First Principles Study Of Structural And Superconducting Properties Of Aromatic Hydrocarbon Systems

Superconducting materials have always been a hot research topic in physics and materials sciences due to its important applications in energy,information,quantum devices and other advanced technologies.Researchers have been devoted to the design and synthesis of new superconducting materials,the understanding of superconducting mechanism and the exploration of superconductors with higher critical temperature.The discovery of superconductivity in metal doped polycyclic aromatic hydrocar-bons?PAHs?has revitalized the research interest in the field of superconductors.PAH compounds are light and low-dimensional,making them promising candidates for su-perconductors with high superconducting transition temperature.In addition,the strong correlation effects of electrons play an important role in the superconducting behaviors.So PAHs can provide a novel system and an opportunity to understand the supercon-ducting mechanism of organic superconductors.On the other hand,PAH compounds are widely applied to manufacturing organic light-emitting devices?OLEDs?which have been considered as the next generation of flat panel displays.Despite vital theoretical research value and potential application,precise nature of PAH superconducting phases,such as the structural characteristics induced by alkali and rare-earth metal doping and superconducting mechanism,are still unknown.In this thesis,the crystal structures,growth evolution,electronic properties,magnetic charac-teristics,electronic correlation effects and superconducting mechanism of metal doped?A= K,Sm?PAHs are systematically discussed using first-principles density functional theory simulations.Our work can be summarized as follows.1.Based on the first-principles calculations with the Van der Waals functional cor-rection,we predicted the geometrical and the electronic structures of Kx-doped chry-sene?x=1,2,3 and 4?.We found that the K doping results in the phase transition from C₂/c to P2₁ symmetry.The result of the formation energies shows that K₂-doped chry-sene is the most stable and can easily be fabricated.K₂chrysene is still a semiconductor,but K₂ chrysene with small charge fluctuation can behave as a metal and is thermody-namically stable in a certain range of K chemical potential.The charge density differ-ence distribution indicates that K dopant only plays a role to supply electrons.So we suggest that the superconductive chrysene shall occur at the phase region of K₂C₁₈H₁₂ with a small chargefluctuation.2.The crystal,electronic structures and magnetic characteristics of Sm-doped chrysene have been investigated by the first-principles calculation using the generalized gradient approximation?GGA?plus U method,also including van der Waals correction. We find that Sm-doped chrysene with C₂/c space group is the most stable where doped Sm atoms stay on the relative middle of chrysene molecules.This material is stabilized at antiferromagnetic ground-state with the metallic feature.The spin moment and or-bital moment of Sm atom at the ground-state is respectively 5.9 ?B and-1.6 ?B,which shows that two electrons are transferred from Sm atom to chrysene molecule.Within the framework of GGA+U,the C-2p electronic states mainly contribute to the Fermi sur-face.Electronic correlation effects are significant to understand the superconductivity in Sm-doped chrysene.3.We studied the crystal structure and electronic properties of the pristine and doped p-Terphenyl by the first-principles calculations.The total energy calculations indicate that both K₁C₁₈H₁₄ and K₂C₁₈H₁₄ are stabilized at P2₁ symmetry while K₃C₁₈H₁₄ at P2₁/c symmetry.The K-doped p-Terphenyl in the considered range is found to be metallic with two bands crossing Fermi level.The doping results in the expansion of system volume.The charge transferred to p-Terphenyl molecule from K atom is concentrated on C-C bonds within the ring closing to the bridge bonds.The formation energy indicates that the doping of two electrons into p-Terphenyl is easier to be fabricated and corresponding to the 7.2 Kelvin superconducting phase in experiment.4.The crystal structures,electronic properties and doping evolution characteristics of K doped phenanthrene in surface adsorption situation are systematically discussed using slab model.We build slab model to explore whether and how K atoms are doped into the phenanthrene crystals by DFT calculations.We found that the crystal structures where K atoms are adsorbed on the surface are unstable compared with the ones that K atoms are doped into the bulk phase.So we speculate that K atoms can be doped into the phenanthrene bulk phase and can locate in intralayer region of phenanthrene molecules.We obtain that K doped phenanthrene in the surface adsorption situation can also be metallic from the view of density of states.Electrons of K atoms transfer to organic molecules and thus can induce superconductivity in the K doped phenanthrene system.The crystal structures,magnetic characteristics,electronic properties and doping evolution characteristics of aromatic hydrocarbons are systematically discussed in this study.Our research is helpful for achieving a better understanding of the superconduct-ing behavior in PAHs and provides a route to obtain high temperature superconductors.