One-dimensional $SU(3)$ Heisenberg Model, the non-linear sigma model, $\theta$-term

Question

Let's consider a one dimensional $SU(N)$ antiferromagnetic Heisenberg Model with an irreducible representation and its conjugate on alternating sites, such that they correspond to a Young tableaux with $n$ columns and 1 row, $n$ columns and (N-1) rows respectively. In the "semiclassical limit" $n\to\infty$ this model can be mapped onto the non linear sigma model with an additional topological term in which $\theta=\pi n$.

If we consider the case N=2, we get the familiar SU(2) Heisenberg chain of spin $S$, with $S=\frac{n}{2}$. This model is massive for $S$ integer given that the topological term vanish $(\theta=0)$. For $S$ half-integer ($\theta=\pi$) the Lieb-Schultz-Mattis (LSM) theorem assure us that the model has to be either gapless or have degenerate ground states. We know that in this case our model has to be gapless.

If now we consider the case N=3, we find that for $n$ even ($\theta=0$) the model is massive. What can be said in general for the case $n$ odd? Is there an equivalent LSM theorem for SU(3)? Can we still say that the corresponding model has to be either gapless or have degenerate ground states? I know that if $n=1$, the model spontaneously breaks the parity symmetry and has two degenerate ground states and is massive. Does this result follow from the semiclassical limit above even though $n$ is not large?

Answer 1

Let me just answer the LSM part. To find the potential LSM theorem you treat the system as having symmetry $\mathbb{Z} \times PSU(N)$, where the generator of $\mathbb{Z}$ is a lattice translation times a complex conjugation (the complex conjugation is necessary for $N > 2$ because of the alternating of representations and their conjugates). I don't know if any papers in the literature have considered this case specifically, but adapting a standard argument, one finds that there is an LSM theorem whenever the representation of $PSU(N)$ at each site is projective. I believe this happens whenever $n$ is not an integer multiple of $N$.