Combined experimental and theoretical studies on glasslike transitions in the frustrated molecular conductors θ-(BEDT-TTF)_{2}MM^{′}(SCN)_{4}

Abstract

We present results of the coefficient of thermal expansion for the frustrated quasi-two-dimensional molecular conductor θ-(BEDT-TTF)_{2}RbZn(SCN)_{4} for temperatures 1.5≤T≤290 K. A pronounced first-order phase transition anomaly is observed at the combined charge-order/structural transition at 215 K. Furthermore, clear evidence is found for two separate glasslike transitions at T_{g}=90–100 K and T_{g}^{†}=120–130 K, similar to previous findings for θ-(BEDT-TTF)_{2}CsZn(SCN)_{4} and θ-(BEDT-TTF)_{2}CsCo(SCN)_{4} reported by Thomas et al. [Phys. Rev. B 105, L041114 (2022)2469-995010.1103/PhysRevB.105.L041114], both of which lack the charge-order/structural transition. Our findings indicate that these glasslike transitions are common features for the θ-(BEDT-TTF)_{2}MM^{′}(SCN)_{4} family, with M=(Rb, Cs) and M^{′}=(Co, Zn), irrespective of the presence or absence of charge order. These results are consistent with our model calculations on the glasslike dynamics associated with the flexible ethylene end groups of the BEDT-TTF molecules for various θ-(BEDT-TTF)_{2}MM^{′}(SCN)_{4} salts, predicting two different conformational glass transitions. Moreover, calculations of the hopping integrals show a substantial degree of dependence on the end groups' conformation, suggesting a significant coupling to the electronic degrees of freedom. Our findings support the possibility that the glassy freezing of the ethylene end groups could drive or enhance glassy charge dynamics.