Comment on 'optical conductivity of high T_c superconductors: from underdoped to overdoped'

In a letter published recently, Puchkov et al, argued that the optical conductivity of some overdoped high T_c superconductors (HTSC) exhibits an unexpected behavior: the total low energy spectral weight does not change upon doping. As a comment, the present report points out that such intriguing behavior is not universal to HTSC. Taking as an example the IR transmission of overdoped Bi2212, it is shown that the conductivity increases uniformly over the whole frequency range, consistent with the increased carrier concentration observed in dc transport measurements.