Tuning the electronic states and superconductivity in alkali fulleride films

Abstract

The successful preparation of superconducting alkali fulleride (AxC60, A = K, Rb, Cs) films using state-of-the-art molecular beam epitaxy overcomes the disadvantages of the air-sensitivity and phase separation in bulk AxC60, enabling for the first time a direct investigation of the superconductivity in alkali fullerides on the molecular scale. In this paper, we briefly review recent cryogenic scanning tunneling microscopy results of the structural, electronic, and superconducting properties of the fcc AxC60 films grown on graphitized SiC substrates. Robust s-wave superconductivity is revealed against the pseudogap, electronic correlation, non-magnetic impurities, and merohedral disorder. By controlling the alkali-metal species, film thickness, and electron doping, we systematically tune the C60x− orientational orderings and superconductivity in AxC60 films and then complete a unified phase diagram of superconducting gap size vs electronic correlation and doping. These investigations are conclusive and elucidated that the s-wave superconductivity retains in alkali fullerides despite of the electronic correlation and presence of pseudogap.

Organic quantum materials: A review

“…155 Positive dependence of T c on the lattice constant suggested phonon-mediated pairing in these FSCs [156][157][158] : as the volume of per C 60 3− anion increased, which led to a larger interfullerene separation, T c was enhanced as the consequence of the increase in N(ε F ). 159 Besides, observation of Hebel-Slichter peaks in NMR [160][161][162] and μSR spectra 163,164 and isotopic effects 165 supported the phonon mechanism. While in A15 Cs 3 C 60 , unexcepted results were obtained.…”

Ternary superconducting hydrides stabilized via Th and Ce elements at mild pressures

“…Since the discovery of the superconductivity of mercury in 1911 [1] , searching for room-temperature superconductors is an ultimate goal in condensed matter physics and materials science. Within 110 years of exploration, researchers discovered many high critical temperature ( T c ) superconductors [2] , [3] , [4] , [5] , [6] , such as cuprate, iron-based, interface, and organic superconductors. The highest T c of 164 K was achieved in Hg-Ba-Ca-Cu-O [7] through diamond anvil cell (DAC) compression to 31 GPa, but the T c value remains lower than room temperature.…”

Understanding the Mott insulating state in 1T-TaS2 and 1T-TaSe2

“…When the ratio of Coulomb repulsion U over the bandwidth W exceeds some critical value, the density of states (DOS) around Fermi level will be split into the upper Hubbard band (UHB) and the lower Hubbard band (LHB), resulting in a Mott insulating gap [1,2]. Proximity to the Mott insulator can lead to different exotic states, such as the superconducting state in cuprates [4], alkali fulleride films [5], and magic-angle graphenes [6,7]. The transition metal dichalcogenide (TMD) 1T-TaS 2 and 1T-TaSe 2 are candidates of Mott insulators [8,9].…”

“…Fig 5. Two examples of single-step areas in bulk 1T-TaS 2 , with a small-gap and metallic spectra on the top layer.…”