FACTORIZABILITY OF THE BCS WAVE-FUNCTION

A BCS-type wave function for an ordinary superconductor is discussed that incorporates a new variational parameter that allows writing the wave function as a product of singlets that is more general than the standard BCS form using Cooper pairs. In spite of the extra variational freedom, it is shown that the minimum-energy state reduces to the ordinary BCS ground state for the ordinary BCS Hamiltonian. However, in the presence of higher-order terms generically present in a Hamiltonian, the factorization symmetry of the wave function is broken. © 1990 The American Physical Society.