We investigated the precise crystal structures and electronic states of a quasi-two-dimensional molecular conductor $\alpha \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{I}}_3$ at ambient pressure. The electronic resistivity of this molecular solid shows metal-to-insulator (MI) crossover behavior at $T_{\mathrm{MI}}=50\mathrm{K}$. Our x-ray diffraction and ${}^{13}\mathrm{C}$ nuclear magnetic resonance experiments revealed that $\alpha \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{I}}_3$ maintains the inversion symmetry below $T_{\mathrm{MI}}$. First-principles calculations found a pair of anisotropic Dirac cones at a general $k$ point, with the degenerate contact points at the Fermi level. The origin of the insulating state in this system is a small energy gap of $\sim 2\mathrm{meV}$ opened by the spin-orbit interaction. The $Z_2$ topological invariants indicate that this system is a weak topological insulator. Our results suggest that $\alpha \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{I}}_3$ is a promising material for studying the bulk Dirac electron system in two dimensions.

Hydrogen-induced modification of electronic structures, magnetic states, and crystal structures of transition metal ($\text{TM}=\text{Fe}$, Co, and Ni) monohydrides was investigated using TM $K$-edge x-ray magnetic circular dichroism (XMCD), x-ray diffraction, and first-principles calculations. The TM hydrides undergo narrowing of TM $3d$ density of states (DOS) and significant shifts in the Fermi energy $E_{\text{F}}$ due to hydrogenation, which is responsible for the magnetic properties of the TM hydrides: ferromagnetic FeH, ferromagnetic CoH, and paramagnetic NiH. The reconstruction of the electronic structure is mainly attributed to the appearance of bonding and antibonding states together with hydrogen-induced volume expansion. We demonstrate that the characteristic XMCD profile of the TM hydrides near the absorption edge probed the reconstruction of the electronic structure above $E_{\text{F}}$. The pressure dependence of XMCD revealed that the ferromagnetic state of FeH is less stable than that of CoH under pressure. The different hydrogenation processes between CoH and NiH at room temperature are reported by means of x-ray diffraction.

The electronic structure of an isostructural series of molecular conductors, {\beta}'-X[Pd(dmit)_2]_2 is systematically studied by a first-principles method based on the density-functional theory. The calculated band structures are fitted to the tight-binding model based on Pd(dmit)_2 dimers on the triangular lattice. We find systematic variation in the anisotropy of the transfer integrals along the three directions of the triangular lattice taking different values. The transfer integral along the face-to-face stacking direction of Pd(dmit)_2 dimers is always the largest. Around the quantum spin liquid, X = EtMe_3Sb, the other two transfer integrals become comparable. We also report sensible differences in the distribution of wavefunctions near the Fermi level between the two dmit ligands of the Pd(dmit)_2 molecule.Comment: 4 pages, 5 figure

We systematically derive low-energy effective Hamiltonians for molecular solids ${\beta }^{\prime }\text{−}X{\left[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2\right]}_2$ ($X$ represents a cation) using ab initio density functional theory calculations and clarify how the cation controls the interdimer transfer integrals and the interaction parameters. The effective models are solved using the exact diagonalization method and the antiferromagnetic ordered moment is shown to be significantly suppressed around the spin-liquid candidate of $X=\mathrm{Et}{\mathrm{Me}}_3\mathrm{Sb}$, which is reported in experiments. We also show that both the geometrical frustration and the off-site interactions play essential roles in the suppression of antiferromagnetic ordering. This systematic derivation and analysis of the low-energy effective Hamiltonians offers a firm basis to clarify the nature of the quantum spin liquid found in ${\beta }^{\prime }\text{−}\mathrm{Et}{\mathrm{Me}}_3\mathrm{Sb}{\left[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2\right]}_2$.