Neutron Spectroscopy of Superconducting Fullerides
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NEUTRON SPECTROSCOPY OF SUPERCONDUCTING FULLERIDES KOSMAS PRASSIDES*, CHRISTOS CHRISTIDES*, JOHN TOMKINSON**, MATTHEW J. ROSSEINSKY***, D. W. MURPHY---, AND ROBERT C. HADDON*** *School of Chemistry and Molecular Sciences, University of Sussex, Brighton BN1 9QJ, UK "**RutherfordAppleton Laboratory, Didcot, Oxon OX1 1 0QX, UK ***AT&T Bell Laboratories, Murray Hill, NJ 07974 ABSTRACT The phonon spectra of pristine fullerene, superconducting K3 C 6 0 and saturation-doped Rb 6 C 6 0 measured by inelastic neutron scattering in the energy range 2.5 - 200 meV at low temperatures reveal substantial broadening of five-fold degenerate Hg intramolecular vibrational modes both in the low-energy radial and the high-energy tangential part of the spectrum. This provides strong evidence for a traditional phonon-mediated mechanism of superconductivity in the fullerides but with an electron-phonon coupling strength distributed over a wide range of energies (33-195 meV) as a result of the finite curvature of the fullerene spherical cage. INTRODUCTION Early efforts in fullerene research focussed in characterising the materials and confirming the originally proposed structures [1]: truncated icosahedral (lh) for C6 0 and ellipsoidal (Dsh) for C 70. At room temperature, solid C60 adopts a face-centred cubic crystal structure. At low temperatures (T25 meV; however, at high energy transfers (>150 meV), the resulting Debye-Waller factors are so large that they tend to wash out some of the vibrational features. RESULTS AND DISCUSSION The vibrational spectra of C6 0 and its reduced forms, C603 - and C 606 - are extremely rich in structure (Fig. 1). There are 174 intramolecular modes giving rise to 46 fundamentals; four are infrared active (Tiu) and ten are Raman active (Ag, Hg), leaving 32 optically inactive modes. INS measurements are not restricted by the usual optical selection rules and consequently, the full vibrational spectrum of C 60 n- (n=0, 3, 6), may be recorded. In addition, there is information on the intermolecular (external) low-energy modes. The intramolecular modes are not expected to show significant dispersion, permitting direct comparisons between the INS data and the available zone-centre theoretical and experimental data to be made. The vibrational spectra [6-10] may be divided into two principal regions : 0-30 meV (lattice modes) and 30-2 10 meV (intramolecular modes). There is an energy gap of the order of - 10-15 meV separating inter- from intramolecular modes. The intramolecular part of the spectrum extends smoothly from 30 to 210 meV. C60 lacks a central atom and consequently, radial forces are weak and the cage vibrations may be distinguished into radial (,-30-110 meV) and tangential (- 110-200 meV) ones. As a result of the carbon cage reduction C 60 -- C603 --- C 60 6-, the high energy cut-off in the vibrational DOS softens substantially from 205.0 meV through 198.0 meV (3.4%) to 190.2 meV (7.2%), whereas the low-energy onset is barely affected, showing a hardening (0.8 meV) for C6 06 -; this reflects the pronounc
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