Optical and thermodynamic properties of the high-temperature superconductor HgBa₂CuO_4+δ

In- and out-of-plane optical spectra and specific heat measurements for the single layer cuprate superconductor HgBa₂CuO_4+δ (Hg-1201) at optimal doping (T_c=97 K) are presented. Both the in-plane and out-of-plane superfluid density agree well with a recently proposed scaling relation ρ_s∝σ_dcT_c. It is shown that there is a superconductivity induced increase of the in-plane low frequency spectral weight which follows the trend found in underdoped and optimally doped Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) and optimally doped Bi₂Sr₂Ca₂Cu₃O_10+δ (Bi-2223). We observe an increase of optical spectral weight which corresponds to a change in kinetic energy ΔW≈0.5 meV∕Cu which is more than enough to explain the condensation energy. The specific heat anomaly is 10 times smaller than in YBa₂Cu₃O_6+δ (YBCO) and 3 times smaller than in Bi-2212. The shape of the anomaly is similar to the one observed in YBCO showing that the superconducting transition is governed by thermal fluctuations.