Concerning the superconducting gap symmetry in YBa₂Cu₃O_{7 − δ}, YBa₂Cu₄O₈, and La_{2 −x}Sr_xCuO₄determined from muon spin rotation in mixed states of crystals and powders

Abstract

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 YBa 2 Cu 3 O 7-δ (  0.05), YBa 2 Cu 4 O 8 and La 2x Sr x CuO 4 (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 YBa 2 Cu 3 O 7-δ and YBa 2 Cu 4 O 8 . For YBa 2 Cu 3 O 7-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 YBa 2 Cu 4 O 8 and La 1.83 Sr 0.17 CuO 4 ). Inconsistencies contained in multiple and noded gap interpretations of crystal data, and observed differences between bulk μ + SR and surface-sensitive measurements are discussed.

Time-reversal invariant and fully gapped unconventional superconducting state in the bulk of the topological compound Nb0.25Bi2Se3

“…This leaves a nodeless or fully gapped state as the most plausible bulk SC pairing state in the bulk of Nb 0.25 Bi 2 Se 3 . It should be noted, though, that saturation of the muon depolarization rate can arise under certain conditions even in nodal superconductors [45][46][47][48][49] and that in such cases extensions beyond Eq. (2) may be required to extract the superfluid density.…”

Charge compensation and optimal stoichiometry in superconducting (CaxLa1−x)(BaHarshman1, Fiory2 2012Phys. Rev. B101340View full textAdd to dashboardCite

“…Results for   /  , where   = 8.514×10 5 G 1 s 1 is the muon gyromagnetic ratio, provide estimates of the superconducting magnetic penetration depth  through the theoretical relation,   /  = (1.26  10 8 G cm 2 )  2 , where for ceramic specimens of (Ca x La 1x )(Ba 1.75x La 0.25+x )Cu 3 O y , as well as YBa 2 Cu 3 O 7-δ ,  2 is obtained as a crystallographic average dominated by the basal (a-b) plane component. [14][15][16] Under optimal experimental and materials conditions, which require a nearly perfectly formed and static lattice of fluxons within a homogeneous clean superconducting material, 17 the zero-temperature limit, denoted as (0), yields the London penetration depth  L = (m*c 2 /4n S e 2 ) 1/2 , reflecting the intrinsic superconducting carrier density n S and an effective mass m*. 18 On the other hand, the presence of disorder and inhomogeneity introduces residual normal conductivity that increases the measured (0) relative to the intrinsic penetration depth (discussed, e.g., in Ref.…”

High-TCsuperconductivity in Cs3C60compounds governed by local Cs–C60Coulomb interactions

“…Another inconsistency is found near the metal-insulator-transition, where NMR T 1 indicates strong coupling for fcc Cs 3 C 60 and Rb 0.35 Cs 2.65 C 60 , whereas weak coupling agrees with the specific heat jump in the alloy [13,49]. While findings of strong coupling are noted hallmarks of high-T C superconductivity [50], such discrepancies remain to be explained.…”