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.

The crystal structure of the p-cyanophenyl nitronyl nitroxide [2-(4-cyanophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1-H-imidazol-1-oxyl 3-N-oxide, abbreviated as p-CNPNN] radical has been solved. It belongs to the orthorhombic system, space group Ic2a, with a= 10.482(2)Å, b= 26.615(5)Å, c=9.903(2)Å and Z= 8. The asymmetric unit contains two independent radical molecules. The intermolecular contacts between the cyano phenyl groups and the nitroxide groups form a quasi-square lattice. The magnetic susceptibility has been measured in the temperature range 1.8–300 K. The temperature dependence of the paramagnetic susceptibility above 4 K can be explained by the ferromagnetic squarelattice Heisenberg model with J/k_B= 0.75 K. Two-dimensionality of the crystal is also confirmed by single-crystal electron paramagnetic resonance (EPR) spectra.

The orbital degree of freedom of electrons greatly influences the physical properties of materials such as magnetic order and unconventional superconductivity. An orbital is a minimal unit of “shape,” and the orbital state can be unraveled by observing the spatial anisotropic distribution of electrons. However, it is difficult to experimentally extract the orbital information in a crystal because of various technical problems. Here, the $\mathrm{Ti}\text{−}3d$ orbital state in perovskite-type oxides $R\mathrm{Ti}{\mathrm{O}}_3$ ($R=\mathrm{Y}$, Sm, and La) is directly determined by a core differential Fourier synthesis (CDFS) method using synchrotron x-ray diffraction. The valence electron-density distribution, including information on the anisotropy and the hybridization between atomic orbitals, can be extracted from the CDFS analysis. Our study not only demonstrates the relationship between the magnetic- and orbital-ordered states called the Kugel-Khomskii model, but also provides a nontrivial picture of the orbital state reconstructed by the orbital hybridization.