Evolution of Superconducting Transition Temperature (Tc) upon Intercalation of HgBr2 into the Bi2Sr1.5-xLaxCa1.5Cu2Oy

Intercalation of HgBr 2 into Bi 2 Sr 1.5 - x La x Ca 1.5 Cu 2 O y (0.0 ≤ x ≤ 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 HgBr 2 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 HgBr 2 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 (Δ T c ) and lattice expansion (Δ d ) between the iodine intercalate (Δ T c ≈ 10 K and Δ d ≈ 3.6 Å) and the HgBr 2 one (Δ T c ≈ 6 K and Δ d ≈ 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.