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Compression of purified, unoriented, highly crystalline single-wall carbon nanotube material reveals an exceptionally large and reversible volume reduction. Density increases rapidly with increasing pressure, approaching that of graphite, and recovers completely upon pressure release. The reversible work done in compressing to 29thinspthinspkbar is 0.18 eV/C atom. We attribute this effect to crushing, or flattening the tube cross section from circular to elliptical. {copyright} {ital 1999} {ital The American Physical Society}

The effect of carrier concentration on fulleride superconductivity is studied by x ray, Raman, magnetization, and ESR of two new cubic families: ${\mathrm{Na}}_{2}{\mathrm{Cs}}_{x}{\mathrm{C}}_{60}$ $(0lxl1)$ and ${M}_{3\ensuremath{-}y}{\mathrm{Ba}}_{y}{\mathrm{C}}_{60}$ ( $0.2lyl2$, $M\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\mathrm{K}$, Rb, or Cs). These are isostructural with known $\mathrm{Pa}\overline{3}$ and $\mathrm{Fm}\overline{3}m$ trivalent superconductors, respectively, but with variable valence $2lnl5$. ${T}_{c}$ is peaked at or very near $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3$, decreasing rapidly in either direction to $l0.5\mathrm{K}$ for $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2.5$ and 5. These results underscore the failure of both one-electron and strong correlation models to explain the dependence of ${T}_{c}$ on band filling.

Synchrotron-x-ray powder-diffraction and differential-scanning-calorimetry measurements on solid ${\mathrm{C}}_{60}$ reveal a first-order phase transition from a low-temperature simple-cubic structure with a four-molecule basis to a face-centered-cubic structure at 249 K. The free-energy change at the transition is approximately 6.7 J/g. Model fits to the diffraction intensities are consistent with complete orientational disorder at room temperature, and with the development of orientational order rather than molecular displacements or distortions at low temperature.

We measured thermoelectric power S of bulk single-wall carbon nanotube materials p doped with acids. In contrast to oxygen-exposed or degassed samples, S is very small at the lowest temperatures, increases superlinearly above a characteristic and sample-dependent T, and then levels off. We attribute this unusual behavior to 1D phonon drag, in which the depression of the Fermi energy cuts off electron-phonon scattering at temperatures below a characteristic T0. This idea is supported by a model calculation in which the low temperature behavior of phonon drag is specifically related to the one-dimensional character of the electronic spectrum.

The vibrational density of states of single-wall carbon nanotubes (SWNT) was obtained from inelastic neutron scattering data from 0 to 225 meV. The spectrum is similar to that of graphite above 40 meV, while intratube features are clearly observed at 22 and 36 meV. An unusual energy dependence below 10 meV is assigned to contributions from intertube modes in the 2D triangular lattice of SWNT bundles, and from intertube coupling to intratube excitations. Good agreement between experiment and a calculated density of states for the SWNT lattice is found over the entire energy range.

Two structural phase transition in K${\mathrm{C}}_{24}$ have been observed by x-ray diffraction: an order-disorder transition at ${T}_{U}=126.0\ifmmode\pm\else\textpm\fi{}0.5$ K and a structural transition ${T}_{L}$ at about 93 K. Details of these transitions are presented, as well as the stacking sequences of the intermediate and low-temperature phases.

The Schumacher-Slichter technique was utilized to determine the absolute Pauli spin susceptibility of stage-1 and stage-2 graphite-As${\mathrm{F}}_{5}$ intercalation compounds. The results lead to a determination of the densities of states at the Fermi energy, 2.8\ifmmode\times\else\texttimes\fi{}${10}^{21}$ and 3.7\ifmmode\times\else\texttimes\fi{}${10}^{21}$ states/eV ${\mathrm{cm}}^{3}$, respectively; substantially lower than in metals with comparable conductivity.

Angle-resolved inverse photoemission with tunable photon energies has been used to map out the unoccupied bands of graphite and lithium-intercalated graphite. At the Brillouin zone center the bottom of the lowest band is found at 4.0\ifmmode\pm\else\textpm\fi{}0.5 eV above the Fermi level in graphite. This band shows strong dispersion normal to the basal plane in excellent agreement with recent self-consistent band-structure calculations. A similar three-dimensional band is found in lithium-intercalated graphite shifted 3 eV to lower energy.

We report the first observation of a marked temperature dependence in the optical properties of TTF-TCNQ, based on thermoreflectance spectroscopy. The structure just above the plasma edge exhibits an abrupt 25% drop with $\stackrel{\ensuremath{\rightarrow}}{\mathrm{e}}\ensuremath{\parallel}\stackrel{\ensuremath{\rightarrow}}{\mathrm{b}}$ as the temperature is lowered past 45-50 K; simultaneously, a strong response for this transition with $\stackrel{\ensuremath{\rightarrow}}{\mathrm{e}}\ensuremath{\perp}\stackrel{\ensuremath{\rightarrow}}{\mathrm{b}}$ emerges. This supports the suggestion that a librational distortion of the TCNQ ion may be responsible for a Peierls transition in the complex.

Dramatic differences in structure, elastic properties, and order-disorder temperatures are observed for stage-2 Li-graphite of different in-plane density, indicating the importance of long-range interactions in cohesive properties. LiC/sub 12/ is three-dimensionally ordered up to approx.500 K with ..sqrt..3 x ..sqrt..3 Li superlattice and AA graphite stacking, whereas LiC/sub 18/ is disordered at 300 K and has AB staking. LO and LA (00l) phonon energies are 30% greater in the former, which can be understood in terms of electrostatic effects.