Noriaki Matsunaga

We performed electrical resistivity and $^{13}\mathrm{C}$ nuclear magnetic resonance (NMR) measurements on $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BEDSe}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{FeCl}}_{4}$ under various pressures to obtain insights into the effect of $\ensuremath{\pi}\text{\ensuremath{-}}d$ interactions on the antiferromagnetic (AF) phase. The resistivity shows that the insulating state is maintained up to 2.0 GPa. $^{13}\mathrm{C}$ NMR measurements reveal that the AF transition occurs at 28, 31, and 23 K under 0.5, 1.0, and 2.0 GPa, respectively, indicating that a single AF phase in $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BEDSe}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{FeCl}}_{4}$ is observed under various pressures, whereas two AF phases were observed in $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BEDSe}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{GaCl}}_{4}$. The difference in phase diagrams between the magnetic anion salt, $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BEDSe}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{FeCl}}_{4}$, and the nonmagnetic anion salt, $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BEDSe}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{GaCl}}_{4}$, is attributed to $\ensuremath{\pi}\text{\ensuremath{-}}d$ interactions. These results provide insight into a possible microscopic role of $\ensuremath{\pi}\text{\ensuremath{-}}d$ interactions in the Fe salt. The magnetic properties of $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BEDSe}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{FeCl}}_{4}$ are described by the isolated cluster comprising two dimers and two anions with positive and negative $\ensuremath{\pi}\text{\ensuremath{-}}d$ interactions, suggesting antiferromagnetic coupling between two dimers in the cluster. We also discuss the possible AF structures of $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BEDSe}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{FeCl}}_{4}$.

External pressure and associated changes in lattice structures are key to realizing exotic quantum phases such as high-$T_{\rm c}$ superconductivity. While applying external pressure is a standard method to induce novel lattice structures, its impact on organic molecular crystals has been less explored. Here we report a unique structural phase transition in (DMET-TTF)$_2$AuBr$_2$ under pressure. By combining advanced high-pressure techniques and $ab$ $initio$ calculations, we elucidate that (DMET-TTF)$_2$AuBr$_2$ undergoes a transition from a quasi-one-dimensional lattice to a nearly perfect rectangular lattice at 0.9 GPa. This transition leads to the realization of an antiferromagnetic Mott insulator with $T_{\rm N}=66$ K, the highest $T_{\rm N}$ in low-dimensional molecular crystal solids to date. Upon increasing the pressure, the antiferromagnetic ordering is suppressed, and a superconducting phase with $T_{\rm c}=4.8$ K emerges around 6 GPa. Our study reveals the significant impact of external pressure on lattice structures of organic molecular crystals and highlights the intricate relationship between geometrical frustration and superconductivity. Our findings also pave the way for realizing functional organic molecular crystals through changes in lattice structures by pressure.

Studies of the quasi-two-dimensional organic conductors $\lambda \text{−}{\left(D\right)}_{2}M{\mathrm{Cl}}_4$ [$D=\text{donor molecules}$, $M=\mathrm{Ga},\mathrm{Fe}$] have shown that $\lambda \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{GaCl}}_4$ [BETS = bis(ethylenedithio)tetraselenafulvalene] undergoes an unconventional superconducting transition and $\lambda \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{FeCl}}_4$ undergoes a field-induced superconducting transition. In $\lambda$-type salts, the interactions between donor molecules and ${\mathrm{FeCl}}_4^{-}$ ($\pi \text{−}d$ interactions) are important. To investigate $\pi \text{−}d$ interaction, a pair of magnetic $M=\mathrm{Fe}$ and nonmagnetic $M=\mathrm{Ga}$ salts which have the same ground state in the donor layer is desired. However, no such pair has been found, and few experimental studies have considered $\pi \text{−}d$ interaction paths. $\lambda \text{−}{\left(\mathrm{BEST}\right)}_{2}M{\mathrm{Cl}}_4$ [BEST = bis(ethylenediseleno)tetrathiafulvalene] are obtained for both anions, and $M=\mathrm{Ga}$ salt shows an antiferromagnetic transition, but the ground state has not been analyzed in $M=\mathrm{Fe}$ salt. We perform x-ray diffraction, magnetic susceptibility measurement, and Mössbauer spectroscopy in $\lambda \text{−}{\left(\mathrm{BEST}\right)}_2{\mathrm{FeCl}}_4$. We find that a magnetic transition is observed at around 26 K. The $\lambda \text{−}{\left(\mathrm{BEST}\right)}_{2}M{\mathrm{Cl}}_4$ system is a system in which both ${\mathrm{FeCl}}_4^{-}$ and ${\mathrm{GaCl}}_4^{-}$ salts show antiferromagnetic transitions. In addition, the ethylene motions observed at room temperature are ordered around 108 K, resulting in the establishment of the $\pi \text{−}d$ interaction path between chalcogens and the anion, and low-field magnetization suggests that the $\pi \text{−}d$ interaction in $\lambda \text{−}{\left(\mathrm{BEST}\right)}_2{\mathrm{FeCl}}_4$ is smaller than that in $\lambda \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{FeCl}}_4$. Our results show that the inner chalcogen of donor molecules is important as the path of the interaction.

At ambient pressure, studies of resistivity, magnetic susceptibility, and carbon-13 nuclear magnetic resonance (NMR) were conducted on the quasi-one-dimensional organic conductor ${\left(\text{DMET-}\mathrm{TTF}\right)}_2{\mathrm{AuBr}}_2$. Resistivity measurements showed a broad minimum at approximately 160 K, and the insulator behavior below this temperature and magnetic susceptibility results revealed a dip structure at 22 K. At the same temperature, a sharp peak in the temperature dependence of $T_1^{-1}$ associated with the antiferromagnetic (AFM). transition was found, along with drastic splitting of the NMR spectra, indicating a commensurate AFM structure. The amplitude of the magnetic moments was determined to be $0.06{\mu }_B$/molecule from the hyperfine coupling constant tensor and the angular dependence of the internal field in the AFM phase. The small magnetic moment signifies the AFM nesting type, i.e., commensurate spin density wave. An antiferromagnetic ordering of $(\uparrow \uparrow \downarrow \downarrow )$ along the one-dimensional chain is expected from the $2k_{\mathrm{F}}$ instability. This behavior can be explained by the strong dimerization of the one-dimensional DMET-TTF chain.

Magnetic susceptibility and proton nuclear magnetic resonance $\left({}^1\mathrm{H}\text{−}\mathrm{NMR}\right)$ measurements were performed for the quasi-two-dimensional $\pi \text{−}d$ interacting system $\lambda \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{STF}\right)}_2{\mathrm{FeCl}}_4$ at ambient pressure. Magnetic susceptibility arising from the $3d$ spins of the ${\mathrm{FeCl}}_4$ anion show an anisotropy at low temperature and its temperature dependence for the external field parallel to the $c$ axis is described as a broad peak structure at 8 K. A sharp peak in the temperature dependence of $T_1^{-1}$ associated with the antiferromagnetic (AF) transition is observed at $T_{\mathrm{AF}}=16$ K, together with the drastic splitting of the NMR spectrum below $T_{\mathrm{AF}}$. The relation between the static susceptibility and the splitting of the NMR shift suggests the existence of the relatively strong $d\text{−}d$ AF interaction. These results can be explained by the model considering the AF-coupled $d$-spin system in the AF long-range-ordered $\pi$-spin system. We find that the AF phases in $\lambda$-type salts can be universally explained by this model.