title: Universal behavior of coupled order parameters below three dimensions.
abstract: We explore universal critical behavior in models with two competing order parameters, and an O(N) ⊕O(M) symmetry for dimensions d≤3. In d=3, there is always exactly one stable renormalization group fixed point, corresponding to bicritical or tetracritical behavior. Employing pseudospectral techniques to solve functional renormalization group equations in a two-dimensional field space, we uncover a more intricate structure of fixed points in d<3, where two additional bicritical fixed points play a role. Towards d=2, we discover ranges of N=M with several simultaneously stable fixed points, indicating the coexistence of several universality classes.
authors.affiliation: Theoretisch-Physikalisches Institut, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, D-07743 Jena, Germany.
authors.forename: Julia
authors.initials: J
authors.lastname: Borchardt
authors.affiliation: Blackett Laboratory, Imperial College, London SW7 2AZ, United Kingdom.
authors.forename: Astrid
authors.initials: A
authors.lastname: Eichhorn