Structural aspects and the conduction mechanism of Ln(Ba_2–xPr_x)Cu₃O_7+δ (Ln = Tm, Lu)

Abstract

TmBa 2-x Pr x Cu 3 O 7+δ and LuBa 2-x Pr x Cu 3 O 7+δ samples with 0.0 < x ≤ 1.0 have been prepared by a solid-state reaction technique. The occurrence of the orthorhombic to tetragonal phase transition, increasing of oxygen content with the increase of Pr doping, and the existence of solubility limit in both systems confirm the correct substitution of Pr at Ba sites. The solubility limit for Pr in the Lu-based samples is less than 0.7, which is less than x = 0.90 for the Tm-based samples. The T c depression versus doping content in both systems shows that depression of superconductivity by Pr at Ba sites is mostly due to the holefilling/localization effects. Also, in the Tm-based system, the metal -insulator transition occurs at x c MIT ≈ 0.30, and the superconductor -insulator transition occurs at x c SIT = 0.45, but in the Lu-based system, x c MIT ≈ 0.35 and x c SIT = 0.50. In the normal state, for both systems with x < x c MIT , the Coulomb gap phase is dominant, whereas for the samples with Pr content x c MIT ≤ x < x c c SIT , two-dimensional variable-range hopping, and for x > x c SIT , three-dimensional variable-range hopping are the dominant conduction mechanisms.