Semiclassical and field theoretic studies of Heisenberg antiferromagnetic chains with frustration and dimerization

The Heisenberg antiferromagnetic spin chain with both dimerization and frustration is studied. The classical ground state has three phases (a Neel phase, a spiral phase and a colinear phase), around which a planar spin-wave analysis is performed. In each phase, we discuss a non linear sigma model field theory describing the low energy excitations. A renormalization group analysis of the SO(3) matrix-valued field theory of the spiral phase leads to the conclusion that the theory becomes $SO(3) \times SO(3)$ and Lorentz invariant at long distances. This theory is analytically known to have a massive spin-1/2 excitation. We also show that $Z_2 ~$ solitons in the field theory lead to a double degeneracy in the spectrum for half-integer spins