Order-Disorder
This is a general feature of order-disorder transitions
The ordered phase becomes unstable towards the formation of defects
Ising model: defects are domain walls dc=1
--> no finite T phase transition in d=1
similar to 1-d percolation where pc=1
that is, any defect in 1-d destroys order
for d=2 defects are compact clusters and a finite temperature phase transition occurs