DIRECT CALCULATION OF THE SPIN STIFFNESS IN THE J(1)-J(2) HEISENBERG-ANTIFERROMAGNET

We calculate the spin stiffness s for the frustrated spin-1/2 Heisenberg antiferromagnet on a square lattice by exact diagonalizations on finite clusters of up to 36 sites followed by extrapolations to the thermodynamic limit. For the nonfrustrated case, we find that s=(0.183 0.003)J1, in excellent agreement with the best results obtained by other means. Turning on frustration, the extrapolated stiffness vanishes for 0.4 J2/J1 0.6. In this intermediate region, the finite-size scaling works poorly an additional sign that there is neither Néel nor collinear magnetic order. Using a hydrodynamic relation, and previous results for the transverse susceptibility, we also estimate the spin-wave velocity in the Néel-ordered region. © 1995 The American Physical Society.