Landau suggested that the low-temperature properties of metals can be understood in terms of long-lived quasiparticles with all complex interactions included in Fermi-liquid parameters, such as the effective mass m^⋆. Despite its wide applicability, electronic transport in bad or strange metals and unconventional superconductors is controversially discussed towards a possible collapse of the quasiparticle concept. Here we explore the electrodynamic response of correlated metals at half filling for varying correlation strength upon approaching a Mott insulator. We reveal persistent Fermi-liquid behavior with pronounced quadratic dependences of the optical scattering rate on temperature and frequency, along with a puzzling elastic contribution to relaxation. The strong increase of the resistivity beyond the Ioffe–Regel–Mott limit is accompanied by a ‘displaced Drude peak’ in the optical conductivity. Our results, supported by a theoretical model for the optical response, demonstrate the emergence of a bad metal from resilient quasiparticles that are subject to dynamical localization and dissolve near the Mott transition.

$\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_{2}X$ salts exhibit a wide range of properties from superconductivity to magnetic ordering. The phase diagram of this system has been understood as the competition between anti-ferromagnetic insulating behavior and superconductivity. The $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_{2}X$ system has been well explained using a parameter, U/W, as well as the high $T_c$ cuprates. $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{Cu}}_2{\left(\mathrm{CN}\right)}_3$, however, behaved as an insulator without magnetic ordering. This salt cannot be described by the phase diagram described by the competition between superconductivity and magnetism. It is interesting to explore the nature of this insulating phase with an eye toward understanding the universal phase diagram of the $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_{2}X$ system. Magnetic frustration due to the triangular spin lattice has recently been reported for $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{Cu}}_2{\left(\mathrm{CN}\right)}_3$ from ${}^1\mathrm{H}$-nuclear magnetic resonance (NMR) study. To investigate the insulating properties of this salt, we characterized it using ${}^{13}\mathrm{C}\text{−}\mathrm{NMR}$. In contrast to previous ${}^1\mathrm{H}\text{−}\mathrm{NMR}$ studies, at low temperatures, we observed line broadenings that were proportional to the hyperfine coupling constants, which suggested inhomogeous electron localization. We also observed antiferromagnetic fluctuations which, as is the case with other salts, intensified with decreasing temperature. To understand the electronic properties of $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_{2}X$ system, in addition to the U/W, the electron localization effect from the relatively narrow bandwidth is important.

To verify the effect of geometrical frustration, we artificially distort the triangular lattice of the quasi-two-dimensional organic conductor $\kappa -{(\mathrm{BEDT}-\mathrm{TTF})}_2{\mathrm{Cu}}_2{\left(\mathrm{CN}\right)}_3$ [BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene] by analogous-molecular substitution and apply ${}^{13}\mathrm{C}$ NMR of bulk and substituted sites, electric conductivity, and static magnetic susceptibility measurements. The results indicate that the magnetic characteristics of the substituted sample are quantitatively similar to those of the pure sample. Moreover, the magnetic characteristics at the substituted sites are also the same as in the bulk. These results suggest that the observed magnetic properties may not be due to the geometrical frustration but the importance of disorder.

We measured the ${}^{13}\mathrm{C}$-NMR spectrum and $T_1$ of the quasi-two-dimensional organic superconductor $\kappa \mathrm{\text{−}}(\mathrm{BEDT}\mathrm{\text{−}}\mathrm{TTF}{)}_2\mathrm{Cu}(\mathrm{NCS}{)}_2$ under pressure. This material was thought to show a relationship between $T_c$ and the effective cyclotron mass $m_c^{*}$, obtained from the Shubnikov–de Haas (SdH) effect. We found that $\kappa \mathrm{\text{−}}(\mathrm{BEDT}\mathrm{\text{−}}\mathrm{TTF}{)}_2\mathrm{Cu}(\mathrm{NCS}{)}_2$ behaved as a Fermi liquid at low temperature under all pressures, and antiferromagnetic fluctuations were expected. The pressure dependence of the Korringa factor is similar to that of the effective cyclotron mass $m_c^{*}$, suggesting that antiferromagnetic fluctuations contribute to the superconductivity of this material. We also found that, under pressure, $T^{*}$ was shifted to 150 K, the temperature characteristic of the shift from bad metal to good metal.

In order to investigate the nature of the insulator phase of $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{Cu}}_2{\left(\mathrm{C}\mathrm{N}\right)}_3$ [where BEDT-TTF represents bis(ethylenedithio)terathiofulvalene] and why this salt is a superconductor under pressure but not under ambient pressure, we examined the ${}^{13}\mathrm{C}$ NMR spectra and spin lattice relaxation rate of $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{Cu}}_2{\left(\mathrm{C}\mathrm{N}\right)}_3$ under pressure. The spin susceptibility evaluated from the Knight shift decreased with increase in pressure, and line broadening was suppressed above a critical pressure. The temperature dependence of ${\left(T_{1}T\right)}^{-1}$ for $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{Cu}}_2{\left(\mathrm{C}\mathrm{N}\right)}_3$ above a critical pressure was that for $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_{2}X$, where $X$ was $\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{C}\mathrm{N}\right)}_2\right]\mathrm{Br}$ or $\mathrm{Cu}{\left(\mathrm{N}\mathrm{C}\mathrm{S}\right)}_2$. These results suggest that the electronic structure of $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{Cu}}_2{\left(\mathrm{C}\mathrm{N}\right)}_3$ under pressure is similar to that of other $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_{2}X$ salts. The results support our model that electron inhomogeneity and hopping conduction, which are common features of semiconductivity in $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_{2}X$ at high temperatures, are responsible for the insulator phase in $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{Cu}}_2{\left(\mathrm{C}\mathrm{N}\right)}_3$. The electron inhomogeneity suppresses the semiconductor-metal transformation and superconductivity observed in other $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_{2}X$ salts.

In the temperature-pressure phase diagram of the organic superconductor (TMTSF)2PF6 (TMTSF: tetramethyltetraselenafulvalene), the superconducting phase and the spin-density-wave (SDW) phase are adjacent to each other. This salt exhibits non-Fermi-liquid (NFL) behavior and superconductivity under pressure. Its superconductive property does not exist at higher pressures where Fermi-liquid (FL) behavior is exhibited. In order to investigate the origin of NFL behavior, systematic C13-NMR measurement of this salt has been assessed under pressure in a wide temperature range. At low temperatures, (T1T)-1 increases, and this increase is suppressed by the increasing pressure. These results suggest that applying pressure alters the electron system from the NFL to the FL state, and that antiferromagnetic fluctuations contribute to the origin of NFL behavior. The connection between the emergence of FL behavior and the disappearance of superconductivity suggests that the SDW fluctuation relates to the mechanism of the superconductivity in (TMTSF)2PF6.