We examined high-pressure electronic structure of a single-component molecular conductor [Pd(dddt)$_2$] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) at room temperature, based on the crystal structure determined by single crystal synchrotron X-ray diffraction measurements at 5.9 GPa. The monoclinic unit cell contains four molecules that form two crystallographically independent molecular layers. A tight-binding model of 8 $\times$ 8 matrix Hamiltonian gives an electronic structure as a Dirac electron system. The Dirac point describes a loop within the first Brillouin zone, and a nodal line semimetal is obtained. The noticeable property of the Dirac cone with a linear dispersion is shown by calculating density of states (DOS). The Dirac cone in this system is associated with the crossing of HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) bands, which originates from the direct interaction between different molecular layers. This is a newly found mechanism in addition to the indirect one [J. Phys. Soc. Jpn., {\bf 86}, 064705 (2017)]. The Dirac points emerge as a line, when the HOMO and LUMO bands meet on the surface and the HOMO-LUMO couplings are absent. Such a mechanism is verified using a reduced model of 4 $\times$ 4 matrix Hamiltonian. The deviation of the band energy ($\delta E$) at the Dirac point from the Fermi level is very small ($\delta E < $ 0.4meV). The nodal line is examined by calculating the parity of the occupied band eigen states at TRIM (Time Reversal Invariant Momentum) showing that the topological number is 1.

中文翻译：

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高压下单组分分子导体 [Pd(dddt)2] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) 的电子结构

我们研究了单组分分子导体 [Pd(dddt)$_2$]（dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate）在室温下的高压电子结构，基于在 5.9 GPa 下通过​​单晶同步加速器 X 射线衍射测量确定的晶体结构。单斜晶胞包含形成两个晶体学独立分子层的四个分子。8 $\times$ 8 矩阵哈密顿量的紧束缚模型给出了作为狄拉克电子系统的电子结构。狄拉克点描述了第一布里渊区内的一个环，并获得了一个节点线半金属。通过计算态密度 (DOS) 显示了具有线性色散的狄拉克锥的显着特性。该系统中的狄拉克锥与 HOMO（最高占据分子轨道）和 LUMO（最低未占据分子轨道）带的交叉有关，这源于不同分子层之间的直接相互作用。除了间接机制之外，这是一种新发现的机制 [J. 物理。社会。Jpn., {\bf 86}, 064705 (2017)]。当 HOMO 和 LUMO 带在表面相遇并且不存在 HOMO-LUMO 耦合时，狄拉克点显示为一条线。使用 4 $\times$ 4 矩阵哈密顿量的简化模型验证了这种机制。狄拉克点的能带能量 ($\delta E$) 与费米能级的偏差非常小 ($\delta E < $ 0.4meV)。