Abstract:
The fluctuation theory of phase transitions is used to study tricritical behavior in the
isotropic phase of two models in which the order parameter is a $p$-dimensional
symmetric traceless tensor. For arbitrary values of $p$, differential equations are
obtained for the vertex functions from the second to the sixth orders as functions of
the reciprocal susceptibility. The cases $p=1,2,3$ are studied. For $p=1,2$,
the models exhibit tricritical behavior. For $p=3$, allowance for fluctuations leads
in the general case to the appearance of instability in the critical behavior of the
models and to the replacement of tricritical behavior by a phase transition of the
first kind.
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