  We study critical phenomena associated with a spontaneous chiral symmetry
breaking in current-driven non-equilibrium steady states by using holography.
We find that the critical exponents $(\gamma, \nu)$ at the tricritical point
are asymmetric between the chiral symmetry restored phase and the broken phase.
Their values in the broken phase are different from those of the mean-field
theory, whereas other critical exponents are the mean-field values. The phase
diagram with respect to temperature and current density shows a re-entrant
structure: the broken chiral symmetry is restored again at low temperatures in
the presence of current density.
