Strange-metal behavior in correlated molecular conductors

The $\kappa$-BEDT-TTF salts are prototypical Mott systems, realizing a correlation-driven electronic transition from a Fermi-liquid state to a Mott insulator. The metallic compounds with spatially modulated Hg-containing anions, $\kappa$-(BEDT-TTF)${}_4$Hg${}_{3-\delta }{X}_8$ ($X$ = Br, and Cl), however, fall outside this classification. Our temperature-dependent transport and broad-range optical investigations reveal a $\rho (T)\propto T$ dependence of the resistivity up to elevated temperatures and a power-law behavior of the frequency-dependent conductivity $\mathrm{\mid }\widehat{\sigma }(\omega )\mathrm{\mid }\propto {\omega }^{-0.7}$; well-known hallmarks of ``strange metals’'. The linear-in-frequency scattering rate of these strange metals scales to the Planckian dissipation. We demonstrate that this response only occurs in systems with incommensurate anion layers and establish a direct link to large-scale spatial modulations. Our conclusions can be extended to other strongly correlated strange-metal compounds