HTSC Cuprates Research Articles

On the synthesis and characterization of superconducting films prepared through spray pyrolysis

HTSC films have been synthesized by reacting Ba - Ca - Cu - O precursor films deposited by spray pyrolysis with Hg - Pb vapour in an evacuated sealed quartz tube at for 4 h. The films with x ranging from 0.1 to 0.3 exhibit high-temperature superconductivity at temperatures falling in the range from 115 to 125 K. The sample with optimum doping of Pb of x = 0.2 shows the highest . Ag admixing at low concentrations has been found to improve . Thus for , i.e. , decreased minimally from 125 K to 124 K and the critical current density has been found to increase to from . However, higher Ag concentrations, e.g. (y > 0.05), in contrast to the case of other HTSC cuprates, have been found to be detrimental to superconducting properties, particularly . The microstructural details as revealed through scanning electron microscopy exhibit the curious characteristic of the existence of spiral-like growth features for lower Ag concentrations, which are found to disappear for higher Ag concentrations.

The Strong Electron Correlation Effects in XAFS Spectra of HTSC Cuprates

Theoretical XAFS spectra of highly correlated copper oxides have been presented as a product of a single-electron part obtained by the SCF Xa- SW method, and a multi-electron part obtained by the multi-band multi-electron p-d model. The main line of CuK-spectrum of La 2 CuO 4 was aligned to the Cud 10 L - configuration and the single satellite line was aligned to the Cud 9 . Comparison of the theoretical data with the experimental ones shows that the ground state of the two-holes in the CUO 4 doped cell is triplet. The last conclusion has been confirmed by analysis of CuL 2,3 - spectra of two-hole impurity states of doped CUO 4 cell. Another interesting future of CuL 2,3 - spectra - a peak 2.8 eV higher the threshold was aligned to the transitions into quasi-stationary states due to existence of a high barrier in the Cud- state potential.

Lattice effects in HTSC cuprates observed by neutron scattering

Various local structural distortions have been observed in HTSC cuprates by pulsed neutron PDF analysis, while the strong softening of the zone-boundary LO phonon due to doping was observed by inelastic neutron scattering. It is suggested that they may share the origin in the strong electron-lattice coupling enhanced by electron correlation. Such a strong coupling could be relevant to the mechanism of superconductivity.

Anharmonical bistability of the oxygen sublattice as a reason for high T c and thermal hysteresis in HTSC cuprates

The hysteresis seen experimentally in the thermal conductivity of both superconducting and nonsuperconducting YBa 2Cu 3O 7−x is explained in terms of a bistable behavior of the sublattice of O(4) apical oxygen atoms.

Energy and length scales in the superconducting phase diagram for HTSC cuprates

The universal phase behaviour of high T c superconductors is considered in a space of temperature, field and hole-concentration and its essential features are explained in terms of several energy and length scales.

Phase separation and dielectric correlations in HTSC

A general description of the phase separation (PS) in HTSC is presented based on a concept of “soft” dielectric ordering with respect to the addition of collectivized charge carriers. The possibility of the PS crucially depends on the properties of the screening ion subsystem. The interaction of electron and ion subsystems determines a large variety of manifestations of the PS under different experimental conditions. It is shown that the assumption of the existence of the PS makes it possible to explain on common grounds a number of experiments on different HTSC, which seem to be quite separate from each other, such as anomalous magnetoresistance at T< T c in Ba 1- x K x BiO 3, the appearance of midgap states in cuprate HTSC, the existence of two plateaus in the dependence of T c on composition x in YBa 2Cu 3O 6+ x , the appearence of photoinduced superconductivity in yttrium superconductors and others.

Nonlinear excitations in the polarization layers of HTSC's

For layered cuprate HTSC's, various deformation potentials are calculated on the basis of LDA-density functional theory. The special role of apec and chain oxygens is stressed. Using these results bipolarons within a “diatomic” one-dimensional model for the M xO y chains present in the doping subsystem of the HTSC's (M = Cu, Bi, TI) are considered in connection with a recently proposed s-channel mechanism.

Rb 3C 60 seen as a further example of chemical “negative-U” high temperature superconductivity : Comparison and contrast with the bismuthates and cuprates

Through a comparison of the electronic structure of C 60 with diamond and graphite, the means whereby “negative U” driven superconductivity can arise in Rb 3C 60, etc. is expounded. In these materials the action is seen as resulting from the filling of antibonding homopolar bonds, a significantly different though related action to that seen as occuring in the cuprate HTSC materials. There it has been earlier proposed that the action results from filling of the antibonding pd σ∗d x 2- y 2 orbital and the resulting shell closure. A short outline is given of the status of - U theoretical modelling.

Anomalous magnetic behaviours of the cuprate superconductors-Meissner effect Hc1 and Hc2

In order to elucidate whether the superconductivity of the cuprate HTSC is 'normal' or 'abnormal', systematic studies have been performed on the Meissner effect, lower and higher critical fields. The Meissner fraction is strongly dependent on the size of sample and on the field, which is explained by a novel model considering the sharp increase in the vortex pinning force while the flux expulsion process proceeds as T is lowered. Another new method has been proposed to determine Hc1 from the magnetization curve above Hc1, based on which the abrupt upturn of the Hc1-T curve frequently reported in the low T region has been attributed to the error induced by the strong pinning. The ambiguity associated with the Hc2-T curve is discussed in terms of the 'giant fluctuation' of superconductivity.

Superconducting gap equation for strongly energy dependent gap functions

A two-band model consisting of dθ and dε states, for the HTSC cuprates, leads to a negative value of the McMillan coupling constant λ. With a Frohlich-type potential, this yields physically acceptable, oscillating solutions for the gap function. We use the Green's function method to justify the use of this kernel. Properties of the solution are summarized.