  Metallic systems exhibit plasmons as elementary charge excitations. This
fundamental concept was reinforced also in high-temperature cuprate
superconductors recently, although cuprates are not only layered systems but
also strongly correlated electron systems. Here, we study how such ubiquitous
plasmons leave their marks on the electron dispersion in cuprates. In contrast
to phonons and magnetic fluctuations, plasmons do not yield a kink in the
electron dispersion. Instead, we find that the optical plasmon accounts for an
emergent band -- plasmarons -- in the one-particle excitation spectrum;
acoustic-like plasmons typical to a layered system are far less effective.
Because of strong electron correlations, the plasmarons are generated by
bosonic fluctuations associated with the local constraint, not by the usual
charge-density fluctuations. Apart from this physical mechanism, the plasmarons
are similar to those discussed in alkali metals, Bi, graphene, monolayer
transition-metal dichalcogenides, semiconductors, diamond, two-dimensional
electron systems, and SrIrO3 films, establishing a concept of plasmarons in
metallic systems in general. Plasmarons are realized below (above) the
quasiparticle band in electron-doped (hole-doped) cuprates, including a region
around (pi,0) and (0,pi) where the superconducting gap and the pseudogap are
most enhanced.
