  The possibility to have diquark configuration in heavy baryons, such as
Lambda_c and Lambda_b, is examined by a nonrelativistic potential model with a
heavy quark and a light scalar diquark. Assuming that the Lambda_c and Lambda_b
baryons are composed of the heavy quark and the point-like scalar-isoscalar ud
diquark, we solve the two-body Schrodinger equation with the Coulomb plus
linear potential and obtain the energy spectra for the heavy baryons. Contrary
to our expectation, it is found that the potential determined by the quarkonium
spectra fails to reproduce the excitation spectra of the Lambda_c and Lambda_b
in the quark-diquark picture, while the Lambda_c and Lambda_b spectra is
reproduced with a half strength of the confinement string tension than for the
quarkonium. The finite size effect of the diquark is also examined and it is
found that introduction of a finite size diquark would resolve the failure of
the spectrum reproduction. The Xi_c excitation energy is also calculated and is
found to be smaller than Lambda_c in the quark-diquark model. This is not
consistent with the experimental observation.
