Square Patterns in Rayleigh-Bénard Convection with Rotation about a Vertical Axis

We present experimental results for Rayleigh-Bénard convection with rotation about a vertical axis at dimensionless rotation rates $0\le \Omega \le 250$ and $\epsilon \equiv \Delta T/\Delta T_c-1\mathrm{}\lesssim 0.2$. Critical Rayleigh numbers and wave numbers agree with predictions of linear-stability analysis. For $\Omega \gtrsim 70$ and small $\epsilon$ the patterns are cellular with local fourfold coordination and differ from the theoretically expected Küppers-Lortz unstable state. Stable as well as intermittent defect-free square lattices exist over certain parameter ranges. Over other ranges defects dynamically disrupt the lattice but cellular flow and local fourfold coordination is maintained.