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In a theory with two chiral fields , ψ1 and ψ2 with a global symmetry relating the relative phase s of both fields, but at low temperatures, the correlation function is nonzero, then we say a fermion condensate (also called chiral condensate) has formed. The VEV above is the order parameter. See order-disorder. For example, in QCD, there is an approximate (so there's no real spontaneous symmetry breaking; the VEV will always be aligned in a fixed direction) axial symmetry which is broken because the quarks form a chiral condensate. See Technicolor (physics) for another example. BCS in superconductivity is another.To get a "feel" for chiral condensates, a good toy model to start with is the Schwinger model.
Quantum field theory
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