Hg-based superconductors of HgBa 2 Ca 2 Cu 3 Re x O y ( x =0.15 denoted as Hg–Re0.15, x =0.18 as Hg–Re0.18 and x =0.2 as Hg–Re0.2) were fabricated by high vacuum sealing and sintering method using a precursor. Grain alignment was formed magnetically under external magnetic field of 10 T. AC susceptibilities and DC magnetizations were measured using a SQUID magnetometer and a PPMS susceptometer at temperature range 5–150 K under magnetic fields up to 14 T. The critical temperatures are 132 K for Hg–Re0.15, 130 K for Hg–Re0.18 and 133 K for Hg–Re0.2 determined by the temperature dependence of AC susceptibilities. The imaginary parts of AC susceptibilities show the double peak structure at low magnetic fields. This double peak structure is confirmed to suggest different depinning mechanisms. The irreversibility fields B irr estimated from the upper peaks of imaginary parts in Hg–Re0.18 sample for B ∥ ab are the largest. Theoretical analysis using the flux creep theory based on a depinning mechanism caused by thermally activated flux creep is favorably compared with the temperature dependence of B irr .

Temperature dependence of irreversibility fields in Re doped Hg-1223 superconductors

Expressions for renormalized phonon density of states (RPDOS), renormalized electron density of states (REDOS), phonon heat capacity (PHC) and electronic heat capacity (EHC) have been derived based on quantum dynamical many body double time thermodynamic Green’s function (GF) theory of phonons and electrons using the effects of phonons, electrons, point impurities and anharmonicities. The theory thus developed is applied to the representative high temperature superconductor (HTS) Y Ba 2 Cu 3 O 7 − δ through numerical computation. It is found that the PHC can be resolved into diagonal and non-diagonal contributions, which are significantly influenced by defects and anharmonicities. The EHC contribution is much smaller than that of PHC. A successful agreement between theory and experiment has been observed for Y Ba 2 Cu 3 O 7 − δ . The presence of electron-phonon coupling constant in each term of EHC is an additional and new feature of the theory.

The phonon and electron heat capacities of cuprate superconductors

The upper boundary of the pseudogap state has been derived as a function of the doping within the impurity mechanism of high-T
 c
 superconductivity under the assumption that, in the region bordering the superconducting phase on the side of the doping less than the optimum level (the pseudogap region), there exist finite superconducting clusters and the boundary of the superconducting phase corresponds to the threshold of the existence of an infinite superconducting cluster. The position of this boundary is in agreement with experiment. The condition imposed on the doping level at which the giant proximity effect should be observed has been derived. It means essentially that the thickness of the nonsuperconducting layer should be small compared to the average size of the superconducting clusters.

Superconducting clusters in the pseudogap region

The impurity mechanism proposed earlier is used to interpret the magnetic properties of high-temperature superconductors. The appearance of superconductive granules and the existence of weak bonds in grains of ceramics and in single crystals is explained. The irreversibility line is obtained and its new interpretation as the line of thresholds of appearance of infinite nonsuperconductive cluster is given. The relaxation time of the magnetic susceptibility in alternating magnetic fields is calculated. The decrease of the Meissner fraction with increase in magnetic field and decrease in doping, and other magnetic properties are explained.

Magnetic properties of high-temperature superconductors: Analysis within the framework of the impurity mechanism

A new interpretation of photoemission spectroscopy data is proposed. It is based on the impurity mechanism of high-temperature superconductivity proposed earlier. The band observed near the Fermi level in this interpretation is the impurity band, and the observed large “Fermi surface” is the surface of the mobility edge, different in different directions. Many other details are also explained naturally.

Photoemission spectroscopy of high-Tc superconductors : Proofs in favor of the impurity mechanism

The impurity mechanism proposed earlier is used to interpret the heat properties of hightemperature superconductors. The expression for the heat-capacity jump describing its temperature and magnetic field dependences is obtained. An explanation of the dependence of the Sommerfeld constant on the stoichiometric composition is proposed.

I. A. Chaban

Papers

Heat capacity of high-temperature superconductors: Analysis within the framework of the impurity mechanism