Effect of Magnesium Doping to Reduce the Charge Reservoir Layer in Cu_0.5Tl_0.5(Ba_2−xMg_x)Ca₂Cu₃O_y (x = 0, 0.15, 0.25, 0.35) Superconductors

The charge reservoir layer plays a crucial role in the charge transfer mechanism in high-temperature superconductors. The effect of reducing the thickness of the Cu_0.5Tl_0.5Ba₂O_4−δ charge reservoir by replacing Ba with Mg atom in Cu_0.5Tl_0.5-1223 crystal has been studied. Cu_0.5Tl_0.5(Ba_2−xMg_x)Ca₂Cu₃O_y (x = 0, 0.15, 0.25, 0.35) superconductor samples were synthesized by a two-step solid-state reaction method. The as-grown samples showed orthorhombic crystal structure in which the c-axis lattice parameter and unit cell volume were systematically suppressed with increasing incorporation of Mg into the final compound, thereby resulting in suppression of the charge reservoir. The room-temperature resistivity increased with the Mg doping content, while the T_c and T_c-onset values decreased according to both resistivity and alternating-current (AC) susceptibility measurements. Fourier-transform infrared (FTIR) absorption measurements revealed three phonon modes related to vibrations of Tl–O_A–Cu(2), Cu(1)–O_A–Cu(2), and CuO₂ planar oxygen atoms at around 420 cm⁻¹, 480 cm⁻¹ to 540 cm⁻¹, and 580 cm⁻¹, respectively. The apical oxygen mode of Tl–O_A–Cu(2) type was hardened whereas the position of the latter two modes remained unchanged with increasing Mg doping in the final compound. Excess conductivity analyses showed a significant decrease in the onset temperature for Cooper pair formation (T*^2D−SW). The coherence along the c-axis ξ_c(0), the interlayer coupling J, and the Fermi velocity v_F of superconducting carriers increased with increasing Mg doping at Ba sites. These result suggest that the transfer mechanism of charge carriers from the Cu_0.5Tl_0.5Ba₂O_4−δ charge reservoir layer to the conducting CuO₂ planes became more efficient, as evidenced by the increased coherence length ξ_c(0) and Fermi velocity v_F of the carriers. However, the suppression of parameters such as B_c0(T), B_c1(T), J_c0(0), and τ_φ indicates that the density of pinning centers was suppressed with increasing Mg doping in the final compound.