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Aromatic superconductors are a new type of high-temperature superconductor discovered in recent years. The superconducting transition temperature (Tc) increases with the size of aromatic molecule increasing, which has attracted widespread attention of experimental and theoretical researchers. The driving mechanism for such a superconductivity, whether it is dominated by electron-phonon coupling or electronic correlation effects, has aroused great interest of many research groups. This paper briefly introduces the rich superconducting phenomena of metal doped aromatic compounds. From the perspectives of electron-phonon coupling or electronic correlations, the superconductivity of aromatic compounds is discussed, which is helpful in exploring aromatic superconductors with higher Tc. The challenges currently faced in the field are also introduced. The rest of this paper is organized as follows. We first introduce the existence of abundant superconducting phases in the experiment of metal doped aromatic compounds. Different doping concentrations of metal cause superconducting phases with different Tc values, especially the highest Tc value of the superconducting phase increases with the size of aromatic molecule increasing. Theoretical prediction shows that all aromatic hydrocarbon superconductors have a low-Tc superconducting phase in a range of 5–7 K, which is a common feature. For systems with few benzene rings (such as benzene, naphthalene, and phenanthrene crystals), only low-Tc phase of 5–7 K exists, while in systems with multiple benzene rings (such as picene, dibenzopentacene, and others with the number of benzene rings more than 5), there are multiple superconducting phases; the highest Tc in long-benzene-ring system depends not only on the number of benzene rings, but also on the chain size of organic molecule. Further research indicates that low-Tc phase is induced by doping about 2 electrons and has good stability, while high-Tc phase results from doping 3 electrons and has slightly poorer stability. Then, the electron-phonon coupling characteristics and electron-electron exchange correlation effects in aromatic compound superconductors are discussed. For low-Tc phases, the values of electronic density of states at the Fermi level are comparable to each other and relatively low, resulting in weak electron-phonon interactions. However, the Tc value predicted by this electron-phonon coupling mechanism is in good agreement with experimental value, indicating that the electron-phonon coupling is sufficient to describe the superconductivity of low-Tc phases. For high-Tc phases, the big values of electron density of states at the Fermi level imply strong electron-phonon interactions, and this electron-phonon coupling increases with the size of organic molecule increasing. However, the Tc value predicted only by the electron-phonon mechanism is lower than the experimental value. The study of electron-electron exchange correlation effect of aromatic compounds shows that the electronic correlation effect increases with the size of aromatic molecule increasing, which is consistent with the increase of Tc maximum value with the size of aromatic molecule increasing in a long-benzene-ring system. This indicates that the superconductivity of high-Tc phase is driven by both the electron-phonon mechanism and the electronic correlation effect. This understanding of superconductivity is significant for exploring and discovering aromatic superconductors with higher transition temperatures. Finally, comprehensive physical models and methods are required in this paper in order to gain a thorough understanding of the superconductivity of aromatic compound. -
Keywords:
- aromatic superconductor /
- superconductivity /
- electron-phonon coupling /
- electronic correlations
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Google Scholar
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图 1 (a) 超导转变温度Tc与芳香分子晶体中有机分子所含苯环数的关系[9]; (b) 电-声耦合常数随有机分子中碳原子数的变化[12]; (c) 电-声相互作用随有机分子中碳原子数的变化, 插图表示电-声相互作用与碳原子倒数呈线性关系[13]
Figure 1. (a) The relationship between the superconducting transition temperature Tc and the number of benzene rings in organic molecules in aromatic molecular crystals[9]; (b) the variation of the electron-phonon coupling constant with the number of carbon atoms in organic molecules[12]; (c) the electron-phonon interaction varies with the number of carbon atoms in organic molecules, and the inset shows a linear relationship between the electron-phonon interaction and the reciprocal of carbon atoms[13].
图 2 各种芳香分子晶体在金属掺杂后的超导转变温度Tc随苯环数n的变化[11], 灰色区域暗示了5—7 K的超导转变温度区间, 实心红色方块表示林海青等[11]的预测结果, 而空心红色方块表示Casula等[14]的预测结果, 其他数据来自实验
Figure 2. The superconducting transition temperature Tc of aromatic molecular crystals doped by metal varies with the number of benzene rings n [11]. The gray area indicates a superconducting transition temperature region of 5—7 K. The solid red squares represent the prediction results of Lin et al.[11], while the hollow red squares represent the prediction results of Casula et al.[14]. Other data come from experiments.
图 3 在芳香分子晶体中, 有效在位库仑能与带宽的比值(Ueff/W)随介电常数的变化. 有机分子的右上标I指分子中苯环呈zigzag排列, II指有机分子构型类似于1, 2:8, 9-二苯并五苯[11]
Figure 3. In aromatic molecular crystals, the ratio of effective on-site Coulombic energy to bandwidth (Ueff/W) varies with the dielectric constant. The superscript I of organic molecules refers to the zigzag arrangement of the benzene rings in the molecule, while II refers to the configuration of organic molecules similar to 1, 2:8, 9-dibenzopentacene [11].
表 1 采用标准的密度泛函(DFT)方法预测芳香有机分子晶体的带隙(Eg)小于实验值. 采用杂化密度泛函(HSE)方法预测获得与实验一致的带隙, 所需的精确交换作用参数υ. 有机分子的右上标I指分子中苯环呈zigzag排列, II指有机分子构型类似于1, 2:8, 9-二苯并五苯[11]
Table 1. The band gap (Eg) of aromatic organic molecular crystals predicted by standard density functional theory (DFT) method is smaller than the experimental values. υ is the adopted precise exchange interaction parameters when obtaining the Eg which is consistent with experimental values. The superscript I of organic molecules refers to the zigzag arrangement of the benzene rings in the molecule, while II refers to the configuration of organic molecules similar to 1, 2:8, 9-dibenzopentacene [11].
C14H10I C18H12I(II) C22H14I C22H14II C26H16I C26H16II C30H18I C30H18II Eg (expt.)/eV 3.16 3.3 3.3 3.3 3.15 3.2 3.2 3.2 Eg (DFT)/eV 2.80 2.40 2.31 2.20 2.07 1.82 2.04 1.03 υ 0.10 0.26 0.30 0.40 0.36 0.55 0.37 0.91 
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[1] Schilling A, Cantoni M, Guo J D, Ott H R 1993 Nature 363 56
Google Scholar
[2] Drozdov A P, Kong P P, Minkov V S, Besedin S P, Kuzovnikov M A, Mozaffari S, Balicas L, Balakirev F F, Graf D E, Prakapenka V B, Greenberg E, Knyazev D A, Tkacz M, Eremets M I 2019 Nature 569 528
Google Scholar
[3] Somayazulu M, Ahart M, Mishra A K, Geballe Z M, Baldini M, Meng Y, Struzhkin V V, Hemley R J 2019 Phys. Rev. Lett. 122 027001
Google Scholar
[4] Little W A 1964 Phys. Rev. 134 A1416
Google Scholar
[5] Ginzburg V L 1964 Phys. Lett. 13 101
Google Scholar
[6] Taniguchi H, Miyashita M, Uchiyama K, Satoh K, Mori N, Okamoto H, Miyagawa K, Kanoda K, Hedo M, Uwatoko Y 2003 J. Phys. Soc. Jpn. 72 468
Google Scholar
[7] Mitsuhashi R, Suzuki Y, Yamanari Y, Mitamura H, Kambe T, Ikeda N, Okamoto H, Fujiwara A, Yamaji M, Kawasaki N, Maniwa Y, Kubozono Y 2010 Nature 464 76
Google Scholar
[8] Wang X F, Liu R H, Gui Z, Xie Y L, Yan Y J, Ying J J, Luo X G, Chen X H 2011 Nat. Commun. 2 507
Google Scholar
[9] Xue M Q, Cao T B, Wang D M, Wu Y, Yang H X, Dong X L, He J B, Li F W, Chen G F 2012 Sci. Rep. 2 389
Google Scholar
[10] Kubozono Y, Mitamura M, Lee X, He X, Yamanari Y, Takahashi Y, Suzuki Y, Kaji Y, Eguchi R, Akaike K, Kambe T, Okamoto H, Fujiwara A, Kato T, Kosugi T, Aoki H 2011 Phys. Chem. Chem. Phys. 13 16476
Google Scholar
[11] Zhong G H, Chen X J, Lin H Q 2019 Front. Phys. 7 52
Google Scholar
[12] Kato T, Yoshizawa K, Hirao K 2002 J. Chem. Phys. 116 3420
Google Scholar
[13] Kato T, Kambe T, Kubozono Y 2011 Phys. Rev. Lett. 107 077001
Google Scholar
[14] Casula M, Calandra M, Profeta G, Mauri F 2011 Phys. Rev. Lett. 107 137006
Google Scholar
[15] Giovannetti G, Capone M 2011 Phys. Rev. B 83 134508
Google Scholar
[16] Kim M, Min B I 2011 Phys. Rev. B 83 214510
Google Scholar
[17] Durand P, Darling G R, Dubitsky Y, Zaopo A, Rosseinsky M J 2003 Nature Mater. 2 605
Google Scholar
[18] Wang X H, Zhong G H, Yan X W, Chen X J, Lin H Q 2017 J. Phys. Chem. Solids 104 56
Google Scholar
[19] Zhong G H, Yang D Y, Zhang K, Wang R S, Zhang C, Lin H Q, Chen X J 2018 Phys. Chem. Chem. Phys. 20 25217
Google Scholar
[20] Wang R S, Gao Y, Huang Z B, Chen X J 2017 Superconductivity in P-Terphenyl arXiv: 1703.05803
[21] Yan J F, Zhong G H, Wang R S, Zhang K, Lin H Q, Chen X J 2019 J. Phys. Chem. Lett. 10 40
Google Scholar
[22] Huang G, Zhong G H, Wang R S, Han J X, Lin H Q, Chen X J 2019 Carbon 143 837
Google Scholar
[23] Wang R S, Cheng J, Wu X L, Yang H, Chen X J, Gao Y, Huang Z B 2018 J. Chem. Phys. 149 144502
Google Scholar
[24] Peng D, Wang R S, Chen X J 2020 J. Phys. Chem. C 124 906
Google Scholar
[25] Wang R S, Zhang K, Zhong G H, Chen X J 2023 Mater. Sci. Eng. B 288 116155
Google Scholar
[26] Wang R S, Gao Y, Huang Z B, Chen X J 2017 Superconductivity at 43 K in a single C-C bond linked terphenyl arXiv: 1703.05804
[27] Wang R S, Gao Y, Huang Z B, Chen X J 2017 Superconductivity above 120 Kelvin in a Chain Link Molecule arXiv: 1703.06641
[28] Zhong G H, Wang X H, Wang R S, Han J X, Zhang C, Chen X J, Lin H Q 2018 J. Phys. Chem. C 122 3801
Google Scholar
[29] Li H, Zhou X, Parham S, Nummy T, Griffith J, Gordon K N, Chronister E L, Dessau D S 2019 Phys. Rev. B 100 064511
Google Scholar
[30] Neha P, Bhardwaj A, Sahu V, Patnaik S 2018 Physica C 554 1
Google Scholar
[31] Liu W H, Lin H, Kang R Z, Zhu X Y, Zhang Y, Zheng S X, Wen H H 2017 Phys. Rev. B 96 224501
Google Scholar
[32] Pinto N, Di Nicola C, Trapananti A, Minicucci M, Di Cicco A, Marcelli A, Bianconi A, Marchetti F, Pettinari C, Perali A 2020 Condens. Matter 5 78
Google Scholar
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