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Fullerene Superconductors
Published in David A. Cardwell, David C. Larbalestier, I. Braginski Aleksander, Handbook of Superconductivity, 2023
Yoshihiro Iwasa, Kosmas Prassides
Taking account of the energy scales of the bandwidth, electron–phonon and electron–electron interactions, the superconductivity as well as the other electronic ground states of fullerides should be understood in terms of the competition or cooperation between the electron–phonon interaction and the electron–electron interaction [23, 24]. This is currently an important issue in solid-state physics in general. Also, in light of the importance of light elements for achieving high Tc superconductivity, the chemical and physical criteria for the occurrence of fullerene superconductivity is of great importance for future explorations and design of new high Tc superconductors.
X-Ray Studies: Chemical Crystallography
Published in David A. Cardwell, David C. Larbalestier, Aleksander I. Braginski, Handbook of Superconductivity, 2022
Lance D. Cooley, Roman Gladyshevskii, Theo Siegrist
Other structures adopted by classical superconductors contain well-defined subunits with an important amount of localized bonding, contrasting with the overall metallic character. Many of the superconducting borides and carbides, for instance, contain pairs of covalently bonded non-metal atoms. This is the case for Lu0.75Th0.25Rh4B4 (CeCo4B4 type), Y1.4Th0.6C3.1 (Pu2C3 type) and for YCl0.75Br0.25 (ls-GdCBr type). The structure of LuNi2B2C contains linear B-C-B units, which interconnect relatively rigid Ni2B2 slabs to form a porous framework that accommodates the rare-earth metal atoms. LuNi2B2C and related borocarbides and boronitrides form a homologous structure series of the general formula RnT2B2 (C/N)n. The crystal structures of this family are strongly reminiscent of those of the high-Tc superconducting oxides. The superconducting fullerides contain the characteristic C60 “balls” in a cubic close-packed arrangement. In K3C60, the cations occupy both the octahedral and the tetrahedral interstices in the framework. The crystal structures of the well-known Chevrel phases, e.g. Mo6Se8 and Pb0.92Mo6S8, also contain well-defined subunits, in this case, clusters where a Mo6 octahedron is surrounded by eight chalcogen atoms forming a concentric cube. See Figure G1.1.7. The basic structure can host atoms of such different elements as alkaline, alkaline-earth, rare-earth metals or transition metals and metals such as lead or tin in the voids between the clusters. The Mo6 octahedron contracts in agreement with bond-valence rules, when the oxidation state or the concentration of interstitial cations increases. The ideal structure is rhombohedral with a cell angle ranging from 88 to 96°, reflecting the degree of delocalization of the cations.
Ab initio study of nitrogen and boron doped dimers
Published in Molecular Physics, 2022
Sandeep Kaur, Hitesh Sharma, V. K. Jindal, Vladimir Bubanja, Isha Mudahar
Physical distortions of molecules play a key role in electronic properties of alkali metal fullerides, which range from insulating to metallic [33], to high-temperature superconducting [34]. Pure is insulating, with the fivefold degenerate highest occupied molecular orbital (HOMO) and the threefold degenerate lowest unoccupied molecular orbital (LUMO) being roughly 2 eV above it. Additional electrons donated by alkali metal atoms are transferred into LUMO producing half-filled conduction band of metallic alkali metal fullerides, e.g. [35]. On the other hand, in case of , spontaneous molecular distortion arises from the coupling of degenerate electronic orbitals to certain vibrational molecular modes. This, Jahn-Teller effect, splits the LUMO into two occupied, lower, degenerate levels and an empty level about 0.1 eV above them. Metallic, , and insulating, , phases were investigated by mapping the local density of states directly beneath the STM tip of both occupied and unoccupied molecular orbitals and observing the presence or absence of an energy gap at the Fermi level [35]. Very different symmetries were observed for the occupied and unoccupied states. This is a signature of the Jahn-Teller effect distortion, which affects the two states in different ways, in contrast to the non-distorted molecules.