D D Shivagan

Simplest nontoxic double-layered cuprate Ba2CaCu2O4(O,F)2 superconductor with a transition temperature of 108K

We report synthesis and properties of the simplest double-layered cuprate superconductor Ba2CaCu2O4(O,F)2. A superconducting transition temperature (Tc) has been achieved as high as 108K for a sample with a nominal fluorine content y=1.6 in Ba2CaCu2O6−yFy. The Tc is the highest among double-layer cuprates that are free of toxic elements such as Hg, Tl, and Pb so that the nontoxic Ba2CaCu2O4(O,F)2 will be promising for application. As y decreases from approximately 2, the a-axis length decreases and the Tc increases from 40to108K. The material is underdoped at large y and as y decreases, the doping moves toward an optimally doped state.

Phase diagram of a lattice of vortex molecules in multicomponent superconductors and multilayer cuprate superconductors

The Abrikosov lattice in the multilayer cuprate superconductor CuBa2Ca3Cu3Oy (Cu-1223) has been experimentally and theoretically demonstrated to be composed of vortex molecules. Cu-1223 is considered to be a typical multicomponent superconductor. We show that in such a system the rotational freedom around the axis of the vortex molecular tube generates orientational disorder and the orientational glass (or crystal) phase, which is never present in conventional vortex lattices consisting of axisymmetric vortices. The emergence of the orientational glass phase and orientational order phase with orthorhombic distortion is a general property of vortex molecule lattices of the multiband type of multicomponent superconductors.

Irreversibility line and flux pinning properties in a multilayered cuprate superconductor of Ba2Ca3Cu4O8(O,F)2 (Tc = 105 K)

Irreversibility line (IL) and flux pinning properties were investigated for a Ba2Ca3Cu4O8(O,F)2 (F-0234) multilayered cuprate superconductor with a Tc of 105 K. The intragrain critical current density (Jc) and irreversibility field (Birr) were determined by using Beans critical state model for the grain-aligned sample (nominal composition Ba2Ca3Cu4O8.7F1.3). The irreversibility line (IL) of F-0234 is much lower than that of (Cu,C)Ba2Ca3Cu4Oy ((Cu, C)-1234) and HgBa2Ca3Cu4Oy (Hg-1234) in spite of the spacing between the superconducting blocks of F-0234 (7.3 A) being much thinner. The double logarithmic plot of Birr field versus [1−(T/Tc) ] analysis hints that the flux line melting model has been adopted. An anisotropy factor of 65 was calculated from a 3D to 2D crossover field of about 0.95 T. Due to the high anisotropy of this system, a low IL has resulted. The flux pinning force density Fp ( ≈JcB) exhibits scaling behaviour when the magnetic field B is normalized by the Birr field. Analysis of the normalized pinning force reveals that a surface pinning mechanism is dominant and the reduced magnetic field bmax = 0.2 agrees with surface pinning mechanism with closely spaced pins.

Iodine intercalation into Ba2Ca3Cu4O8(O,F)2 multilayered superconductors

We have synthesized an iodine-intercalated superconductor IxBa2Ca3Cu4O8(O,F)2. The intercalation was performed by heating a host compound of Ba2Ca3Cu4O8(O,F)2 with I2 in an evacuated quarts tube. The Tc significantly increased from 82 K to 121 K by the iodine intercalation. The Tc is the highest in four CuO2 layered cuprates excluding Hg- and Tl-system. The iodine will be intercalated between Ba-(O,F) layers. Composition analysis indicated that nearly stage-I iodine intercalation (about χ = 0.9) was realized. The iodine atoms expand the spacing between the double Ba-(O,F) layers by about 4.0 A which is comparable to the value of 3.5 A for the iodine intercalation to Bi-system.

Material synthesis of HgBa2Can-1CunOy multilayered cuprates under high pressure

Multilayered cuprates having crystallographically inequivalent CuO2 planes are worthful materials to realize the mechanism of high-Tc superconductivity as well as a stage to ascertain new phenomena. The material synthesis of HgBa2Can-1CunOy multilayered cuprates by employing high-pressure synthesis technique is demonstrated. A starting mixture with a composition ratio of Hg:Ba:Ca:Cu:O = 1:2:n-1:n:2n+2+? was synthesized by mixing the precursor of Ba2Ca2Cu3Oy (y~7.5) with an appropriate amount of HgO, Ca2CuO3, CuO and Cu2O (or Cu). The oxygen content of the mixture was adjusted incisively by ascertaining a ratio of CuO, Cu2O (or Cu) to obtain samples. Almost single-phase samples up to n = 6 were synthesized. The X-ray diffraction analysis of the grain-aligned sample with nominal compositions of n ? 7 revealed that the sample simultaneously comprises from n = 6 up to 16 phases.

Critical current densities and irreversibility field of high-Tc Ba2Ca3Cu4O(O,F)2 superconductor

The superconductivity in apical fluorine system of Ba2Ca3Cu4O10-yFy: the fourth member of Ba2Can-1CunO2n(O,F)2 homologous series is significant to study as it deals with the novel phenomenon of superconductivity like co-existence of superconductivity and antiferromagnetism, in-equivalent and non-similar charge distribution in the CuO2 planes in a unit cell etc. The polycrystalline samples of Ba2Ca3Cu4O8(O,F)2 (F-0234) were synthesized under high pressure and showed sharp superconducting transitions temperature at 105 K. From the DA magnetization hysteresis loops, we determined the intragrain critical current density (Jc) and irreversibility field (Birr) using Beans critical state model. The irreversibility line(IL) of F-0234 is higher than that for optimally-doped Bi-2212. However, IL much lower than that of Cu-1212 (Y-123) although the spacing of F-0234 (7.3 A) is much thinner than that of Cu-1212.

Vortex dynamics in Hg-based multi- and super-multi-layered cuprates

Hg-based multi- and super-multi-layered cuprates HgBa2Can-1CunOy (Hg:12(n-1)n) (n?4) are composed of the charge reservoir layer (CRL) HgBa2Ox, and of CuO2 planes: outer planes (OP) with pyramidal oxygen coordination and inner planes (IP) with square oxygen coordination, with different charge distribution. This fact gives rise to very interesting phenomena, like coexistence of superconductivity (SC) in OPs with antiferromagnetism (AF) in IPs in Hg:1245. The dependence of Tc on the number n of CuO2 planes is also interesting: for n increasing from 4 to 5 and to 6, Tc decrease from 125 to 108 and then to about 103 K; further increase in n keeps Tc constant at about 103 K. In Hg:1245 we show that the AF ordering at TN of 60 K has no influence on vortex dynamics, and there is a quite robust c-axis supercurrent. Vortex melting lines of various samples showed that Hg-based multi-layered cuprates (n = 4, 5) behaves in the usual way (increasing n resulted in decrease of Josephson coupling (JC) and a less sharp melting line) and are well described by the usual anisotropic GL-based theory. Samples with n ? 6 have all the same melting line, suggesting that, in this case, the short-range JC becomes much less important than the long-range magnetic coupling (MC). However, since the JC between two pancakes in OPs of adjacent unit cells (pancakes separated by the thin CRL) is large, we suggest that the vortex matter in Hg-based super-multi-layered cuprates is composed of magnetically-coupled pancake-vortex-molecules.