  The excitation energy spectra of heavy baryons consisting of a heavy quark
and two light quarks are investigated by using diquark models in order to
examine the nature of the diquark as a constituent of single heavy baryons. We
consider two diquark models; in model A the diquark is treated as a point-like
particle, while it has a spatial size in model B. We determine the masses of
scalar and axial vector diquarks by the mass difference of the ground state
charmed baryons to the $\Lambda_{c}$ baryon, while the mass of the $ud$ scalar
diquark in the $\Lambda_{c}$ baryon is assumed to be 500 MeV as a reference.
The parameters of these models are fixed by the $1p$ excitation energy of
$\Lambda_{c}$. We find that model A reproduces well the excitation energy
spectra of the charmed and bottomed baryons, although the string tension of the
confinement potential in model A should be a half of that of the charmonium,
while Model B suggests degeneracy of the $2s$ and $1d$ states, which is not
seen in the $\Lambda_{c}$ spectrum.
