Shuhei Fukuoka

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 $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BEDSe}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{FeCl}}_{4}$.

The effects of a magnetic field on the Mott transition, a longstanding issue in condensed matter physics, are studied using magnetoresistance measurements for organic Mott insulators, $\lambda \text{−}{\left(\text{BETS}\right)}_2{\mathrm{GaBr}}_x{\mathrm{Cl}}_{4-x}$, up to 55 T. We demonstrate that an insulator-to-metal transition with a first-order nature and a successive superconductor-to-insulator-to-metal transition are induced by magnetic fields. We discuss the difference in magnetic susceptibility between the insulating and metallic phases, which is a key factor in inducing these transitions. Field induced transitions in experimentally feasible fields under ambient pressure conditions lead to further experimental verification of Mott transitions.

We performed ${}^{13}\mathrm{C}$ NMR measurements on $\lambda \text{−}{\left(\mathrm{BEDSe}\text{−}\mathrm{TTF}\right)}_2{\mathrm{FeCl}}_4$[BEDSe-TTF = bis(ethylenediseleno) tetrathiafulvalene] to investigate the antiferromagnetic transition in this material and evaluate the exchange interaction $J_{\pi \mathit{d}}$ between the donor molecules and ${\mathrm{FeCl}}_4^{-}$ anions. Based on the NMR spectrum and the nuclear spin-lattice relaxation rate, we determined that the antiferromagnetic transition in the donor layers occurs at 25 K. The structure of the NMR spectra in the antiferromagnetic phase resembles that of $\lambda \text{−}{\left(\mathrm{BEDSe}\text{−}\mathrm{TTF}\right)}_2{\mathrm{GaCl}}_4$. We used the angle dependence of the NMR shift in the paramagnetic phase and the experimentally determined demagnetization factor to estimate $J_{\pi \mathit{d}}$ of $\lambda {\text{-(BEDSe-TTF)}}_2{\mathrm{FeCl}}_4$ as 3.4(7) $\mathrm{T}/\mu {}_{\mathrm{B}}$ . This result shows that the $\pi \text{−}d$ interaction of $\lambda {\text{-(BEDSe-TTF)}}_2{\mathrm{FeCl}}_4$ is weaker than that of $\lambda \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{FeCl}}_4$. The different signs of $J_{\pi \mathit{d}}$ between $\lambda \text{−}{\left(\mathrm{BEDSe}\text{−}\mathrm{TTF}\right)}_2{\mathrm{FeCl}}_4$ and $\lambda \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{FeCl}}_4$ could be attributed to the different paths for the $\pi \text{−}d$ interaction in the two systems.

We have performed ${}^{13}\mathrm{C}$ NMR measurements on $\lambda \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{STF}\right)}_2{\mathrm{Fe}}_x{\mathrm{Ga}}_{1-x}{\mathrm{Cl}}_4$ and evaluated Fe content dependence of the magnetic properties. We found that the angle dependence of the Knight shift of $\lambda \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{STF}\right)}_2{\mathrm{Fe}}_x{\mathrm{Ga}}_{1-x}{\mathrm{Cl}}_4$ changes depending on the Fe content, which is explained by the exchange fields through the $\pi \text{−}d$ interaction and dipole fields. An antiferromagnetic ordering was observed in all Fe content samples except for $\lambda \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{STF}\right)}_2{\mathrm{GaCl}}_4$ ($x=0$). This indicates that the antiferromagnetic ground state is stabilized by the interaction between $\pi$ and $3d$ spins. From the linewidths of the NMR spectra measured at the lowest temperature, we confirmed that the size of the ordered $\pi$ spin moment is independent of the Fe content. Based on the results, we discuss the role of $\pi \text{−}d$ interaction for the stabilization of the antiferromagnetic ordering and the mechanism of the unconventional magnetism of $3d$ spins observed in $\pi \text{−}d$ systems.

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.

We investigate the physical properties of the Shastry-Sutherland lattice material ${\mathrm{BaNd}}_2{\mathrm{ZnO}}_5$. Neutron diffraction, magnetic susceptibility, and specific heat measurements reveal antiferromagnetic order below 1.65 K. The magnetic order is found to be a $2\text{−}\mathit{Q}$ magnetic structure with the magnetic moments lying in the Shastry-Sutherland lattice planes comprising the tetragonal crystal structure of ${\mathrm{BaNd}}_2{\mathrm{ZnO}}_5$. The ordered moment for this structure is 1.9(1) ${\mu }_B$ per Nd ion. Inelastic neutron scattering measurements reveal that the crystal field ground state doublet is well separated from the first excited state at 8 meV. The crystal field Hamiltonian is determined through simultaneous refinement of models with both the LS coupling and intermediate coupling approximations to the inelastic neutron scattering and magnetic susceptibility data. The ground state doublet indicates that the magnetic moments lie primarily in the basal plane with magnitude consistent with the size of the determined ordered moment.