Doping Dependence of Upper Critical Field of High-T_c Cuprate Bi_2+xSr_2−xCaCu₂O_8+δ Estimated from Irreversibility Field at Zero Temperature

We investigated the temperature (T) dependence of the irreversibility field H_irr(T) in high-critical-temperature cuprate Bi_2+xSr_2−xCa_1−yY_yCu₂O_8+δ (Bi-2212) single crystals over a wide range of hole doping level (p). H_irr(T) was evaluated by measuring the magnetization hysteresis loop. The value of H_irr(T) extrapolated to T = 0 K [H_irr(0)], is either equal to or sets the lower boundary for the upper critical field at T = 0 K [H_c2(0)]. T_c shows a parabolic p-dependence (peak at p = 0.16), whereas μ₀ H_irr(0) increases monotonically with p by approximately one order of magnitude, from 19 T for the most underdoped sample (p = 0.065, T_c = 24 K) to 209 T for the most overdoped sample (p = 0.200, T_c = 75 K). The present results qualitatively agree with H_c2(0) values evaluated from the specific heat measurements. The observed p-dependence of H_irr(0) in Bi-2212 is distinct from those in YBa₂Cu₃O_7−δ and HgBa₂CuO_6+δ, in which a pronounced dip structure appears in the underdoped region. Considering that the dip structures observed in these two systems are likely associated with the formation of competing orders (most likely field-induced charge orders), the present results indicate that the influence of the competing order in Bi-2212 is less prominent than that in the other two systems.