Evolution of superconducting transition temperature (T_c) upon intercalation of HgBr₂ into the Bi₂Sr_1.5-xLa_xCa_1.5Cu ₂O_y

Intercalation of HgBr₂ into Bi₂Sr_1.5-xLa_xCa_1.5Cu ₂O_y (0.0 ≤ r ≤ 0.4) superconductor has been carried out in order to elucidate the origin of T_c evolution upon intercalation. The T_c's obtained from the dc magnetic susceptibility measurements were plotted against x. The plot of T_c vs x for the pristines showed the parabolic feature with overdoped (0.0 ≤ x < 0.1), optimally doped (x = 0.1), and underdoped (0.2 ≤ x ≤ 0.4) regions. The T_c's of the HgBr₂ intercalates in the overdoped region were reduced less than ∼6 K but increased by 4-6 K in the underdoped one compared with nonintercalated samples. Such changes in T_c upon intercalation indicate hole doping from intercalant to host lattice. An attempt of semiempirical calculation was made to determine the hole concentration doped by intercalation. Upon HgBr₂ intercalation, the amount of hole doping was estimated to be ∼0.2 hole per formula unit of the sample with x = 0.0, whereas the doping of ∼0.3 hole was estimated for the iodine intercalated sample. Considering the T_c depression (ΔT_c) and lattice expansion (Δd) between the iodine intercalate (ΔT_c ≈ 10 K and Δd ≈ 3.6 Å) and the HgBr₂ one (ΔT_c ≈ 6 K and Δd ≈ 6.3 Å), it can be concluded that the change in T_c upon intercalation clearly depends on the hole concentration due to the charge transfer between host and guest, rather than the interblock electronic coupling due to the lattice expansion.