Combined experimental and theoretical studies on glasslike transitions in the frustrated molecular conductors $\theta$-(BEDT-TTF)${}_{2}M{M}^{\mathrm{\prime }}$(SCN)${}_4$

We present results of the coefficient of thermal expansion for the frustrated quasi-two-dimensional molecular conductor $\theta$-(BEDT-TTF)${}_2$RbZn(SCN)${}_4$ for temperatures 1.5 K $\le T\le$ 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 $T_{\mathrm{g}}$ = 90-100 K and $T_{\mathrm{g}}^{†}$ = 120-130 K, similar to previous findings for $\theta$-(BEDT-TTF)${}_2$CsZn(SCN)${}_4$ and $\theta$-(BEDT-TTF)${}_2$CsCo(SCN)${}_4$, reported in T. Thomas et al., Phys. Rev. B 105, L041114 (2022), both of which lack the charge-order/structural transition. Our findings indicate that these glasslike transitions are common features for the $\theta$-(BEDT-TTF)${}_{2}M{M}^{\mathrm{\prime }}$(SCN)${}_4$ family with $M$ = (Rb, Cs) and $M^{\mathrm{\prime }}$ = (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 endgroups of the BEDT-TTF molecules for various $\theta$-(BEDT-TTF)${}_{2}M{M}^{\mathrm{\prime }}$(SCN)${}_4$ salts, predicting two different conformational glass transitions. Moreover, calculations of the hopping integrals show a substantial degree of dependence on the endgroups’ conformation, suggesting a significant coupling to the electronic degrees of freedom. Our findings support the possibility that the glassy freezing of the ethylene endgroups could drive or enhance glassy charge dynamics.