Tunable lower critical fractal dimension for a non-equilibrium phase transition

Abstract: We theoretically investigate the role of spatial dimension and driving frequency in a non-equilibrium phase transition of a driven-dissipative interacting bosonic system. In this setting, spatial dimension is dictated by the shape of the external driving field. We consider both homogeneous driving configurations, which correspond to standard integer-dimensional systems, and fractal driving patterns, which give rise to a non-integer Hausdorff dimension for the spatial density. The onset of criticality is characterized by critical slowing down in the excited density dynamics as the system asymptotically approaches the steady state, signaling the closing of the Liouvillian frequency gap. We show that the lower critical dimension -- below which the non-equilibrium phase transition ceases to occur -- can be controlled continuously by the frequency detuning of the driving field.