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

We present experimental results for Rayleigh-B\'enard convection with rotation about a vertical axis at dimensionless rotation rates 0≤Ω≤250 and ε≡ΔT/ΔTc-10.2. Critical Rayleigh numbers and wave numbers agree with predictions of linear-stability analysis. For Ω70 and small ε the patterns are cellular with local fourfold coordination and differ from the theoretically expected K\"uppers-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.