  In order to discuss the spin-gap formation in a multiorbital system, we
analyze an e_g-orbital Hubbard model on a geometrically frustrated zigzag chain
by using a density-matrix renormalization group method. Due to the appearance
of a ferro-orbital arrangement, the system is regarded as a one-orbital system,
while the degree of spin frustration is controlled by the spatial anisotropy of
the orbital. In the region of strong spin frustration, we observe a finite
energy gap between ground and first-excited states, which should be called a
spin-orbital gap. The physical meaning is clarified by an effective Heisenberg
spin model including correctly the effect of the orbital arrangement influenced
by the spin excitation.
