Effect of Magnesium Doping to Reduce the Charge Reservoir Layer in Cu 0.5 Tl 0.5 (Ba 2− x Mg x )Ca 2 Cu 3 O 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 0.5 Tl 0.5 Ba 2 O 4− δ charge reservoir by replacing Ba with Mg atom in Cu 0.5 Tl 0.5 -1223 crystal has been studied. Cu 0.5 Tl 0.5 (Ba 2− x Mg x )Ca 2 Cu 3 O 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 CuO 2 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 0.5 Tl 0.5 Ba 2 O 4− δ charge reservoir layer to the conducting CuO 2 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 c 0 ( T ), B c 1 ( T ), J c 0 (0), and τ φ indicates that the density of pinning centers was suppressed with increasing Mg doping in the final compound.