary mass. Explicit bosonized forms for the Lagrangian and the current are obtained in terms of a vector gauge field. Results for the corresponding expressions are also obtained in the case of a free massive fermion. Finally, a comment on the derivation of the current algebra directly from the bosonized expressions is included.
PION AND KAON DISSOCIATION IN HOT QUARK MEDIUM
Authors: BHATTACHARYYA ABHIJIT , GHOSH SANJAY K. , RAHA SIBAJI
Published: May 2003

Pion and kaon dissociation in a medium of hot quark matter is studied in the Nambu–Jona-Lasinio model. The decay widths of pion and kaon are found to be large but finite at temperatures much higher than the so-called critical temperature of chiral or deconfinement transition, kaon decay width being larger. Consequently, pions and even kaons (with a lower density compared to pions) should coexist with quarks and gluons at such high temperatures. The implication of the above result in the study of quark–gluon plasma is discussed.
SOLITON DECAY, TIME DELAYS AND BREATHERS IN THE CHIRAL EQUATION
Authors: AL-SOLAMY FALLEH , BEGGS EDWIN J.
Published: May 2003

This paper exhibits soliton decay and shifts in the world lines of colliding solitons in the principal chiral model, by means of pictures and calculations. To do this the energy and momentum density are defined from the Morse functions in the inverse scattering picture.
INFRARED QUASI FIXED POINTS AND MASS PREDICTIONS IN THE MSSM II: LARGE tan β SCENARIO
Authors: JURČIŠIN M. , KAZAKOV D. I.
Published: May 2003

We consider the infrared quasi fixed point solutions of the renormalization group equations for the Yukawa couplings and soft supersymmetry breaking parameters in the MSSM in the large tan β regime. The existence of ir quasi fixed points together with the values of gauge couplings, third generation quarks, lepton and Z-boson masses allows one to predict masses of the Higgs bosons and SUSY particles as functions of the only free parameter, m1/2, or the gluino mass. The lightest Higgs boson mass for M SUSY ≈1 TeV is found to be mh=128.2-0.4-7.1± 5 GeV for μ>0 and mh= 120.6-0.1-3.8±5 GeV for μ<0.
THE HIGGS MASS BOUND IN THE SUSY MULTI-HIGGS-DOUBLET MODEL
Authors: SAKAMURA YUTAKA
Published: May 2003

The upper bound on the mass of the lightest Higgs boson is provided in the supersymmetric standard model with multi-Higgs doublets, up to two-loop order. Relatively large corrections are expected from the experimentally unconstrained extra Yukawa couplings. We calculate it taking into account the two-loop leading-log contributions as a function of the scale Λ, below which the theory remains perturbative. Even for Λ=104 GeV, we obtain the upper bound of 140 GeV, which is heavier than that of the MSSM by 10 GeV.
ENERGY BARRIER IN THE TWO-HIGGS MODEL
Authors: KLEIHAUS B.
Published: May 2003

The electroweak model is extended by a second Higgs doublet and a numerical investigation of static, finite energy classical solutions is performed. The results indicate that for a large domain of the parameters of the Higgs potential, the energy barrier between topologically distinct vacua of the Lagrangian is constituted by a bisphaleron.
AHARONOV–BOHM EFFECT AND COORDINATE TRANSFORMATIONS
Authors: CAMACHO A.
Published: May 2003

Resorting to a Gedankenexperiment which is very similar to the famous Aharonov–Bohm proposal it will be shown that, in the case of a Minkowskian space–time, we may use a nonrelativistic quantum particle and a noninertial coordinate system and obtain geometric information of regions that are, to this particle, forbidden. This shows that the outcome of a nonrelativistic quantum process is determined not only by the features of geometry at those points at which the process takes place, but also by geometric parameters of regions in which the quantum system cannot enter. From this fact we could claim that geometry at the quantum level plays a nonlocal role. Indeed, the measurement outputs of some nonrelativistic quantum experiments are determined not only by the geometry of the region in which the experiment takes place, but also by the geometric properties of space–time volumes which are, in some way, forbidden in the experiment.
RENORMALIZATION GROUP METHODS IN CONSTRUCTIVE FIELD THEORIES
Authors: WATANABE HIROSHI
Published: May 2003

Mathematical construction of quantum field theory is reviewed with emphasis on the conceptual structure of the construction and on the role of rigorous renormalization group analysis without technical details. After explaining a rigorous formulation of a renormalization group method in a weak coupling region, a new approach in a strong coupling region is proposed in the context of the hierarchical approximation. An idea of proving triviality in d≥4 dimensions utilizing this new proposal concludes this review.
AN INTRODUCTION TO THE THEORY OF SELF-SIMILAR STOCHASTIC PROCESSES
Authors: EMBRECHTS PAUL , MAEJIMA MAKOTO
Published: May 2003

Self-similar processes such as fractional Brownian motion are stochastic processes that are invariant in distribution under suitable scaling of time and space. These processes can typically be used to model random phenomena with long-range dependence. Naturally, these processes are closely related to the notion of renormalization in statistical and high energy physics. They are also increasingly important in many other fields of application, as there are economics and finance. This paper starts with some basic aspects on self-similar processes and discusses several topics from the point of view of probability theory.
CHARGED BLACK HOLES IN ASYMPTOTICALLY ANTI-DE SITTER SPACETIME
Authors: MITRA P.
Published: May 2003

Unlike the extremal Reissner–Nordström black hole in ordinary spacetime, the one in anti-de Sitter spacetime is a minimum of action and has zero entropy if (functional integral) quantization is carried out after extremalization. However, if extremalization is carried out after quantization, then the entropy is a quarter of the area as in the usual case. This is because the functional integral is dominated by non-extremal configurations of lower action. However, some special non-extremal black holes have higher action and may decay into extremal configurations.

An attempt is also made to see if the anti-de Sitter version can be described by a microscopic model consisting of a one-dimensional gas of massless particles as in the usual case. Such a description is possible only in the limit of vanishing cosmological constant.
REGULARISATION OF CHIRAL GAUGE THEORIES
Authors: DASS N. D. HARI
Published: May 2003

This article gives a review of the topic of regularising chiral gauge theories and is aimed at a general audience. It begins by clarifying the meaning of chirality and goes on to discussing chiral projections in field theory, parity violation and the distinction between vector and chiral field theories. It then discusses the standard model of electroweak interactions from the perspective of chirality. It also reviews at length the phenomenon of anomalies in quantum field theories including the intuitive understanding of anomalies based on the Dirac sea picture as given by Nielsen and Ninomiya. It then raises the issue of a non-perturbative and constructive definition of the standard model as well as the importance of such formulations. The second Nielsen–Ninomiya theorem about the impossibility of regularising chiral gauge theories under some general assumptions is also discussed. After a brief review of lattice regularisation of field theories, it discusses the issue of fermions on the lattice with special emphasis on the problem of species doubling. The implications of these problems to introducing chiral fermions on the lattice as well as the interpretations of anomalies within the lattice formulations and the lattice Dirac sea picture are then discussed. Finally the difficulties of formulating the standard model on the lattice are illustrated through detailed discussions of the Wilson–Yukawa method, the domain wall fermions method and the recently popular Ginsparg–Wilson method.
A MICROSCOPIC APPROACH TO SPIN GAP AND SINGLE CuO2 PLANE SUPERCONDUCTIVITY
Authors: BASKARAN G.
Published: May 2003

We sharpen the RVB mechanism of superconductivity proposed by Anderson in 1987 and explain the spin gap phenomenon as well as superconductivity in a single CuO 2 layer. We state our mechanism in the form a theorem, and heuristic proof, in which the neutral spinon pairing in the spin gap phase frustrates the single electron kinetic energy leading to a new "singular" Landau parameter, which is a finite "tomographic" BCS interaction in an underlying tomographic Luttinger liquid.
LINEAR RESPONSE THEORY FOR DISORDERED BOSONS
Authors: NISAMANEEPHONG P.
Published: May 2003

Within the hard-core boson model and the 1/S spin wave approximation, we have constructed the linear response theory for disordered boson system. Our numerical results shows that the instability of the superfluidity occurs when the paramagnetic part cancels out the diamagnetic part with a critical finite amout of disorder.
DUALITY ROTATIONS FOR ABELIAN BORN-INFELD LAGRANGIANS
Authors: ASCHIERI PAOLO
Published: May 2003

We present the Born–Infeld Lagrangian with n Abelian gauge fields and show that it has an Sp(2n, ℝ) duality symmetry. This involves solving a unilateral matrix equation and more generally an algebraic equation over a non-commutative ring.
A THEORY OF ALGEBRAIC INTEGRATION
Authors: CASALBUONI R.
Published: May 2003

In this paper we study the problem of quantizing theories defined over a nonclassical configuration space. If one follows the path-integral approach, the first problem one is faced with is the one of definition of the integral over such spaces. We consider this problem and we show how to define an integration which respects the physical principle of composition of the probability amplitudes for a very large class of algebras.
COTANGENT AND TANGENT MODULES ON QUANTUM ORBITS
Authors: AKUESON P. , GUREVICH D.
Published: May 2003

Let [Formula: see text] be the "coordinate ring" of a quantum sphere. We introduce the cotangent module on the quantum sphere as a one-sided [Formula: see text]-module and show that there is no Yang–Baxter type operator converting it into a [Formula: see text]-bimodule which would be a flatly deformed object w.r.t. its classical counterpart. This implies non-flatness of any covariant differential calculus on the quantum sphere making use of the Leibniz rule. Also, we introduce the cotangent and tangent modules on generic quantum orbits and discuss some related problems of "braided geometry".
DISTANCES IN NONCOMMUTATIVE GEOMETRY
Authors: IOCHUM BRUNO
Published: May 2003

In this review, we give examples of explicitely computed distances including classical geometries, finite noncommutative geometries and applications to particle physics.
NONCOMMUTATIVE GEOMETRY, STRINGS AND DUALITY
Authors: LIZZI FEDELE
Published: May 2003

In this talk, based on work done in collaboration with G. Landi and R. J. Szabo, I will review how string theory can be considered as a noncommutative geometry based on an algebra of vertex operators. The spectral triple of strings is introduced, and some of the string symmetries, notably target space duality, are discussed in this framework.
ON THE DUALITY OF QUANTUM LIOUVILLE FIELD THEORY
Authors: O'RAIFEARTAIGH L. , PAWLOWSKI J. M. , SREEDHAR V. V.
Published: May 2003

It has been found empirically that the Virasoro centre and 3-point functions of quantum Liouville field theory with potential e2bΦ(x) and external primary fields exp(αΦ(x)), are invariant with respect to the duality transformations ℏα→q-α where q=b-1+b. The steps leading to this result (via the Virasoro algebra and 3-point functions) are reviewed in the path-integral formalism. The duality stems from the fact that the quantum relationship between the α and the conformal weights Δα is two-to-one. As a result the quantum Liouville potential may actually contain two exponentials (with related parameters). It is shown that in the two-exponential theory the duality appears in a natural way and that an important extrapolation which was previously conjectured can be proved.
BICOMPLEXES, INTEGRABLE MODELS, AND NONCOMMUTATIVE GEOMETRY
Authors: DIMAKIS ARISTOPHANES , MÜLLER-HOISSEN FOLKERT
Published: May 2003

We discuss a relation between bicomplexes and integrable models, and consider corresponding noncommutative (Moyal) deformations. As an example, a noncommutative version of a Toda field theory is presented.
CRYSTAL STRUCTURE DATA ON A NEW CUBIC PHASE ACTING AS ANTAGONIST IN THE SYNTHESIS OF Hg(Re)-1223 SUPERCONDUCTOR
Authors: CELOTTI G. , TAMPIERI A. , CAVALLARO A.
Published: May 2003

During the attempts to improve sealed-quartz tube synthesis of Hg(Re)-1223 superconductor, a new compound was systematically identified in certain unsuccessful experimental conditions. It was characterized by a well distinct powder diffraction pattern with cubic diamond-like extinctions and lattice parameter 8.48-8.53 Å. The composition was determined as Hg 2 Re 0.5 Ba 4 Ca 1.5 O 8.5 and an univocal interpretation was found in terms of the high-symmetry space group Fm3m(225), with Z=2, (= 6.70 g/cm 3; the reliability factor was 5.0%. This phase, Cu-free and not superconductive, rather than a precursor can be considered as an unwanted accident in Hg(Re)-1223 preparation route. Its existence was confirmed by a successful synthesis starting from the above reported stoichiometry.
ANELASTIC AND 139La NQR RELAXATION STUDY OF La2-xSrxCuO4 AROUND THE CRITICAL Sr CONTENT x = 0.02
Authors: CAMPANA A. , CANTELLI R. , CORDERO F. , CORTI M. , RIGAMONTI A.
Published: May 2003

Anelastic relaxation and 139 La NQR relaxation measurements in La 2-x Sr x CuO 4 with x ≃ 0.02 are discussed in terms of spin and lattice excitations, ordering processes and thermal depinning of charged stripes. Two possible scenarios are considered. On one hand, the phase diagram of La 2-x Sr x CuO 4 at the boundary xc ~ 0.02 between the antiferromagnetic (AF) and the spin-glass phase could be more complicate than previously thought, and a peak of magnetic origin in the NQR rate near 150 K could signal an unconventional transition to a quasi-long range ordered state. On the other hand, the 139 La NQR spectra favour a transition to a conventional AF phase around 50 K. In this case, the peak of the NQR rate around 150 K at 12 MHz would correspond to an anelastic peak around 80 K in the kHz range, and they would constitute the first direct evidence of the freezing of charged stripes involving simultaneously lattice and spin excitations. The barrier of thermal depinning and the frequencies of the stripes motions are thus estimated.
XAS STUDY OF (BaCuO2)2/(CaCuO2)n SUPERLATTICES
Authors: COLONNA S. , ARCIPRETE F. , BALZAROTTI A. , BALESTRINO G. , MEDAGLIA P. G. , PETROCELLI G.
Published: May 2003

EXAFS (Extended X-ray Absorption Fine Structure) and XANES (X-Ray Absorption Near Edge Structure) spectroscopies have been used to probe the local bonding of the artificially layered superconducting (BaCuO2)2/(CaCuO2)n (n = 2, 3, 4) superlattices. Fluorescence-yield measurements have been performed above the Cu and Ba K-edges. This study shows that the charge reservoir (CR) block of the Ba/Ca superlattice contains oxygen vacancies randomly distributed in the CuO2 planes and that the two apical oxygen atoms are displaced out of the Ba plane giving rise to buckled BaO planes. These oxygen atoms have a distorted pyramidal and octahedral coordination around Cu at Ba/Ca interface and in the (BaCuO2) block, respectively.
ULTRATHIN SUPERCONDUCTING YBa2Cu3O7-δ FILMS OBTAINED BY HIGH OXYGEN PRESSURE DC SPUTTERING TECHNIQUE
Authors: BOFFA M. A. , CUCOLO A. M.
Published: May 2003

Ultrathin YBa 2 Cu 3 O 7-x films with thicknesses ranging from 24 Å to 300 Å have been grown by high oxygen pressure sputtering technique on (100)K SrTiO 3 substrates without any additional buffer layer. These samples have been characterized by means of X-Ray diffraction and transport measurements. The thickness was calibrated by using the low angle X-Ray diffraction thecnique. The superconducting properties indicated no degradation of the resistive T c (ρ=0)=89 K for the 300 Å samples, while T c (ρ=0)=35.3 K was measured on the 3 unit cell thick films.
Growth and characterization of NdBa2Cu3O7-x thin films deposited by d.c. sputtering
Authors: CUCOLO M. C. , CUCOLO A. M.
Published: May 2003

Among the ReBa 2 Cu 3 O 7-x (ReBCO, Re=rare earth) cuprate family the NdBa 2 Cu 3 O 7-x system (NdBCO) is known to have the highest critical temperature and critical current. Moreover, in comparison with the YBa 2 Cu 3 O 7-x compound, NdBCO shows a better surface stability and morfology that are basic requirements for the realization of high temperature superconducting (HTS) devices. We have investigated the fabrication procedure of NdBCO films on (001) SrTiO 3 substrates using a dc sputtering technique in high oxygen pressure. The effect of several deposition parameters on the structural and superconducting properties of the NdBCO thin films, has been investigated. The x-ray diffraction patterns showed that the deposited films were c-axis oriented without any impurity phase. The surface morfology was studied by a standard Scanning Electron Microscopy analysis (SEM). T c (ρ=0) ≥ 91 K and J c (77 K ) ≤ 0.6 106 A/cm 2 have been obtained at the optimized sputtering conditions.
OVERVIEW ON THE ELECTROCHEMICAL DEPOSITION OF Bi-2212 FILMS
Authors: GIUNCHI G. , VILLARD G. , LEGENDRE F. , RÉGNIER P. , BIFULCO C. , POISSONNET S.
Published: May 2003

In the framework of the coated tape superconductors, the electrolytic deposition is one of the chemical routes to obtain highly textured films either of the precursors or of the fully superconducting oxides. The technology will be reviewed with particular emphasis on the growth of Bi 2 Sr 2 CaCu 2 O 8+δ (Bi-2212) film on Ag tape. For this system, recent advancements have been achieved combining the electrolytic deposition with intermediate thermal treatments and with an adequate optimisation of the initial stoichiometry of the deposited elements.

A careful control of the morphology and amount of the not superconducting alkaline earth cuprates phases, grown in the film, allows to reach high critical current density, up to 40.000 A cm -2, in self-field at 77 K.

A qualitative analysis by optical and electron microscopy, as well as a more quantitative analysis by x-ray diffraction reveals that the alignment of the Bi-2212 lamellas with the substrate as well as the decrease of the amount of the cuprates correlate with the superconducting transport properties.
RESEARCH INTO A NEW PRECURSOR FOR SUPERCONDUCTING MERCURY CUPRATES
Authors: COSTA G. A. , CARNASCIALI M. M.
Published: May 2003

By studying the influence of Hg stoichiometry on the Tc of the superconducting compound HgBa 2 CuO 4-δ (Hg-1201), we have started to re-examine the composition of the only intermediate phase reported in the Cu-Hg diagram. The oxidation of this phase could lead to a very unstable Cu-Hg oxide to employ with BaO 2 in the synthesis of superconducting mercury cuprates or could be used as a source of mercury in the reaction with the precursor Ba 2 CuO 3.
DEPOSITION OF (Ba, La)CuO2/CaCuO2 SUPERCONDUCTING MULTILAYERS BY PULSED LASER ABLATION
Authors: BRACCINI V. , MARRE' D. , MOLLICA A. , GRASSANO G. , SIRI A. S.
Published: May 2003

Superconducting multilayers, whose growth and structure are determined by the kinetic deposition instead by thermodynamics, are a powerful tool for investigating high T c superconductors (HTSC) properties. By means of Pulsed Laser Deposition (PLD), we grew such materials alternating infinite layer phases both pure and doped, to supply CuO 2 planes and charge reservoir (CR) blocks respectively. We found out that using chemically doped CR leads to a semiconducting behavior, whereas oxygen doping seems to be much more effective, since the BaCuO 2 infinite phase, grown in high oxygen partial pressure, does actually bring about a superconducting behavior. We deposited BaCuO 2/ CaCuO 2 superlattices with various periodicities and studied their transport and structural properties. By varying the deposition parameters, we reached both kinetic and thermodynamic growth and we observed semiconducting and superconducting behaviors.
ON THE ROLE OF Nd/Ba DISORDER ON THE SUPERCONDUCTING PROPERTIES OF Re1(NdxBa2-x)Cu3O7-δ (Re=Nd, Y) THIN FILMS
Authors: SALLUZZO M. , ANDREONE A. , CASSINESE A. , IAVARONE M. , MAGLIONE M. G. , PALOMBA F. , PICA G. , VAGLIO R.
Published: May 2003

The role of disorder on the superconducting properties of Re 1( Nd x Ba 2-x ) Cu 3 O 7-δ (Re=Nd, Y) epitaxial thin films has been studied. The films are deposited by Ar+O 2 magnetron and diode dc sputtering from targets characterised by different x (0, 0.08 and 0.12). In situ X-ray Photoemission Spectroscopy (XPS), Scanning Tunneling Microscopy (STM), and non-conventinal X-ray diffraction measurements have been used to determine the exact composition and the structural properties of each sample. The temperature dependence of the ab-plane penetratin depth of highly c-axis epitaxial samples, characterised by different Nd/Ba ratios, has been determined by an inverted microstrip resonator technique. Results on the Nd 1+x Ba 2-x Cu 3 O 7-δ system show that only stoichiometric films exhibit a linear penetration depth at low temperature while Nd-rich films show a T 2 law. Preliminary measurements on the Y 1( Nd x Ba 2-x ) Cu 3 O 7-δ system confirm these results. The data are analysed in the framework of the d-wave model taking into account the effect of impurities on the superconducting properties.
LOCAL LATTICE FLUCTUATIONS AS A FUNCTION OF CHEMICAL PRESSURE
Authors: SAINI N. L. , BIANCONI A. , LANZARA A. , AGRESTINI S. , DI CASTRO D. , OYANAGI H.
Published: May 2003

Here we report local lattice fluctuations as a function of chemical pressure in the high T c superconductors at the optimum doping (δ = 0.16). The chemical pressure is determined by the mismatch between the rock-salt M-O (M=Ba, Sr, La) layers and the CuO 2 layers. High k-resolution Cu K-edge EXAFS has been used to measure the strain of the Cu-O bond lengths and the amplitude of local lattice distortions probed by the Debye-Waller factor of the Cu-O bonds in different materials under increasing chemical pressure. We find that in the normal metallic state, showing non-Fermi liquid transport behavior, the Debye-Waller factor reaches a maximum at a critical value of the strain η c of the CuO 2 plane. The results provide a direct evidence that at the maximum critical temperature T c the system has diverging local lattice fluctuations.
Upper Critical Field and Irreversibility Line in Bi2Sr2CuO6+δ/CaCuO2 Superconducting Superlattices Obtained by MBE
Authors: Salvato Matteo , Attanasio Carmine , Carbone Gerardina , Fittipaldi Rosalba , Di Luccio Tiziana , Mercaldo Lucia V. , Montella Albino , Prischepa Serghej L. , Maritato Luigi
Published: May 2003

Resistivity measurements in external applied magnetic field up to 8.5T have been performed on Bi2Sr2CuO6+δ/CaCuO2 superconducting superlattices obtained by MBE. The magnetic field (H) vs. temperature (T) phase diagrams have been determined and the experimental data have been compared with that obtained in the case of Bi2Sr2CuO6+δ thin films deposited with the same technique. A reduction of the anisotropy has been obtained in the case of the superlattices with respect to the case of Bi2Sr2CuO6+δ thin films and a three dimensional behavior has been observed by paraconductivity measurements.
89Y AND 63Cu NMR-NQR AND SUSCEPTIBILITY STUDY OF UNDERDOPED SUPERCONDUCTING Y1-xCaxBa2Cu3O6.1
Authors: CAMPANA A. , CARRETTA P. , CORTI M. , LASCIALFARI A. , RIGAMONTI A. , LICCI F.
Published: May 2003

89 Y NMR relaxation has been used to study the spin freezing process in Y 1-x Ca x Ba 2 Cu 3 O 6.1. Substituting Ca 2+ for Y 3+ in the parent antiferromagnetic YBa 2 Cu 3 O 6.1 leads the system in the underdoped superconducting phase. In the normal state of the sample at x=0.15 the spin-lattice relaxation rate, which is not affected by the antiferromagnetic correlation of the Cu 2+ magnetic moments, still evidences a decrease of (1/ T 1 T ) on cooling, indicative of a pseudo-gap opening in the density of states around the Fermi energy. The most relevant result is found in the superconducting phase, where the recovery of the 89 Y NMR signal displays two distinct laws, pointing out different relaxation mechanisms without common spin temperature. One exponential recovery law yields a decay rate which decreases on cooling, as expected in the superconducting phase. The second relaxation process turns out to be described by a stretched exponential, with fast relaxation rates, strongly increasing on cooling, with a characteristic spin fluctuations frequency reaching the 20 MHz range at about 8 K. This observation is interpreted as the direct experimental evidence of the coexistence, at mesoscopic level, of superconductive and spin-glass like phases, the spin freezing process being described by the fast rate of the nuclear relaxation.
EFFECTS OF PINNING ANISOTROPY ON YBCO FILM CURRENT-VOLTAGE CHARACTERISTICS
Authors: CAMERLINGO C. , NAPPI C. , RUSSO M.
Published: May 2003

The dissipative properties of (103)/(031) oriented films YBCO films grown on (110) SrTiO3 substrates were investigated in the vortex creep motion regime and compared with characteristics of c-axis oriented films deposited on both and (001)SrTiO3 and (001)NdGaO3 substrates. The current-voltage characteristics were measured at different temperatures, using a proper geometrical bridge configuration, able to test space anisotropy. The in-plane anisotropy due to the layered structure of the superconductor was clearly observed in the (103)/(031) oriented films. Experimental data are consistent with a thermal activated mechanism of vortex motion on a periodic pinning potential, attributed to intrinsic layer pinning. The intrinsic pinning energy per unit length of magnetic field line is evaluated as U0≈400 KeV/cm, in the low temperature limit. Consistently with theory, this energy is weakly dependent on temperature, the U/KBT ratio fading out at T ≈ Tc, where flux flow mechanism is activated for vortex motion.
COMPLEX RESISTIVITY OF YBa2Cu3O7-δ AT 48 GHz IN MAGNETIC FIELDS
Authors: Rogai R. , Marcon R. , Silva E. , Fastampa R. , Giura M. , Sarti S. , Boffa M. , Cucolo A. M.
Published: May 2003

We present measurements of the magnetic field dependent complex resistivity at 48 GHz in a nearly-optimally doped thin YBa 2 Cu 3 O 7-δ film. Below Tc the application of the magnetic field leads to a detectable magnetoresistivity. The real part is clearly measurable from our lowest temperature up to Tc, while the imaginary part can be distinguished from zero only for T>87 K . The magnetoresistivity arises from the combined effects of fluxon motion and field-induced superfluid depletion. We discuss whether our data can be described or not by models for the magnetic response in an s-wave or d-wave superconductor.
ELECTRON-PHONON COUPLING ORIGIN OF THE RESISTIVITY IN YNi2B2C SINGLE CRYSTALS
Authors: GONNELLI R. S. , MORELLO A. , UMMARINO G. A. , STEPANOV V. A. , BEHR G. , GRAW G. , SHULGA S. V. , DRECHSLER S. -L.
Published: May 2003

Resistivity measurements from 4.2 K up to 300 K were made on YNi 2 B 2 C single crystals with T c = 15.5 K . The resulting ρ(T) curve shows a perfect Bloch-Grüneisen (BG) behavior, with a very small residual resistivity which indicates the low impurity content and the high cristallographic quality of the samples. The value λ tr = 0.53 for the transport electron-phonon coupling constant was obtained by using the high-temperature constant value of d ρ/ d T and the plasma frequency reported in literature. The BG expression for the phononic part of the resistivity ρ ph (T) was then used to fit the data in the whole temperature range, by approximating [Formula: see text] with the experimental phonon spectral density G(Ω) multiplied by a two-step weighting function to be determined by the fit. The resulting fitting curve perfectly agrees with the experimental points. We also solved the real-axis Eliashberg equations in both s- and d-wave symmetries under the approximation [Formula: see text]. We found that the value of λ tr here determined in single-band approximation is quite compatible with T c and the gap Δ experimentally observed. Finally, we calculated the normalized tunneling conductance, whose comparison with break-junction tunnel data gives indication of the possible s-wave symmetry for the order parameter in YNi 2 B 2 C .
LOCAL STRUCTURE OF THE CHARGE ORDERED La0.5Ca0.5MnO3
Authors: AGRESTINI S. , SAINI N. L. , LANZARA A. , NATALI F. , BIANCONI A.
Published: May 2003

Local lattice of the charge ordered La 0.5 Ca 0.5 MnO 3 has been studied by temperature dependent Mn K-edge x-ray absorption measurements, using fluorescence detection method, to explore how the local lattice distortions are associated with the charge ordering in this system. The x-ray absorption near edge structure (XANES) reveals an abrupt change in the local MnO 6 octahedral geometry across the charge ordering temperature. The extended x-ray absorption fine structure (EXAFS) analysis shows largely distorted MnO 6 octahedra with coexisting Q 3-type and Q 2-type of Jahn-Teller (JT) modes at low temperature in the commensurate charge ordered phase. We find that small JT distortions persist also in the charge disordered phase at high temperature.
THE SCALING OF THE R(T) RESPONSE IN BSCCO (2212) FILMS AT LOW MAGNETIC FIELDS
Authors: PARACCHINI C. , ROMANO' L.
Published: May 2003

The ratio V/I in three BSCCO (2212) films is studied at low magnetic fields (0.5-300 G). In the field-dependent region the data of R vs. Θ=[T-To(H))]/[Tm-To(H))] overlap on one line, where To(H) and Tm(H) are the limits of the process. The result is similar to the R(T,I) dependence in several planar superconductors and this coincidence indicates that low current and field play the same role in the dissipation of these materials. The R(T,I,H) dependence is approximated by R=Ro exp (- b /Θ0.5), suggesting that in these conditions the dissipation may be attributed to a single process due to the de-pairing of thermally excited vortex-antivortex pairs.
THE FIRST SECONDS IN THE MAGNETIC RELAXATION ON MULTIFILAMENTARY BSCCO(2223)/AG TAPES
Authors: ZOLA D. , POLICHETTI M. , PACE S.
Published: May 2003

The first 60 seconds of the magnetisation decay M(t) of a multifilamentary BSCCO/Ag tape has been studied by a vibrating sample magnetometer. Measurements were performed at different temperatures T [4.2 K ÷ 45 K] and DC magnetic fields up to 11 T. The behaviour in the first seconds after the external field is stopped are often neglected due to the presence of far from equilibrium magnetic states, which are function of the initial conditions or of the sample characteristics. We have observed a fast decay in the first 10 seconds, which has been analysed at different temperatures and several external fields. Finally a possible mechanism, responsible of the observed behaviour is discussed.
Microwave Electrodynamics of low TC and high TC Systems with Coexisting Superconductivity and Magnetism
Authors: Mercaldo Lucia V. , Talanov Vladimir V. , Anlage Steven M. , Attanasio Carmine , Maritato Luigi
Published: May 2003

We investigate the microwave electrodynamics of both artificial low T C superconducting/magnetic (S/M) layered structures and high T C cuprates. In particular we focus on Nb/CuMn (superconducting/spin-glass) bilayers, and on GbBa 2 Cu 3 O 7-δ (GBCO) thin films, which show coexistence of superconductivity and long range ordered antiferromagnetism below T N = 2.2 K . In both cases we are interested in the influence of magnetism on superconducivity. Moreover, in the GBCO case, we want to shed light on the problem of the determination of the pairing symmetry in the cuprates. First we show surface impedance data at 10 GHz on Nb/CuMn bilayers. We extract information about the induced order parameter in the magnetic layer and we compare it with the exotic behavior predicted for S/M proximity systems. Then we present microwave surface impedance data at three different frequencies on GBCO c-axis oriented epitaxial thin films. Both the resistance and the reactance data on GBCO show an unusual low temperature behavior, mainly due to change in magnetic permeability. This result indicates that the paramagnetism of the rare-earth ions has to be taken into account when extracting the superconducting penetration depth as a function of temperature, and thus determining the pairing state symmetry of the cuprates.
A SURVEY OF NONADIABATIC SUPERCONDUCTIVITY IN CUPRATES AND FULLERIDES
Authors: CAPPELLUTI EMMANUELE , GRIMALDI CLAUDIO , PIETRONERO LUCIANO , STRÄSSLER SIGFRID
Published: May 2003

High-Tc superconductors are characterized by very low carrier densities. This feature leads to two fundamental consequences: on one hand the Fermi energies are correspondingly small and they can be of the same order of phonon frequencies. In such a situation nonadiabatic corrections arising from the breakdown of Migdal's theorem can not be longer neglected. In addition, small carrier densities imply poor screening and correlation effects have to be taken into account. We present a comprehensive overview of the theory of superconductivity generalized into the nonadiabatic regime which is qualitatively different from the conventional one. In this framework some of the observed properties of the cuprates and the fullerene compounds can be naturally accounted for, and a number of theoretical predictions are proposed that can be experimentally tested.
DISTANCE-DEPENDING ELECTRON-PHONON INTERACTIONS FROM ONE- AND TWO-BODY ELECTRONIC TERMS IN A DIMER
Authors: ACQUARONE M. , NOCE C.
Published: May 2003

For a dimer with a non-degenerate orbital built from atomic wave functions of Gaussian shape we evaluate all the electron-phonon couplings derived from the one-body and two-body electronic interactions, considering both the adiabatic and extreme non-adiabatic limit. Not only the values of the coupling parameters in the two limits, but also the expressions of the corresponding terms in the Hamiltonian differ. Depending on the distance between the dimer ions, some of the two-body couplings are comparable, or even larger than the one-body ones.
GAP AND PSEUDOGAP EVOLUTION IN UNDERDOPED CUPRATES
Authors: BENFATTO L. , CAPRARA S.
Published: May 2003

We describe the spectral properties of underdoped cuprates as resulting from a momentum-dependent pseudogap in the normal-state spectrum. Such a model accounts, within a BCS approach, for the doping dependence of the critical temperature and of the leading-edge shift observed in the cuprates. We also reproduce the Tc-T* bifurcation near optimum doping.
QUANTUM PERCOLATION AND CHARGE TRANSPORT IN HIGH Tc SUPERCONDUCTORS
Authors: DALLACASA V. , DI SIA P.
Published: May 2003

We have investigated the consequences of internal fields on transport properties in insulators close to a metal-insulator-superconductor transition. An interpolation formula for the resistivity as a function of temperature describing insulating, metallic and superconducting regimes has been deduced in the framework of a quantum percolation model in which hopping processes on localised states occur. The contribution to the internal fields on carriers has been taken into account up to quadrupole terms. The resistivity follows a law ρ ∪(T0)δ T e(T0/T)1/2 where T0 corresponds to a coulomb gap and exhibits anisotropic quadrupole angular dependence, suggesting a correlation with the pseudo-gap observed in superconductors in the underdoped regime.
FLUX FLOW EFFECTS IN VERY LONG JOSEPHSON JUNCTIONS
Authors: CIRILLO M. , MERLO V. , RUSSO R. , CIKMACS P.
Published: May 2003

We report on measurements on very long, L ≃ 30λj, NbAlOxNb underdamped in-line junctions, on which we observed displaced linear slope (DLS) generated by the application of an external magnetic field. We study the behaviour of the branches as a function of the applied magnetic field in terms of both current amplitude and voltage position. The DLS is seen to shift rigidly towards higher voltages when increasing the field, spanning a region roughly centred around the Josephson plasma frequency. We discuss the behaviour of linear branches in terms of one dimensional flux-flow along the extended side of the junction, comparing our data with the results of numerical modeling; from these calculations the non-resonant nature of DLS is very evident, in contrast to the resonant regime giving rise to Fiske steps (FS).
DEVELOPMENT OF A Nb/Al TECHNOLOGY FOR SNS JOSEPHSON JUNCTIONS
Authors: LACQUANITI V. , MAGGI S. , MONTICONE E. , POLCARI A. , STENI R. , ANDREONE D.
Published: May 2003

We report here the status of development of our Nb/Al/Nb SNS Josephson junctions to be used as superconducting D/A converters. The electrical behaviour and the morphology of these Nb/Al/Nb junctions are strongly dependent on the deposition rate of the Al barrier. Junctions fabricated at a high rate of 5 nm/s have stable and reproducible properties. A possible interpretation of the effect of the Al morphology on the electrical behaviour of the junctions is given.
MACROSCOPIC QUANTUM COHERENCE IN JOSEPHSON SYSTEMS
Authors: RUGGIERO B. , CORATO V. , ESPOSITO E. , GRANATA C. , RUSSO M. , SILVESTRINI P. , STODOLSKY L.
Published: May 2003

We present our recent experiments on macroscopic tunneling in the quantum regime for underdamped Josephson junctions with different classical-quantum crossover temperatures. These experiments are performed towards the observation of macroscopic quantum coherence in Josephson systems. A new method to measure coherence in a two-level system based on the adiabatic inversion in an rf squid, is presented. This approach could open new perspectives in view of realization of an elementary quantum bit.
ESCAPE FROM THE ZERO-VOLTAGE STATE IN HYSTERETIC DC-SQUIDS WITH DIFFERENT DIMENSIONLESS INDUCTANCE
Authors: CASTELLANO M. G. , INTELISANO A. , LEONI R. , MILANESE N. , TORRIOLI G. , COSMELLI C. , CARELLI P. , CHIARELLO F.
Published: May 2003

We studied the escape process from the zero-voltage state to the running state in hysteretic dc-SQUIDs made with trilayer Nb-AlOx-Nb junctions. We analyze the experimental results, according to the thermal activation model, for devices having similar parameters except for the dimensionless inductance β L , which however is always very small. The interest in hysteretic dc-SQUIDs is in the possibility of using them to perform non-invasive measurements of the flux states in rf-SQUIDs: this achievement would be very important both in tests of macroscopic quantum properties of SQUIDs and in measurements on a class of quantum bits that has been recently proposed, based on magnetic flux states in SQUIDs.
ASYMMETRIC DC-SQUIDS: MAGNETIC AND NOISE PROPERTIES
Authors: TESTA G. , PAGANO S. , SARNELLI E. , CALIDONNA C. R. , RUSSO M. , FURNARI M. MANGO
Published: May 2003

SQUID configurations with an asymmetry in both the critical current and the normal resistance of the two Josephson junctions have been numerically analyzed.

The analysis has been carried out on a high performance computer, by choosing parameters typical of low critical temperature SQUIDs. In particular the dependence of both the magnetic flux noise S Φ and the flux to voltage transfer function V Φ on the bias current and the normalized SQUID inductance β have been investigated. Thermal noise effects, including contributions of both junction and damping resistance have been introduced in the calculations. The results show that an increase of the asymmetry leads to higher V Φ and lower S Φ values. This suggests to use asymmetric dc-SQUIDs to improve the device performance for both small and large inductance values.