Anisotropic superconductivity in the Emery model

We have investigated anisotropic superconductivity originating from intersite pairing between holes in nearest and next nearest neighboring sites in the Emery model. Strong local Coulomb correlations among holes in copper orbitals have been taken into account within the Hubbard I approximation scheme. The superconducting transition temperature has been evaluated as a function of the hole concentration. It has been shown that with the onset of superconductivity, pairing among oxygen-like quasiparticles in the mixeds-wave+d-wave channel plays the dominating role, being replaced by pairing in the extendeds-wave channel for higher concentration of holes. Superconducting correlations are mostly effective for a rather narrow range of the model parameter values, close to values derived from band structure calculations. Therefore, the coupling betweens-wave andd-wave channels seems to be a general feature of superconductivity in CuO₂ planes.     

Room-Tc Superconductivity on Whispering Mode in Quasi-1D Composite of Superconducting Nanotubes: Is It Possible?

Combining Little's and Ginzburg's ideas with recent progress in nanotubes research, a novel type of material is advanced as a perspective high-T _c superconductor on a base of a close-packed lattice of quasi-1D superconducting nanotubes. Idea is offered that superconducting coaxial multilayer nanotubes of the correlation length in diameter is an ideal and natural trap for pinning of Abrikosov vortex. Nanotube should be layered superconductor, such as LuNiBC. Mechanism of superconductivity was proposed and substantiated quantitatively on a base of a whispering mode, which is shown to be responsible for a strong enhancement of electron–phonon interaction and for an increase of critical temperature. Nanocomposite built from such quasi-1D nanotubes when coinciding with vortex lattice provides ideal conditions for the pinning, resonance, distortion, ordering, and Little–Parks effects, the joint action of which is suggested to result in synergetic effect increasing the superconductivity. Such quasi-1D or 2D nanotubular crystal is proposed to synthesize by template approach using zeolite-like membrane.     

Lattice Effects in High-Temperature Superconducting Cuprates Revealed by Thin Film Single Crystals

In contrast to conventional superconductivity where phonons lead to the formation of Cooper pairs, in high-temperature superconductivity (HTSC), the role of electron–phonon coupling has long been neglected. The in-plane Cu–O bonds in HTSC cuprates show unconventional broadening at low temperature as carriers are doped. Here, we focus on the high-quality polarized X-ray absorption spectroscopy (XAS) data for a model HTSC system, (La,Sr)₂CuO₄ (LSCO). Thin film single crystal samples were prepared by state-of-the-art MBE, precisely controlling compositions. High-quality data was obtained by use of a segmented X-ray detector. The in-plane Cu–O radial distribution function (RDF) in LSCO (x=0.15) shows broadening as temperature is lowered, which shows a sharp drop at the critical temperature, which is followed by a gradual increase (disorder). Comparing the data with resistivity, we find a remarkable coincidence between the sharpening and the onset of superconductivity. Since the sharpening of RDF is interpreted as correlated motion of oxygen atoms (phase coherence due to superconductivity), the results demonstrate that the superconducting state directly relates to the unconventional oxygen displacements in a bond stretching mode. The results will be discussed in relation to local models of distortion of the different nature (metallic vs. insulating), that is, strongly influenced by strain.      

Superconductivity characterization by a magnetic pendulum

A magnetic pendulum device was designed, developed, and used to characterize high-T_c materials. A correlation between the volume fraction of superconducting phase in a composite and the force of expulsion was found. The device is sensitive enough to detect a volume of 0.5% superconducting phase in a composite. The application of the device for characterizing composites, incipient superconductivity, and films has been discussed.     

Crystal-chemical anomalies in nonsuperconducting PrBa2Cu3O7

Studies of the crystal chemistry of nonsuperconducting PrBa₂Cu₃O₇ indicate that this compound is strictly isostructural with its superconducting RBa₂Cu₃O₇ (R = Y, rare earth) analogs. Crystallographically, Pr is present in the trivalent state according to the structural trends exhibited by the RBa₂Cu₃O₇ series as a function of R³⁺ ionic radius. The sole structural anomaly attributable to the presence of Pr³⁺ in the YBa₂Cu₃O₇ structure is a next-next-nearest neighbor effect and consists of an unusually short axial Cu-O distance, i.e., a short bond length between the in-plane copper and the chain oxygen. The correlation of this anomaly with the nonmetallic/nonsuperconducting properties of PrBa₂Cu₃O₇ supports a variety of literature reports, both theoretical and experimental, suggesting that the apical oxygen in the YBa₂Cu₃O₇ structure plays a critical role in mediating the appearance of superconductivity. The mechanism by which the f-electrons in Pr³⁺ (f ²) interact with the Cu-O manifold to produce the nonmetallic behavior of PrBa₂Cu₃O₇ remains unknown; however, superconductivity is turned back on for Nd³⁺ (f ³), immediately next to Pr and just slightly smaller. Careful comparative studies of superconducting NdBa₂Cu₃O₇ and nonmetallic PrBa₂Cu₃O₇ are needed to elucidate the critical difference in the behavior of the f-electrons and may shed light on the fundamental mechanism of high-temperature superconductivity in copper oxides.     

Cu0.5Tl0.5Ba2Ca3Cu4−y Zn y O12−δ (y=0, 1.0, 2.0, 3.0, 3.5): Superconductor with Four ZnO2 Planes

A new Cu_0.5Tl_0.5Ba₂Ca₃Cu_4−y Zn _y O_12−δ (y=0, 1.0, 2.0, 3.0, 3.5) superconductor with four ZnO₂ planes is reported. The structure of the material remains tetragonal for all Zn doping concentration. The substitution of Zn at CuO₂ planar site was carried out following Cu_0.5Tl_0.5Ba₂Ca₃Cu_4−y Zn _y O_12−δ (y=0, 1.0, 2.0, 3.0, 3.5) formula. Contrary to all previous studies of Zn doping in all copper oxide high temperature superconductors, the zero resistivity critical temperature T _c(R=0), critical current density and quantity of diamagnetism increase with increased Zn concentration. The onset temperature of superconductivity in these samples was observed at 128 K and T _c(R=0) at 122 K for y=3.5. The volume of the unit cell observed through X-ray diffraction scan is found to decrease with increase Zn doping; promoting an increase in Fermi vector K _F and effective density of states which results in enhanced superconductivity parameters. The synthesis of Cu_0.5Tl_0.5Ba₂Ca₃Cu_4−y Zn _y O_12−δ material by this method is highly reproducible.      

Structural instabilities and superconductivity in La-214 compounds

In La_2–x Ba _x CuO₄ (LBCO) the transition to a low-temperature tetragonal phase and the suppression of superconductivity occur at the carrier concentration p 1/8 per copper. We will discuss the roles of various material parameters that control this instability. An unusual lattice softening has been found by ultrasonic measurement on La_2–x Sr _x CuO₄ (LSCO). This softening is present only in an in-plane shearing mode and is ascribed to the growth of structural fluctuations in the normal state.These phenomena are closely related because both the structural change in LBCO and the applied strain in LSCO lift the degeneracy of in-plane oxygen sites. They clarify the importance of strong coupling between the normal-state electronic system and the lattice by a Peierls-type mechanism.     

Effect ofin Situ cooling on superconductivity of artificially layered BSCCO films with 2223 structure

Thin BSCCO films with the 2223 structure were deposited onto MgO substrate kept at 665°C by applying a sequential sputtering method using metallic Bi and Sr-Ca-Cu-O targets. The resultant films were then cooled under various oxygen partial pressures and cooling rates. It was found that the oxygen partial pressure and substrate temperature during cooling affect significantly the development of superconductivity of the as-deposited films. When the films were cooled stepwise under a critical oxygen partial pressure around 0.3 torr, the as-deposited films showed either superconductivity or nonsuperconductivity behavior. HRTEM images of the films showed no essential difference in the modulated structure, suggesting that the excess oxygen in the Bi-O double layers is the same and not the cause of the different superconducting behavior in the as-deposited films.     

151Eu Mössbauer studies on Zn-doped EuBa2Cu3O7−y

¹⁵¹Eu Mössbauer studies have been performed on the compounds EuBa₂(Cu_1?x Zn _x )₃O_7?y withx=0·0, 0·025, 0·05, 0·075 and 0·1. The parent compound, EuBa₂Cu₃O_7?y is superconducting with a transition temperature (T _c ) of 88 K.T _c is depressed as Zn is substituted for Cu in this system and the compounds withx>0·05 do not show superconductivity down to 12 K.¹⁵¹Eu Mössbauer studies at 295 K show a single Mössbauer line in all the compounds (whether superconducting or not) with isomer shift value typical of Eu^{3 +} ion. Further, the isomer shift values are nearly independent ofx and the temperatures down to 10 K. These observations imply that the Cu-O network responsible for superconductivity is very weakly coupled to the Eu sublattice.     

Comparison of Superconductivity and Structure for Lanthanum Replacing for Barium and Copper in YBa2Cu3Od

X-ray diffraction studies and the resistivity measurements are used to characterize the structure and the superconductivity of the nominal composition of YBa₂Cu_{3 – x} La _x O _d (YBCLO) cuprates with x 0.30. There was a BaCuO₂ impurity phase detected with x 0.10. The structure of the main phase (123) has the orthorhombic form with Pmmm symmetry in the whole doping range. With increasing content of lanthanum, x, the lattice constants increase for x < 0.04, and decrease for x 0.04. Rietveld refinements for X-ray diffraction show that the dopant of lanthanum substitutes for copper in the lower doping level, and replaces for both barium and copper in the high doping level. The zero-resistance temperature T _c0 first increases with the increase of the content of lanthanum in YBCLO as x 0.04 and then decreases with x as x 0.04. We compared the results with those of La-doped YBa_{2 – z} La _z Cu₃O _y cuprates. The different relationship in superconductivity dependence of lanthanum content may result from the strains due to the different occupancy of lanthanum in the unit cell of YBa₂Cu₃O _d .