arXiv subjects

Anthony T. Fiory

Publications and source records attributed to Anthony T. Fiory.

21 records · Page 2Linked to original sources

arXiv2012

Theory of High-Tc Superconductivity: Transition Temperature

It is demonstrated that the transition temperature (Tc) of high-Tc superconductors is determined by their layered crystal structure, bond lengths, valency properties of the ions, and Coulomb coupling between electronic bands in adjacent, spatially separated layers. Analysis of 31 high-Tc materials (cuprates, ruthenates, rutheno-cuprates, iron pnictides, organics) yields the universal relationship for optimal compounds, kBTc0 = β/\ellζ, where \ell is related to the mean spacing between interacting charges in the layers, ζ is the distance between interacting electronic layers, β is a universal constant and Tc0 is the optimal transition temperature (determined to within an uncertainty of +/- 1.4 K by this relationship). Non-optimum compounds, in which sample degradation is evident, e.g. by broadened superconducting transitions and diminished Meissner fractions, typically exhibit reduced Tc < Tc0. It is shown that Tc0 may be obtained from an average of Coulomb interaction forces between the two layers.

cond-mat.supr-con↗

arXiv2012

High-Tc Superconductivity in Ultra-thin Crystals: Implications for Microscopic Theory

High transition temperature (high-Tc) superconductivity is associated with layered crystal structures. This work considers superconductivity in ultra-thin crystals (of thickness equal to the transverse structural periodicity distance d for one formula unit) of thirty-two cuprate, ruthenate, rutheno-cuprate, iron pnictide, organic, and transuranic compounds, wherein intrinsic optimal (highest) transition temperatures Tc0 (10 - 150 K) are assumed. Sheet transition temperatures Tcs = αTc0, where α < 1, are determined from Kosterlitz-Thouless (KT) theory of phase transitions in two-dimensional superconductors. Calculation of α involves superconducting sheet carrier densities NS derived theoretically from crystal structure, ionic valences, and known doping, a two-fluid model for the temperature dependence of the superconducting magnetic penetration depth, and experimental data on KT transitions; α is on average 0.83 (varying with standard deviation 0.11). Experiments on several thin crystal structures of thickness dF approaching d are shown to be consistent with calculations of Tc0 from microscopic superconductivity theory and with Tcs from KT theory, where the presence of disorder is also taken into account; careful analyses of these thin film studies indicate a minimum thickness dF \approx d for superconductivity

cond-mat.supr-con↗

arXiv2012

Concerning the superconducting gap symmetry in YBa2Cu3O7-δ, YBa2Cu4O8, and La2-xSrxCuO4 determined from muon spin rotation in mixed states of crystals and powders

Muon spin rotation (μ+SR) measurements of square-root second moments of local magnetic fields σ in superconducting mixed states, as published for oriented crystals and powder samples of YBa2Cu3O7-δ (δ {\approx} 0.05), YBa2Cu4O8 and La2-xSrxCuO4 (x ~ 0.15-0.17), are subjected to comparative analysis for superconducting gap symmetry. For oriented crystals it is shown that anomalous dependences of σ on temperature T and applied field H, as-measured and extracted a- and b-axial components, are attributable to fluxon depinning and disorder that obscure the intrinsic character of the superconducting penetration depth. Random averages derived from oriented-crystal data differ markedly from corresponding non-oriented powders, owing to weaker influence of pinning in high-quality crystals. Related indicators for pinning perturbations such as non-monotonic H dependence of σ, irreproducible data and strong H dependence of apparent transition temperatures are also evident. Strong intrinsic pinning suppresses thermal anomalies in c-axis components of σ, which reflect nodeless gap symmetries in YBa2Cu3O7-δ and YBa2Cu4O8. For YBa2Cu3O7-δ, the crystal (a-b components, corrected for depinning) and powder data all reflect a nodeless gap (however, a-b symmetries remain unresolved for crystalline YBa2Cu4O8 and La1.83Sr0.17CuO4). Inconsistencies contained in multiple and noded gap interpretations of crystal data, and observed differences between bulk μ+SR and surface-sensitive measurements are discussed.

cond-mat.supr-con↗