CAN THE UNIVERSAL JUMP BE OBSERVED IN 2-DIMENSIONAL XY MAGNETS

The universal jump in the spin-wave stiffness is a characteristic property of the Kosterlitz-Thouless-Berezinskii phase transition in an infinitely large 2D-XY magnet. It has recently been demonstrated [S. T. Bramwell and P. C. W. Holdsworth, J. Phys. Condens. Matt. 5, L53 (1993)] that finite-size rounding is much more important in two-dimensional than in three-dimensional systems. This is because the properties of the phase transition depend logarithmically, rather than algebraically, on the system size. Consequently, finite-size rounding will be dominant in any real, macroscopic 2D-XY magnet. How the rounded universal jump still exhibits observable universal properties is discussed.