= Critical region of a phase transition
The critical region is the range of control parameters near a <thermodynamic critical point> where correlated <order parameter> fluctuations invalidate a simple <mean-field approximation>. For short-range scalar quartic theory below four dimensions, the dimensionless <Ginzburg criterion> ratio grows as $g\xi^{4-D}$ in fixed microscopic units. The range where this ratio is of order one or larger is fluctuation dominated. Its width depends on nonuniversal microscopic coefficients even though the limiting <critical exponents> can be universal.
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