The role of electron–vibron interaction and local pairing in conductivity and superconductivity of alkali-doped fullerides

Abstract

b Metrolohichna str. Kiev-03680, Ukraine.We investigate the competition between the electron-vibron interaction (interaction with the Jahn-Teller phonons) and the Coulomb repulsion in a system with the local pairing of electrons on the 3-fold degenerate lowest unoccupied molecular orbital (LUMO). The el.-vib. interaction and the local pairing radically change conductivity and magnetic properties of alkali-doped fullerides AnC60, which would have to be antiferromagnetic Mott insulators: we have shown that materials with n = 1, 5 and A = K, Rb are conductors but not superconductors; n = 3 and A = K, Rb are conductors and superconductors at low temperatures, but with A = Cs they are Mott-Jahn-Teller insulators at normal pressure; n = 2, 4 are nonmagnetic Mott insulators. Thus superconductivity, conductivity and insulation of these materials have common nature. Using this approach we obtain the phase diagram of A3C60 analytically, which is the result of interplay between the local pairing, the el.-vib. interaction, Coulomb correlations, and formation of small radius polarons.

Merohedral disorder and impurity impacts on superconductivity of fullerenes

“…Distinct from a conventional superconductor, however, in charged fullerenes the t1u-derived conduction band of  0.5 eV is narrow and comparable to the electron-vibron interactions, thereby causing a breakdown of the Migdal's theorem [51][52][53] . As a result, superconductivity with local nonretarded attractive interactions 29,30 would be less sensitive to distribution of the electronic DOS in conduction band 54 , and instead determined by some ensemble averaged DOS 55 . This differs from the usual BCS superconductor where Tc is essentially governed by the DOS at EF, and happens to match the non-correlated superconductivity with the merohedral disorder that significantly change the t1u-derived DOS distribution (Fig.…”

“…In the local pairing mechanism, the key ingredients for high-Tc superconductivity are the strong coupling of the t1u electrons to intramolecular JT phonons in trivalent fullerides [55][56][57] . The phonon-mediated unusual multiorbital (attractive) interactions lead to an effectively inverted Hund's coupling (S = 1/2) 32 and a local spin-singlet s-wave pairing on the same orbital 30 , further enhanced via a coherent tunneling of pairs between orbitals (the Suhl-Kondo mechanism) 58,59 .…”

Direct Investigation of Excited C60 Dianion and Its Intramolecular Electron Transfer Behaviors

“…39,47 The lowest unoccupied molecular orbital (LUMO) of C 60 is identified as the 3-fold degenerate t 1u orbital that accommodates up to six electrons. 48 magnetic susceptibility data from analytically pure dianionic salts once indicated a paramagnetic nature in the context of C 60 2− spin multiplicity. 49 Both ground singlet and triplet states of C 60 2− appeared to have similar energies and were thermally populated even at helium temperatures, 49 although these findings contradicted theoretical predictions and pulsed EPR measurements.…”

“…Fullerenes, on the other hand, have emerged as fundamental components in a wide range of organic functional materials, from organic field-effect transistors and organic solar cells to organic light-emitting diodes, benefiting from their remarkable electron affinities and superior electron transport properties. − Moreover, fullerene anions have drawn considerable attention for their implications in relevant performances such as the superconductivity of doped semiconducting materials and the ferromagnetism of fullerene-included devices. , As a consequence, diverse chemical and electrochemical methods have been deployed to produce fullerene anion species, with visible/near-infrared absorption, Raman, and electron spin resonance spectroscopies (EPR) demonstrating their characteristics. − In spite of the volume of research surrounding fullerene anions, the elucidation of their essential features, including electronic structures, is still under debate. For instance, the quasi-spherical structure of the C 60 molecule has been reported to possess icosahedral symmetry, which causes electronic level splitting. , The lowest unoccupied molecular orbital (LUMO) of C 60 is identified as the 3-fold degenerate t 1u orbital that accommodates up to six electrons . Therefore, magnetic susceptibility data from analytically pure dianionic salts once indicated a paramagnetic nature in the context of C 60 2– spin multiplicity .…”

Caged-Electron States in Endohedral Li Fullerenes

“…The already wide range of applicability of fullerenes has dramatically further grown since the discovery of endohedral fullerenes. − Owing to their broader range of electronic properties, they may even play a more important role than their parent fullerenes and exhibit electronic phenomena that the parent fullerenes cannot have, such as, for instance, interatomic Coulombic decay . Among the many applications of fullerenes and their derivatives, we just briefly mention organic solar cells, − supercapacitors, − catalyzers, − and superconductive agents. − …”