Combined experimental and theoretical studies on glasslike transitions in the frustrated molecular conductors \(\theta\)-(BEDT-TTF)\(_2MM'\)(SCN)\(_4\)

• Published in: arXiv.org 2023-10
• Main Authors: Saito, Yohei, Ganter, Owen, Shang, Chao, Hashimoto, Kenichiro, Sasaki, Takahiko, Winter, Stephen M, Müller, Jens, Lang, Michael
• Format: Article
• Language: English
• Subjects: Conductors; Coupling (molecular); Ethylene; Freezing; Mathematical analysis; Phase transitions; Thermal expansion
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2310.16637

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 \(\leq T \leq\) 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 \(T_{\mathrm{g}}^\dagger\) = 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)\(_2MM^\prime\)(SCN)\(_4\) family with \(M\) = (Rb, Cs) and \(M^\prime\) = (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)\(_2MM^\prime\)(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.