We argue that recently observed superconductivity-induced blue shift of the
plasma frequency δωpl​ in Bi2​Sr2​CaCu2​O8+δ​ is
related to the change in the integrated dynamical structure factor associated
with the development of the spin resonance below Tc​. We show that the
magnitude of δωpl​ is consistent with the small integrated
spectral weight of the resonance, and its temperature dependences closely
follow that of the spin resonance peak.Comment: 5 pages, 3 figure