Title: Non-reciprocal phase transitions
presenter Dr. RYO HANAI (APCTP)
Date: 10AM, 29th Mar. E6 #1501/zoom
13:30 PM, 30th Mar. E6 #2502/zoom
회의 ID: 876 2332 4709
Phase transitions are ubiquitous in nature. For equilibrium cases, the celebrated Landau theory has provided great success in describing these phenomena on general grounds. Even for nonequilibrium transitions such as optical bistability, flocking transition, and directed percolation, one can often define Landau’s free energy in a phenomenological way to successfully describe the transition at a meanfield level. In such cases, the nonequilibrium effect is present only through the noise-activated spatial-temporal fluctuations that break the fluctuation-dissipation theorem. Here, by generalizing the Ginzburg-Landau theory to be applicable to driven systems, we introduce a novel class of nonequilibrium phase transitions [1-2] and critical phenomena  that does not fall into this class. Remarkably, the discovered phase transition is controlled by spectral singularity called the exceptional points that can only occur by breaking the detailed balance and therefore has no equilibrium counterparts. The emergent collective phenomena range from active time (quasi)crystals to exceptional point enforced pattern formation, hysteresis, to anomalous critical phenomena that exhibit anomalously large phase fluctuations (that diverge at d≤4) and enhanced many-body effects (that become relevant at d<8) . The inherent ingredient to these is the non-reciprocal coupling between the collective modes that arise due to the drive and dissipation.
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 R. Hanai, A. Edelman, Y. Ohashi, and P. B. Littlewood, Non-Hermitian phase transition from a polariton Bose-Einstein condensate to a photon laser. Phys. Rev. Lett. 122, 185301 (2019).
 R. Hanai and P. B. Littlewood, Critical fluctuations at a many-body exceptional point. Phys. Rev. Res. 2, 033018 (2020).