Doping effect on the penetration depth of HgBa₂CuO_{4 + δ}

We have investigated systematically by magnetization measurements the penetration depth λ of HgBa₂CuO_{4 + δ} with doping 0.1 ≤ δ ≤ 0.3, ranging from the underdoped region of δ < 0.22 to the overdoped region of δ > 0.22. The penetration depth was found to vary with temperature quadratically for the underdoped samples below the transition temperature (T_c) over the entire temperature region examined, while linearly for the optimally doped and overdoped samples below 0.5 T_c. The observation suggests that the temperature dependence of λ is affected by doping in a complex way and its inference to the pairing symmetry should be taken only with extreme care.