• Volume 14, Issue 4

      August 1991,   pages  889-1170

    • Foreword

      P Rama K J Rao S K Joshi C N R Rao S V Subramanyam

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    • Upper critical field of Bi1·5-Pb0·5-Ca2-Sr2-Cu3-Ox superconductor

      V Prasad S V Subramanyam

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      The highTc superconductor of nominal composition Bi1·5-Pb0·5-Ca2-Sr2-Cu3-Ox was prepared by solid-state reaction method. The upper critical fieldHc2 of the material with a zero-resistance transition at 110K was investigated by observing variations in resistance with temperature down to 90 K and with a magnetic field up to7T. The slope ofHc2 with temperature was about −0·41T/K for zero resistance transition and −6·86T/K for onset of superconductivity.Hc2 values at 0K were estimated to be 31·3T and 563T for zero-resistance transition and onset of superconductivity respectively.

    • Double-dimensional crossover in layered superconductor

      A S Sidorenko V I Dediu A G Sandler

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      The critical magnetic fieldsHc2 of superconducting layered structures V/Cu were investigated. The double-dimensional crossover 3D-2D-3D was observed on the temperature (Hc2(θ)) dependences of critical magnetic fields. The field crossover 3D-2D is caused by strong temperature dependence of superconducting coherence length ξs nearTc. The second crossover 2D-3D is provided by temperature dependence of normal metal coherence length ξN and reflects the 3D isotropization of layered structure V/Cu at low temperature.

    • Critical current density and magnetization studies on Bi(Pb)SrCaCuO

      P K Mishra Shailendra Kumar G Ravikumar P Chaddah B A Dasannacharya Ram Prasad N C Soni

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      HighTc phase (Tc ∼ 110 K) has been obtained in Bi-Sr-Ca-Cu-O system by partially substituting Bi by Pb. Magnetic hysteresis has been measured as a function of temperature. Critical current densities have been measured at 77 K both by transport and a.c. magnetization method in bulk samples for various concentrations of Pb. The results show that substitution of 15 at % Pb for Bi is most preferable for higher critical current density.

    • Critical current density of the superconductor YBa1·5Ca0·5Cu3O7\t-\gd

      R B Jotania S M Joshi G J Baldha H H Joshi R G Kulkarni

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      The real and imaginary (lossy) components of a.c. susceptibility of high-Tc superconductor YBa1·5Ca0·5Cu3O7−δ were measured as a function of temperature for different a.c. fields and frequencies. From the lossy component of the a.c. susceptibility the transport critical current density and its temperature dependence have been computed on the basis of Kim’s critical state model.

    • Interplay of low and highTc phases in Bi-Sr-Ca-Cu-O superconductor

      P L Upadhyay K C Nagpal R G Sharma

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      In the Bi-Sr-Ca-Cu-O system stringent conditions of heat-treatment lead to the formation of a mixture of both the low and highTc phases and obtaining a single-phase material becomes extremely difficult. This study reports preparation of samples with single superconducting transitions at ∼ 75 K and ∼ 108 K; the compositions of which correspond ton=2,3 in the series Bi2Sr2Can−1CunO4 + 2n. X-ray diffraction studies show that the lowerTc material is a relatively pure phase while the higherTc phase only co-exists with the lowerTc phase. The most obvious effect of doping the system with lead is to make the reaction take place faster and thereby increase the volume fraction of the 110K phase.

    • Role of lead in the growth of the highTc phase in the Bi2−xPbxSr2Ca2Cu3Oy systems

      L S Vaidhyanathan A M Niraimathi P Majumdar G Rangarajan M Muralidhar D Mangapathi Rao M Nagabhushanam V Hari Babu

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      Bi2−xPbxSr2Ca2Cu3Oy superconducting samples with 0<x<0·3 have been synthesized and characterized using X-ray powder diffraction.Tc and superconducting volume fraction have been measured using a.c. magnetic susceptibility, d.c. electrical resistivity as well as X-band microwave surface resistance in the normal state. The data indicate the growth of the highTc (2223) phase with corresponding reduction of the lowTc (2122) phase with increasingx, up to 0·25. Beyond this value ofx there is a slight deterioration of the superconducting behaviour.

    • AC susceptibility studies on Bi-Sr-Ca-Cu-O system

      Arani Chakravarti C D Mukherjee R Ranganathan N Chatterjee A K Raychaudhuri

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      We report the low field a.c. susceptibility data χ′(T, Hrms), χ″(T, Hrms) for the newly prepared superconducting system BixSr4Ca2Cu4Oy (x=0·5, 1·0 and 1·5) containing Bi at concentrations lower than that in the conventional Bi-based system. Our experimental results are discussed in the light of existing theories.

    • Magnetic and microstructural properties of YBa2Cu3O7−x-Ag composite

      D Sen D Bhattacharya SK Ghatak KL Chopra

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      The magnetic response of YBa2Cu3O7−x−5 mol% Ag composite to low-frequency magnetic field and its microstructure have been studied. Microstructural analysis shows evidence of platelet-type grain growth and silver fills the intergranular regions. The granular nature of the sample is revealed from the strong decrease in a.c. response in the presence of d.c. magnetic field. The intergranular shielding current estimated from the complex response and using the Bean’s model sharply increases with temperature below transition temperature.

    • DC magnetization studies of (PrxY1−x)Ba2Cu3O7−y

      L S Vaidhyanathan C K Subramaniam G Rangarajan

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      Magnetization was measured on Pr-doped YBa2Cu3O7−y for fields up to 5·5T. The paramagnetic behaviour is correlated to the free ion values of Pr3+ and Pr4+. Hysteresis experiments were done for superconducting composition for fields up to 1100 Gauss. The intragrain critical current density is calculated using Bena’s formula.

    • Influence of various parameters on the a.c. diamagnetic susceptibility of Bi-based superconductors

      M Nicolas J P Burger

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      We present some results of a.c. susceptibility measured on pure and doped with indium and potassium Bi-compounds. For both the 80K and 110K phases, we follow the χ′ and χ″ variations as a function of (a) the annealing treatment, (b) the Sr/Ca ratio and (c) the doping. Indium doping has either a positive or a negative effect on the 80K material, depending on the Sr/Ca ratio. It induces an important increase of the superconductive volume of the 110K material. Effects due to potassium are opposite. The experimental results agree with the fact thatTc goes through a maximum as the hole density increases.

    • Time relaxation of magnetization in type-II superconductors

      G Ravikumar P Chaddah K V Bhagwat

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      We assume that currents induced by isothermal changes of magnetic field decay logarithmically with time. Incorporating this time dependence into the critical state model, we obtain logarithmic relaxation rate of magnetization as a function of field for the case of an infinite cylinder. We compare these calculations with our earlier calculations on infinite slab geometry.

    • Crystal structure and resistivity studies on PrBa2 −xPrxCu3O7 −y oxide system

      S K Malik C V Tomy Ram Prasad N C Soni K Adhikari

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      The compound PrBa2Cu3O7 −y is not superconducting while most other RBa2Cu3O7 −y (R=rare earth) compounds exhibit superconductivity in the 90K range. The system PrBa2 −xPrxCu3O7 −x has been prepared to study the effect of excess Pr at the Ba site on the structure, resistivity and magnetic behaviour of this system. It is observed that single-phase compounds in the above series form forx=0·8—that is up to the composition Pr1·8Ba1·2Cu3O7 −y. While stoichiometric PrBa2Cu3O7 −y is orthorhombic, the compounds with excess Pr show tetragonal structure. Four-probe dc resistivity measurements show that all the single-phase compounds in the above series do not exhibit superconductivity and are semiconducting down to 12 K. Magnetic susceptibility measurements reveal deviation from Curie-Weiss behaviour starting at a characteristic temperature, which is taken to be the ordering temperature (TN) of the Pr moments. BothTN and overall resistivity decrease with increasingx and may have a common origin.

    • Scaling behaviour of relaxation dependences in metaloxide superconductors

      A S Sidorenko G I Panaitov A M Gabovich D P Moiseev V M Postnikov

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    • Electronic structure of YBa2Cu3O7 and YBa2Cu3O6—charge-self-consistent band structure calculations

      V A Pai A P Sathe V R Marathe

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      Charge self-consistent LCAO band structure (CSCBS) calculations are reported for orthorhombic YBa2Cu3O7 and tetragonal YBa2Cu3O6 assuming ordered vacancy models. The effective atomic charges are used to study the charge transfer. In YBa2Cu(1)1Cu(2)2O7, the two types of copper atoms have their energy bands almost overlapping with effective valency of each copper as 7/3 (or effective valency of each oxygen as approximately — 13/7), so that electron hopping can take place without any loss or gain of energy while in YBa2Cu(1)1Cu(2)2O6, Cu(1)1 is monovalent and Cu(2)2 are divalent with significant difference in their bands. Therefore, YBa2Cu3O7 should conduct much better compared to YBa2Cu3O6. This corroborates the experimental observations that YBa2Cu3O7 is a (super)conductor while YBa2Cu3O6 is not. The calculated effective charges and DOS support the above view.

    • Phonon dispersion relations and densities of states in La2CuO4 and La2NiO4

      Narayani Choudhury K R Rao S L Chaplot

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      Rigid-ion model calculations of phonon dispersion relations, densities of states and partial densities of states of the highTc superconductor La2CuO4 and its isostructural compound La2NiO4 have been carried out both in the tetragonal and orthorhombic phases of La2CuO4 and the tetragonal phase of La2NiO4. The calculations are in fair agreement with reported experiments. The computed phonon dispersion in the tetragonal phase of La2CuO4 reproduces the soft mode behaviour for the lowest Σ4 TO branch which is found to harden in the orthorhombic phase, consistent with experimental data.

    • Interatomic potential, phonon spectrum and molecular-dynamics simulation up to 1300K and 10 GPa in YBa2Cu3O7\t-\gd for δ=0 to 1for δ=0 to 1

      S L Chaplot

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      An empirical interatomic potential for YBa2Cu3O7−δ is determined for different oxygen contents (δ=0 to 1), consisting of Coulomb and short-range interactions. The calculated structure and phonon spectrum, and the results of molecular-dynamics computer simulation on the orthorhombic-to-tetragonal phase transition are in fair agreement with reported experiments.

    • Electronic and vibrational excitations in layered highTc superconductors

      Bal K Agrawal S Agrawal P S Yadav S Kumar J S Negi Namrata Varshney

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      In the series of the layered highTc superconductors (A)1,2 B2Can−1CunO2n+3,4 (A=Tl or Bi;B=Ba or Sr), withn=number of consecutive Cu-O layers, the electron energy states of some of the Tl-based systems have been investigated. The electron and phonon dispersion curves have been obtained. The electronic states near the Fermi level are dominated by the hybridized Cu(d) and the O(p) orbitals. The dispersion curves are highly two-dimensional with very small dispersion alongc-axis. The number of bands is enhanced with increase in the number of the consecutive Cu-O planes (n). The present results agree with those obtained earlier for other superconducting phases. The phonons are overall dominated by the vibrations of the light mass oxygen atom modes both lying in or outside the Cu-O planes. The low-frequency phonons involve the motions of the heavier Tl, Ba atoms etc.

    • Collective mode dynamics of the vortex lattice in a highTc superconductor

      B N Narahari Achar Chao Liu

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      Dispersion relations have been obtained numerically for the collective modes of the flux line lattice of a highTc superconductor in the London limit and the effect of pinning forces is included. It is found that the waves propagating along the magnetic field the dispersion consists of two branches and that the low frequency branch may interact with other low-frequency excitations in the lattice.

    • Some properties of vortices in layered superconducting structures

      A F Volkov

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      Some properties of vortices in layered superconductors and superconducting structures with the Josephson interaction between S-layers are studied theoretically. The Josephson vortex lattice is formed in the system if the magnetic fieldHe is parallel to the layers. Long-wave oscillations of this lattice have a sound-like spectrum; the velocity of “magnetic sound” propagating across the layers is small and diminishes with increase ofHe. Using an analogy with electron-hole plasma in semiconductors, we investigate also the behaviour of the layered structure in the fieldHe perpendicular to the layers at temperatures close to the Kosterlitz-Thouless transition temperatureTKT. The spatial dependence ofH(r) and the impedance of the system are determined atT>TKT.

    • Gibbs’ energy of formation of YBa2Cu3O7-x (tetragonal)

      A M Azad O M Sreedharan K T Jacob

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      The high temperature ceramic oxide superconductor YBa2Cu3O7-x (1–2–3 compound) is generally synthesized in an oxygen-rich environment. Hence any method for determining its thermodynamic stability should operate at a high oxygen partial pressure. A solid-state cell incorporating CaF2 as the electrolyte and functioning under pure oxygen at a pressure of 1·01 × 105 Pa has been employed for the determination of the Gibbs’ energy of formation of the 1–2–3 compound. The configuration of the galvanic cell can be represented by: Pt, O2, YBa2Cu3O7−x, Y2BaCuO5, CuO, BaF2/CaF2/BaF2, BaZrO3, ZrO2, O2, Pt. Using the values of the standard Gibbs’ energy of formation of the compounds BaZrO3 and Y2BaCuO5 from the literature, the Gibbs’ energy of formation of the 1–2–3 compound from the constituent binary oxides has been computed at different temperatures. The value ofx at each temperature is determined by the oxygen partial pressure. At 1023 K for O content of 6·5 the Gibbs’ energy of formation of the 1–2–3 compound is −261·7 kJ mol−1.

    • Investigation of the many-particle spectrum of superconductor YBa2Cu3O7

      G A R Lima A Fazzio R Mota

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      We investigate the electronic structure of superconductor YBa2Cu3O7 through a molecular cluster approach. The calculations are performed self-consistently through a semi-empirical LCAO technique, and correlation effects are taken into account by a configuration interaction procedure (INDO-CI). Our results for the larger clusters yield a density of states (in the valence band) that is in good agreement with the experimental data. We obtain for the ground-state a strongp-d covalency, resulting in a width of around 8 eV for the valence band. The interactions of Cu(1) and (2) with O(1),(2),(3) and (4) are analysed, showing how hybridization occurs. In all cases we calculate the Mott-Hubbard energy U taking into account many-body effects.

    • Relative thermodynamic stabilities of constituent oxides of highTc superconductors

      SK Sundaram

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      A thermodynamic scale of the relative stabilities of constituent oxides can help the choice of stoichiometric multi-component compositions and can provide guidance for judicious selection of heat-treatment conditions for high-Tc superconductors. A thermodynamic analysis was undertaken to study Y2O3-BaO-CuO, BaO-K2O-Bi2O3 and La2O3-SrO-NiO systems. The relative stability of the oxides was expressed in terms of two ratios,Sv andSp computed using free-energy of formation and vapour pressure data. CuO, Bi2O3 and NiO were taken as reference oxides for YBa2Cu3O7\t-\gd, Ba0·6K0·4BiO3 and La1·9Sr0·1NiO4 respectively. Thermodynamically, the reference oxide was found to be the least stable amongst the constituent oxides in each of these systems.

    • Specific-heat anomaly near superconducting transition in high temperature superconductors

      M P Das

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      A brief appraisal of the current state of understanding on the specific heat anomaly nearTc in the oxide superconductors is presented. Spectacular specific heat double transitions are explained with plausible models.

    • Is high-Tc superconductor an undegenerated doped semiconductor?

      A I Golovashkin O M Ivanenko K V Mitsen

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      The model of electron spectrum of HTS is suggested. It is supposed that the main feature of the spectrum is the presence of a narrow local pair level placed near the top of the filled two-dimensional (2D) electron band. Holes in this band are the result of thermal activation of electrons from the band to the pair level. The temperature dependences of resistivity and Hall constant of HTS in this model are in agreement with experiment on YBCO. The possibility of a first-order phase transition in such a system is considered.

    • Interpretation of doping phenomena in high-Tc superconductors

      Leena Chandran Laurens Jansen

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      On the basis of an indirect-exchange pairing mechanism of superconductivity we present a consistent interpretation of doping phenomena in both hole-doped as well as electron-doped high-Tc superconductors. We argue that in all these materials the unifying feature is the existence of a correlated narrow band of electron states formed due to doping. Numerous experimental evidences for the occurrence of such a band (reflectivity, thermoelectric power, electrical resistivity, X-ray absorption, point-contact tunneling etc.) now exist.

      Assuming the existence of such a band it was earlier shown that the indirect-exchange (superexchange) coupling between electrons in this band via closed-shell oxygen anions is attractive in the s-wave channel and leads to high-Tc superconductivity. Within the framework of this pairing mechanism, recent doping experiments (for both types of doping) can be given a unified interpretation. In addition, definitive predictions of the doping conditions under which critical temperatures are expected to enhance, are made.

    • Properties of combined phonon-high-energy boson mechanism

      R Akis J P Carbotte

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      We consider the properties of a superconductor with a joint mechanism. One involves, otherwise unspecified, high-energy excitations and is modelled with a BCS pairing potential. The other is of low energy, presumably involving phonons. When the second contribution is significant, the resulting properties can differ greatly from BCS, and for most but not all properties, in a direction opposite to that expected for conventional strong coupling systems.

    • A new look at the highTc superconductors using BCS-theory

      D N Tripathy L K Mishra

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      Using the conventional phonon-exchange mechanism of superconductivity we have succeeded in reproducing the transition temperatureTc of a large number of newly discovered highTc superconductors by introducing a certain modification to the well-known BCS-formula forTc.

    • Composition dependence of transition temperature in new superconductors: La2 −x(Ba, Sr)xCuO4

      R K Singh A C Sharma N K Gaur Dinesh Varshney

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      We have investigated the variation of transition-temperature and coupling parameter with the composition concentration in La2 −x(Ba, Sr)xCuO4 superconductors using a formulation developed on the basis of an idea of pairing of charge carriers by exchange of both acoustic plasmons and phonons. Reasonably good agreement is found between the recent experimental results and our theoretical results on the superconducting transition temperature.

    • Superconducting gap function and upper critical field in the presence of inhomogeneous magnetic order

      PC Padhi S Panigrahi S Das NC Das

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      Rare earth ternary superconductors are known to exhibit oscillatory magnetic orders below their superconducting transition temperatures. The study of behaviour of superconducting electrons in an inhomogeneous magnetic field is therefore important for such systems. We report here the results of our theoretical study of superconducting gap function Δ(T) and upper critical fieldHc2(T). The results are applied to analyse and explain the variation of Δ(T) andHc2(T) in case of NdRh4B4.

    • Frustration-mediated superconductivity in two-dimensional classical Heisenberg model

      K Y Szeto

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      The two-dimensional frustrated classical Heisenberg model on a square lattice is used to describe the magnetic interaction in La2CuO4. In the slow-hopping regime, the model suggests anXY-like ground state. An analysis of the effective interaction between frustrated bonds shows the possibility of pair formation and superconductivity.

    • Spin glass state in high-temperature superconductors

      V A Cherenkov

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      The glass state of high-temperature superconductors in the models of Josephson weak links is studied. Reversibility phenomena in weak magnetic fields are outlined.

    • A mechanism for pairing of RVB excitations

      S N Behera D Sa

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      The fluctuations in the RVB order parameter over their meanfield values are considered in the BZA theory which mediate the interaction between the RVB excitations. The condition under which the effective interaction could be attractive and result in super-conductivity is investigated. The dependence of the transition temperature on the dopant concentration and the RVB order parameter is calculated.

    • Chemical potential and the self-energy correction of spinors: Further results on the large-U hubbard model

      L Hu K W Yu

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      The spinor-holon effective Hamiltonian proposed by Zou and Anderson is used to calculate the chemical potential and the self-energy correction of spinors, taking into account the effect of spin exchange interaction. In the superfluid phase, the transition temperature is sensitive to various choices of the on-site Coulomb repulsionU and the band parametert. The correction due to spin fluctuations is extremely important for small doping fractions δ. In the region of δ ≤ 8t2U, the effect of paramagnon fluctuations renders the mean field theory invalid. The paramagnetic susceptibility χ in the long wavelength limit is almost independent of temperatureT at high temperatures. For fixedT, χ increases with doping δ while for fixed δ, χ decreases withT, in qualitative agreement with experiments.

    • New effects in highTc superconductors

      P Bhattacharyya

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      We show that a proper interpretation of the critical field measurements can distinguish spin fluctuation mechanism from others in the context of the pairing hypothesis. We can predict the existence of a new mode in the mixed state of a superconductor.

    • Upper critical field of aD-wave superconductor stabilized by antiferromagnetic fluctuations

      M Prohammer J P Carbotte

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      We have derived microscopic equations for the upper critical magnetic field of aD-wave superconductor stabilized by antiferromagnetic spin fluctuations. We present numerical results for the reduced fieldhc2(t) as a function of reduced temperaturet when the magnetic field is along thez axis and also zero-order results when it is along thex ory axis. Two differentD-wave models are considered. The angular dependence ofHc2 forT nearTc is given as a function of polar angleϑ in thez−x orz−y plane.

    • Superconductivity in the two-band hubbard model

      K P Jain R Ramakumar C C Chancey

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      We present a study of superconducting correlations in a two-band Hubbard model with a wide band strongly hybridized with a narrow band in which an attractive on-site interaction is operating. The narrow band pairs can induce superconducting correlations in the wide band through hybridization interaction. A generalized gap function for the induced wide-band pairing is obtained and its properties in the intermediate interaction region are analysed. Relevance of the results to high-Tc superconductors is briefly discussed.

    • Static hole in a two-dimensional antiferromagnet

      D M Gaitonde V Ramesh S N Behera

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      The energetics of static hole in a 2D antiferromagnetic (AF) is studied. AF is described by magnon operators in the LSW approximation, while the holes by fermionic operators. The energy spectrum of the magnons in the presence of the hole is determined from the perturbed Green’s function.

    • Calculations of the effects of point defects on hubbard models of high temperature superconductivity

      J W Halley S Davis P Samsel Robert Joynt

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      We describe studies of the effects of defects on Hubbard models of highTc superconductors. We first present numerical mean field calculations on thet-J model in which disorder due to point defects in treated explicitly. Secondly, we describe variational Monte Carlo calculations at zero temperature in which the full effects of the Gutzwiller projection are included in a calculation of the variational BCS ground state including randomly distributed point defects. Finally we discuss possible microscopic mechanisms by which point defects could enhance pairing in the BCS state.

    • Charge bag model of superconductivity in intercalated layered compounds

      S N Behera D M Gaitonde

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      In certain layered compounds the superconducting transition temperature (Tc) is enhanced on intercalation. The superconductivity in these materials arises over a charge density wave background. To explain this enhancement inTc we propose the charge bag model analogous to the spin bag model proposed earlier by Schrieffer as a mechanism for highTc superconductivity.

    • Role of apex oxygen and redox elements in high temperature superconductors

      R E Amritkar D G Kanhere P V Panat A P B Sinha

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      The presence of valence fluctuating redox ions such as Pb, Tl, Sb, etc can enhance the transition temperature of the new oxide superconductors. We propose the possibility of the existence of negative Hubbard parameter (U) on redox ion which results in effective negativeU on apex oxygen bridging redox ion and the CuO2 planes. This enhancesTc.

    • Interlayer coupling in peroxitonic model of superconductivity

      R Rai

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      The unusual ESR spectra of YBa2Cu3O7−x is ascribed to a Cu2+ ion, whose one ligand in CuO2 plane is involved in a peroxiton O−Cu+−O bond. The analysis shows that the presence of a ligand in O substantially increases the energies ofd-hole orbitals. Since this increase for YBa2Cu3O7\t-\gd, orbital is much larger than that ford3z2r2 orbital, their separation is drastically reduced and there occurs a significant orthorhombic mixing between the two. This charge transfer from a planardx2r2 orbital to ad3z2r2 orbital, which lies mainly perpendicular to the plane, provides a mechanism for interlayer coupling. The effect of this coupling on superconducting transition temperature is discussed.

    • Electron structure and correlation effects in high-Tc superconductors and transition metal oxides

      V I Anisimov M A Korotin V R Galakhov E Z Kurmaev

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    • Oxide glass filaments and their superconducting phase by annealing

      B K Chaudhuri K K Som

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      Bi4Ca3Sr3CuxOy (4334) oxide glasses with different concentrations of the transition metal copper have been prepared by rapid quenching technique from respective melts. These glasses have been characterized from electrical, magnetic, dielectric and other properties. The semiconducting glasses are not superconducting even down to 4·2 K. However, by properly annealing the glasses in air or in oxygen atmosphere they can be converted into the corresponding superconducting phases with superconducting transition temperatures between 70 and 110 K depending on the Cu ion concentrations. The dc conductivity of the glasses could be explained with small polaron adiabatic hopping mechanism. Analysis shows that the correlated barrier hopping (CBH) mechanism is the most appropriate model for explaining the a.c. conductivities of these glasses. Addition of a small amount (about 2 wt%) of B2O3 makes it possible to draw glass filaments which could also be converted into the superconducting phase withTc values lower than those obtained from the respective pure glasses.

    • Critical current studies on metal clad R1Ba2Cu3O7−x wires

      S R Shukla Y S Reddy R G Sharma

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      Silver clad wires of R1Ba2Cu3O7−x (R=Y, Gd, Sm, Dy and Ho) have been fabricated following the powder-in-tube method and cold-rolling. The critical current densityJc at 77 K and zero magnetic field is restricted to 66 A.cm−2. In a separate experiment we have studied the effect of silver sheath thickness onJc of the Y1Ba2Cu3O7−x wires. Interestingly,Jc is higher for larger sheath thickness indicating that large sheath thickness prevents oxygen loss during sintering. Uniaxial pressing of the wires into flat tapes results in an increase of theJc. MaximumJx is, however, limited to 169 A.cm−2. No grain alignment is found in the core material of our wires which is essential for highJc. Several options are being tried.

    • Slip processing of highTc superconductors

      R K Sinha R K Sinha

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      Slip-processing technique has been used to fabricate tapes and alumina-supported films of superconducting YBa2Cu3O7−x. Good densification and connectivity are revealed by scanning electron microscopy. Both the tape and film show superconductivity well above the liquid nitrogen temperature with a transition range of 3° and 5°K respectively.

    • YBaCuO ceramics-based DC interferometer with current-formed weak links

      S I Bondarenko A V Lukashenko A A Shablo E N Tilchenko Yu K Neimirok

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      The results of the study of dc interferometers and SQUIDS from the thick-film YBa1·5Ca0·5Cu3O7\t-\gd ceramics are presented here. It is shown that under the influence of high-density currents through the film constriction the differential resistance of interferometers thus formed increases by 2–3 orders of magnitude and the voltage-field characteristics become more regular. The frequency dependence of intrinsic noise in dc SQUIDs based on thick-film interferometers is investigated. It is demonstrated that in the white noise frequency region (1–10 Hz) the energy sensitivity is withinδɛn=10−28 to 10−29 J/Hz.

    • Study of Josephson effects in microbridges of Bi-Sr-Ca-Cu-O films made by spray pyrolysis

      D K Walia A K Gupta G S N Reddy V N Ojha V S Tomar Neeraj Khare N D Kataria K C Nagpal

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      Superconducting Bi-Sr-Ca-Cu-O (2212) films were prepared by spraying stoichiometric aqueous solutions of nitrates of bismuth, strontium, calcium and copper on heated MgO (100) substrates and subsequent annealing in air. TheR-T curves of the films show metallic behaviour above the superconducting transition temperature.Tc (R=0) is observed between 80 and 85 K. Annealing temperature has a profound effect onTc (R=0) and on the orientation of the film. Annealing in air in near-melting region yields highly oriented films withc-axis perpendicular to the substrate. These films show a sharp superconducting transition with zero resistance at 85 K. Microbridges of the dimensions of 50 µm × 50 µm have been patterned photolithographically followed with chemical etching. The 1 V characteristics of the microbridges show Josephson effects due to the presence of grain boundary weaklinks. The temperature dependence of the critical current for these microbridges suggest formation of superconductor-normal-superconductor type weaklinks.

    • Josephson effects and dc SQUID behaviour in break junction of Y-Ba-Cu-O at 77 K

      S K Arora A K Gupta Neeraj Khare N D Kataria G S N Reddy V N Ojha V S Tomar

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      Break junctions of Y-Ba-Cu-O superconductor were realized by creating fresh crack in the bar shape bulk samples mounted in a specially designed probe kept in the liquid nitrogen bath. While the bulk sample was kept static, a sharp tip of a shaft was moved downward from the outside of the dewar in order to break the sample. It was possible to change the critical current of the break junctions by adjusting pressure on the tip of the shaft. The effects of magnetic field and microwave power on the I-V characteristic of the junction were studied. The junction was found to show periodic V-B behaviour and the microwave-induced Shapiro steps were clearly observed at 77 K. The flux noise spectrum of the junction was also studied.

    • Performance of a 2-hole Y-Ba-Cu-O rf SQUID

      S K Arora A K Gupta Neeraj Khare V S Tomar V N Ojha

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      A two-hole rf SQUID has been fabricated out of bulk YBCO by drilling two holes and careful erosion of the wall between the holes. Commercial SQUID electronics is inductively coupled to the SQUID through a copper coil glued in one of the holes. Periodic oscillations in V-B characteristic of the SQUID are observed at 77 K. The spectral density of the flux noise in the white noise region is 5·5 × 10−4Φ0 / √Hz. The flux noise is frequency-dependent below 200 Hz.

    • Applications of superconducting magnetic energy storage in electrical power systems

      S Banerjee T N Saha

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      Fast-acting energy storage devices can effectively damp electromechanical oscillations in a power system, because they provide storage capacity in addition to the kinetic energy of the generator rotor, which can share the sudden changes in power requirement. The present paper explores the means of reducing the inductor size for this application so that the use of high-Tc superconducting materials becomes feasible.

    • Propagation of a.c. magnetic field through high-Tc coatings

      J Karthikeyan A S Paithankar P Chaddah N Venkatramani K P Sreekumar V K Rohatgi

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      Studies on the propagation of AC magnetic field through plasma-sprayed superconducting Y1Ba2Cu3O7−x coatings show that complete shielding is achieved up to a certain critical magnetic field strengthH0. Increase in the thickness orJc of the specimen increases theH0 value. Flux-trapping occurs in the specimen at high frequencies and the frequency at which it occurs increases with increase in specimenJc.

    • Shape-forming and densification of ceramic superconductors

      O M Prakash R M Anklekar N K Khosla Sunil Kumar B Chaudhuri C A Thomas A P Magadum M Ramesh B T Rao T R Rama Mohan

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      Processing studies on varied shape-forming and densification of bulk ceramic superconductor, YBa1·5Ca0·5Cu3O7\t-\gd, are reported in this paper. Polyvinyl butyral-polyethylene glycol-trichloroethylene has been found to be the best binder-plasticizer-solvent system in plastic shape-forming. The effect of initial particle morphology on final densification has been the most sensitive single parameter as compared to compaction pressure and final sintering durations at ∼930°C. 1-2-3 powders of mean particle size ∼ 1·94 µm have yielded sintered densities ∼92% T.D. albeit with lower oxygen intake O6·7.

    • Polymeric encapsulation of 1-2-3 ceramic superconductors

      B Chaudhuri R M Anklekar C A Thomas T R Rama Mohan O M Prakash

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      YBa2Cu3O7−x pellets have been found to degrade and lose superconducting properties when exposed to the normal atmosphere for long durations. Hence, coating the superconducting pellets with a polymer to protect it from moisture and CO2 of the ambient atmosphere is highly desirable. In this paper the criteria of selection of a polymer and some preliminary studies on polymeric encapsulation of 1-2-3 pellets are reported. Fevitite epoxy and stycast have been found to show encouraging results.

    • Grain orientation studies in YBCO superconductors by molten salt synthesis

      Sudhakar Gopalakrishnan Walter A Schulze

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      The molten salt or the flux method is utilized in this study to fabricate a grainoriented YBa2Cu3O7−x superconductor. The formation of YBa2Cu3O7−x in the presence of molten salts of Na, K, Li belonging to chloride and iodide systems does not appear feasible due to the instability of the superconducting phase in these salt systems. Studies using the “green phase,” Y2BaCuO5, as seed crystals suggest a two-stage approach in the formation of YBa2Cu3O7−x. The process uses Y2BaCuO5 formed by molten salt synthesis which has been observed to be stable in water and against most of the salts. The 211 crystals can be mixed with oxides and converted to 123. Analysis of X-ray data and SEM micrographs indicates a certain degree of grain orientation, which can be further enhanced by tape-casting or not-forging.

    • Study of galvanomagnetic effects in Bi-Sr-Ca-Cu-O films

      Ramesh Chandra Neeraj Khare A K Gupta D K Walia Sangeeta Chaudhry V N Ojha V S Tomar

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      Galvanomagnetic effect has been studied at 77 K on Bi-Sr-Ca-Cu-O films as a function of d.c. bias current. These films were prepared by spray pyrolysis and screen-printing techniques, withTc (R=0) 80 K and 100 K respectively. Magnetic field dependence ofIc of sprayed-film showed very small hysteresis whereas screen-printed film showed greater hysteresis. Application of small magnetic field on these films destroyed the zero-resistance state and a finite resistance was developed. The slope of the resistance vs magnetic field curveΔR/ΔH depended on the current flowing through the sample. To increaseΔR/ΔH, meander-shape pattern was prepared on the sprayed film. In generalΔR/ΔH increased by a factor of 103 after sample patterning. The possibility of using these films as a sensitive magnetic field sensor is discussed.

    • YBa2Cu3O7 superconductor as a mild, deep oxidation catalyst

      Y Srinivasa Rao K Sai Prasad Rao V S Subrahmanyam C S Sunandana

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      Several Y-Ba-Cu-O compositions including the superconducting YBa2Cu3O7 are scanned for their oxygen uptake using low temperature oxygen chemisorption (LTOC) technique as the principal probe at 195 K. BET surface area and pore size distribution were also determined. The results indicate their potential for use as mild, deep oxidation catalysts even under ambient and sub-ambient conditions.

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