Evidence for endohedral muonium inKxC60and

Abstract

Positive muons injected into solid C6p, K4C6p, and K6C6p form vacuumlike muonium (tt e ) with a (6-12)% probability. Observation of coherent spin precession of muonium establishes that all three materials are nonmagnetic The experiment was performed on the M15 and M13 beamlines at TRIUMF which provide nearly 100% spinpolarized positive muons with a momentum of 28 MeVjc. The starting material of high-purity C60 powder was prepared using standard techniques [2] and showed no detectable infrared lines attributable to solvent molecules. High-resolution powder x-ray diA'raction yielded a crystallite size of greater than 1500 A and high-performance liquid chromatography showed better than 99.5% pure C6p with no trace of C600. The process for K doping is described elsewhere [20]. In order to prevent any exposure to air, the K4C60 and K6C6p powdered samples were

Coulomb couplings in positively charged fullerene

“…For n = 2 holes the ground state remains a triplet, with a very small gap to the lowest singlet, while low-spin ground states prevail for n ≥ 3. For electrons, the total effective J = −25 meV indicates that low-spin anti-Hund states are to be expected for C n− 60 , as is indeed observed experimentally in many electron-doped C 60 compounds (Kiefl et al 1992, Zimmer et al 1995, Lukyanchuk et al 1995, Prassides et al 1999.…”

“…In conclusion, both the Coulomb repulsion and the phonon-mediated attraction calculated within LDA are likely to need a rescaling by a similar factor of order two. Thus the balance between the two opposing interactions remains delicate in both C n− 60 [as demonstrated by the presence of both high-spin and low-spin local ground states in different chemical environments (Brouet et al 2001, Kiefl et al 1992, Zimmer et al 1995, Lukyanchuk et al 1995, Prassides et al 1999, Schilder et al 1994, Arovas and Auerbach 1995] and even more so in C n+ 60 , where however the high-spin states should be more favored. Moreover, especially in the hole doped case, we find multiplet splittings which are comparable to the theoretical bandwidth of solid-state fullerene (about 0.5 eV), indicating that Hund-rule intramolecular interactions are an important ingredient in C 60 ions.…”

Electron-vibron interactions in charged fullerenes. II. Pair energies and spectra

“…As an adsorbate on a metal surface, the electronegative C 60 molecule naturally picks up electrons [23,24], and recent evidence has been provided of charge transfer which can be as large as n=6 [17]. In the solid state, finally, there are compounds, covering a wide range of charge transfers, from n=1, as in TDAE + -C − 60 [25] or Rb 1 C 60 [26], n=3, as in K 3 C 60 or Rb 3 C 60 [27], n=4 as in K 4 C 60 [28], n=6 as in Rb 6 C 60 [29], or even higher as in Li 12 C 60 [30,16].…”

Role of alkali atoms inA4C60

“…While A 3 C 60 are metals 1 and superconductors, 2,3 A 4 C 60 are insulators. [4][5][6] The t 1u band of A 4 C 60 is only partly filled, and in contrast to A 6 C 60 , A 4 C 60 is not a band insulator, since band structure calculations predict a metal with a large density of states at the Fermi energy. 7,8 This raises interesting questions about what makes A 4 C 60 an insulator.…”