Xun-Wang Yan

Van der Waals density functional study of the structural and electronic properties of La-doped phenanthrene

By the first principle calculations based on the van der Waals density functional theory, we study the crystal structures and electronic properties of La-doped phenanthrene. Two stable atomic geometries of La₁phenanthrene are obtained by relaxation of atomic positions from various initial structures. The structure-I is a metal with two energy bands crossing the Fermi level, while the structure-II displays a semiconducting state with an energy gap of 0.15 eV, which has an energy gain of 0.42 eV per unit cell compared to the structure-I. The most striking feature of La₁phenanthrene is that La 5d electrons make a significant contribution to the total density of state around the Fermi level, which is distinct from potassium doped phenanthrene and picene. Our findings provide an important foundation for the understanding of superconductivity in La-doped phenanthrene.

Ba2phenanthrene is the main component in the Ba-doped phenanthrene superconductor

We have systematically investigated the crystal structure of Ba-doped phenanthrene with various Ba doping levels by the first-principles calculations combined with the X-ray diffraction (XRD) spectra simulations. Although the experimental stoichiometry ratio of Ba atom and phenanthrene molecule is 1.5:1, the simulated XRD spectra, space group symmetry and optimized lattice parameters of Ba1.5phenanthrene are not consistent with the experimental ones, while the results for Ba2phenanthrene are in good agreement with the measurements. The strength difference of a few XRD peaks can be explained by the existence of pristine phenanthrene. Our findings suggest that instead of uniform Ba1.5phenanthrene, there coexist Ba2phenanthrene and undoped phenanthrene in the superconducting sample. The electronic calculations indicate that Ba2phenanthrene is a semiconductor with a small energy gap less than 0.05 eV.

Structural and electronic properties of potassium-doped 1,2;8,9-dibenzopentacene superconductor: comparing with doped [7]phenacenes

ABSTRACT Polycyclic aromatic hydrocarbons doped by metal have exhibited the potential of high temperature superconductivity. Understanding the basic properties of materials is the key to reveal the superconductivity. Here, a systemically theoretical study has been done to explore crystal structures and electronic properties of pristine and potassium-doped 1,2;8,9-dibenzopentacene, compared with [7]phenacenes case. We determined that vdW-DF2 functional is more suitable to describe the non-local interaction in a molecular crystal. Based on this functional, we predicted the crystal structures and investigated in detail the K atomic positions in a system. It was found that the intralayer doping leads to lower total energy. From the calculated formation energy, for each 1,2;8,9-dibenzopentacene molecule, the doping of two electrons is more stable under the relatively K-poor condition while the doping of four electrons is more stable under the K-rich condition. Between these two phases, the three-electron doping phase stabilises in a narrow region of K chemical potential. Combining with the electronic states at Fermi level, we analysed the reasons of superconductivity enhancement in doped 1,2;8,9-dibenzopentacene. This work further deepens the understanding of 1,2;8,9-dibenzopentacene superconductor.

Magnetic moment and spin state transition on rare monovalent iron ion in nitridoferrate Ca6Li0.5Fe0.5Te2N3

By first principles simulation, we studied the magnetic properties of Ca6(Li0.5Fe0.5)Te2N3 including the spin moment, orbital moment, and pressure-driven spin state transition of Fe ion from the high-spin (S = 3/2) to the low-spin (S = 1/2) state. Our spin-polarized calculations have correctly revealed that Ca6(Li0.5Fe0.5)Te2N3 is a magnetic semiconductor with a band gap of 0.49 eV. The spin moment around the Fe ion with a 3d7 configuration is 3.0 μB, and the orbital moment is 0.15 μB, which is contrary to the large orbital moment suggested by experiment. With increasing pressure, a high-spin to low-spin state transition with an Fe moment collapse from 3 μB to 1 μB occurs at 18 GPa, accompanied by a semiconductor to metal transition. The underlying mechanism is explained in terms of the interplay between the crystal field and the electron population of the Fe 3d orbitals.

Pressure-induced ferromagnetic half-metallicity in cobaltocene

The electronic and magnetic properties in organometallic compound cobaltocene under pressure have been investigated by the first-principles calculations based on the van der Waals density functional theory. At ambient pressure, cobaltocene lies in the paramagnetic state, which is consistent with the experimental measurements. With increasing pressure, the paramagnetic phase evolves into the ferromagnetic semiconducting phase. When pressure exceeds 60 GPa, the closing of gap between valent and conducting bands results in the ferromagnetic half-metallicity in cobaltocene. The formation of the metallic state can be understood in terms of the orientation and hybridization of Co d xz , d yz and C p z orbitals. We also find that the ferromagnetic half-metallicity arises at much lower pressure upon doping rhodocene into cobaltocene. Our results provide a new route to realize the half-metallicity in cobaltocene and similar metallocene compounds.

The atomic structures and electronic properties of potassium-doped phenanthrene from a first-principles study

The atomic structures and electronic properties of potassium-doped phenanthrene (PHA) with various doping levels have been investigated using first principles calculations based on van der Waals density functional theory (DFT). Our results show that the possible stoichiometric content of potassium in KxPHA compounds is x = 1 or x = 2, corresponding to the structural phases K1-A, K1-B or K2-A. The formation energy of the K2-A phase is −0.32 eV per K atom, which is comparable to the energy of KC8. These K3PHA phases, mentioned in previous reports, have a positive formation energy larger than +0.24 eV per K atom, which should not exist in samples. K2PHA in the K2-A phase is a metal with three bands crossing the Fermi level, and the Fermi surface sheet along Γ–Z has a cylinder shape associated with the two-dimensional nature of the electronic states. Our calculations suggest that the K2-A phase is the most stable and reasonable structure in K-doped PHA compounds.