Visualizing molecular orientational ordering and electronic structure in CsnC60 fulleride films

Abstract

Alkali-doped fullerides exhibit a wealth of unusual phases that remain controversial by nature.Here we report a cryogenic scanning tunneling microscopy study of the sub-molecular structural and electronic properties of expanded fullerene C n− 60 films with various cesium (Cs) doping. By varying the discrete charge states and film thicknesses, we reveal a large tunability of orientational ordering of C n− 60 anions, yet the tunneling conductance spectra are all robustly characteristic of energy gaps, hallmarks of Jahn-Teller instability and electronic correlations. The Fermi level lies halfway within the insulating gap for stoichiometric Cs doping level of n = 1, 2, 3 and 4, apart from which it moves toward band edges with concomitant electronic states within the energy gap. Our findings establish the universality of Jahn-Teller instability, and clarify the relationship among the doping, structural and electronic structures in CsnC60 fullerides.

Entropy and electronic orders of the three-orbital Hubbard model with antiferromagnetic Hund coupling

“…The maximum T c at n = 1.5 and 2.5 is comparable to the maximum T c ≈ 0.01 reached at half filling [6]. In fcc fulleride superconductors, AFO is suppressed by geometrical frustration, and SC has been recently found also in doped compounds [20,21], although a more careful modeling would be needed to account for the thin-film nature of these systems. Our results suggest that also the quarter-filled fulleride compounds should exhibit unconventional SC with a high T c .…”

“…This model is relevant for the description of fulleride compounds, where the small bare J is overscreened by a coupling to Jahn-Teller phonons [8,15,16]. Recent experimental progress [20,21] enables a systematic study of chemically doped thin films of these unconventional superconductors. Our work is not meant to be a realistic study of fulleride compounds, but the simple model considered captures the qualitative physics of this class of materials, as has been shown in recent investigations of the half-filled system [11,17,18].…”

“…Last year, the first single crystals (thin films) of fulleride compounds were grown epitaxially, and this experimental technique allows one to control the filling over a wide range via doping [20,21]. It is thus interesting to extend the theoretical studies of the three-orbital Hubbard model with negative Hund coupling to the doped regime.…”

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] .…”

“…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] . 显然地, 这样普 遍存在的绝缘基态是电子强关联效应和姜-泰勒效应 共同作用的结果(图18(b)插图) [149][150][151]158,159] .…”