Direct calculation of spin stiffness for spin-1/2 Heisenberg models

Abstract
The spin stiffness of frustrated spin-1/2 Heisenberg models in one and two dimensions is computed by exact diagonalizations on small clusters that implement spin-dependent twisted boundary conditions. Finite-size extrapolation to the thermodynamic limit yields a value of 0.14±0.01 for the spin stiffness of the unfrustrated planar antiferromagnet. We also present a general discussion of the linear-response theory for spin twists, which ultimately leads to the moment sum rule.
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