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We have measured the Raman spectrum of individual single walled carbon nanotubes in solution and compare it to that obtained from the same starting material where the tubes are present in ordered bundles or ropes. Interestingly, the radial mode frequencies for the tubes in solution are found to be approximately 10 cm (-1) higher than those observed for tubes in a rope, in apparent contradiction to lattice dynamics predictions. We suggest that there is no such contradiction, and propose that the upshift is due rather to a decreased energy spacing of the Van Hove singularities in isolated tubes over the spacings in a rope, thereby allowing the same laser excitation to excite different diameter tubes in these two samples.

Thin-film multilayer structures with up to 20 repeat layers have been grown in a high-vacuum chamber by sequential deposition of aluminum (Al) and fullerene (${\mathrm{C}}_{60}$) onto room-temperature substrates. The periodicity of the layers is confirmed by x-ray-diffraction and in situ resistance measurements. The presence of underlying layers of ${\mathrm{C}}_{60}$ reduces the critical thickness at which Al becomes conducting from \ensuremath{\sim}35 to \ensuremath{\sim}20 \AA{}. In addition, there is a sudden increase in resistance that occurs when each Al layer is covered by a monolayer of ${\mathrm{C}}_{60}$. These observations, together with the measurement of a downward shift in frequency of a considerably broadened Raman-active ${\mathit{A}}_{\mathit{g}}$(2) pentagonal-pinch mode, imply that up to six electrons per ${\mathrm{C}}_{60}$ are transferred from the Al to the ${\mathrm{C}}_{60}$ layer. This demonstration of charge transfer across planar metal-${\mathrm{C}}_{60}$ interfaces suggests that multilayers may be a useful vehicle for forming fullerene interface compounds in two-dimensional structures.

We present evidence for and study the properties of coreless Josephson vortices (parallel to the superconducting layers) in the organic superconductor (BEDT-TTF${)}_{2}$Cu(NCS${)}_{2}$, using ac susceptibility measurements in the mixed state. We observe (1) extremely weak-pinning restoring forces for flux motion parallel to the layers, due to the absence of the vortex normal core; (2) single vortex (noncollective) pinning, due to weak interactions between Josephson vortices; (3) the ``lock-in'' of the vortices parallel to the layers when the dc field is applied at an arbitrary angle; and (4) highly nonlinear response to the ac field h in the lock-in state, above a threshold of h\ensuremath{\sim}0.5 G. The behavior of tilted flux lines is dominated by the much stronger pinning and collective interactions of two-dimensional Abrikosov ``pancake'' vortices, and linear response is largely restored when the flux lines unlock from the layers. We measure the Josephson penetration depth to be ${\ensuremath{\lambda}}_{\mathrm{\ensuremath{\perp}}}$(T=5 K)\ensuremath{\sim}200 \ensuremath{\mu}m, yielding a penetration depth anisotropy of \ensuremath{\gamma}=${\ensuremath{\lambda}}_{\mathrm{\ensuremath{\perp}}}$/${\ensuremath{\lambda}}_{\mathrm{\ensuremath{\parallel}}}$\ensuremath{\sim}160-350.

We report the two-dimensional angular-correlation-of-annihilation-radiation measurement of the ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ (pair) momentum distribution in the organic superconductor \ensuremath{\kappa}-(BEDT-TTF${)}_{2}$Cu(NCS${)}_{2}$. Our data, when compared with tight-binding (H\"uckel) electronic-band calculations, show features consistent with predicted Fermi surfaces. The structure around Z in our spectrum supports the results from earlier de Haas--van Alphen and Shubnikov--de Haas measurements. In addition, our data show features consistent with the predicted open Fermi surface that runs nearly parallel to YM.

We report on the electrical resistivity of K[sub 3]C[sub 60] and Rb[sub 3]C[sub 60] thin films. These films, grown at elevated temperatures, are highly textured and consist of large single-crystal grains. The films exhibit metallic behavior up to 500 K, with residual resistivities of 1.2 m[Omega] cm. The low-temperature ([ital T]) resistivity exhibits a [ital T][sup 2] dependence. We suggest that it is dominated by electron-electron interactions, and we find quantitative agreement not only with estimates based on the electron density and the bandwidth, but also with other narrow band systems. The electron-phonon interactions only become important for the resistivity above room temperature, because these phonons correspond to high-frequency vibrations of the C[sub 60] molecule. Nevertheless, virtual excitations of these same high-energy phonons are responsible for the superconductivity. Using similar criteria of conventional organic superconductors, we find that these materials are dominated at all temperatures by electron-phonon interactions.

The muon-spin-relaxation (\ensuremath{\mu}SR) technique has been used to study the internal field modulation dynamics of fluxons in crystals of \ensuremath{\kappa}-[BEDT-TTF${]}_{2}$Cu[NCS${]}_{2}$ (${\mathit{T}}_{\mathit{c}}$\ensuremath{\approxeq}10.5 K). The data exhibit a strong field dependence for applied fields in the range 129 Oe\ensuremath{\le}\ensuremath{\Vert}${\mathbf{H}}_{\mathrm{ext}}$\ensuremath{\Vert}\ensuremath{\le}4 kOe, which is attributed to a combination of vortex motion, finite core size effects, and pinning. In low fields (\ensuremath{\Vert}${\mathbf{H}}_{\mathrm{ext}}$\ensuremath{\Vert}=129 Oe), the data exhibit a flux-pinning transition at ${\mathit{T}}_{\mathit{x}}$\ensuremath{\sim}5 K, as evidenced by a peak in the second moment of the local field distribution at \ensuremath{\sim}5 K accompanied by an enhanced minimum in the first moment at the same temperature. In higher fields (\ensuremath{\Vert}${\mathbf{H}}_{\mathrm{ext}}$\ensuremath{\Vert}=750 Oe, 3 kOe, and 4 kOe), where the effects of motion and pinning are minimized, the symmetry of the underlying gap function is confirmed as s wave. Corrections for finite fluxon core, finite mean free path, and flux-lattice disorder were applied in determining the London penetration depth, ${\ensuremath{\lambda}}_{\mathit{L}}^{(\mathit{b}\mathit{c})}$(0)=7680\ifmmode\pm\else\textpm\fi{}700 \AA{}.

Crystalline films of C60 with ~1 μm grain size and a preferred [111] texture have been doped with K and Rb to form superconducting compositions that have sharp zero-resistance transitions. The temperature dependence of the resistance above Tc for both K3C60 and Rb3C60 films reveals metallic behavior up to temperatures as high as 520 K without any evidence of saturation. For Rb3C60, electronic mean free paths significantly shorter than nearest-neighbor C60 distances and anomalously high values of the electron-phonon coupling strength are inferred. These results suggest that there is a strong interaction between the conduction electrons and the intramolecular vibrational modes and also bring into question the assumption that all of the donated charge (three electrons per C60) is itinerant.

We report the observation and assignment of the doping-induced features in the UV-visible and near-IR optical spectra of ${\mathrm{C}}_{60}$ thin films and their direct correlations with electrical resistivity. A pronounced bleaching and several induced absorptions are observed upon doping with the alkali metals, potassium and rubidium. The absence of a strong dependence of the induced spectra upon intercalant suggests that the induced features observed are intrinsic to the host fullerene film. The evolution of these transitions, as doping proceeds from pristine ${\mathrm{C}}_{60}$ to ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$, is consistent with a two-domain picture for the formation of the metallic state. As the intercalation proceeds beyond the resistance minimum, an overall change in spectral shape begins to occur and energy shifts of many of the features become apparent. These observations indicate changes in the electronic bands accompanying the structural phase transformations from ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ to ${\mathrm{K}}_{6}$${\mathrm{C}}_{60}$. There is no spectral evidence for the presence of ${\mathrm{K}}_{4}$${\mathrm{C}}_{60}$ during doping.

A new stable compound ${\mathrm{Ba}}_{3}$${\mathrm{C}}_{60}$ is reported in the Ba-${\mathrm{C}}_{60}$ phase diagram. Rietveld refinement of x-ray powder diffraction data shows that this compound has the A15 structure with a lattice constant of 11.34 \AA{}. The Pm3\ifmmode\bar\else\textasciimacron\fi{}n space group implies a perfect alternating orientational order for the ${\mathrm{C}}_{60}$ molecules, not previously observed in doped fullerite structures. The relative stability of the A15 phase over the fully intercalated fcc structure can be explained by a simple model involving the Madelung energy differences, orientation dependence of the ${\mathrm{C}}_{60}$-cation interaction energy, and distortion-induced relaxational energy gains.

A formulation within the London approximation of magnetic ring currents is applied to C60, C70, and their hexa-anions. Contrary to previous approaches, this formulation does not require the identification of closed loops and can therefore be applied to these molecules despite their topological complexity. Knowledge of the ring currents provides a better understanding of their magnetic ring-current susceptibilities. Probe dipoles are used to investigate local spatial variations of the magnetic field induced by the ring currents. In an attempt to evaluate how the carbon chemical shifts are affected by the ring currents, we separate the current into a component circulating above and one below the molecular surface. Assuming a displacement of the negatively charged electronic clouds towards the outside of the molecule, we axe able to understand the relative positions of the carbon NMR lines of C60 and C60(6-).

We have measured the low-energy inelastic neutron scattering spectra of superconducting k 3 C 60 and insulating Rb 6 C 60 . Well-defined peaks are observed at room temperature at 1.59(4) meV [full width at half maximum (FWHM)=1.11(15) meV] and 5.49(18) meV [FWHM=1.41(44) meV], respectively. They harden with decreasing temperature and the dependence of their intensities on the scattering vector shows that they are due to small-amplitude librational motion

We have measured the normal-state susceptibility \ensuremath{\chi} for both ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_{3}$${\mathrm{Cu}}_{60}$ and the specific heat jump \ensuremath{\Delta}C at the superconducting ${\mathit{T}}_{\mathit{c}}$ for ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$. Within a phonon-mediated pairing model, the combined effects of \ensuremath{\chi} and \ensuremath{\Delta}C are shown to give strong support to pairing via high-energy modes (g1000 K) associated with intramolecular vibrations. In addition, the small \ensuremath{\Delta}C along with the known upper critical field slope provide evidence for an extreme dirty-limit superconductor, with mean free path l\ensuremath{\approxeq}10 \AA{}.

We report the longitudinal resistivity and Hall-effect data of thin films of ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ in the normal and superconducting states in magnetic fields up to 12.5 T. The resistivity is 2.5 m\ensuremath{\Omega} cm at room temperature and near the metal-to-insulator transition. The Hall coefficient is small as expected for a half-filled conduction band, and changes sign at 220 K. The resistivity is interpreted in terms of the granularity of the film which leads to zero-dimensional superconductivity in these systems with a length scale of 70 \AA{}. We find a superconducting coherence length of \ensuremath{\sim}26 \AA{}, which we interpret as a single-grain property. The Pippard coherence length, i.e., in the absence of granularity, is estimated to be \ensuremath{\sim}150 \AA{}.

We characterize the molecular reorientational dynamics in two phases of solid ${\mathrm{C}}_{60}$ with $^{13}\mathrm{C}$ NMR measurements. A change in the nature of the dynamics, indicated by a change in kinetic parameters extracted from spin-lattice relaxation data, occurs at the phase transition at 260 K. Above the transition, the molecules appear to execute continuous rotational diffusion; below the transition, they appear to jump between symmetry-equivalent orientations. This interpretation is consistent with the x-ray-diffraction results of Heiney et al. as well as our NMR relaxation and spectral data.

Crystals of C60 or C70 cocrystallized with n-pentane grow as elongated, ten-sided columns. X-ray diffraction shows ordering of C60 or C70 molecules along the column and a remarkable tenfold symmetry normal to the column. The ratio of the x-ray-diffraction vectors of the two lowest-order diffraction spots is nearly equal to the “golden ratio,” τ. Despite these similarities with decagonal, quasicrystalline order, the diffraction indicates crystalline order with a twinned unit cell.

The proton spin-lattice relaxation rate of the organic bifunctional dithiazolyl biradical 1,3-[(${\mathrm{S}}_{2}$${\mathrm{N}}_{2}$C)${\mathrm{C}}_{6}$${\mathrm{H}}_{4}$(${\mathrm{CN}}_{2}$${\mathrm{S}}_{2}$)] was measured at two NMR frequencies, 15.6 and 31.0 MHz. Data in the temperature range 350\char21{}480 K exhibit an enhancement in the rate consistent with magnetic-susceptibility results. The relaxation rate has a frequency-dependent part attributed to slow molecular motions, and a frequency-independent and strongly temperature-dependent spin motion contribution associated with free radicals. In addition to the increase in spin number, our results show an increase in the rate of spin fluctuation with temperature. A quantification of the motional rate extracted from these data does not support the speculation that these spins are the carriers that cause the increase in the electrical conductivity observed in the same temperature range.

Complementary $^{77}\text{S}\text{e}$ nuclear magnetic resonance (NMR) and electrical transport have been used to correlate the spin-density dynamics with the subphases of the field-induced spin-density wave (FISDW) ground state in ${(\text{TMTSF})}_{2}{\text{ClO}}_{4}$. We have found that the peaks in the spin-lattice relaxation rate $1/{T}_{1}$ appear within the metal-FISDW phase boundary and/or at first-order subphase transitions. In the quantum limit above 25 T, the NMR data have given an insight into the FISDW electronic structure.

We have performed a torque magnetometry study of an organic superconductor. The results indicate that the superconductivity of \ensuremath{\kappa}-BEDT-${\mathrm{TTF}}_{2}$Cu(NCS${)}_{2}$ is extremely anisotropic, and suggest a lower bound of 4\ifmmode\times\else\texttimes\fi{}${10}^{4}$ for the superconducting effective-mass anisotropy. Our data have also allowed us to make a detailed test of the predictions of the three-dimensional (3D) model for the torque and also those of a recently suggested ``2D'' approach. Our results are in better agreement with the predictions of the 3D model.

We report the first direct measurement of the effective magnetic penetration depth in oriented single crystals of {kappa}-(BEDT-TTF){sub 2}Cu(NCS){sub 2}, with {ital T}{sub {ital c}}(5 G){approx}9 K. Results yield an effective in-plane value of {lambda}{sub {ital e}{ital f}{ital f}}{sup {ital b}{ital c}}(0){approx}9800 A (for {ital H}{sub ext}{approx}3 kG), and a temperature dependence consistent with conventional {ital s}-wave pairing. Comparison with the London penetration depth, {lambda}{sub {ital L}}(0) (estimated to be {approx}5100 A), indicates a tendency toward dirty-limit superconductivity, with the ratio of coherence length over mean free path of {xi}{sub 0}{sup {ital b}{ital c}}/{ital l}{sub {ital b}{ital c}}{approx}2.7. From our results, it appears unnecessary to invoke any unconventional pairing schemes to explain the superconductivity in this material.

Dans la phase metallique, la magnetoconductivite est proportionnelle a Hα ou α est inferieur a 2, et α decroit lorsque le champ magnetique tourne autour de l'axe c * . Dans les etats d'onde de densite de spin induite par champ, il y a une augmentation des anomalies aux transitions de phase lorsque le champ tourne

Measurements of the superconducting ${\mathit{T}}_{\mathit{c}}$ and Raman spectra in isotopically modified ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$ are reported. A decrease in ${\mathit{T}}_{\mathit{c}}$ of 0.65\ifmmode\pm\else\textpm\fi{}0.10 K in samples with (75\ifmmode\pm\else\textpm\fi{}5)% substitution of $^{13}\mathrm{C}$ for $^{12}\mathrm{C}$ yields an exponent of \ensuremath{\alpha}=0.37\ifmmode\pm\else\textpm\fi{}0.05. This interpreted as evidence for phonon-mediated pairing.

Meissner-effect and microwave-absorption measurements on bulk samples show that ${\mathrm{Rb}}_{\mathit{x}}$${\mathrm{C}}_{60}$ is superconducting with a maximum transition temperature of 28 K. This is a 10-K (60%) increase over the K-doped material. Only ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_{3}$ and the cuprate superconductors have higher transition temperatures.

Mesures en haute resolution de la chaleur massique pour des champs magnetiques allant jusqu'a ST. Observation d'un saut ΔC a la transition supraconductrice. L'application du champ elargit la transition mais la decroit legerement. Les valeurs du champ critique sont superieures a celles obtenues par mesure de magnetoconductivite