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

The integrable quantum group splq(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.