  We propose a phenomenological QCD sum rule with an explicit diquark field to
investigate the essential ingredients inside the hadrons. Introducing the mass
(m_\phi) and the condensate (\phi^2) for the diquark field as parameters in the
model, we find that the sum rule works well for Lambda, Lambda_c and Lambda_b.
This implies that these Lambda baryons can be represented by a diquark and a
quark configuration. We also find that there is a duality relation among the
parameters (m_\phi, \phi^2), for which the sum rule is equally good. In the
limit when \phi^2 =0 we find m_\phi=0.4 GeV, which can be thought as the
constituent diquark mass.
