bluuebunny/crossref_metadata_2025 huggingface dataset:
STOCHASTIC RESONANCE IN COUPLED THRESHOLD ELEMENTS WITH INPUT SIGNALS SHIFTED IN PHASE
Authors: KRAWIECKI A. , SUKIENNICKI A. , KOSIŃSKI R. A.
Published: April 2003

Stochastic resonance in a system of two coupled threshold elements (neurons) forming a small artificial neural network is investigated. The elements have either antisymmetric or logistic (binary) response function and are driven by periodic signals and independent noises. Periodic signals at their inputs have equal amplitudes and frequencies but are shifted in phase. Depending on the response function and the phase shift, enhancement of stochastic resonance in individual elements, characterized by the output signal-to-noise ratio, and stochastic resonance with a spatiotemporal input signal, characterized by the correlation function between the input and output signals, are observed for proper coupling between elements.
FORMATION AND LIFE-TIME OF MEMORY DOMAINS IN THE DISSIPATIVE QUANTUM MODEL OF BRAIN
Authors: ALFINITO ELEONORA , VITIELLO GIUSEPPE
Published: April 2003

We show that in the dissipative quantum model of brain the time-dependence of the frequencies of the electrical dipole wave quanta leads to the dynamical organization of the memories in space (i.e., to their localization in more or less diffused regions of the brain) and in time (i.e., to their longer or shorter life-time). The life-time and the localization in domains of the memory states also depend on internal parameters and on the number of links that the brain establishes with the external world. These results agree with the physiological observations of the dynamic formation of neural circuitry which grows as brain develops and relates to external world.
RANDOM WALK LEARNING MACHINE
Authors: ANDRECUT MIRCEA
Published: April 2003

A simple random walk learning algorithm for associative memories is described. The Hebbian memory matrix optimized by the random walk algorithm leads to a perfect learning in associative memories. Also, in the special case of a binary memory matrix, the random walk learning algorithm leads to an increase of the critical storage density from α c = 0.102 to α c = 0.25.
COOPER–PAIR SUPERFLUID STATE IN HIGH-TEMPERATURE SUPERCONDUCTORS
Authors: CHEN ZENG-BING
Published: April 2003

We propose a lattice model of high-temperature superconductors based on their intrinsic granular-like structure. Due to the shortness of the coherent length in high-temperature superconductors, the superconducting order parameter can be well defined locally only in small regions, in which the number and the phase of the Cooper–pairs will experience strong quantum fluctuations. The "superconducting order parameter" in terms of the local number and phase operators of the Cooper–pairs is then regarded as an effective quantized bosonic field described by a discretized Ginzberg–Landau-type effective Hamiltonian, first proposed for granular superconductors and recently for high-temperature superconductors. The effective Hamiltonian also describes the quantum fluctuations of the superconducting order parameter. The global superconducting order sets in when the Cooper–pairs are in the superfluid state. This is in contrast to the previous picture, in which the phase coherence drives the system into the superconducting state when only the phase fluctuations of the superconducting order parameter are considered.
RENORMALIZATION GROUP APPROACH TO THE PROBLEM OF FLOW THROUGH IRREGULAR PACKED BEDS
Authors: VOLCHENKOV DMITRI , LIMA RICARDO
Published: April 2003

We have calculated the asymptotics of Green's function of the differential equation of nonlinear diffusion in the microscopic range with strong porosity fluctuations in the problem of flow through irregular packed beds for the arbitrary dimension of space and arbitrary porosity fluctuations covariance.
SCALING LAWS OF REVERSIBLE AGGREGATION IN COMPACT CLUSTER SYSTEMS
Authors: ZHUANG GUOCE , WANG JIAFU , WANG WEI
Published: April 2003

The reversible cluster–cluster aggregation processes in compact cluster systems are studied via a scaling argument and Monte Carlo simulations. To describe the detail effects of fragmentations from tree-like fractal to compact clusters a relative breakup probability [Formula: see text] with an exponent β is introduced. The mean-field rate equation and numerical simulation results indicate that the critical exponent y, which is defined as  ~ k-y, has a value of (α + ξ - β + 2)-1. It is shown that the scaling properties of the cluster size distributions are determined by the selections of the exponents α, β and ξ.
OPTICAL PROPERTIES OF NANOCRYSTALLINE YTTERBIA DOPEDC ZIRCONIA THIN FILMS
Authors: HARTRIDGE A. , KRISHNA M. GHANASHYAM , BHATTACHARYA A. K.
Published: April 2003

Thin films of Zr 1-x Yb x O 2-y where x ranges from 0 to 0.3 have been coated onto quartz substrates using an inorganic sol–gel approach at low temperature. The films were fully crystalline and continuous as deposited, with a fluorite structure. On heating over 900°C, films with less than 16% doping had a tetragonal structure whilst those above retained a cubic structure. Crystallite size measurements indicated that the films were nanocrystalline, the size decreasing as a function of increasing dopant concentration. The films are transparent in the region 500 to 1500 nm with very low optical losses. The film refractive index seems relatively independent of the dopant concentration. The optical band gap is dependent on the dopant concentration and is in the range 5.6 to 5.7 eV. There is an increase in refractive index and decrease in optical absorption with increase in annealing temperature from 600°C to 1050°C.
QUANTUM DYNAMICS OF COUPLED BOSONIC WELLS WITHIN THE BOSE–HUBBARD PICTURE
Authors: FRANZOSI ROBERTO , PENNA VITTORIO , ZECCHINA RICCARDO
Published: April 2003

We relate the quantum dynamics of the Bose–Hubbard model (BHM) to the semiclassical nonlinear equations that describe an array of interacting Bose condensates by implementing a standard variational procedure based on the coherent state method. We investigate the dynamics of the two-site BHM from the purely quantum viewpoint by recasting first the model within a spin picture and using then the related dynamical algebra. The latter allows us to study thoroughly the energy spectrum structure and to interpret quantally the classical symmetries of the two-site dynamics. The energy spectrum is also evaluated through various approximations relying on the coherent state approach.
QUANTUM STATISTICS OF A NEW TWO MODE SQUEEZE OPERATOR MODEL
Authors: ABDALLA M. SEBAWE , OBADA A.-S. F.
Published: April 2003

In this paper we introduce a new squeeze operator which is a combination of the two single photon squeezing operator and two mode correlated operator. We construct the general coherent-state wave function, and study the effects of this operator on the photon number sum and difference. We find in absence of the squeezing parameters λ1 and λ2 both the sum and the difference are conserved by the two mode correlated operator. The squeezing phenomenon is discussed, and the minimum uncertainty state is found for small value of the squeezing parameter λ. Besides the Glauber second order correlation function, the statistical investigations are carried out for the quasi-probability distribution functions (P-representation, Wigner function and Q-function). The discussion related to the phase space distribution function is given, and the matrix elements of the density operator are calculated. The photocount distribution is also considered.
NEW MACROSCOPIC QUANTUM STATE IN PEROVSKITE-CRYSTALLINE CONDUCTOR WITH LOCALIZED CARRIER SYSTEM
Authors: SUGAHARA M. , OGI S. , ARAKI K. , UEDA Y. , SHIMODA R. , SUGIURA R.
Published: April 2003

The large zero-point oscillation of energy ~0.1–1 eV of strong coupling hole carriers in layered perovskite (LP) conductor is shown to provide effective field B* ~ 103–4 T causing fractional quantum Hall effect (FQHE) when space charge and moderate localization are present. The quasiparticle excitation spectrum of the FQHE system is calculated. The FQHE state is a macroscopic quantum state where macroscopic interference effect is expected. Using (001) La 2-x Sr x CuO 4-y (LSCO) film, we observed "dc dielectric interference" and the interference effect at 28 THz, which are explicable by the FQHE in the layered perovskite film.
COHERENT STATES, DISPLACED NUMBER STATES AND LAGUERRE POLYNOMIAL STATES FOR su(1, 1) LIE ALGEBRA
Authors: WANG XIAO-GUANG
Published: April 2003

The ladder operator formalism of a general quantum state for su(1, 1) Lie algebra is obtained. The state bears the generally deformed oscillator algebraic structure. It is found that the Perelomov's coherent state is a su(1, 1) nonlinear coherent state. The expansion and the exponential form of the nonlinear coherent state are given. We obtain the matrix elements of the su(1, 1) displacement operator in terms of the hypergeometric functions and the expansions of the displaced number states and Laguerre polynomial states are followed. Finally some interesting su(1, 1) optical systems are discussed.
EFFICIENCY AND CURRENT REVERSALS IN SPATIALLY INHOMOGENEOUS RATCHETS
Authors: DAN DEBASIS , JAYANNAVAR A. M. , MAHATO MANGAL C.
Published: April 2003

Efficiency of generation of net unidirectional current in an adiabatically driven symmetric periodic potential system is studied. The efficiency shows a maximum, in the case of an inhomogeneous system with spatially varying periodic friction coefficient, as a function of temperature. The ratchet is not most efficient when it gives maximum current. The direction of current may also be reversed as a function of noise strength when, instead, an asymmetric periodic potential is considered.
STRUCTURE AND INTERACTIONS OF CLUSTERS
Authors: HAUCK J. , MIKA K.
Published: April 2003

Ordered structures, which are characterized by the numbers T1 and T2 of nearest and next-nearest neighbors of the same kind in T1, T2 structure maps, can be classified in enthalpy and entropy stabilized structures. 111 three-dimensional structures MxNy with identical Ti values of all M and N particles were determined for the three-dimensional lattices. The numbers of shells were extended to i=11–18 to compare the values for closed shells with the magic numbers x and y of isotopes (M = proton , N = neutron ) or small clusters MxNy or Mx□y (□=vacancy). Electroneutral clusters were derived for 42 structures with x ≠ y. Colloidal DNA or proteins are clusters with different Ti values of each structural unit within long chains.
ON THE PHASE TRANSITIONS OF SECOND-ORDER
Authors: PARK YUNG
Published: April 2003

This paper is no longer available. Please contact "ijmpb@wspc.com"
MAGNETIC INSTABILITY IN A PARITY INVARIANT TWO-DIMENSIONAL FERMION SYSTEM
Authors: ELIASHVILI M. , TSITSISHVILI G.
Published: April 2003

We consider the parity invariant QED2+1 where the matter is represented as a mixture of fermions with opposite spins. It is argued that the perturbative ground state of the system is unstable with respect to the formation of magnetized ground state. Carrying out the finite temperature analysis we show that the magnetic instability disappears in the high temperature regime.
CHERN–SIMONS THEORY AND QUANTUM FIELDS IN THE LOWEST LANDAU LEVEL
Authors: ELIASHVILI M. , TSITSISHVILI G.
Published: April 2003

By considering the area preserving geometric transformations in the configuration space of electrons moving in the lowest Landau level (LLL) we arrive at the Chern–Simons type Lagrangian. Imposing the LLL condition, we get a scheme with the complex gauge fields and transformations. Quantum theory for the matter field in LLL is considered and formal expressions for Read's operator and Laughlin wave function are presented in the second quantized form.
EPITAXIAL SrTiO3 BUFFER LAYERS FOR YBa2Cu3O7-δ SUPERCONDUCTING THIN FILMS ON MgO SUBSTRATE
Authors: MORI Z. , NAKAGAMA T. , HIGO S. , DOI T. , KOBA S. , HAKURAKU Y.
Published: April 2003

SrTiO 3 (STO) thin film was prepared on single crystal MgO (100) substrate by rf magnetron sputtering as a buffer layer of superconducting thin film. Sputtering conditions influence to the structural property were investigated. Highly epitaxial a-axis oriented STO thin film with extremely smooth surface was obtained by optimizing the sputtering parameters. YBa 2 Cu 3 O 7-δ (YBCO) thin film with critical temperature (T c ) of 89.8 K was obtained on STO buffer. Surface morphology of YBCO thin film was also improved. The effects of STO buffer were discussed in comparison with YBCO thin films on MgO substrate.
AN ALGEBRAIC APPROACH TO THE INTERACTION OF A THREE-LEVEL ATOM INTERACTING WITH QUANTIZED RADIATION FIELD
Authors: ZOU XU-BO , XU JING-BO , GAO XIAO-CHUN , FU JIAN
Published: April 2003

The system of a three-level atom in the Ξ configuration coupled to two quantized field modes with arbitrary detuning and density-dependent multiphoton interaction is studied by dynamical algebraic method. With the help of an su(3) algebraic structure, we diagonalize the Hamiltonian by making use of unitary transformations and obtain the eigenvalues, eigenstates and time evolution operator for the system. Based on this su(3) structure, we also show that the system of a three-level atom in the Ξ configuration can be exactly transformed to an effective two-level Hamiltonian by an unitary transformation. Finally, we show that there exist an su (N) algebraic structure in the system of a N-level atom interacting with N-1 field modes.
A COMPUTER SIMULATION OF SLOW-POSITRON IMPLANTATION DEPTHS IN ALUMINUM
Authors: BOUARISSA N. , WALKER A. B.
Published: April 2003

The mean penetration depths and stopping profiles of positrons with incident energies up to 10 keV impinging on semi-infinite aluminum with normal and oblique angles of incidence are stochastically modelled within a Monte-Carlo framework. The elastic scattering cross sections have been obtained from a partial wave expansion. To model inelastic core and valence electron excitation, we have used the Gryzinski's expression. Our simulated results agree very well with the available experimental data and clearly demonstrate that the mean penetration depths are not adequately described by the simple power law commonly used in defect profiling. This suggests that both mean penetration depths and stopping profiles cannot generally be modelled by material-independent parameters as was proposed previously. The mean penetration depth decreases non-monotonically with increasing the angle of incidence from 0° to 80°. This can be seen from the reduction of the width of the implantation profiles.
KONDO IMPURITY IN A DISORDERED METAL: ANDERSON'S THEOREM REVISITED
Authors: CHAKRAVARTY SUDIP , NAYAK CHETAN
Published: April 2003

We consider a local moment which is coupled by a non-random Kondo J to a band of conduction electrons in a random potential. We prove an analog of Anderson's theorem in a large-N limit of this model. The theorem states that when the disorder is weak, the disorder-averaged low-temperature thermodynamics is independent of the strength of the disorder; remarkably, it further states that fluctuation effects in the long-time limit are independent even of the realization of the disorder. We discuss the relationship of this theorem to theoretical and experimental studies of similar problems.
COMPUTER SIMULATION STUDY OF A NEMATOGENIC LATTICE-GAS MODEL
Authors: ROMANO S.
Published: April 2003

We have considered a classical lattice-gas model, consisting of a three-dimensional simple-cubic lattice, whose sites host three-component unit vectors; pairs of nearest-neighbouring sites interact via the nematogenic potential [Formula: see text] here P2(τ) denotes the second Legendre polynomial, νj=0,1 are occupation numbers, uj are the unit vectors (classical spins), and ∊ is a positive quantity setting energy and temperature scales (i.e. T*=k B T/∊); the total Hamiltonian is given by [Formula: see text] where ∑{j<k} denotes sum over all distinct nearest-neighbouring pairs of lattice sites. The saturated-lattice version of this model defines the extensively studied Lebwohl–Lasher model, possessing a transition to an orientationally ordered phase at low temperature; according to available rigorous results, there exists a μ0<0, such that, for all μ>μ0, the system supports an ordering transition at a finite, μ-dependent, temperature. We have studied here the case μ=0, and found evidence of a transition, taking place at a lower temperature, and possessing a more pronounced first-order character than its Lebwohl–Lasher counterpart; a Mean Field treatment has also been worked out, and found to yield results in qualitative agreement with simulation.
THE INHOMOGENEOUS HEISENBERG CHAIN WITH THE INTERACTION OF CLASSICAL AND QUANTUM SPINS
Authors: SADOVNIKOV B. I. , INOZEMTSEVA N. G. , INOZEMTSEV V. I.
Published: April 2003

The thermodynamics of an inhomogeneous 1D Heisenberg chain with alternating classical and quantum spins is studied. The explicit expressions for the free energy, specific heat, thermal spin correlations and linear susceptibility are found.
FIELD BEHAVIOR OF AN ISING MODEL WITH APERIODIC INTERACTIONS
Authors: GHOSH ANGSULA , HADDAD T. A. S. , SALINAS S. R.
Published: April 2003

We derive exact renormalization-group recursion relations for an Ising model, in the presence of external fields, with ferromagnetic nearest-neighbor interactions on Migdal–Kadanoff hierarchical lattices. We consider layered distributions of aperiodic exchange interactions, according to a class of two-letter substitutional sequences. For irrelevant geometric fluctuations, the recursion relations in parameter space display a nontrivial uniform fixed point of hyperbolic character that governs the universal critical behavior. For relevant fluctuations, in agreement with previous work, this fixed point becomes fully unstable, and there appears a two-cycle attractor associated with a new critical universality class.
MÖSSBAUER STUDY OF In3+ AND Al3+ ADDITIVES IN Mg–Mn FERRITES
Authors: SINGH M. , CHAUHAN B. S.
Published: April 2003

The effect of In 3+ and Al 3+ substitution in Mg–Mn ferrite has been studied from Mössbauer analyzes of Mg 0.9 Mn 0.1 M x Fe 2-x O 4(0 ≤x≤0.9) and Mg 0.9 Mn 0.1 Fe 2-y O 4(0≤y≤0.5) ferrites. The internal magnetic field has been found to decrease at tetrahedral and octahedral sites when the concentration of In 3+ and Al 3+ ions is successively increased. A ferromagnetic relaxation followed by paramagnetic transition is observed for higher concentrations of In 3+ ions, it has been inferred that a larger concentration of In 3+ ions enhances relaxation effects. The observed values of isomer shift corresponding to Fe 3+ ions at A as well as B sites in both the series show an insignificant variation with increase in the concentration of In 3+ and Al 3+ ions. It is also inferred that all the samples show negligible value of quadrupole splitting with in experimental errors. The occupancy ratio increases with increase of In 3+ ions and the occupancy ratio decreases with increase of concentration of Al 3+ ions. The results of occupancy of Fe 3+ ions at B to A site, as obtained from Mössbauer studies are in conformity with the cation distribution used to explain variation of 4πMs and μ0.
FRACTIONAL DIMENSIONALITY ON SELF-TRAPPED FRENKEL EXCITONS IN MOLECULAR CRYSTALLITES
Authors: TAKESHIMA MASUMI , MATSUI ATSUO H.
Published: April 2003

Self-trapped exciton characteristics in microcrystallites are discussed in terms of the fractional dimension. The concept of the fractional dimension is introduced since microcrystallites are formed with various side lengths where the concept of the integral dimensions (1D, 2D and 3D) are not significant. The theory is based on the tight-binding approach. Uniqueness in self-trapping appears in the magnitude of the fractional dimension. Exceptionally high threshold for self-trapping is found when (1) a thin crystallite is made of two layers, or (2) a crystallite is formed by two molecular chains, or (3) a linear molecular chain is made of four molecules. The fractional dimension analysis thus offers a wider overview of the self-trapped exciton character than the analysis that can be done using intergral dimensions only.
GREEN'S FUNCTION THEORY OF THE DOPED ANISOTROPIC ANTIFERROMAGNET
Authors: CHANG ZHE
Published: April 2003

The Green's function theory of the t–J model is generalized to investigate phase diagram and magnetic properties of the doped anisotropic antiferromagnet, which is believed to describe the elementary properties of the high-temperature superconductors. Antiferromagnetic–metalic phase transition is shown. The explicit dependence of the Néel temperature and sublattice magnetization of the anisotropic doped antiferromagnets on anisotropic parameter ς, doping density δ and parameters of the t–J model are obtained for small doping concentration.
EFFECT OF INELASTIC SCATTERING OF HOT ELECTRONS ON THERMIONIC COOLING IN A SINGLE-BARRIER STRUCTURE
Authors: ZHANG C.
Published: April 2003

One of the important problems in thermionics using layered structures is the inelastic scattering of hot electrons in the electrodes and in the barrier region. Scattering in these systems is mainly via the electron–phonon interaction, or indirectly via the electron–electron interaction. In semiconductor heterostructures at room temperature, the LO-phonon plays a crucial role in thermalising electrons. In this work we study the effect of electron–phonon scattering on thermionic cooling in a single-barrier structure. Because of the asymmetry of the barrier under a bias, a larger fraction of the total energy loss will be dissipated in the hot electrode. As a result, we find that the theoretical thermal efficiency can increase due to limited electron–phonon scattering.
OPTICAL PROPERTIES OF RANDOM SEMICONDUCTING ALLOYS
Authors: GANGULI BIPLAB , MOOKERJEE ABHIJIT
Published: April 2003

A formalism has been developed for the study of optical properties of random semiconducting alloys. The effects of randomness and clustering in the optical transition matrix element may be incorporated within our formalism.
SCATTERING RATES IN A SEMICONDUCTOR HETEROSTRUCTURE: A VARIATIONAL WAVEFUNCTION APPROACH
Authors: GONDAR J. L. , COMAS F. , CASTRO F.
Published: April 2003

We calculate the conduction electron scattering rates in a model for a semiconductor heterostructure (SH) of nanometric dimensions. Electron interaction with polar optical phonons (POP) is considered by applying POP modes analyzed in previous papers. The SH is modeled as a two interfaces slab, one of them mechanically free and the other one an interface with a semi infinite semiconductor. Only intraband electron transitions within the first subband are studied and a variational approach is introduced for the electron wavefunction. Comparisons with previous works on the subject are made.
HOW DOES A QUADRATIC TERM IN THE ENERGY DISPERSION MODIFY THE SINGLE-PARTICLE GREEN'S FUNCTION OF THE TOMONAGA–LUTTINGER MODEL?
Authors: BUSCHE TOM , KOPIETZ PETER
Published: April 2003

We calculate the effect of a quadratic term in the energy dispersion on the low-energy behavior of the Green's function of the spinless Tomonaga–Luttinger model (TLM). Assuming that for small wave-vectors q= k-k F the fermionic excitation energy relative to the Fermi energy is v F q+q2/(2m), we explicitly calculate the single-particle Green's function for finite but small values of λ=q c /(2k F ). Here k F is the Fermi wave-vector, q c is the maximal momentum transfered by the interaction, and v F =k F /m is the Fermi velocity. Assuming equal forward scattering couplings g2=g4, we find that the dominant effect of the quadratic term in the energy dispersion is a renormalization of the anomalous dimension. In particular, at weak coupling the anomalous dimension is [Formula: see text], where γ is the anomalous dimension of the TLM. We also show how to treat the change of the chemical potential due to the interactions within the functional bosonization approach in arbitrary dimensions.
SIMPLE QUANTUM CONFINEMENT THEORY FOR EXCITON IN INDIRECT GAP NANOSTRUCTURES
Authors: OANH NGUYEN THI VAN , VIET NGUYEN AI
Published: April 2003

We propose in this work a simple quantum confinement theory for excitons based on the effective mass approximation, for investigation of optical properties of indirect gap nanostructures. We show that using this simple model, we can get the analytic solutions and reobtain the main tight-binding approximation numerical results of Hill et al.1 for silicon nanostructures: blue shift of band gap and increase overlap between the states at the band edges when the nanostructures size in decreased.
HEAT TREATMENTS OF TANZANIA RUBY AS MONITORED BY ESR SPECTROSCOPY
Authors: WINOTAI P. , WICHAN T. , TANG I. M. , YAOKULBODE J.
Published: April 2003

The main purpose of heat treatment is to find the optimum condition to either enchance or reduce the color in order to increase value of ruby. It was found that by heating a slightly bluish Tanzania ruby in oxygen gas atmosphere could lead to a more perfect ruby. While X-ray diffractometer found the maximum decrease in c/a ratio of the hexagonal structure after the heat treatment at 1200°C, the number of Fe 2+ ions converted to Fe 3+ was detected by electron spin resonance spectrometer and found to increase with heating temperatures. These were clearly two factors that reduced blue color in the Tanzania ruby. The best heat treating temperature to enhance the quality of the ruby was at 1300°C.
SPATIAL DISTRIBUTION OF ATOMS IN GAS-COVERED Pd-X NANOPARTICLES (X=Ag, Cu, Ni, Pt)
Authors: MENON MAHESH , KHANRA BADAL C.
Published: April 2003

A Monte-Carlo (MC) simulation procedure has been developed where the pair bond energies are allowed to take into account the various coordination numbers of surface atoms and the presence of adsorbates. The pair bond energies are calculated from partial bond energies of atoms which, in turn, are calculated from modified tight binding model in the second moment approximation. The model has been applied to study the role of adsorption of hydrogen, oxygen, carbon monoxide and nitric oxide on the surface composition and surface bond geometry of bimetallic Pd -X(X= Ag , Cu , Ni , Pt ) nanoparticles having fcc cubo-octahedral geometry with 586 atoms. The results are compared with the known experimental results. Importance of the results in studying reactions on supported bimetallic catalysts has been highlighted.
HALL EFFECT IN DIFFERENT MAGNETIC PHASES OF Fe-RICH γ-FeNiCr ALLOYS
Authors: SINHA G. , MAJUDMAR A. K.
Published: April 2003

In this paper we present the Hall effect data of Fe 80-x Ni x Cr 20(14≤x≤30) alloys in four different magnetic phases within the fcc γ-phase. In the spin-glass (SG) phase (x=19) the nonlinear behaviour of the Hall resistivity (ρ H ) with field arises from the spin-orbit scattering. The temperature variation of ρ H shows broad peaks around T g for lower fields (<0.1 T) but they disappear at higher fields (1 T). We separate the ordinary (OHC) and the extra-ordinary (EHC) Hall coefficients in the ferromagnetic sample (x=30) and show their temperature variation. In the reentrant spin-glass (RSG) (x=26), again ρ H shows a nonlinear variation with field. The temperature variation of ρ H shows anomaly near both T g and T c . In the antiferromagnetic phase (AF) (x=14), ρ H increases more or less linearly with field and its temperature variation shows broad peaks around T N for lower fields.
ANOMALOUS BEHAVIORS OF NON-RESONANT MICROWAVE ABSORPTIONS OF SUPERCONDUCTORS
Authors: SUGAWARA K. , ARAI N. , KOUZUKI A. , ICHIMURA S. , NAOI H. , HOTTA K. , HIROSE H.
Published: April 2003

The non-resonant microwave absorption (NRMA) measurements have been intensively performed for superconducting thin films of YBa 2 Cu 3 O y fabricated on MgO (100) substrates and powder samples of LaSrCuO systems. In order to complement the study, we also review the NRMA of BiSrCaCuO system. A particular attention has been paid to the following phenomena: (i) phases, (ii) effect of current, (iii) directional effect of applied magnetic field, (iv) hysteresis, (v) microwave power absorbed as a function of magnetic field, (vi) linewidth in the vicinity of T c , and (vii) modulation amplitude effect. A preliminary study on ( La 0.98 Dy 0.02)1.85 Sr 0.15 CuO 4 was also reported.
FORMATION OF TWO LAYERS IN Pr1-xCaxBa2Cu3O7-y(0≤x≤0.5) THIN FILMS DEPOSITED AT DIFFERENT TEMPERATURES
Authors: DÍAZ J. A. , CONTRERAS O. , SIQUEIROS J. M.
Published: April 2003

We have grown epitaxial Pr 1-x Ca x Ba 2 Cu 3 O 7-y(0≤x≤0.5) thin films on SrTiO 3 and Yttrium stabilized zirconia (YSZ) substrates by pulsed laser deposition at different temperatures. X-ray diffraction and transmission electron microscopy analysis revealed epitaxial growth on YSZ substrates for x=0 along the a and c axis directions for T=570° C , where first the film grows oriented along a-axis and afterward, it undergoes a shift in the orientation, growing in the c-axis direction from then on. The cell parameters were c=1.166 nm and a~b=0.386 nm .
QUANTUM THEORY OF THE SEEBECK COEFFICIENT IN METALS
Authors: FUJITA S. , HO H.-C. , OKAMURA Y.
Published: April 2003

Based on the idea that different temperatures generate different carrier densities and the resulting diffusion causes the thermal emf, a new formula for the Seebeck coefficient S is obtained: [Formula: see text], where q, n, εF, [Formula: see text]. and [Formula: see text]. are respectively charge, carrier density, Fermi energy, density of states at ∊F and volume. Ohmic and Seebeck currents are fundamentally different in nature. This difference can cause significantly different transport behaviors. For a multi-carrier metal the Einstein relation between the conductivity and the diffusion coefficient does not hold in general. Seebeck (S) and Hall (RH) coefficients in noble metals have opposite signs. This is shown to arise from the Fermi surface having "necks" at the Brillouin boundary.
ANISOTROPIC PSEUDOGAP IN THE HALF-FILLING TWO-DIMENSIONAL HUBBARD MODEL AT FINITE TEMPERATURE
Authors: SAIKAWA TAIICHIRO , FERRAZ ALVARO
Published: April 2003

We have studied the pseudogap formation in the single-particle spectra of the half-filling two-dimensional Hubbard model. Using a Green's function with the one-loop self-energy correction of the spin and charge fluctuations, we have numerically calculated the self-energy, the spectral function, and the density of states in the weak-coupling regime at finite temperatures. Pseudogap formations have been observed in both the density of states and the spectral function at the Fermi level. The pseudogap in the spectral function is explained by the non-Fermi-liquid-like nature of the self-energy. The anomalous behavior in the self-energy is caused by both the strong antiferromagnetic spin fluctuation and the nesting condition on the non-interacting Fermi surface. In the present approximation, we find a logarithmic singularity in the integrand of the self-energy imaginary part. The pseudogap in the spectral function is highly momentum dependent on the Fermi surface. This anisotropy of the pseudogap is produced by the flatness of the band dispersion around the saddle point rather than the nesting condition on the Fermi level.
SYMMETRY CLASSES OF MEAN FIELD SOLUTIONS IN THE DOUBLE DEGENERATE eg HUBBARD MODEL OF MANGANESE OXIDES
Authors: MASAGO AKIRA , MORI SHOJI , OZAKI MASA-AKI
Published: April 2003

This paper describes symmetry properties of mean field (unrestricted Hartree–Fock) solutions of spin and orbital ordered phases in a double degenerate eg Hubbard model including antiferomagnetic interaction with t2g electrons in manganese oxides. We consider spin (axial along z axis) and orbital ordered states with four types of ordering vectors QF=Q0=(0, 0, 0), QA= Q1=(0, 0, π), QC=Q2 =(π, π, 0) and QG=Q3=(π, π, π) without charge order, spin canting and Jahn–Teller distortion. We obtained invariance groups G(ij)(i, j=F, A, C, G) for 4 (for spin orders)×4 (for orbital orders)=16 states. The canonical form of the mean field Hamiltonian for G(ij) state is obtained from its symmetry condition for the invariance group G(i, j). We found that the mean field Hamiltonian includes Hamiltonian elements besides eg spin operators [Formula: see text] and orbital pseudo-spin operators T. We performed numerical calculations of 16 ordered states for eg electron numbers per site ne=1.0~0.1 at zero temperature and found sequential change of the spin ordered structures such as A→F→A→C→G with hole doping for a parameter set. Spin canted states derived from non-canted G(ij)(i, j=F, A, C, G) states are discussed.
INDUCTIVE CHARACTERIZATION OF SUPERCONDUCTING NANO-ENGINEERED ARTIFICIAL CUPRATES
Authors: MEDAGLIA P. G. , ORGIANI P. , ARUTA C. , BALESTRINO G. , TEBANO A.
Published: April 2003

Ultrathin artificial high temperature superconducting structures were grown by pulsed laser deposition stacking in sequence epitaxial layers of ( Ba 0.9 Nd 0.1) CuO 2+x and CaCuO2. Critical temperature Tc and critical current density Jc for such nano-engineered structures consisting of a single (CaCuO2) block sandwiched between two ( Ba 0.9 Nd 0.1) CuO 2+x charge reservoir blocks were measured inductively. The onset of superconducting transition is higher than 50 K. The purely intralayer critical current density was measured at 4.2 K and resulted to be larger than 108 A/cm2.
SOME ASPECTS OF MATERIAL PREPARATION IN MAGNESIUM DIBORIDE: THIN FILMS GROWTH AND TAPES FABRICATION
Authors: FERDEGHINI C. , BRACCINI V. , BELLINGERI E. , CIMBERLE M. R. , FERRANDO V. , GRASSO G. , MALAGOLI A. , MANFRINETTI P. , MARRÈ D. , MODICA M. , PALENZONA A. , PALLECCHI I. , PELLEGRINO L. , PUTTI M. , RAMADAN W. , RONCALLO S. , TUMINO A. , SIRI A. S.
Published: April 2003

In this paper a short description of the problems related to the preparation of magnesium diboride samples in form of bulk, thin films and tapes is given. The principal problem is that MG is highly volatile: this fact imposes severe constraints to the sample preparatation. We briefly review the activity in this field of the INFM-LAMIA laboratory. A method has been developed to prepare very clean and dense bulk samples with good transport properties in a single step, starting from pure elements. In the tapes fabrication, very promising results are obtained with a powder in tube procedure: the low cost and the good results obtained in term of critical current density make this compound interesting for the applications. The various thin film deposition techniques developed will be also shortly reviewed with particular regard to the in situ ones.
STATUS AND PERSPECTIVES FOR MAGNET SYSTEMS BASED ON HTS CONDUCTORS
Authors: MATRONE A.
Published: April 2003

High Temperature Superconducting (HTS) materials will be applied to magnet systems if they will satisfy engineering specifications about performances and if their costs will be "acceptable" (i.e. justified by the benefits of their usage). Although there are now more than 50 known HTS materials, only Bi-2212 and Bi-2223 are available as long-length conductors usable for prototypes of cryocooler cooled magnets, high field insert coils, transformers, etc. Conductor performances of importance in magnet systems design will be addressed like: engineering critical current, anisotropy, strain tolerance, AC losses, resistive transition and n-value. Achieved costs and performances of HTS magnet systems will be reviewed and possible future improvements will be discussed by considering also YBCO coated conductors and MgB2 wires under development.
SUPERCONDUCTIVITY IN THE VICINITY OF A 2D ELECTRONIC TOPOLOGICAL TRANSITION
Authors: ANGILELLA G. G. N. , PIEGARI E. , VARLAMOV A. A.
Published: April 2003

We review some results concerning the superconducting properties of a 2D superconductor in the proximity to an electronic topological transition (ETT). In contrast to the 3D case, we find that the superconducting gap at T = 0 is characterized by a nonmonotonic behavior, with maxima occurring close to the ETT, both for s- and for d-wave pairing. Such a result is in good qualitative agreement with the phenomenological trend recently observed for [Formula: see text] as a function of the hopping ratio t′/t for several cuprate compounds. We also derive a nonmonotonic dependence of the quasiparticle inverse lifetime due to impurity scattering. We further analyze the effect of an ETT on the Ginzburg-Landau stiffness η, for which we recover an expression analogous to the case of an isotropic dispersion relation.
SCANNING TUNNELING SPECTROCOPY ON MgB2 THIN FILMS
Authors: BOBBA F. , LAMY R. , RODITCHEV D. , SACKS W. , KLEIN J. , FERRANDO V. , FERDEGHINI C. , GIUBILEO F. , CUCOLO A. M.
Published: April 2003

Low temperature Scanning Tunneling Microscopy/Spectroscopy (STM/STS) was used to study the superconducting properties of as grown MgB2 thin films. Two different Pulsed Laser Ablation deposition procedures were used to fabricate the samples. Depending on growth conditions, STS/STM analysis revealed either superconducting or normal metallic behavior on the sample surfaces. In the first case the tunneling spectra with well-pronounced superconducting gap of 2.6-3.2 meV were observed at T = 4.2 K. STS spectra were found to be position independent, thus showing a very high spatial homogeneity of the films. No evidence of double gap structures was found. Moreover, in contrast with a bulk TC = 35.7K of the samples, the superconductivity with a damped TC was observed on the surface. In the second case, the superconducting features were completely absent in the tunneling spectra. We associate such a behaviour of the samples of second type with the presence of a thick metallic cap layer all over the surface.
HIGH DENSITY MgB2 OBTAINED BY REACTIVE LIQUID Mg INFILTRATION
Authors: GIUNCHI GIOVANNI
Published: April 2003

A new route to the sintering of MgB2 has been identified, based on a reactive infiltration of liquid Mg on a powdered B preform. The technique allows to obtain large bulk manufacts in an inexpensive way, without the need of high pressure apparatus. The best of the obtained samples shows a transport current density of 3 kA/cm2 at 4.2K and 9 T. The critical aspects of the technology are presented, together with the recent achievements and the perspective applications.
FABRICATION AND CHARACTERIZATION OF Ni-SHEATHED MgB2 SUPERCONDUCTING TAPES
Authors: MALAGOLI A. , GRASSO G. , TUMINO A. , MODICA M. , BRACCINI V. , RONCALLO S. , BELLINGERI E. , FERDEGHINI C. , SIRI A. S.
Published: April 2003

The preparation route for Nickel-sheathed MgB2 tapes has been studied in order to achieve improved transport properties at temperatures above that of liquid helium. Superconducting tapes have been manufactured by the Powder-In-Tube method, that consists in the cold working of a Nickel tube filled by reacted MgB2 powders, and in a final heat treatment carried out in Argon atmosphere. The could working procedure has been tuned in order to achieve the highest degree of MgB2 density, while limiting the formation of sausaging of the superconducting core cross section. The effect of the final heat treatment has been studied both on the superconducting and microstructural properties of the MgB2 tapes. The critical current of the reacted MgB2 tapes has been measured as a function of the magnetic field and of the temperature on short samples as well as on a small pancake coil. Finally, the microstructural and XRD analyses have revealed the clear presence of a MgB2Ni2.5 reaction layer between the Nickel sheath and the Nickel sheath and the MgB2 superconducting core in the reacted samples.
IN-SITU DEPOSITION OF YBCO/CeO2 ON BIAXIALLY TEXTURED Ni ALLOY TAPES BY THERMAL COEVAPORATION
Authors: BINDI M. , GIANNI L. , ZANNELLA S. , BOTARELLI A. , BALDINI A. , GAUZZI A. , TUISSI A.
Published: April 2003

We report on the in-situ preparation and characterization of coated conductors consisting of a ≈ 400 nm thick YBCO superconducting film grown by thermal co-evaporation on a ≈ 200 nm thick CeO2 buffer layer grown by e-beam evaporation on biaxially textured Ni-Cr and Ni-V substrates. We studied the degree of texture of both YBCO and CeO2 layers as a function of substrate temperature during deposition and of oxygen partial pressure. X-ray diffraction patterns reveal that the highest degree of (100) texture of the buffer layer, necessary for the subsequent growth of the YBCO film with the desired c-axis orientation, is achieved at substrate temperatures of the order of 670-730°C. The backfilling of the chamber with oxygen during the cooling-down of the substrates after deposition of the CeO2 layer greatly improves the degree of crystallization of the buffer layer. YBCO films show good superconducting properties with critical temperatures Tc of the order of 87 K. Work is in progress to scale up the process for long tapes.
ELECTRICAL, THERMAL AND MECHANICAL STABILITY OF AFM BSCCO-2223 COMPOSITE CONDUCTORS FOR CURRENT LEAD APPLICATIONS
Authors: MARTINI L.
Published: April 2003

In this work, the relevant properties of AFM BSCCO-2223/AgAu composite conductors and the most crucial technical problems faced in the design and manufacturing of current leads (CLs) are presented and discussed. Important issues such as thermal stability of AFM conductors, their effective mechanical reinforcement and the viable technical solutions for kA-class CLs are addressed. AFM BSCCO-2223 composite conductors and kA-class AFM bundles went through an extensive electrical, thermal and mechanical characterisation in order to evaluate and possibly improve their overall performances and to increase their potential and range of application. Experimental results on ageing tests on AFM specimens, with and without mechanical reinforcement, after thermal cycling in liquid nitrogen and quench experiments, induced by overcurrents and/or lacking of cooling, are reported and discussed in detail.
ELECTRON-PHONON INTERACTION IN N-DOPED CUPRATES: AN INELASTIC X-RAY SCATTERING STUDY
Authors: D'ASTUTO M. , MANG P. K. , GIURA P. , SHUKLA A. , MIRONE A. , KRISCH M. , SETTE F. , GHIGNA P. , BRADEN M. , GREVEN M.
Published: April 2003

Inelastic x-ray scattering (IXS) with very high (meV) energy resolution has become a valuable spectroscopic tool, complementing the well established coherent inelastic neutron scattering (INS) technique for phonon dispersion investigations. In the study of crystalline systems IXS is a viable alternative to INS, especially in cases where only small samples are available. Using IXS, we have measured the phonon dispersion of Nd1.86Ce0.14CuO4 + δ along the [ξ, 0, 0] and [ξ, ξ, 0] in-plane directions. Compared to the undoped parent compound, the two highest longitudinal optical (LO) phonon branches are shifted to lower energies because of Coulomb-screening effects brought about by the doped charge carriers. An additional anomalous softening of the highest branch is observed around q = (0.2, 0, 0). This anomalous softening, akin to what has been observed in other compounds, provides evidence for a strong electron-phonon coupling in the electron-doped high-temperature superconductors.
QUANTITATIVE MAGNETO-OPTICAL ANALYSIS OF SUPERCONDUCTORS
Authors: LAVIANO F. , BOTTA D. , CHIODONI A. , GERBALDO R. , CHIGO G. , GOZZELINO L. , MEZZETTI E. , BINDI M. , ZANNELLA S.
Published: April 2003

A magneto-optical equipment has been set up and carefully calibrated. In this paper, we shortly review the main characteristic of this equipment, then we focus on the analysis to be performed in order to extract from magneto-optical images the local values of magnetic induction and electrical current density along the superconducting sample. This implies the numerical inversion of the Biot-Savart law. A detailed analysis modeling the interaction of the spontaneous magnetization across the indicator film with the magnetic fields in the specimen region has been performed. In particular, we take into account the generally disregarded interaction of the indicator film with in-plane magnetic fields, i.e. with the component parallel to the indicator surface. A set of experimental data on YBCO films is presents to show the actual impact of the new analysis method, for what concerns both the local values of the magnetic induction and the electrical current density evaluation. A critical comparison of the new obtained data with known electrodynamical models in made. It results that, in the Meissner state, a simple electrodynamical model nicely describes the electrical current obtained from our experimental data. On the contrary, in the critical state, the Bean model does not account well for the observel local variations of the critical current along the sample.
EVIDENCE OF CHARGED POLARON ARRAYS FROM THE OPTICAL CONDUCTIVITY OF LANTHANUM-STRONTIUM CUPRATE
Authors: LUCARELLI A. , ORTOLANI M. , PERLA A. , LUPI S. , MASELLI P. , CALVANI P.
Published: April 2003

The optical conductivity of a series of LSCO single crystals with 0 < x < 0.15 is measured as a function of doping and temperature from 20000 down to 15 cm -1. The behaviour of the polaron conductivity is isolated and analyzed through its first frequency moment. A softening of the polaron binding energy EP with cooling and doping is observed, and an explanation is proposed in terms of polaron stripe formation.
NUMERICAL INVESTIGATION ON AC PROPERTIES IN HIGH TC SUPERCONDUCTING TAPES
Authors: TEBANO RICCARDO , MELE RENATA , BOFFA VINCENZO , GÖMÖRY FEDOR , STRYCEK FRANTISEK , SEILER EUGEN
Published: April 2003

Reduction of AC losses for large-scale applications of superconductors is a critical issue. Therefore, the quantitative evaluation of AC losses is important for the development of superconductors and their applications to electrical power systems. The development of numerical models that simulate the electromagnetic phenomena inside superconductors allows to understand the electromagnetic behavior of superconductors and to evaluate the AC loss properties. Following an approach proposed by Brandt in several papers, a numerical model was developed in order to study the AC properties of superconducting tapes in different geometrical arrangements and with time dependent current and magnetic field. Here we show an example for simple single-layer model cables to show how this rather simple and versatile numerical approach allows optimizing configurations for actual operational conditions.
THIRD-HARMONIC EMISSION IN MgB2 SUPERCONDUCTOR AT MICROWAVE FREQUENCIES
Authors: GALLITTO A. AGLIOLO , BONSIGNORE G. , LI VIGNI M.
Published: April 2003

We report a set of experimental results on microwave third-harmonic emission by MgB 2 superconductor. We have investigated third-harmonic signals as a function of temperature, input power level and intensity of the static magnetic field. We show that different mechanisms are responsible for third-harmonic emission at temperatures near T c and far below T c , similarly to what has been obtained in other high- T c superconductors.
STATIC PROPERTIES OF THE BOSON-FERMION MODEL: SCALE ENERGY ANALYSIS
Authors: CUOCO M. , NOCE C. , RANNINGER J. , ROMANO A.
Published: April 2003

The analysis of the pseudogap phase in high-T c superconductors is often performed in terms of scenarios where itinerant electrons are coupled to bosonic excitations. Within this context, we have investigated the static properties of the boson-fermion model on a ring of finite size length by means of the thermal Lanczos method. As a result, we have determined the temperature dependence of the specific heat and the static susceptibility for different values of the exchange coupling associated to processes in which bosons decey into pairs of itinerant electrons, and vice-versa. The nature and the scale of energies of the relevant excitations which control the temperature behaviour of these thermodynamic quantities are also discussed.
NEW COMPARISONS FOR LOCAL QUANTITIES OF THE TWO-DIMENSIONAL HUBBARD MODEL
Authors: AVELLA ADOLFO , MANCINI FERDINANDO
Published: April 2003

We have compared the results of out approximation scheme, the composite operator, method, for the double occupancy and the internal energy of the two-dimensional Hubbard model with numerical data obtained by means of the Lanczos1and quantum Monte Carlo schemes.2The agreement is very good at both half-filling and away from it showing how reliable is the approximation scheme.