SCALE-INDEPENDENT FLUCTUATIONS OF SPIN STIFFNESS IN THE HEISENBERG-MODEL AND ITS RELATIONSHIP TO UNIVERSAL CONDUCTANCE FLUCTUATIONS

It is shown that, at low temperatures, the absolute thermal fluctuations of the spin-stiffness constant in the classical Heisenberg model are independent of the scale L for d < 4, if L is larger than the lattice spacing but smaller than the correlation length, while the relative fluctuation is proportional to L4-2d, where d is the dimensionality, and is thus scale independent in d = 2 (with logarithmic corrections). The phenomenon is strikingly similar to the universal conductance fluctuations known for the problem of an electron in a random potential.