  The effects of electron correlation in the quasi-two-dimensional organic
conductor \alpha-(BEDT-TTF)2I3 are investigated theoretically by using an
extended Hubbard model with on-site and nearest-neighbor Coulomb interactions.
A variational Monte Carlo method is applied to study its ground-state
properties. We show that there appears a nonmagnetic horizontal-stripe charge
order in which nearest-neighbor correlation functions indicate a tendency
toward a spin-singlet formation on the bonds with large transfer integrals
along the charge-rich stripe. Under uniaxial pressure, a first-order transition
from the nonmagnetic charge order to a zero-gap state occurs. Our results on a
spin correlation length in the charge-ordered state suggest that a spin gap is
almost unaffected by the uniaxial pressure in spite of the suppression of the
charge disproportionation. The relevance of these contrasting behaviors in spin
and charge degrees of freedom to recent experimental observations is discussed.

Strongly Correlated Electrons