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Cooperative Mercury Motion in the Ionic Conductor Cu2HgI4

In glass-forming materials particles increasingly move together as the glass transition is approached . Such cooperativity is also found in other arrested systems and seems to be intimately connected to the slow dynamics . Here we report on the observation of large-scale dynamic correlations in a distinctly non-glassy system – the conductive phase of the ionic conductor Cu2HgI4 [1] . Using carefully designed nuclear magnetic resonance experiments we prove that mercury ions are the main contributors to conduction (establishing Cu2HgI4 as the first known mercury conductor), and show that mercury diffusion is anomalous . Nonlinear conductivity measurements, used as a probe for dynamical heterogeinity, reveal a characteristic correlation timescale tau_corr . The same timescale is confirmed by stimulated echo NMR measurements, where the fraction of ions trapped after tau_corr can be directly measured . To explain the cooperativity we propose a simple model with two essential ingredients – disorder and existence of two kinds of particles, slow (copper) and fast (mercury). We compare the results with recent studies of arrested and glass-forming materials [2, 3], thus establishing an unexpected connection between seemingly different fields.

ionic conductor; NMR; nonlinear conductivity; cooperativity

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