Abstract
The discovery of superconductivity above 30 K in aromatic hydrocarbons in the past years has been an exciting event in condensed matter physics, chemistry, and materials. Superconductivity was realized by simply introducing electrons in the network made by carbon and hydrogen - the two components for organic systems. However, the accurate amount of doped electrons and their positions have not been established experimentally. No agreement has been reached for the mechanism of their superconductivity. Here, the first principles theory with the combination of electron correlations is used to address these problems. Our calculations show that both the single-ring compound (benzene) and the two-benzene rings compound (naphthalene) are superconductors with transition temperatures (Tc’s) of 6.2 K and 5.8 K respectively after upon doing of potassium. By examining the existing experimental data carefully, we find that there exists a unique superconducting phase with Tc in the range of 5 - 7 K range for all aromatic hydrocarbons. The almost constant low density of states at Fermi level with about two electrons doping accounts for this unified phase. The high density of states at Fermi level upon three-electron doping seems responsible for the high-Tc phase in these hydrocarbons with long benzene rings. Meanwhile, the electronic correlations also increase with increasing the number of benzene rings. These findings offer clues to the understanding of the superconductivity as well as to the search of new superconductors in this family.
Highlights
Organic materials with a crystal structure made primarily of a complex network based on carbon and/or hydrogen were postulated to superconduct with high transition temperature (Tc) and may even above room temperature [1]
The projector augmented wave (PAW) method [19] was adopted, and the exchange correlation energy was described by the Ceperley-Alder local density approximation (LDA) [20] as parameterized by Perdew and Zunger [21], which has been confirmed to be adequate by previous studies [22]
As a comparison and a check on accuracy, we start our investigation by looking at solid benzene
Summary
Organic materials with a crystal structure made primarily of a complex network based on carbon and/or hydrogen were postulated to superconduct with high transition temperature (Tc) and may even above room temperature [1]. Superconductivity in Polycyclic Aromatic Hydrocarbons transition temperature Tc, and the highest Tc seems to increase with the number of benzene rings within the unit cell [6,7,8,9]. These compounds all have a superconducting phase with Tc ∼ 5 − 7 K. The crystal structures of potassium-intercalated PAHs have been experimentally determined [11] This information is very useful in theoretical studies on the superconducting phase and for understanding the driving force for superconductivity. Will its properties mimic those of all aromatic hydrocarbons? What factor is important for higher Tc? Exploring structures of doped solid benzene and examining corresponding phase stability, followed by the study of superconductivity, are the tasks of this work
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