Relationship between combinative energy and superconductivity in Tl2Ba2Can-1CunO2n+4

The non-monotonic relationship of Tc with the number of Cu-O planes (n) per unit cell for compounds of Tl2Ba2Can-1CunO2n+4 (n = 1, 2, 3, and 4) is investigated from the perspective of cohesive energy. The unit cell of the Tl superconductors is treated as two blocks: the perovskite block where the Cu-O planes are located and the rock salt block, which is considered as a charge-reservoir. We proposed a model to calculate the combinative energy of the two blocks. It is found that the combinative energy between the two blocks is closely related to the value of Tc. The relation demonstrates an interesting way to understand the nonlinear change of Tc with the number of Cu-O planes in the layered superconductors. This means that the interaction between the two blocks plays an important role in superconductivity

Structure of Bi2Sr2CaCu2Oy single crystals in different atmosphere

Structures of Bi2Sr2CaCu2Oy single crystals, as grown, annealed in O2 and Ar at 400°C for 48 h, respectively, have been examined by X-ray diffraction. It is found that the crystal after annealing in O2 structurally has good quality, while after annealing in Ar it has bad quality. Most important, multi-layer structure is found from the high resolution rocking curve of the crystal annealed in Ar. Considering the change of the Tc value of the crystals after annealing in different atmospheres, we suggest besides oxygen content, structural modification may play an important role in the superconductivity of Bi2Sr2CaCu2Oy

Similarity of crystalline structure of BSCCO single crystal and multi-layer film

Bi2Sr2CaCu2Oy single crystal was studied by means of high-resolution X-ray diffraction and the local work function measurement with scanning tunneling microscope (STM). The results show that the single crystal is natively inhomogeneous. The rocking curve of 00l reflection indicates that the c-axis lattice constant of the crystal varies in a narrow range, clearly showing a multi-layer character. The reflectivity measurement supports this result. It shows an obvious multi-layer character too. The small angle scattering indicates that although the c-axis lattice constant is little different in the crystal, it shows a long period of about 340 A. The local work function measurement with STM demonstrates that the crystal is inhomogeneous. Some part in the crystal acts like thin film and some part like substrate, supporting the results of X-ray analyses

Significant improvement of flux pinning and irreversibility field of nano-Ho2O3 doped MgB2

Mg1-x(Ho2O3)xB2 alloys are prepared by in situ solid state reaction to investigate the magnetic Ho2O3 dopant on the flux pinning behaviour in MgB2. Ho2O3doping does not change the crystal structure, Tc, and Hc2of MgB2, however, Jc and Hirr have been significantly enhanced. Jc of 1.2 x 105 A/cm2 at 5 K in 5 T field is achieved for the best sample, significantly higher than the values achieved by the non-magnetic impurities, such as Ti-, Zr, Y2O3-doped MgB2. The observed magnetic HoB4 particles with a size between 5 and 10 nm were attributed to be the source for the enhanced flux pinning effects

Grain boundary doping effect on critical current density in YBa2Cu3O7 polycrystalline materials

Diffusion method has been developed to preferentially dope the grain boundaries (GBs) in textured YBa2Cu3O7 bicrystals and polycrystalline materials. Ag, Ca, Fe, Pb, etc. have been used as the dopants in this study. The distribution of the dopants is found to be highly localized around the GBs. The Jc of YBa2Cu3O7 textured polycrystalline samples are significantly enhanced by doping Ag or Ca in the GBs. The Ca doping effect has been explained by charge-carrier-compensation in the GBs, which reduces the GB/junction resistance and thus increases the GB critical current. The Ag doping effect has been explained by the partially repairing of the broken Cu–O bonds, which transforms an extended geometric distortion in CuO2 planes to a localized electronic distortion. Four mechanisms for repairing GB of HTSs are proposed, which are charge-carrier compensation, grain boundary cleaning, weak-link to pinning-center transformation, and concurrent doping

Evaluation of integrals over STOs on different centers and the complementary convergence characteristics of ellipsoidal-coordinate and zeta-function expansions

One-electron integrals over three centers and two-electron integrals over two centers, involving Slater-type orbitals (STOs), can be evaluated using either an infinite expansion for 1/r12 within an ellipsoidal-coordinate system or by employing a one-center expansion in spherical-harmonic and zeta-function products. It is shown that the convergence characteristics of both methods are complimentary and that they must both be used if STOs are to be used as basis functions in ab initio calculations. To date, reports dealing with STO integration strategies have dealt exclusively with one method or the other. While the ellipsoidal method is faster, it does not always converge to a satisfactory degree of precision. The zeta-function method, however, offers reliability at the expense of speed. Both procedures are described and the results of some sample calculation presented. Possible applications for the procedures are also discussed

Self-heating and ac losses in superconductors

Self-heating induced by ac loss in superconductors under different thermal boundary conditions has been investigated numerically. Self-heating effect results in non-uniform distributions of temperature and critical current density in superconductors. The self-heating effect is also sensitive to the thermal boundary conditions. A low-field instability of the critical current density is found when the external magnetic field decreases to near zero after fully penetrating the superconductors, leading to low-field flux jumps as observed in MgB2 and YBa2Cu3O7 superconductors