Remarks on exotic superconductivity mechanisms

A generalization of a recent theory of the superconductingT _c for non-phonon interaction mechanisms is presented; it incorporates strong coupling effects in a simple way. In addition it is pointed out that the solution of the integral equation for the kernel of the new gap equation to all orders in the interaction is necessary, feasible, and significant (with regards to highT _c ) in certain cases; this is illustrated by an approximate solution for a simple model of a typical exotic interaction mechanism.     

Density of states and superconductivity in ScTc2 and ScRe2

The low-temperature specific heats of ScTc₂ and ScRe₂ are reported. The newly discovered record highT _c for a Laves phase (10.9 K) in ScTc₂ is explained qualitatively by its factor of two largerN(0) than in ScRe₂ (T _c=0.93 K). Comparison of the data for ScTc₂ with that for HfV₂ (T _c=8.4 K), which has three times theN(0) as for ScTc₂, suggests that the electron-phonon coupling strength is also an important factor in the highT _c of ScTc₂.     

CESR in superconductors: Relevance of highT c copper oxide systems

CESR is a highly effective tool to study the interaction among conduction electrons in normal metals. In superconductors belowT _c it can reveal vital information concerning the pairing interactions. A comparative study of highT _c and conventional superconductors is presented and it is shown that the disappearance of CESR in the superconducting state is a feature common to both conventional and highT _c superconductors and establishes the importance of exchange interactions in pairing. This is supported by experimental observations.     

Influence of sintering time and quenching on the formation of highT c phase

Influence of sintering time and quenching in Bi_{2 −x} Pb _x Ca₂Sr₂Cu₃O _y (x=0.0, 0.1, 0.2, 0.25, 0.3 and 0.4) samples have been studied by resistance and XRD measurements. In samples sintered at 850°C for 4 days,T _c(0) increases with Pb concentration.T _c(0) increased from 81 K forx=0.0 to 109 K inx=0.30 sample and then decreased. Increasing the sintering time to 10 days decreased theT _c Quenching further decreased theT _c(0). From X-ray diffraction patterns, the intensity peaks of low and highT _c phases have been measured. The addition of Pb promotes highT _c-phase. Sintering time, slow cooling and rapid quenching studies show that there is an optimum sintering time and cooling rate to produce a highT _c-phase.     

Structure and superconductivity in ternary systems of compounds

A survey of the known highT _c ternary systems of compounds is presented. Special emphasis has been placed on the part played by the crystal structure and bonding in the exhibition of highT _c superconductivity. The superconducting and other physical properties of the ternary borides have been discussed in relatively more detail. Finally, the available guidelines from crystal and solid state chemistry points of view, have been reiterated for a search for new highT _c materials.     

Sintering period dependence of composition and superconducting properties of (BiPb)SrCaCu,1.8Ox system

The material system (Bi_0.7Pb_0.3)Sr₁ Ca₁ Cu_1.8O _x forms, at 840° C, two major phases having a highT _c (100K) and a lowT _c (70 K) both of which consist of platelets and a non-superconducting minor phase which has a rod-like shape and is isolated by the major phases. As the sintering period increases, the amount of highT _c phase increases accompanying the decrease in lowT _c phase while the amount of the non-superconducting phase is independent of sintering period, resulting in a superconductor withT _c of 100 K. Changes in compositions of each phase also occur during sintering due to evaporation of bismuth and lead.     

Derivation of a T cequation from the theory of strong coupling superconductors

Based on the theory of strong coupling superconductors, we have derived aT _cequation. This equation is more accurate than, and seems to lack the disadvantages of, otherT _cequations, and has features similar to the exact solution.     

A new look at the highT c superconductors using BCS-theory

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

Study of the impact of HgO addition and low-field magnetic relaxation behaviour in granular high-T c superconductors

A series of high-T _c superconductors have been prepared with the HgO addition/substitution. Significant improvement in theT _conset as well asT _c0 was observed in all the cases. Substitution of Hg at the Sr site and Ba site in the case of (Bi, Pb)-Sr-Ca-Cu-O and Y-Ba-Cu-O systems, respectively over a range 0·01–0·6 at% helps in constructing an entire spectrum: improvement ofT _c0 up to 0·4 at % in the case of Bi-system and up to 0·03 at % in the case of Y123 system and slight drop inT _c thereafter. Such improvement is the result of abundant supply of highly reactive nascent oxygen all through the bulk. HgO decomposes and provides oxygen which helps in maintaining proper oxygen stoichiometry throughout the bulk. No Hg or Hg-based impure phases were observed in the X-ray diffraction spectra. Low-field (10–100 Oe) magnetic relaxation studies reveal faster relaxation of the intergranular critical state in the case of silver added samples as the grain boundary coupling energyE _J becomes quite uniform across the entire bulk which leads to smaller flux pinning energy. The distribution of the pinning energy is evaluated from the observed relaxation pattern and is found to be narrower in the case of silver added samples. It was also observed that the transportJ _c ∼ exp[−ΔT _c/T _c0] and the flux pinning energyU ∼ ΔT _c, where ΔT _c is the transition width and is a measure of the inhomogeneity within the sample. Such relationships may help in devising a strategy for achieving highJ _c, highU yet low ΔT _c. Silver addition turns out to be an effective tool in tailoring the sample properties depending on requirement.     

Collective response of YBa2Cu3O7/Ag weak-link junction arrays

The collective response of superconducting thin-film arrays of 300×300 coupled YBa₂Cu₃O₇/Ag weak-link junctions is investigated as a function of external magnetic fields. Our highT _c arrays, which were fabricated using electron-beam and photolithography techniques, are based on a substrate-induced defect method. In this method, a square array of thin-film Ag lines produces a two-dimensional lattice of junctions in an overlaying thin-film array of highT _c lines. We find that the zero-field resistive transition of the array is governed by a Kosterlitz-Thouless-type transition. Below the super-conducting transition, we observe quantum oscillations in magnetoresistance curves at both integer and half-integer multiples of the flux quantum. These results suggest that long-range phase coherence may be established in large highT _c arrays.     

Dielectric Properties of Na<Subscript>0.7</Subscript>CoO<Subscript>2</Subscript> and of the Superconducting Na<Subscript>0.3</Subscript>CoO<Subscript>2</Subscript>·1.3H<Subscript>2</Subscript>O

We present dielectric properties of ceramic anhydrous Na_0.7CoO₂ and the superconducting Na_0.3CoO₂·1.3H₂O materials. The presence of water which induces superconductivity also may increase the dielectric constant (ε) of the hydrated material. This is consistent with the predicted relationship between the highε and the enhancement ofT _c in highT _c superconductors. The anhydrous sample is porous and the transport is due to some percolation via the pores. The porosity is much higher for the hydrated material and the transport is ionic inside bulk water.     

Preparation and degradation behaviour of Bi-cuprate superconductors

The role of preparation conditions and the effect of addition of Pb in Bi-Sr-Ca-Cu-O (BSCCO) superconductor, on theT _c,J _c and grain orientation have been studied. Calcination at a temperature higher than the melting point of Bi₂O₃ after the prereaction at 800°C leads to formation ofc-axis oriented nearly single phase material. The presence of lead yields a nearly highT _c phase (2223) exhibiting a maximum transition temperatureT _c=110 K and ΔT _c=2 K. The addition of lead and grain orientation together result in an increase inJ _c by nearly two orders of magnitude. The degradation behaviour was studied in terms of changes inT _c,J _c and structure on exposure to atmosphere up to 250 days. Samples with 0.6 Pb were found to be superconducting withT _c=97 K even after 250 days. The highT _c phase was stable against degradation. Our studies indicate that the degradation of leaded BSCCO is a surface phenomenon rather than bulk phenomenon.     

Preparation and characterization of non-random YBa2Cu3O7-Ag composite

Silver addition to the highT _c superconductor, YBa₂Cu₃O₇ (YBCO) is known to improve its mechanical properties and critical current (J _c) characteristics. We have synthesized the superconductor-metal composite YBCO-Ag by an electroless process at room temperature (using sucrose or hydrazine hydrate in an alkaline medium). The latter method produces a non-random coating whereby each grain of YBCO is coated uniformly by silver. The intrinsic highT _c of 90 K of YBCO can be recovered by compaction of the composite and subsequent heat treatment (900°C and 600°C) in oxygen.     

Comment on the specific heat of superfluid 3He

We present numerical results for the specific heat of ³He-B in the weak-coupling-plus model. We find that the full temperature dependence of the specific heat is accurately determined by the specific heat jump at T _c .This provides a test of the T _c /T _F expansion scheme for calculating strong coupling effects in superfluid ³He.     

Charge bag model of superconductivity in intercalated layered compounds

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

Growth of highT c Bi-Sr-Ca-Cu-O thick films

Thick films of Bi-Sr-Ca-Cu-O were deposited on (100) MgO substrates by screen-printing technique with the starting composition 1112. To attain the superconducting state, the films were subjected to two-step heat-treatment. R-T and XRD have been studied for films annealed at different durations of the second step. InitiallyT _c (R=0) increased from 77 to 103 K as the annealing duration was increased after whichT _c decreased. Kinetics of the growth of highT _c phase is discussed in the light of our results.     

Implications of Tc-Variation in UBe13 for a Possible Fulde–Ferrell–Larkin–Ovchinnikov Phase

We measure the heat capacity of a UBe₁₃ sample with an unusually low T _c for a polycrystal. We find an upturn in the upper critical field H _c2(T) below about T _c/2, much as for higher-T _c samples. Comparing the critical fields in our sample and in samples with higher T _c's shows that the low temperature limit of H _c2 is proportional to T _c(H = 0), as expected if the upturn comes from an FFLO phase and strong coupling.     

Superconductivity in rapidly quenched Mo-P-B alloys

Superconducting transition temperatures (T _c) were measured for a series of Mo-P-B alloys in the Mo-rich corner of the ternary system. The alloys were studied in two forms: (a) slowly cooled after induction melting, and (b) rapidly quenched in an arc hammer apparatus. The rapid quenching produced an amorphous structure, but most of the alloys also contained some crystalline material, indicating that this system is not conducive to easy glass formation. Ternary slowly cooled crystalline alloys in the concentration range 20 to 30 at % phosphorus and 10 to 20 at % boron had relatively highT _c values of 9.6 to 9.9 K; higher than for either the Mo-P or Mo-B binary systems.T _c values up to 9.02 K were observed for completely amorphous foils, consistent with the previously reported maximumT _c values for amorphous alloys in this system. This remains the highestT _c reported for a metallic glass. All of the rapidly quenched foils were brittle.     

Superconductivity behaviour of screen-printed LnBa2Cu3O7 (Ln=Eu,Y) films

Thick films of the highT _csuperconducting oxides, LnBa₂Cu₃O₇, Ln = Eu, Y, have been fabricated by screen printing on alumina and SrTiO₃ substrates. Conditions for optimum superconductivity behaviour of the films have been established.T _c ^onset varies from 90–94 K for all the films but zero resistance was observed only in a few cases.     

Effective single-band Hamiltonian for electron-phonon coupling in cuprate superconductors

We derive an effective single-band Hubbard type Hamiltonian for CuO₂ planes. The Hamiltonian includes both electron-electron repulsion and electron-phonon coupling to oxygen vibrational modes. We start with first-principles density functional theory parameters and then map onto a single-band model. Unlike previous mappings to a single-band Hamiltonian, ours explicitly preserves the Fermi surface shape and matrix elements of the many-band Hamiltonian. We consider both in-plane oxygen breathing modes as well as out-of-plane tilting modes. The latter modes have a quadratic electron-phonon coupling, and are also highly anharmonic in La₂CuO₄ based superconductors. The coupling to breathing modes is too small to account for highT _c, while the coupling to quadratic modes is much stronger even though they would be neglected in a standard Migdal-Eliashberg approach to superconductivity.