  Motivated by the observation that there may exist hadronic excitations even
in the quark-gluon plasma (QGP) phase, we investigate how the properties of
quarks, especially within the quasi-particle picture, are affected by the
coupling with bosonic excitations at finite temperature (T), employing
  Yukawa models with a massive scalar (pseudoscalar) and vector (axial-vector)
boson of mass m. The quark spectral function and the quasi-dispersion relations
are calculated at one-loop order. We find that there appears a three-peak
structure in the quark spectral function with a collective nature when T is
comparable with m, irrespective of the type of boson considered. Such a
multi-peak structure was first found in a chiral model yielding scalar
composite bosons with a decay width. We elucidate the mechanism through which
the new quark collective excitations are realized in terms of the Landau
damping of a quark (an antiquark) induced by scattering with the thermally
excited boson, which gives rise to mixing and hence a level repulsion between a
quark (antiquark) and an antiquark-hole (quark-hole) in the thermally excited
antiquark (quark) distribution. Our results suggest that the quarks in the QGP
phase can be described within an interesting quasi-particle picture with a
multi-peak spectral function. Because the models employed here are rather
generic, our findings may represent a universal phenomenon for fermions coupled
to a massive bosonic excitation with a vanishing or small width. The relevance
of these results to other fields of physics, such as neutrino physics, is also
briefly discussed. In addition, we describe a new aspect of the plasmino
excitation obtained in the hard-thermal loop approximation.
