Gaps and excitations in fullerides with partially filled bands : NMR study of Na2C60 and K4C60

We present an NMR study of Na2C60 and K4C60, two compounds that are related by electron-hole symmetry in the C60 triply degenerate conduction band. In both systems, it is known that NMR spin-lattice relaxation rate (1/T1) measurements detect a gap in the electronic structure, most likely related to singlet-triplet excitations of the Jahn-Teller distorted (JTD) C60^{2-} or C60^{4-}. However, the extended temperature range of the measurements presented here (10 K to 700 K) allows to reveal deviations with respect to this general trend, both at high and low temperatures. Above room temperature, 1/T1 deviates from the activated law that one would expect from the presence of the gap and saturates. In the same temperature range, a lowering of symmetry is detected in Na2C60 by the appearance of quadrupole effects on the 23Na spectra. In K4C60, modifications of the 13C spectra lineshapes also indicate a structural modification. We discuss this high temperature deviation in terms of a coupling between JTD and local symmetry. At low temperatures, 1/T$_1$T tends to a constant value for Na2C60, both for 13C and 23Na NMR. This indicates a residual metallic character, which emphasizes the proximity of metallic and insulting behaviors in alkali fullerides.Comment: 12 pages, 13 figure

The degenerate 3-band Hubbard model with "anti-Hund's rule" interactions; a model for AxC60

We consider the orbitally degenerate 3-band Hubbard model with on-site interactions which favor low spin and low orbital angular momentum using standard second order perturbation theory in the large Hubbard-U limit. At even integer filling this model is a Mott insulator with a non-degenerate ground state that allows for a simple description of particle-hole excitations as well as gapped spin and orbital modes. We find that the Mott gap is generally indirect and that the single particle spectrum at low doping reappears close to even filling but rescaled by a factor 2/3 or 1/3. The model captures the basic phenomenology of the Mott insulating and metallic fullerides AxC60. This includes the existence of a smaller spin gap and larger charge gap at even integer filling, the fact that odd integer stoichiometries are generally metallic while even are insulating, as well as the rapid suppression of the density of states and superconducting transition temperatures with doping away from x=3.Comment: Revised with additional reference

High-pressure optical microspectroscopy study on single-walled carbon nanotubes Encapsulating C60

The high-pressure behavior of single-walled carbon nanotubes (SWCNTs) filled with C-60 molecules, so-called C-60@SWCNT peapods, has been investigated by optical spectroscopy using nitrogen and argon as pressure transmitting media. Peapods with a high filling ratio were prepared by the sublimation method and characterized by high-resolution transmission electron microscopy. Optical transmission measurements under high pressure were conducted on films of SWCNTs and C-60@SWCNT peapods. As with SWCNTs, the absorption bands observed for peapods exhibit a red-shift under pressure. The relative energy shifts of the optical transitions under pressure are higher than those for empty SWCNTs, demonstrating enhanced hybridization and/or symmetry breaking effects. We find an anomaly in the pressure-induced shifts of the optical transitions at P-c1 approximate to 6.5 GPa (P-c1 approximate to 5.5 GPa) when using nitrogen (argon) as pressure transmitting medium. The anomaly signals the deformation of the nanotube from circular to an oval shape. The value of Pc, is in good agreement with theoretical predictions of the pressure-induced deformation for highly filled peapods with similar average diameter. A plateau in the pressure-induced shifts with an onset at P-c2 = 12-13 GPa indicates a saturation of the pressure-induced effects above this critical pressure.1991sciescopu

Possible metallization of the Mott insulators TiOCl and TiOBr: Effects of doping and external pressure

We report the results of doping- and pressure-dependent experimental investigations on the low-dimensional Mott-Hubbard insulators TiOCl and TiOBr. As observed by photoelectron spectroscopy, doping with Na and K shifts the valence band to higher binding energies associated with a jump of the chemical potential. With increasing doping, a broad hump develops close to the Fermi energy and the energy gap “softens”. The application of pressure induces the appearance of spectral weight close to the Fermi energy, as probed by infrared transmittance and reflectance measurements. The pressure-induced changes in the electronic properties coincide with a structural phase transition according to powder X-ray diffraction studies under pressure. The experimental findings are compared to theoretical predictions

Polaron physics and crossover transition in magnetite probed by pressure-dependent infrared spectroscopy

The optical properties of magnetite at room temperature were studied by infrared reflectivity measurements as a function of pressure up to 8 GPa. The optical conductivity spectrum consists of a Drude term, two sharp phonon modes, a far-infrared band at around 600 cm 1 and a pronounced mid-infrared absorption band. With increasing pressure both absorption bands shift to lower frequencies and the phonon modes harden in a linear fashion. Based on the shape of the MIR band, the temperature dependence of the dc transport data, and the occurrence of the far-infrared band in the optical conductivity spectrum, the polaronic coupling strength in magnetite at room temperature should be classified as intermediate. For the lower energy phonon mode an abrupt increase of the linear pressure coefficient occurs at around 6 GPa, which could be attributed to minor alterations of the charge distribution among the different Fe sites

Possible metallization of the Mott insulators TiOCl and TiOBr: effects of doping and external pressure

We report the results of doping- and pressure-dependent experimental investigations on the low-dimensional Mott-Hubbard insulators TiOCl and TiOBr. As observed by photoelectron spectroscopy, doping with Na and K shifts the valence band to higher binding energies associated with a jump of the chemical potential. With increasing doping, a broad hump develops close to the Fermi energy and the energy gap “softens”. The application of pressure induces the appearance of spectral weight close to the Fermi energy, as probed by infrared transmittance and reflectance measurements. The pressure-induced changes in the electronic properties coincide with a structural phase transition according to powder X-ray diffraction studies under pressure. The experimental findings are compared to theoretical predictions