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講演者： 花井 亮 氏 (シカゴ大学)
タイトル：”Phase transitions in non-reciprocally interacting matter”
Usually, microscopic objects such as atoms and molecules obey the Newton’s third law, i.e., they interact in a reciprocal way. However, non-reciprocality is a common feature of active systems that arise in a broad context of science ranging from social sciences, biology to physics: A peregrine falcon chases a dove because of their non-reciprocal “interaction”; Neural networks are composed of inhibitory and excitatory neurons that couple non-reciprocally; Social networks are composed of nodes with non-reciprocal connectivity; Synthetic physical non-reciprocal interactions can be realized in complex plasma and optical nanoparticle systems.
Despite its ubiquitous presence in Nature, the collective properties of such non-reciprocally interacting many-body systems are poorly understood. In this talk, I will generalize the Ginzburg-Landau theory of equilibrium phase transitions to be applicable to non-reciprocal matter . I show that non-reciprocity gives rise to unusual many-body phases and transitions controlled by spectral singularities called exceptional points. The emergent collective phenomena range from active time (quasi)crystals to exceptional point enforced pattern formation and hysteresis. I illustrate these phenomena by giving three paradigmatic examples of self-organization generalized to have non-reciprocal interactions: synchronization, flocking, and pattern formation.
If time allows, I will also talk about a quantum many-body system (exciton-polariton system) that exhibits analogous phase transitions  and critical phenomena (“critical exceptional point”), which is associated with a new universality class with anomalously giant phase fluctuations (that diverges at d≤4) and enhanced many-body correlations (that becomes relevant at d<8) .  M. Fruchart*, R. Hanai*, P. B. Littlewood, and V. Vitelli, arXiv:2003.13176.  T. Kato, Perturbation theory for linear operators, 2nd ed. (Springer, 1984).  R. Hanai, A. Edelman, Y. Ohashi, and P. B. Littlewood, Phys. Rev. Lett. 122, 185301 (2019).  R. Hanai and P. B. Littlewood, Phys. Rev. Research 2, 033018 (2020). 講演は英語で行われます。 The talk will be given in English.