Phase transitions in anisotropic superconducting and magnetic systems   with vector order parameters: Three-loop renormalization-group analysis
  The critical behavior of a model with N-vector complex order parameter and three quartic coupling constants that describes phase transitions in unconventional superconductors, helical magnets, stacked triangular antiferromagnets, superfluid helium-3, and zero-temperature transitions in fully frustrated Josephson-junction arrays is studied within the field- theoretical renormalization-group approach in three dimensions. To obtain qualitatively and quantitatively correct results perturbative expansions for \beta-functions and critical exponents are calculated up to three-loop order and resummed by means of the generalized Pade-Borel procedure. Fixed-point coordinates, critical exponent values, RG flows, etc. are found for the physically interesting cases N = 2 and N = 3. Marginal values of N at which the topology of the flow diagram changes are determined as well. In most cases the systems mentioned are shown to undergo fluctuation-driven first-order phase transitions. Continuous transitions are allowed in hexagonal d-wave superconductors, in planar helical magnets (into sinusoidal linearly-polarized phase), and in triangular antiferromagnets (into simple unfrustrated ordered states) with critical exponents \gamma = 1.336, \nu = 0.677, \alpha = -0.030, \beta = 0.347, \eta = 0.026, which are hardly believed to be experimentally distinguishable from those of the 3D XY model. The chiral fixed point of RG equations is found to exist and possess some domain of attraction provided N > 3. Thus, magnets with Heisenberg (N = 3) and XY-like (N = 2) spins should not demonstrate chiral critical behavior with unusual critical exponents; they can approach the chiral state only via first-order phase transitions. 
S. A. Antonenko and A. I. Sokolov (Saint Petersburg Electrotechnical   University, St. Petersburg, Russia)