  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 $T_c$ [Uykur $et al$., Phys. Rev. Lett. 112, 127003
(2014)][Dubroka $et al$., Phys. Rev. Lett. 106, 047006 (2011)]. By carefully
subtracting the normal component from the imaginary part of conductivity
$\sigma_2$($\omega$), 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.

Superconductivity