We consider the Quantum Inverse Scattering Method with a new R-matrix
depending on two parameters q and t. We find that the underlying algebraic
structure is the two-parameter deformed algebra SUq,tβ(2) enlarged by
introducing an element belonging to the centre. The corresponding Hamiltonian
describes the spin-1/2 XXZ model with twisted periodic boundary conditions.Comment: LateX file, 9 pages, Minor changes (including authors` names in the
hep-th heading