Carrier dependences of anisotropy and irreversibility field in Pb₂Sr₂Y_1-xCa_xCu₃O₈ single crystals

Anisotropic properties of the electrical resistivity and the resistive superconducting transition under magnetic fields have been investigated for the single-crystal Pb₂Sr₂Y_1-xCa_xCu₃O₈ with 0 ≦ x ≦ 0.5 in the so-called underdoped region. The anisotropic parameter γ ≡ (m^*_c/m^*_ab) ^{1 2} is found to increase from ∼5 to 25 with decreasing x, namely, with decreasing hole concentration. The irreversibility field H_irr has been studied from the magnetization measurements for H {curly logical or} c-axis. In the high-temperature regime near T_c, H_irr is found to be proportional to 1-T/T_c) ^{3 2} for x∼0.5 and to (1-T/T_c) ^{3 2} or (1-T/T_c)² for smaller x values. In this regime, H_irr is also found to be proportional to γ^-2. In the low-temperature regime, on the other hand, H_irr grows exponentially as approximately expressed as e^-T/T_o, and shows a still stronger growth as e^T_o/T at much lower temperatures for smaller x values. These behaviors are discussed in terms of flux creep, vortex-lattice melting and vortex-glass melting.