Exotic s-wave superconductivity in alkali-doped fullerides

Evidence for Band Renormalizations in Strong-Coupling Superconducting Alkali-Fulleride Films

There has been a long-standing debate about the mechanism of the unusual superconductivity in alkali-intercalated fullerides. In this Letter, using high-resolution angle-resolved photoemission spectroscopy, we systematically investigate the electronic structures of superconducting K_{3}C_{60} thin films. We observe a dispersive energy band crossing the Fermi level with the occupied bandwidth of about 130 meV. The measured band structure shows prominent quasiparticle kinks and a replica band involving the Jahn-Teller active phonon modes, which reflects strong electron-phonon coupling in the system. The electron-phonon coupling constant is estimated to be about 1.2, which dominates the quasiparticle mass renormalization. Moreover, we observe an isotropic nodeless superconducting gap beyond the mean-field estimation (2Δ/k_{B}T_{c}≈5). Both the large electron-phonon coupling constant and large reduced superconducting gap suggest a strong-coupling superconductivity in K_{3}C_{60}, while the electronic correlation effect is suggested by the observation of a waterfall-like band dispersion and the small bandwidth compared with the effective Coulomb interaction. Our results not only directly visualize the crucial band structure but also provide important insights into the mechanism of the unusual superconductivity of fulleride compounds.

Doping Asymmetry and Layer-Selective Metal-Insulator Transition in Trilayer K_{3+x}C_{60}

Thin films provide a versatile platform to tune electron correlations and explore new physics in strongly correlated materials. Epitaxially grown thin films of the alkali-doped fulleride K_{3+x}C_{60}, for example, exhibit intriguing phenomena, including Mott transitions and superconductivity, depending on dimensionality and doping. Surprisingly, in the trilayer case, a strong electron-hole doping asymmetry has been observed in the superconducting phase, which is absent in the three-dimensional bulk limit. Using density-functional theory plus dynamical mean-field theory, we show that this doping asymmetry results from a substantial charge reshuffling from the top layer to the middle layer. While the nominal filling per fullerene is close to n=3, the top layer rapidly switches to an n=2 insulating state upon hole doping, which implies a doping asymmetry of the superconducting gap. The interlayer charge transfer and layer-selective metal-insulator transition result from the interplay between crystal field splittings, strong Coulomb interactions, and an effectively negative Hund coupling. This peculiar charge reshuffling is absent in the monolayer system, which is an n=3 Mott insulator, as expected from the nominal filling.

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

Alkali-fulleride superconductors with a maximum critical temperature T_{c}∼40  K exhibit a similar electronic phase diagram to that of unconventional high-T_{c} superconductors. Here we employ cryogenic scanning tunneling microscopy to show that trilayer K_{3}C_{60} displays fully gapped strong coupling s-wave superconductivity, accompanied by a pseudogap above T_{c}∼22  K and within vortices. A precise control of the electronic correlations and potassium doping enables us to reveal that superconductivity occurs near a superconductor-Mott-insulator transition and reaches maximum at half-filling. The s-wave symmetry retains over the entire phase diagram, which, in conjunction with an abrupt decline of the superconductivity below half-filling, indicates that alkali fullerides are predominantly phonon-mediated superconductors, although the electronic correlations also come into play.

Hydrocarbon-soluble, hexaanionic fulleride complexes of magnesium

Fullerene C₆₀ reacts with dimagnesium(i) compounds LMgMgL, where L is a monoanionic β-diketiminate ligand, to contact ion complexes [(LMg)_nC₆₀], where n is predominantly 2, 4 or 6.

The reaction of the magnesium(i) complexes [{(^Arnacnac)Mg}₂], (^Arnacnac = HC(MeCNAr)₂, Ar = Dip (2,6-iPr₂C₆H₃), Dep (2,6-Et₂C₆H₃), Mes (2,4,6-Me₃C₆H₂), Xyl (2,6-Me₂C₆H₃)) with fullerene C₆₀ afforded a series of hydrocarbon-soluble fulleride complexes [{(^Arnacnac)Mg}_nC₆₀], predominantly with n = 6, 4 and 2. ¹³C{¹H} NMR spectroscopic studies show both similarities (n = 6) and differences (n = 4, 2) to previously characterised examples of fulleride complexes and materials with electropositive metal ions. The molecular structures of [{(^Arnacnac)Mg}_nC₆₀] with n = 6, 4 and 2 can be described as inverse coordination complexes of n [(^Arnacnac)Mg]⁺ ions with C₆₀^n– anions showing predominantly ionic metal–ligand interactions, and include the first well-defined and soluble complexes of the C₆₀^6– ion. Experimental studies show the flexible ionic nature of the {(^Arnacnac)Mg}⁺···C₆₀^6– coordination bonds. DFT calculations on the model complex [{(^Menacnac)Mg}₆C₆₀] (^Menacnac = HC(MeCNMe)₂) support the formulation as an ionic complex with a central C₆₀^6– anion and comparable frontier orbitals to C₆₀^6– with a small HOMO–LUMO gap. The reduction of C₆₀ to its hexaanion gives an indication about the reducing strength of dimagnesium(i) complexes.

Correlation-induced superconductivity dynamically stabilized and enhanced by laser irradiation

Studies on out-of-equilibrium dynamics have paved a way to realize a new state of matter. Superconductor-like properties above room temperatures recently suggested to be in copper oxides achieved by selectively exciting vibrational phonon modes by laser have inspired studies on an alternative and general strategy to be pursued for high-temperature superconductivity. We show that the superconductivity can be enhanced by irradiating laser to correlated electron systems owing to two mechanisms: First, the effective attractive interaction of carriers is enhanced by the dynamical localization mechanism, which drives the system into strong coupling regions. Second, the irradiation allows reaching uniform and enhanced superconductivity dynamically stabilized without deteriorating into equilibrium inhomogeneities that suppress superconductivity. The dynamical superconductivity is subject to the Higgs oscillations during and after the irradiation. Our finding sheds light on a way to enhance superconductivity that is inaccessible in equilibrium in strongly correlated electron systems.

Spontaneous Orbital-Selective Mott Transitions and the Jahn-Teller Metal of A_{3}C_{60}

The alkali-doped fullerides A_{3}C_{60} are half-filled three-orbital Hubbard systems which exhibit an unconventional superconducting phase next to a Mott insulator. While the pairing is understood to arise from an effectively negative Hund coupling, the highly unusual Jahn-Teller metal near the Mott transition, featuring both localized and itinerant electrons, has not been understood. This property is consistently explained by a previously unrecognized phenomenon: the spontaneous transition of multiorbital systems with negative Hund coupling into an orbital-selective Mott state. This symmetry-broken state, which has no ordinary orbital moment, is characterized by an orbital-dependent two-body operator (the double occupancy) or an orbital-dependent kinetic energy and may be regarded as a diagonal-order version of odd-frequency superconductivity. We propose that the recently discovered Jahn-Teller metal phase of Rb_{x}Cs_{3-x}C_{60} is an experimental realization of this novel state of matter.

Orbital disproportionation of electronic density is a universal feature of alkali-doped fullerides

Alkali-doped fullerides show a wide range of electronic phases in function of alkali atoms and the degree of doping. Although the presence of strong electron correlations is well established, recent investigations also give evidence for dynamical Jahn–Teller instability in the insulating and the metallic trivalent fullerides. In this work, to reveal the interplay of these interactions in fullerides with even electrons, we address the electronic phase of tetravalent fulleride with accurate many-body calculations within a realistic electronic model including all basic interactions extracted from first principles. We find that the Jahn–Teller instability is always realized in these materials too. In sharp contrast to the correlated metals, tetravalent system displays uncorrelated band-insulating state despite similar interactions present in both fullerides. Our results show that the Jahn–Teller instability and the accompanying orbital disproportionation of electronic density in the degenerate lowest unoccupied molecular orbital band is a universal feature of fullerides.

Understanding the electronic phases of alkali-doped fullerides is a long-standing and challenging task for material scientists. Here the authors show that Jahn-Teller instability and orbital disproportionation of electronic density in the lowest unoccupied molecular orbital band is universal in these systems.