Study of Carrier Transfer Mechanism When Substituting Strontium at Barium Sites in CuTl-1223 Superconducting Phase

Strontium-doped CuTl-1223 phase has been obtained by employing a nonstoichiometric composition of the form Tl_1/2Cu_1/2(Ba_1−xSr_x)Ca₁Cu₄O_y (x = 0, 0.1, 0.2, 0.3). The superconducting properties of the samples were investigated by x-ray diffraction analysis and four-probe resistivity, alternating-current (AC) susceptibility, and Fourier-transform infrared (FTIR) absorption measurements. The effect of doping strontium atoms on the intrinsic superconductor parameters of these samples was studied by excess conductivity analysis. All the samples showed orthorhombic crystal structure in space group Pmmm, and the cell parameters were determined by using all the planar reflections. The c-axis length and the unit cell volume decreased with Sr doping in the final compound. Suppression of the room-temperature resistivity followed by metallic variations in resistivity versus temperature measurements are typical features for these samples. The zero-resistivity critical temperature and the onset of diamagnetism were suppressed with increasing Sr doping in the final compound. The apical oxygen phonon mode of type Cu(1)–O_A–Cu(2) observed at around 548 cm⁻¹ hardened with increasing Sr doping. The excess conductivity analysis revealed that the coherence length along the c-axis, the interlayer coupling, and the Fermi velocity of the carriers increased for the Sr doping levels of x = 0.1 and 0.2 but decreased for the sample with x = 0.3. The values of B_c0(T), B_c1(T), and J_c0(0) increased with increasing Sr doping in the final compound. It is proposed that this effect arises due to an increase in the superconducting volume fraction. A decrease in the value of the London penetration depth λ_p.d. and the Ginzburg–Landau (GL) parameter shows that the flux-pinning characteristics of the samples were improved by Sr doping. An increase in the mean free time of the carriers and a decrease in the energy required to break apart Cooper pairs result from a decrease in the remanent field scattering induced by the increased population of pinning centers in Sr-doped samples.