  In this article, we discover a fundamental excitation called electromagnon in
the cuprate superconducting states. Doped holes render local inversion
asymmetry which produces hidden Rashba spin-orbit coupling in copper-oxide
plane. Rashba effect leads to Dzyaloshinskii-Moriya (DM) interaction between
neighboring spins, which establishes a general spin Hamitonian combined with
Heisenberg exchange interaction. The physical origin of spin fluctuations has
been revealed from Rashba-induced DM term. Electromagnon excitations are found
to result from spin fluctuations manifested as spin spiral states by hopping of
doped holes in high temperature superconducting states. The frequency of
electromagnon excitations, peak position of dielectric constant spectrum,
decreases linearly with decrease of exchange interaction in various families of
superconductors. Comparisons with superconductivity-induced B1g Raman peaks
show good agreement and thus possibly provide the experimental evidences for
electromagnon excitations in cuprates.
