  To understand orbital-angular-momentum contributions is becoming crucial for
clarifying nucleon-spin issue in the parton level. Twist-two structure
functions b_1 and b_2 for spin-one hadrons could probe orbital-angular-momentum
effects, which reflect a different aspect from current studies for the spin-1/2
nucleon, since they should vanish if internal constituents are in the S state.
These structure functions are related to tensor structure in spin-one hadrons.
Studies of such tensor structure will open a new field of high-energy spin
physics. The structure functions b_1 and b_2 are described by tensor-polarized
quark and antiquark distributions delta_T-q and delta_T-qbar. Using HERMES data
on the b_1 structure function for the deuteron, we made an analysis of
extracting the distributions delta_T-q and delta_T-qbar in a simple x-dependent
functional form. Optimum distributions are proposed for the tensor-polarized
valence and antiquark distribution functions from the analysis. A finite tensor
polarization is obtained for antiquarks if we impose a constraint that the
first moments of tensor-polarized valence-quark distributions vanish. It is
interesting to investigate a physics mechanism to create a finite
tensor-polarized antiquark distribution.
