Strong superconductivity with local Jahn-Teller phonons in C60 solids

We analyze fulleride superconductivity at experimental doping levels, treating the electron-electron and electron-phonon interactions on an equal footing, and demonstrate that the Jahn-Teller phonons create a local (intramolecular) pairing which is surprisingly resistant to the Coulomb repulsion, despite the weakness of retardation in these low-bandwidth systems. The requirement for coherence throughout the solid then yields a very strong doping dependence to T(c), one consistent with experiment and much stronger than expected from standard Eliashberg theory.     

Electron-phonon coupling and its evidence in the photoemission spectra of lead

We present a detailed study of the influence of strong electron-phonon coupling on the photoemission spectra of lead. Representing the strong-coupling regime of superconductivity, the spectra of lead show characteristic features that demonstrate the correspondence of physical properties in the normal and the superconducting state, as predicted by the Eliashberg theory. These features appear on an energy scale of a few meV and are accessible for photoemission only by using modern spectrometers with high-resolution in energy and angle.     

Experimental evidence of a dynamic Jahn-Teller effect in C60(+)

Detailed analysis of the highest occupied molecular orbital band shape in the photoelectron spectrum of gaseous C60 reveals a dynamic Jahn-Teller effect in the ground state of C60(+). The direct observation of three tunneling states asserts a D(3d) geometry for the isolated cation, originating from a strong vibronic coupling. These results show that the ionic motion plays an important role in the electron-phonon interaction.     

Quasiparticle spectra, charge-density waves, superconductivity, and electron-phonon coupling in 2H-NbSe2

High-resolution photoemission has been used to study the electronic structure of the charge-density wave (CDW) and superconducting dichalcogenide, 2H-NbSe2. From the extracted self-energies, important components of the quasiparticle interactions have been identified. In contrast to previously studied TaSe2, the CDW transition does not affect the electronic properties significantly. The electron-phonon coupling is identified as a dominant contribution to the quasiparticle self-energy and is shown to be very anisotropic (k dependent) and much stronger than in TaSe2.     

The Jahn-Teller Instability of the Negatively Charged C60 Molecules

The linear 2 × (E + T) Jahn-Teller problem is relevant to the instability of C₆₀ molecules in threefold degenerate states when the E and T modes become degenerate. An isostationary function method is generalized to identify all extremes on the corresponding potential energy surface for different occupation numbers. For the C₆₀ molecules, the extremes are found to form an equipotential minimal energy trough for all occupation cases. For a nondegenerate case, two different coupling regimes are possible, favoring either pure tetragonal or pure trigonal distortions, depending upon the correponding Jahn-Teller energies.     

High-field ESR in the metallic fullerides RbC60 and CsC60

We discuss field-dependent electron spin resonance (ESR) experiments on the orthorhombic phase of RbC₆₀ and CsC₆₀. X-and W-band measurements have been performed to determine electronic scattering rates in the metallic temperature regime from 50 to 350 K. The insulating low-temperature phase (T < 50 K) has been investigated by X-, Q-, and W-band ESR, i.e., at 9, 34 and 95 GHz, in order to clarify whether collective resonance modes of a possibly magnetic ground state can be observed.     

Jahn-Teller effect in highly resolved optical spectra of the phenalenyl radical

Highly resolved emission and excitation spectra of phenalenyl are reported. Our semiempirical calculations predict E″ symmetry for the lowest excited state. Indeed, Jahn-Teller effects are observed. Force field calculations have been performed to determine the symmetries of vibrations visible in the spectra. The Jahn-Teller coupling is analysed with a scheme of Sloane and Silbey.