Bachelor's defense: Sigurd Anker Laursen Storgaard

Title: Frustration in classical spin systems

Classical spins on triangular and square lattices are studied, and a way of determining the ground state spin configurations as a helical order is argued along with a way of classifying these according to their ground state pitch vectors. Taking up to second nearest neighbouring spins into account, parametrised continuous ground state degeneracies with certain values of the interaction constants are shown. It is shown that due to an order by disorder effect these degeneracies are lifted at finite temperature. A third nearest neighbour interaction is added in the square lattice and it is shown that an infinite degeneracy in the case of an antiferromagnetic second nearest neighbour bond being half of an antiferromagnetic nearest neighbour bond is reduced to a parametrised one.