Renormalization-Group Analysis of Bicritical and Tetracritical Points

Recently developed renormalization-group techniques are summarized and exploited to yield a renormalization-group analysis of bicritical and tetracritical points (which arise in antiferromagnets and boson systems). For $n\lesssim 3$ an isotropic or Heisenberg fixed point dominates and gives bicritical behavior; but for $n\gtrsim 4$ a new "biconical" fixed point with irrational $\epsilon$-expansion coefficients appears. This describes a tetracritical point and may be relevant to displacive phase transitions.