Investigation of Precursor Superconducting State in YBa$_2$Cu$_3$O$_{7-\delta}$ through In-plane Optical Spectroscopy

A precursor of superconductivity has been searched in the in-plane optical spectra of underdoped YBa 2 Cu 3 O y , in which the previous c-axis optical spectra showed the presence of superconducting carriers at a temperature far above T c1,2) . By carefully subtracting the normal component from the imaginary part of conductivity 𝜎 2 (𝜔) , we found a clear in-plane response of superconducting condensate at the temperature consistent with the c-axis optical data. This confirms that the precursory superconductivity developing with reducing a doping level is an intrinsic phenomenon in the cuprates.