Square patterns in Rayleigh-Benard convection with rotation about a vertical axis

We present experimental results for Rayleigh-Benard convection with rotation about a.vertical axis at dimensionless rotation rates 0 less than or equal to Omega less than or equal to 250 and epsilon = Delta T/Delta T-c - 1 less than or similar to 0.2. Critical Rayleigh numbers and wave numbers agree with predictions of linear-stability analysis. For Omega greater than or similar to 70 and small epsilon the patterns are cellular with local fourfold coordination and differ from the theoretically expected Kuppers-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. [S0031-9007(98)06696-4].