16/10/08 09:17:43.14 xW+EldRM.net
>>48 つづき
6. Isotropic Heisenberg model in two dimensions
The isotropic Heisenberg model for a two-dimensional system of spins is quite different
from the xy model. To show the nature of this difference. we consider a large system
with periodic boundary conditions, but similar considerations apply to other boundary
conditions. In the xy model at low temperatures, the direction of magnetization in a
region is defined by a single angle φ which varies slowly in space. Although the angle φ
fluctuates by a large amount in a large system, the number of multiples of 2π it changes
by on a path that goes completely round the system is a topological invariant, so that
式（85）(86)略
(引用終り)