  The excitation spectra of \Lambda_c and \Lambda_b baryons are investigated by
using a quark-diquark model in which a single-heavy baryon is treated as the
bound state of a heavy quark and a scalar diquark. We take two types of
relativistic corrections into account for the quark-diquark potential. In the
first type, we consider the one-gluon exchange between the heavy quark and one
of the light quarks in the diquark. In the second, we consider the one-gluon
exchange between a scalar particle and a heavy quark. We find that there is a
large difference between two types of corrections due to different treatment of
the internal color structure of the diquark. The relativistic corrections are
important for the solution to the string tension puzzle, particularly, the
Darwin term makes a large contribution.
