  Quantum criticality within Dirac fermions harbors a plethora of exotic
phenomena, attracting sustained attention in the past decades. Nevertheless,
the nonequilibrium dynamics therein has rarely been studied. To fill in the
gap, we explore the imaginary-time relaxation dynamics in a typical Dirac
quantum criticality belonging to chiral Heisenberg universality class.
Performing large-scale quantum Monte Carlo simulation, we unveil rich
nonequilibrium critical phenomena from different initial states. Particularly,
a new dynamic exponent characterizing the non-stationary evolution in the
short-time state is determined as $\theta=-0.84(4)$, in sharp contrast with the
prevalent belief that $\theta$ is positive as demonstrated in classical cases.
Furthermore, we propose a universal dynamic scaling theory governing the
fruitful nonequilibrium properties in Dirac quantum criticality. Armed with the
scaling theory, we develop a new framework to investigate fermionic quantum
criticality based on short-time dynamics, paving a promising avenue to
fathoming quantum criticality in diverse fermionic systems with high
efficiency.
