Critical regime and suppression of the pseudo-gap in Cu0.5Tl0.5Ba2Ca3Cu4−yZnyO12−δ superconductors via excess conductivity analyses

Abstract

We have doped Zn at the Cu planar sites in Cu0.5Tl0.5Ba2Ca3Cu4−yZnyO12−δ (y=0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0) superconductors, which decrease the population of small spins of Cu atoms and most likely suppress the anti-ferromagnetically aligned spins in the inner-CuO2 planes (IP). For the observation of such effects, fluctuation induced conductivity (FIC) analyses of the above mentioned samples have been carried out using the Aslamazov–Larkin (AL) theory in the critical regime closer to the transition temperature (Tc) and the Lawrence–Doniach (LD) model closer to Tc and above. The idea behind these analyses is the fact that Cooper pair formation starts well above Tc. The influence of the Cooper-pairs on the normal electrons is determined by the Maki-Thompson (MT) model. From these analyses we have determined the coherence length along the c-axis (ξc(0)), the inter-plane coupling constant (J), the phase relaxation time of the carriers (τφ), the Fermi velocity (VF) of the carriers and the energy required to break apart the electrons pairs (Epb). Using the Ginzburg number (NG) and the Ginzburg–Landau (GL) equations, the thermodynamic critical magnetic field (Bc(0)), the lower critical field (Bc1(0)), the upper critical field (Bc2(0)), the critical current density (Jc(0)) and the penetration depth (λp.d) are also calculated. Four fluctuation regions above Tc are observed for all of the samples, namely the critical (cr), three dimensional (3D), two dimensional (2D) and zero dimensional (0D) fluctuation regions. The values of Bc(0), Bc1(0) and Jc(0) are increased while the values of E and λp.d are suppressed with increasing Zn content at Cu sites. The intersection of 2D LD, and 0D exponents determines a cross-over of 2D LD & MT contributions. Associated with the width of 3D AL/LD regimes, the crossover temperature T3D-2D is shifted to higher values with enhanced Zn doping. It is most likely that decreased density of small spins of Cu atoms induced by increased Zn doping, may suppress the spin scatterings that consequently result in higher values of T3D-2D, critical fields (Bc(0), Bc1(0)), Jc(0) and lower values of Epb and λp.d in the final compound. The decreased density of small spins of Cu atoms in the CuO2/ZnO2 planes is viewed in terms of suppression of the spin gap and, hence, the pseudo-gap in Cu0.5Tl0.5Ba2Ca3Cu4−yZnyO12−δ (y=0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0) superconductors.