Excess conductivity of Cu₀.₅Tl₀.₅Ba₂Ca₃Cu₄₋yZnyO₁₂₋δ superconductors

• Опубліковано в: : Физика низких температур
• Дата: 2012
• Автори: Khan, Nawazish A., Hasnain, S.M.
• Формат: Стаття
• Мова: Англійська
• Опубліковано: Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України 2012
• Теми: Свеpхпpоводимость, в том числе высокотемпеpатуpная
• Онлайн доступ: https://nasplib.isofts.kiev.ua/handle/123456789/116831
• Назва журналу: Digital Library of Periodicals of National Academy of Sciences of Ukraine
• Цитувати: Excess conductivity of Cu₀.₅Tl₀.₅Ba₂Ca₃Cu₄₋yZnyO₁₂₋δ
 superconductors / Nawazish A. Khan, S.M. Hasnain // Физика низких температур. — 2012. — Т. 38, № 1. — С. 28-38. — Бібліогр.: 31 назв. — англ.

Oxide high-Tc superconductors (HTSC) are anisotropic in character since the charge carriers have free moment
 in the conducting CuO₂ planes [1] whereas their motion is impeded by insulating/partially insulating
 MBa₂O₄₋δ (M = Y, Bi, Hg, Tl, CuTl, etc.) charge reservoir layers. In the transport process the charge carriers
 have to tunnel across insulating/partially insulating barriers along the c-axis and across the grain boundaries,
 which promote a fluctuation in the order parameter and in turn to the conductivity of the carriers. The studies of
 such fluctuation conductivity (FIC) may help in understanding the intrinsic mechanism of superconductivity.
 Here the electrical resistivity ρ(T) versus temperature data of as-prepared and oxygen post-annealed
 Cu₀.₅Tl₀.₅Ba₂Ca₃Cu₄₋yZnyO₁₂₋δ (y = 0, 0.5, 1.5, 2.5) samples is studied for FIC analyses in the temperature regime
 well above the critical temperature; such analyses have been carried out by employing Lawrence and
 Doniach (LD) and Maki–Thompson (MT) models. The coherence length, inter-plane coupling, exponent, dimensionality
 of fluctuations and the phase relaxation time of the carriers are determined from such analyses. It is observed
 that the crossover temperature associated with two distinct exponents fits very well with the two-dimensional
 (2D) and three-dimensional (3D) LD equations. The crossover temperature T0 is shifted to higher temperatures
 with enhanced Zn doping. The 3D LD region is shifted to higher temperature with the increased Zn doping. We
 have elucidated from these analyses that lower Tl content in the final compound may increase the charge carrier’s
 doping efficiency of MBa2O4–δ charge reservoir layer, resulting into an increase in the coherence length
 along the c-axis and superconductivity parameters. A small decrease in the coherence length along the c-axis
 ξc(0) is observed in the samples with Zn doping of y = 1.5 whereas ξc(0) increases in the samples y = 0.5, 2.5. In
 comparison with as-prepared samples, the ξc(0) decreases after post-annealing in oxygen atmosphere. It is most
 likely that a decrease in the density of charge carrier’s is promoted by oxygen diffusion in the unit cell may suppress
 the ξc(0). The increase oxygen diffusion is evidenced from the softening of phonon modes after postannealing
 in oxygen atmosphere. The decreased population of small spins of Cu atoms induced by doping of Zn
 is viewed in the terms of suppression of spin gap and hence the pseudo-gap in Cu₀.₅Tl₀.₅Ba₂Ca₃Cu₄₋yZnyO₁₂₋δ
 (y = 0, 0.5, 1.5, 2.5) samples.