Motivated by the recent discovery of Dirac nodal line in the single-component molecular conductor [Pt(dmdt) 2 ], we propose a three-orbital tight-binding model based on the Wannier fitting of the first-principles calculation, and address the problems of edge states, topological properties and magnetic susceptibility. We find that logarithmic peaks of the local density of states emerge near the Fermi energy, owing to pseudo-one-dimensional edge states that appear between the Dirac nodal lines. Magnetic susceptibility calculated in our model can explain the experimental result at a high temperature. In the presence of a realistic spin-orbit coupling, we show that [Pt(dmdt) 2 ] is a topological nodal line semimetal with isolated electron and hole pockets.
We construct three-orbital tight-binding models describing single-component molecular conductors [Pt(dmdt)2] and [Ni(dmdt)2] using first-principles calculations. We show that [Ni(dmdt)2] is a Dirac nodal line system with highly one-dimensional edge states at the (001) edge, similar to [Pt(dmdt)2], as demonstrated in prior studies. To investigate possible edge magnetism, we calculate longitudinal and transverse spin susceptibilities using real-space-dependent random-phase approximation (RPA) in three-orbital Hubbard models in the presence of spin-orbit coupling. We find that the edge spin-density wave (SDW) is induced by the Coulomb repulsion and incommensurate nestings of the Fermi arcs. We also find that the magnetic structure of the edge SDW can be changed via extremely small carrier doping, which is controllable in molecular conductors.
Traditional molecular conductors are composed of more than two chemical species. Two prerequisites for the design of molecular metals have long been considered to be 1) forming of the electronic band and 2) existence of charge carriers created by the intermolecular charge transfer between the molecules constructing the band and other chemical species. On the other hand, a single-component molecular metal, [Ni(tmdt)2] (tmdt = trimethylenetetrathiafulvalenedithiolate), was developed in 2001; it is a planar nickel complex coordinated by the extended-TTF dithiolate ligands, tmdt from both sides. Since then, various types of single-component molecular conductors with a variety of extended-TTF dithiolate ligands have been developed. In this account, we briefly describe the recent progress in research on single-component molecular conductors. First, single-component molecular conductors in isostructural systems, [M(tmdt)2] (M = Ni, Pd, Pt, Au, and Cu) are described. Recent orbital-selective 13C and 1H NMR experiments have genealogically elucidated the differences in the electronic states and physical properties of these systems, that is, their various unusual phenomena are produced from their multi-orbital correlated π or π-d electron systems. Next, we describe [Ni(hfdt)2] (hfdt = bis(trifluoromethyl)tetrathiafulvalenedithiolate), the first single-component molecular superconductor, which was revealed by high-pressure resistivity measurements with a diamond anvil cell (DAC). The superconducting transition occurred around 7.5–8.7 GPa with a maximum Tc (onset temperature) of 5.5 K. Recent theoretical calculation has revealed that [Ni(hfdt)2] will be a new molecular Dirac electron system. In the final section, we briefly introduce molecular Dirac electron systems. Recently, a new series of semimetals, [M(dmdt)2] (M = Pt and Ni; dmdt = dimethyltetrathiafulvalenedithiolate) was synthesized. They belong to a three-dimensional ambient-pressure molecular massless Dirac electron system. The first-principles band structure calculations of [M(dmdt)2] (M = Pt and Ni) revealed that Dirac cones emerge along the a* direction and form Dirac nodal lines.
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