Evidence for two superconducting components in oxygen-annealed single-phase YBaCuO

• Published in: Cryogenics (Guildford) 1987-09, Vol.27 (9), p.475-480
• Main Authors: Goldfarb, R.B, Clark, A.F, Braginski, A.I, Panson, A.J
• Format: Article
• Language: English
• Subjects: a.c. losses; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Exact sciences and technology; high critical temperature superconductors; lower critical field; Physics; Properties of type I and type II superconductors; Superconductivity; superconductors; susceptibility; Transport properties (electric and thermal conductivity, thermoelectric effects, etc.); YBaCuO superconductors
• ISSN: 0011-2275; 1879-2235
• DOI: 10.1016/0011-2275(87)90108-1

The complex susceptibility of a sintered YBaCuO superconductor is strongly dependent on a.c. field amplitude, h. Very small values of h must be used for the real part of susceptibility, χ′, to reach a value corresponding to bulk diamagnetism just below the critical temperature, T c. The imaginary part, χ″, represents hysteresis loss in the sample. Thus, χ″ versus temperature becomes positive when h exceeds the lower critical field, H cl of the superconductor. Annealing the material in oxygen gives rise to two distinct components, a relatively high- T c, high- H cl superconductor (denoted as ‘G’ or ‘good’) and a relatively low-T c, low- H cl superconductor (denoted as ‘B’ or ‘bad’). Curves of susceptibility versus increasing temperature reflect the dual nature of the annealed sample: χ′ has an inflection point at T c of the B component and approaches zero at T c of the G component, while χ″ has a peak at each T c. Both critical temperatures decrease linearly with increasing h, though at very different rates. H cl of the G component is considerably greater than H cl of the B component. The lower critical fields are linearly decreasing functions of temperature. Two models might explain the susceptibility data. In the grain model, the G component consists of superconducting grains and the B component is either intergranular material, unfavourably orientated anisotropic grains, or oxygen-depleted grain boundaries. In the surface model, the G component is in the interior of the sample and the B component is at the sample's surface. This condition could arise if there was oxygen depletion at the surface subsequent to total enrichment during annealing.