对高温超导机理和室温超导材料的追寻，贯穿了凝聚态物理领域近四十年的发展历史。作为拥有最高超导转变温度的大分子超导体，碱金属掺杂的富勒烯体系展示出丰富的物相和非常规的超导配对机制，是研究多体相互作用和高温超导机理的重要平台。目前的研究表明碱金属掺杂富勒烯体系的超导态具有s波对称性，并且在二维薄膜样品中存在赝能隙态。对于A3C60 (A = K, Rb, Cs)，与超导紧密相关的t1u分子轨道在Jahn-Teller效应和洪特耦合作用下产生劈裂，形成了S = 1/2的低自旋基态。考虑到t1u能带的带宽W ~ 0.5 eV以及在位库仑排斥能Ueff ~ 1 eV，则有Ueff/W > 1，显示出富勒烯超导体系中强烈的电子关联效应。此外，在压力或掺杂量的调控下，富勒烯体系的超导相均出现拱形结构，这与铜基氧化物高温超导体非常类似。然而富勒烯体系的s波超导相近邻磁性莫特绝缘相，又与d波铜基超导体的相图有所区别。因此研究碱金属掺杂富勒烯体系的电子结构和多体相互作用，是理解超导机理和相图结构的关键。我们将先进的分子束外延生长技术（MBE）与角分辨光电子能谱技术（ARPES）相结合，搭建了可以实现薄膜原位生长和电子结构测量的MBE-ARPES真空互联系统。我们在外延生长的石墨烯衬底上成功制备了高质量的具有超导电性的富勒烯薄膜样品，并系统地研究了其丰富的电子结构。在K3C60超导薄膜中，我们观测到了超导相干峰的信号，获得了各向同性的超导能隙分布，同时发现了超导转变温度之上赝能隙的存在。通过对费米面附近电子结构的分析，厘清了由Jahn-Teller活性声子模式Hg(2)、Hg(3)和Hg(8)导致的能带弯折和复制带。大的电子-声子耦合常数（λ = 1.2）以及显著高于BCS理论值的约化能隙（2Δ/kBTc = 5 > 3.52）表明K3C60是具有强耦合作用的s波超导体。值得注意的是，较窄的半填充占据态带宽（~130 meV）和光电子谱上大范围（~1.2 eV）的“waterfall”结构，暗示着电子关联效应对于完全理解富勒烯体系的超导机理是不可或缺的。
The pursuits of mechanism for high-temperature superconductivity and room-temperature superconductors last for nearly forty years in the field of condensed matter physics. Recording the highest superconducting transition temperature of all molecular superconductors, alkali-intercalated fullerides exhibit various phases and unconventional superconducting pairing mechanism, which have been important platforms for studying many-body interactions and high-temperature superconducting mechanism.The superconducting state of alkali-intercalated fullerides possesses s-wave symmetry according to various experimental researches up to now, while a pseudogap state emerges in two dimensional thin films. As for A3C60 (A = K, Rb, Cs), degenerate t1u molecular orbitals related to superconductivity split and form a low-spin S = 1/2 ground state in the presence of Jahn-Teller effect and Hund’s coupling. Considering the bandwidth W ~ 0.5 eV of t1u band and on-site Coulomb repulsion Ueff ~ 1 eV, the ratio Ueff/W > 1 indicates strong electronic correlation effects in alkali-intercalated fullerides. Besides, the domed superconducting phase diagram shows a remarkable similarity with cuprate under the controlling parameter of pressure and doping level. Nevertheless, the s-wave superconducting phase proximate to magnetic mott insulating phase is different from that in d-wave cuprate. Hence the electronic structure and many-body interaction in fullerides are crucial for understanding superconducting mechanism and phase diagram.By combination of advanced molecular beam epitaxy (MBE) technique and angle-resolved photoemission spectroscopy (ARPES), we have built an ultra-high vacuum system enabling in-situ film growth and electronic structure measurements. We have successfully synthesized high-quality ultrathin films of alkali-intercalated C60 on epitaxial bilayer graphene and systematically investigated their electronic structures.In the thin film of K3C60, we observe the superconducting coherent peak at low temperature and an isotropic nodeless superconducting gap while a pseudogap persists above the transition temperature. By analyzing the electronic structure near fermi energy, kinks and replica bands caused by Jahn-Teller active phonon modes Hg(2), Hg(3) and Hg(8) are clarified. Large electron-phonon coupling constant (λ = 1.2) and reduced gap (2Δ/kBTc = 5 > 3.52) beyond BCS estimation indicate that K3C60 is a strong-coupled s-wave superconductor. Significantly, narrow bandwidth of half-filled t1u band (~130 meV) and “waterfall” structure in a large energy scale (~1.2 eV) suggest that electronic correlation is indispensable for fully understanding the superconducting mechanism in alkali-intercalated fullerides.
