Spiral orientational order in quantum Hall skyrmion lattices
We investigate the existence of spiral ordering in the planar spin orientation of skyrmions localised on a face centered rectangular lattice (FCRL). We use the non-linear sigma model (NLSM) to numerically calculate the minimum energy configurations of this lattice around the $\nu=1$ quantum Hall ground state. Our variational ansatz contains an angle $\theta$, characterising the FCRL and an angle $q$, characterising the orientational order. As $\nu$ is increased towards one, there is a smooth transition from the triangular lattice (TL) characterised by $(\theta,q) = (120^o,120^o)$ to FCRLs with spiral orientational order. The novel feature we find is that these phases are characterised by $\theta, q)$ values such that $\theta+q = 240^o$ (same as the TL phase). As $\nu$ incresaes further towards one, there is a sharp transition from the FCRLs to the square lattice (SL), characterised by $(\theta,q)=(90^o,180^o)$. Consequently, the parameter $\theta+q$ jumps sharply at the FCRL-SL transition and can serve as an order parameter to characterise it.