Quasiparticle to local moment crossover in bad metals
Non-Fermi-liquid charge transport in the vicinity of electronic instabilities has been intensely studied for decades. Deviations from $\rho_{\rm FL}=\rho_0+AT^2$ in bad and strange metals are commonly ascribed to a breakdown of Landau's quasiparticle (QP) concept. Yet, it remains unclear what mechanism drives the temperature dependence of $\rho(T)$ beyond $\rho_{\rm FL}$. Here, we examine the bad metal upon approaching the Mott metal-insulator transition via chemical pressure in $\kappa$-[(BEDT-STF)$_x$(BEDT-TTF)$_{1-x}$]$\rm _2 Cu_2 (CN)_3$. Through nuclear magnetic resonance (NMR) and transport experiments on the same single crystals, we directly link the onset of deviations from Korringa law $(T_1T)^{-1} = \mathrm{const.}$ with the rise of $\rho(T)$ beyond $\rho_{\rm FL}$. From the NMR relaxation rate, we can identify the gradual crossover between the QP-dominated regime at low $T$ to predominant local moments at higher $T$. By comparing our experimental findings with dynamical mean-field theory calculations, which accurately reproduce the transport data, we reveal how this crossover is reflected in $T$-dependent changes of the QP spectrum. Near the Mott insulator, where $d\rho/dT<0$ at high $T$, an Einstein-relation analysis shows that bad-metal behavior with $d\rho/dT>0$ is driven by the temperature dependence of the electronic compressibility rather than the diffusion constant.