Spin excitations in the integrable open quantum group invariant supersymmetric t-J model

The integrable quantum group spl_q(2, 1)-invariant supersymmetric t-J model with open boundaries is studied via an analytic treatment of the Bethe equations. An su(2) feature is seen to hold for states at or close to half-filling. For these states the eigenvalues of the transfer matrix of the t-J model satisfy a set of su(2) functional relations. The finite-size corrections to the relevant eigenvalues, and thus the surface effect on the spin excitations, have been calculated analytically by solving the functional relations.