Charge glass by supercooling topological-ordered liquid

The charge glass phenomenon, observed in a group of triangular-lattice-based organic compounds, θ-(BEDT-TTF)₂X(SCN)₄, has been drawing considerable attention in recent years [1]. These compounds commonly show charge ordering at low temperatures. However, the ordering transition is prohibited, or considerably delayed, depending on the speed of cooling the system, and this dynamical obstruction of charger ordering is called as a charge glass phenomenon. Several remarkable features accompany this phenomenon. Firstly, charge order grows in an experimentally observable time range, only when the system is cooled to just below the critical temperature. Secondly, the time scale of ordering process has a minimal value at so-called nose temperature, at which the qualitative change of ordering dynamics has also been reported [2]. And thirdly, the initial ordering process proceeds quite slowly compared with typical crystallization process, especially as observed in X=TlZn system [3].

We show that this glassy behavior can be attributed to the topological nature of the normal phase in the high-temperature region, where the geometrical frustration of the lattice structure gives rise to global conserved quantities, which we name as fluxes. To enable the charge ordering, the value of the conserved fluxes must be altered by the motion of fractional charges conjugate to the flux, which we name as triplets. As a consequence, the charge ordering dynamics is severely constrained by the time scale of the diffusive motion of triplets, and it delays the formation of ordered domains.

To verify this scenario, we adopt the V1-V2-V3 Ising model on a triangular lattice, representing the presence/absence of electron charge as an Ising variable, and address its relaxation dynamics after the sudden cooling to low temperatures by kinetic Monte Carlo method. As a result, we could reproduce the time-temperature-transformation (TTT) diagram with the nose temperature, as obtained in experiments. We further showed that the system exhibits unusually slow initial dynamics, as characterized by small Avrami exponent. These findings well explain the main features of the charge glass phenomenon, and provide a basic viewpoint to further explore the mysterious properties of the charge glass phenomenon [4].

[1] F. Kagawa et al., Nature Physics 9, 419–422 (2013).

[2] H. Murase et al., Nature Communications 14, 6011 (2023).

[3] S. Sasaki et al., Science 357, 1381–1385 (2017).

[4] K. Kimata, H. Ikeda, and M. Udagawa, in preparation