Josephson plasma resonance and phonon anomalies in trilayer   Bi_2Sr_2Ca_2Cu_3O_10 superconductor
  The far-infrared (FIR) c axis conductivity of a Bi2223 crystal has been measured by ellipsometry. Below T$_c$ a strong absorption band develops near 500 cm $^{-1}$, corresponding to a transverse Josephson-plasmon. The related increase in FIR spectral weight leads to a giant violation of the Ferrell-Glover-Tinkham sum-rule. The gain in c axis kinetic energy accounts for a sizeable part of the condensation energy. We also observe phonon anomalies which suggest that the Josephson-currents lead to a drastic variation of the local electric field within the block of closely spaced CuO$_2$ planes. 
A.V Boris, D. Munzar, N.N. Kovaleva, B. Liang, C.T. Lin, A. Dubroka,   A.V. Pimenov, T. Holden, B. Keimer, Y. -L. Mathis, an...