  In this paper searches are presented for the pair production of first and
second generation scalar leptoquarks and limits are given on the quark-lepton
compositeness scale from proton-antiproton collision data at a center-of-mass
energy 1.96 TeV, collected with Run II D0 Detector in 2002-2004.
  No evidence for a leptoquark signal has been observed. From the upper bounds
on the product of cross section times branching ratio beta=Br(LQ to lj), a
lower mass limits of M(LQ1) > 241 GeV and M(LQ2) > 247 GeV for the first and
second LQ generation are set for beta=1. These results, combined with those
obtained by D0 in Run I at a center-of-mass energy of 1.8 TeV, allow to exclude
scalar LQ masses up to 256 GeV and 251 GeV (for beta=1) for the first and
second generation, respectively.
  The dilepton mass spectra in pp -> l+l-+X interactions are studied using
dielectron (dimuon) data samples, corresponding to an integrated luminosity of
271 pb-1 (406 pb-1). The mass spectra being a probe for new physics are
examined for new interactions of quarks and leptons from a common composite
structure. No excess of events is found over the expectation from Standard
Model processes. The current experimental lower limits on the compositeness
scale vary, for different chirality channels, from 3.6 to 9.1 TeV for the
(eeqq) and from 4.2 to 9.8 TeV for the (mumuqq) contact interaction.
