HALF-INTEGRAL SPIN-SINGLET QUANTUM HALL-EFFECT

We provide numerical evidence that the ground state of a short-range-i nteraction model at nu=1/2 is incompressible and spin singlet for a wi de range of repulsive interactions. Furthermore it is accurately descr ibed by a trial wave function studied earlier. For the Coulomb interac tion we find that this wave function provides a good description of th e lowest-lying spin-singlet state, and propose that fractional quantum Hall effect would occur at nu=1/2 if this state became the global gro und state. Some conclusions of an earlier paper are invalidated by the present numerical study of a larger system; these will be indicated.