Spin Flop, Supersolids, and Bicritical and Tetracritical Points

A scaling theory is introduced for bicritical points, such as antiferromagnetic spinflop points (with analogies to the upper point in ^4He), where two distinct critical lines meet. Experimentally testable predictions follow from renormalization-group calculations which indicate that the bicritical exponents should be Heisenberg like for systems with n3 components; the crossover exponent (1. 25 for n=3) is directly observable. For n>3 an intermediate ("supersolid") low-temperature phase may appear, the bicritical point then becoming tetracritical.