Molecular Physics An International Journal at the Interface Between Chemistry and Physics ISSN: 0026-8976 (Print) 1362-3028 (Online) Journal homepage: http://www.tandfonline.com/loi/tmph20 Structural and electronic properties of potassium- doped 1,2;8,9-dibenzopentacene superconductor: comparing with doped [7]phenacenes Guo-Hua Zhong, Chao Zhang, Xunwang Yan, Xiaoguang Li, Zheng Du, Gexin Jing & Cencen Ma To cite this article: Guo-Hua Zhong, Chao Zhang, Xunwang Yan, Xiaoguang Li, Zheng Du, Gexin Jing & Cencen Ma (2017) Structural and electronic properties of potassium-doped 1,2;8,9- dibenzopentacene superconductor: comparing with doped [7]phenacenes, Molecular Physics, 115:4, 472-483, DOI: 10.1080/00268976.2016.1274439 To link to this article: http://dx.doi.org/10.1080/00268976.2016.1274439 Published online: 04 Jan 2017. Submit your article to this journal Article views: 23 View related articles View Crossmark data Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=tmph20 Download by: [116.25.97.90] Date: 01 February 2017, At: 07:11 MOLECULAR PHYSICS, VOL. , NO. , – http://dx.doi.org/./.. RESEARCH ARTICLE Structural and electronic properties of potassium-doped ,;,-dibenzopentacene superconductor: comparing with doped []phenacenes Guo-Hua Zhong a, Chao Zhangb,XunwangYanc, Xiaoguang Lid, Zheng Due, Gexin Jinge and Cencen Maf aShenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen, P.R.China; bDepartment of Physics, Yantai University, Yantai,P.R.China;cSchool of physics and electrical engineering, Anyang Normal University, Henan, P.R.China; dInstitute for Advanced Study, Shenzhen University, Shenzhen, P.R. China; eNational Supercomputing Center in Shenzhen, Shenzhen, P.R.China; fShenzhen Energy Guangming Gas Turbine Power Plant Construction Planning and Development Office, Shenzhen, P.R. China ABSTRACT ARTICLE HISTORY Polycyclic aromatic hydrocarbons doped by metal have exhibited the potential of high temperature Received October superconductivity. Understanding the basic properties of materials is the key to reveal the super- Accepted December conductivity. Here, a systemically theoretical study has been done to explore crystal structures and KEYWORDS electronic properties of pristine and potassium-doped 1,2;8,9-dibenzopentacene, compared with Polycyclic aromatic [7]phenacenes case. We determined that vdW-DF2 functional is more suitable to describe the non- hydrocarbons; hydrocarbons; local interaction in a molecular crystal. Based on this functional, we predicted the crystal structures organic superconductor; and investigated in detail the K atomic positions in a system. It was found that the intralayer doping dibenzopentacene 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. 1. Introduction to the Tc enhancement near 40 K. The research of the superconductivity in PAHs renews the interest in organic Organic based compounds are believed to be potential superconductors. As a new kind of superconducting high temperature or room temperature superconductors materials with intriguing properties, the magnetism [10– based on the idea of V. L. Ginsburg that the interac- 12], electronic correlation effects [12–14] and pressure tion of electrons with much higher excitation energy effects [15–19] have received much attention. The fea- than the phonon energy can result in a substantially ture of armchair edge type, which is regarded as a key higher critical temperature T [1]. The recent discovery c factor of superconductivity in such a system, is shared of superconductivity in metal doped polycyclic aromatic in hydrocarbon superconductors in contrast to the zigzag hydrocarbons (PAHs) [2–9]seeminglysupportsthisidea edge type [20]. It has been experimentally observed that because increasing the number of benzene rings has led all of the doped phenanthrenes (C14H10) exhibit the CONTACT Guo-Hua Zhong [email protected] © Informa UK Limited, trading as Taylor & Francis Group MOLECULAR PHYSICS 473 superconductivity of Tc ∼ 5 K, though the dopant is bytheprojectoraugmentedwave(PAW)method,while selectable among K, Rb, Sr, Ba and Sm [4,6,7]. The the monoelectronic valence electrons were expanded in superconductivity is as high as Tc ∼ 15 K in K-doped plane waves with an cutoff energy of 600 eV. For the coronene [3] while only 7 K in K-doped [6]phenacene optimisation, a conjugate-gradient algorithm was used to [9]. The superconductivity reaches to Tc ∼ 18 K in K- relax the ions into their instantaneous ground state. The doped picene (C22H14)[2], even 33 K in K-doped 1,2;8,9- Monkhorst-Pack k-point grids were generated according −1 dibenzopentacene [C30H18(I)] [5]. Combining with the to the specified k-point separation 0.02 A˚ .Andthecon- geometrical shape of these organic molecules, these find- vergence thresholds were set as 10−5 eV in energy and ings above indicate that Tc changes with the number and 0.005 eV/Ainforce.InthestandardDFT,thelocalden-˚ the arrangement of benzene rings. Thus, a new avenue in sity approximation (LDA) [24] and the generalised gra- the quest for organic superconductors has been opened. dient approximation (GGA) of Perdew-Burke-Ernzerhof The higher superconducting transition temperature is version [25]wereadoptedtodescribetheelectronic expected in this kind of materials. exchange-correlation interactions. As a comparison, the So far, the highest Tc of 33.1 K was observed in K- vdW density functional (vdW-DF) [26], the second ver- doped 1,2;8,9-dibenzopentacene [KxC30H18(I), x = 3.45] sion of vdW-DF (vdW-DF2) [27] and the semi-empirical for PAH superconductors [5]. Actually the obtained high- DFT method of Grimme (DFT-D2) [28]wereadoptedto est Tc is far less than the room temperature. One of its investigate the influence of the dispersion interactions to main limitations is that the kind of compound is diffi- the optimisation of crystal structures. cult in fabricating experimentally. The ideal crystalline sample for measurement is lack. However, for C30H18(I) with the long-benzene-ring chain, the result of elec- 3. Results and discussion tron energy-loss spectroscopy [21]hasconfirmedthe 3.1. Functional analysis formation of K-doped phases. Moreover, [n]phenacenes as large as [11]phenacenes have been synthesised [22], In order to obtain the accurate crystal structures of pris- which implies more choices to realise the higher super- tine and doped C30H18(I), we first tested the precision conducting transition temperature in PAHs. Interestedly, of different functionals by comparing optimised crys- multi-T phases exist in PAHs with the long-benzene- c tal parameters of pristine naphthalene (C10H8), C14H10, ring chain such as 6.5, 6.9, 7, 7.4, 8, 17 and 18 K in K- chrysene (C18H12)andC22H14 with their experimen- doped C22H14 [2] as well as 5, 7, 7.4, 28.2 and 33.1 K tal values. The calculated results are summarised in in KxC30H18(I) [5]. Besides the number and the arrange- Table 1, comparing with experimental and previous the- ment of benzene rings, we point out that the supercon- oretical values [29–35]. From the calculated data listed ductivity is related to the doping content and the position in Table 1, we can conclude that vdW-DF2 functional of K atoms which also determines the different electronic produces the crystal parameters closer to experimen- properties of doped system. It is important for under- talones.InthecaseofC14H10,ourdatabasedon standing this fascinating superconductivity to reveal the LDA/GGA is consistent with the previous LDA/GGA crystal and electronic structures of KxC30H18(I). Unfor- calculation [31,32]. However, the optimised lattice con- tunately, the research on basic properties of KxC30H18(I) stantsbasedonLDAarecorrespondinglysmallerthan is still lacking. In this work, therefore, we present a the experimental ones [30]. The contraction of cell vol- detailed theoretical study on the structures and elec- ume reaches 10.8% or over this. Contrary to LDA, GGA tronic properties of KxC30H18(I) using the first-principles leads to the larger lattice constants than experiment [30]. study with van der Waals density functional, which is In particular, the expansions of the lattice constant in simultaneously compared with those of [7]phenacenes b direction and the volume of unit cell reach to ∼ 9% [C30H18(II), which is used to distinguish from C30H18(I)] and ∼ 15%, respectively. As a result, both LDA and GGA with the different armchair edge type. functionals are unsuited to obtain the reasonable crystal parameters. Based on the consideration of vdW interac- tions, the calculation results listed in Table 1 imply the 2. Computational details importance of the dispersion interactions in this kind of Within the framework of the density functional the- molecular crystal. Differing from the previous theoreti- ory (DFT), the Vienna ab initio simulation package cal study [32]inwhichvdW-DFistheoptimalscheme, (VASP) [23]wasemployedtocarryoutthecalculations