Tuning across Universalities with a Driven Open Condensate

Abstract

Driven-dissipative systems in two dimensions can differ substantially from their equilibrium counterparts. In particular, a dramatic loss of off-diagonal algebraic order and superfluidity has been predicted to occur because of the interplay between coherent dynamics and external drive and dissipation in the thermodynamic limit. We show here that the order adopted by the system can be substantially altered by a simple, experimentally viable tuning of the driving process. More precisely, by considering the long-wavelength phase dynamics of a polariton quantum fluid in the optical parametric oscillator regime, we demonstrate that simply changing the strength of the pumping mechanism in an appropriate parameter range can substantially alter the level of effective spatial anisotropy induced by the driving laser and move the system into distinct scaling regimes. These include (i) the classic algebraically ordered superfluid below the Berezinskii-Kosterlitz-Thouless (BKT) transition, as in equilibrium; (ii) the nonequilibrium, long-wavelength-fluctuation-dominated Kardar-Parisi-Zhang (KPZ) phase; and the two associated topological-defect-dominated disordered phases caused by proliferation of (iii) entropic BKT vortex-antivortex pairs or (iv) repelling vortices in the KPZ phase. Furthermore, by analyzing the renormalization group flow in a finite system, we examine the length scales associated with these phases and assess their observability in current experimental conditions.

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Plain Language Summary

One of the most fascinating observations in the study of complex systems is that they exhibit universality: The same behavior can be found in vastly different physical realizations. A liquid boiling into a gas, for example, behaves exactly like a magnet losing its magnetic properties as it heats up. Similar behaviors among disparate systems can be grouped into what are known as universality classes, with each class describing a different behavior. While this classification is well established for systems that are in thermal equilibrium, it is tricky to classify nonequilibrium systems that are perturbed by some external mechanism. We find a way to experimentally realize one of these classes and show that the same system can be moved between different classes by only slightly changing the strength of the external drive.

Driven systems often exhibit behavior that is very different from their equilibrium counterparts. One example, the Kardar-Parisi-Zhang (KPZ) universality class, comprises a colorful diversity of dynamical processes ranging from the growth of liquid crystals and bacterial colonies to the combustion of paper. Despite an abundance of theoretical predictions, observation of KPZ scaling in two spatial dimensions remains elusive. One candidate system is a fluid of exciton-polaritons—mixed light-matter bosonic particles—in semiconductor microcavities. We show that under parametric driving, not only is KPZ scaling within reach of current experimental setups, but it is also possible to tune the system between fundamentally different universal regimes by making the system more anisotropic.

We provide the missing link between the general theory of the two-dimensional KPZ universality class and an actual physical realization, giving experimentalists a clear guideline in the search for KPZ physics in parametrically driven exciton-polaritons.