Séminaire

Mardi 19 Mars 2019 à 11h00.

Experimental evidence for universal behaviours in nonequilibrium systems: From coarsening & percolation in liquid crystals to interface growth in polycrystals

''Different materials and idealised models at thermal equilibrium can share the same large-scale behaviour at a critical point of a phase transition. This fact allows to classify them in terms of universality classes [1]. Similar phenomenology is theoretically expected for certain purely-dissipative and driven nonequilibrium dynamics, although experimental evidence remains comparatively scarce for both cases. In this seminar, I present two series of experiments in which nonequilibrium universal behaviours are observed. The first series concerns the ordering dynamics of twist nematic liquid crystals (TNLC) [2]. The dynamics is electrically triggered by suddenly changing the system state: from a nematic-turbulent (disordered) state - comprising plenty of topological defects - to an ordered state which is nearly degenerated regarding the twist nematic alignment. I show that TNLC ordering dynamics is not only a membership of the model A universality class [3], but it is, simultaneously, in the critical percolation (CP) class. Such model-A-CP coexistence implies a generalisation for continuum ordering theories [3], as it has been recently proposed from discrete model systems [4]. In the second part of the seminar, I turn attention to the growth of polycrystalline (CdTe) thin films. Despite the complexity involved in a polycrystalline growth, CdTe surface fluctuations remarkably display statistics in the Kardar-Parisi-Zhang (KPZ) class [5]. I explore the CdTe-KPZ system to inquire about the effect of the substrate temperature and the substrate order on the appearance of KPZ scaling. I show that the former aspect controls the crossover time to KPZ [5], while a disordered substrate induces novel transients to KPZ that are not theoretically accounted yet. That is the case for the Pseudo-steady state (PSS) [6], a growth regime in which neither fluctuations nor correlations grow in time, thus resembling the genuine state steady of finite-size interfaces. Since PSS seems to arise in microscopically distinct systems, it might be a general type of dynamics occurring in interface growth phenomena. These series of works provide compelling evidence of nonequilibrium universal behaviours; they also show how such universality is affected by additional complexities typically not accounted for by idealised theories. References [1] See e.g., M. Henkel, Conformal invariance and critical phenomena. Springer Science (2013) [2] R. A. L. Almeida, K. A. Takeuchi (In preparation) [3] A. J. Bray, Adv. Phys. 51, 481 (2002) [4] L. F. Cugliandolo, C. R. Physique 18, 5 (2017) [5] R. A. L. Almeida et al., Phys. Rev. B 89, 045309 (2014); Europhys. Lett. 109, 46003 (2015) [6] R. A. L. Almeida et al., Sci. Rep. (Nature) 7, 3773 (2017)''