Direct Observation of Full-Gap Superconductivity and Pseudogap in Two-Dimensional Fullerides

Abstract

Alkali-fulleride superconductors with a maximum critical temperature Tc  40 K exhibit similar electronic phase diagram with unconventional high-Tc superconductors [1][2][3][4][5] where the superconductivity resides proximate to a magnetic Mott-insulating state 3-6 . However, distinct from cuprate compounds, which superconduct through two-dimensional (2D) CuO2 planes 7 , alkali fullerides are attributed to the three-dimensional (3D) members of high-Tc family 8,9 . Here, we employ scanning tunneling microscopy (STM) to show that trilayer K3C60 displays fully gapped strong coupling s-wave superconductivity that coexists spatially with a cuprate-like pseudogap state above Tc  22 K and within vortices. A precise control of electronic correlations and doping reveals that superconductivity occurs near a superconductor-Mott insulator transition (SMIT) and reaches maximum at half-filling. The s-wave symmetry retains over the entire phase diagram, which, in conjunction with an abrupt decline of superconductivity below half-filling, demonstrates that alkali fullerides are predominantly phonon-mediated superconductors, although the multiorbital electronic correlations also come into play.

Atomic-scale preparation and characterization ofhigh-temperature superconducting thin films

“…分子尺度的 扫描隧道显微镜研究也表明, 依赖于所选衬底和碱金 属掺杂量, 外延富勒烯薄膜中的C 60 分子可以形成不同 的旋转序 [158][159][160][161][162][163][164][165] . 场扫描隧道显微镜对其形貌和电子结构进行了原子尺 度上的表征 [166,167] . 不同于金属衬底上的C 60 薄膜 [158][159][160][161][162] , 外延生长在石墨烯衬底上的C 薄膜表现出一个高达 3.5 eV的绝缘能隙(图18(b)) [168] , 对应于分子最高占据 [166] .…”

“…的1×1格子. 不同于体单晶 [167] [167] , 与之 The dI/dV spectrum and its best fit (red curve) to a single isotropic s-wave superconducting gap with Δ=5.4 meV. (f) Spatially averaged tunneling spectra as a function of temperature, presenting a pseudogap above T c ∼22 K (red curve).…”

“…(f) Spatially averaged tunneling spectra as a function of temperature, presenting a pseudogap above T c ∼22 K (red curve). (g) Measured dependence on temperature of coherence peak amplitude (circles) and gap depth (squares) [167].…”

“…联能U, 从而抑制超导电性 [167] . 系统地分析表明, 掺杂 富勒烯的高温超导电性是电子关联效应和电声子耦合 共同作用的结果, 如图24中类似于铜氧化物高温超导 体中极大的约化能隙2Δ/k B T c > 6已被证明与电子强关 联效应密切相关 [147,175,176] 图 25 (a) 3层和(b) 9层K 3 C 60 薄膜上依赖于碱金属K掺杂量x 的超导能隙(V=20 mV, I=200 pA), 测量温度为4.6 K [167] Figure 25 Evolution of the superconducting gap with K doping x in (a) 3 ML and (b) 9 ML K x C 60 films, respectively. All spectra were measured at T=4.6 K (set point: V=20 mV, I=200 pA) [167].…”

“…Phys Rev Lett, 2002, 89: 087002, arXiv: cond-mat/0208053 10 Hsu F C, Luo J Y, Yeh K W, et al Superconductivity in the PbO-type structure α-FeSe. Proc Natl Acad Sci, 2008, 105: 14262-14264图 24 (网络版彩图)掺杂富勒烯K x C 60 薄膜依赖于x的相图, 其中红色和蓝色的虚线分别显示了3层和9层厚薄膜中T c (方 形)、能隙Δ(实心圆)和约化能隙2Δ/k B T c (星形, 下图)随掺杂 量x在4.6 K的演化趋势[167]…”