Redox-controlled potassium intercalation into two polyaromatic hydrocarbon solids.

Alkali metal intercalation into polyaromatic hydrocarbons (PAHs) has been studied intensely after reports of superconductivity in a number of potassium- and rubidium-intercalated materials. There are, however, no reported crystal structures to inform our understanding of the chemistry and physics because of the complex reactivity of PAHs with strong reducing agents at high temperature. Here we present the synthesis of crystalline K2Pentacene and K2Picene by a solid-solid insertion protocol that uses potassium hydride as a redox-controlled reducing agent to access the PAH dianions, and so enables the determination of their crystal structures. In both cases, the inserted cations expand the parent herringbone packings by reorienting the molecular anions to create multiple potassium sites within initially dense molecular layers, and thus interact with the PAH anion π systems. The synthetic and crystal chemistry of alkali metal intercalation into PAHs differs from that into fullerenes and graphite, in which the cation sites are pre-defined by the host structure.

Upper critical field reaches 90 tesla near the Mott transition in fulleride superconductors.

Controlled access to the border of the Mott insulating state by variation of control parameters offers exotic electronic states such as anomalous and possibly high-transition-temperature (Tc) superconductivity. The alkali-doped fullerides show a transition from a Mott insulator to a superconductor for the first time in three-dimensional materials, but the impact of dimensionality and electron correlation on superconducting properties has remained unclear. Here we show that, near the Mott insulating phase, the upper critical field Hc2 of the fulleride superconductors reaches values as high as ∼90 T-the highest among cubic crystals. This is accompanied by a crossover from weak- to strong-coupling superconductivity and appears upon entering the metallic state with the dynamical Jahn-Teller effect as the Mott transition is approached. These results suggest that the cooperative interplay between molecular electronic structure and strong electron correlations plays a key role in realizing robust superconductivity with high-Tc and high-Hc2.

Raman spectroscopic study of the rare-earth fullerides Eu6-xSrxC60.

We present Raman spectroscopic studies of the isostructural and isoelectronic Eu(6-x)Sr(x)C(60) (x = 0, 3, 5, 6) and Ba(6)C(60) compounds. The Raman spectra of the Eu-based fullerides show dramatic changes compared to the pure alkaline-earth systems, including significant broadening, splitting and frequency shifts of the fivefold degenerate H(g) intramolecular modes of C(60). Moreover, the A(g)(2) mode exhibits an even larger downshift and a remarkable broadening. These findings are consistent with distortions of the C(60) molecular cages and a considerable electron-phonon coupling strength-strongly enhanced in the Eu containing systems-originating from the strong orbital hybridization between the metal atom and the C(60) molecule.

Strong electron correlations in the normal state of the iron-based FeSe0.42Te0.58 superconductor observed by angle-resolved photoemission spectroscopy.

We investigate the normal state of the "11" iron-based superconductor FeSe0.42Te0.58 by angle-resolved photoemission. Our data reveal a highly renormalized quasiparticle dispersion characteristic of a strongly correlated metal. We find sheet dependent effective carrier masses between approximately 3 and 16m{e} corresponding to a mass enhancement over band structure values of m{*}/m{band} approximately 6-20. This is nearly an order of magnitude higher than the renormalization reported previously for iron-arsenide superconductors of the "1111" and "122" families but fully consistent with the bulk specific heat.

Intermolecular overlap geometry gives two classes of fulleride superconductor: electronic structure of 38 K Tc Cs3C60.

Superconductivity emerges for the A15 polymorph of the fulleride Cs3C60 upon compression to a pressure of approximately 4 kbar. Using density functional theory we study the bonding in the A15 phase as a function of unit cell volume comparing it to that in the fcc polymorph. We find that, despite its smaller packing density, the bcc-derived A15 phase has both a substantially wider bandwidth for the partially occupied t1u band and a higher density of states at the Fermi level. This result can be traced to the striking differences in the nature of the interanion Tc--the two sphere packings (body centered versus face centered) observed experimentally produce two electronically distinct classes of fulleride superconductors.

Resonant processes and Coulomb interactions in (C59N)2.

We have determined the on-site molecular Coulomb interaction energy U of a (C59N)2 bulk film and find values ranging from 1.10+/-0.10 eV for the highest occupied molecular orbital to 1.35+/-0.10 eV for the deeper lying orbitals, comparable to values found in C60. The on-site Coulomb interaction between a carbon core hole and valence electrons, Uc, is, however, substantially lower than in C60 at 1.35+/-0.07 eV. Resonant photoemission (RESPES) results show a weakened participator decay channel, especially around the N 1s threshold, where resonance of the highest occupied molecular orbital shoulder is absent. Near-edge x-ray absorption fine structure and constant initial state measurements, taken in parallel with the RESPES data, indicate, however, that matrix element effects cannot be ruled out.

Synthesis of biocompatible poly[2-(methacryloyloxy)ethyl phosphorylcholine]-coated magnetite nanoparticles.

A well-defined, double-hydrophilic diblock copolymer comprising poly[2-(methacryloyloxy)ethyl phosphorylcholine]-block-(glycerol monomethacrylate) (PMPC30-PGMA30, where the numbers represent the average degrees of polymerization for each block) was evaluated for the synthesis of colloidally stable ultrafine magnetite sols. Sterically stabilized paramagnetic sols were prepared in aqueous solution by chemical coprecipitation of ferric and ferrous salts in the presence of this block copolymer. The PMPC30-PGMA30-stabilized magnetite sol had a mean transmission electron microscopy (TEM) diameter of 9.4 +/- 1.7 nm and a mean hydrodynamic diameter of 34 nm. This sol exhibited improved colloidal stability with respect to long-term storage and pH variation compared with magnetite sols prepared in the presence of alternative water-soluble homopolymers and diblock copolymers. Fourier transform infrared (FT-IR) spectroscopy, thermogravimetry, electron spectroscopy imaging (ESI), and zeta potential studies indicate that the PMPC30-PGMA30 diblock copolymer was adsorbed onto the surface of the sol via the PGMA30 block, with the PMPC30 chains acting as the stabilizing block. Such sterically stabilized sols are expected to be improved contrast agents for magnetic resonance imaging (MRI) applications.

Low temperature magnetic instabilities in triply charged fulleride polymers

The electronic properties of the C3-60 polymer in Na2Rb0.3Cs0.7C60 were studied by X-band and high field (109.056 GHz) ESR. They are characteristic of a strongly correlated quasi-one-dimensional metal down to 45 K. On further cooling, a pseudogap of magnetic origin opens at the Fermi level below 45 K with three-dimensional magnetic ordering occurring below T(N) approximately 15 K, as confirmed by the observation of an antiferromagnetic resonance mode. The Na2Rb1-xCsxC60 family of polymers offers a unique way to chemically control the electronic properties, as the opening of the gap in this system of predominantly itinerant electrons is an extremely sensitive function of the interchain separation.

Neutron powder diffraction study (T= 4.2-300 K) and polarization analysis of YBaCuFeO5+δ.

The crystal and magnetic structures of the perovskite YBaCuFeO5+δhave been studied in the temperature rangeT = 4.2-300 K by powder neutron diffraction. In addition to the antiferromagnetic ordering transition atTN = 442 K, a commensurate-incommensurate magnetic transition is detected atTN' = 190 K. Below this temperature, two sets of satellite peaks surround the (1/2,1/2,1/2) magnetic peak atd âˆ¼ Å, collapsing into a single set of satellites below 155 K. Polarization analysis has been performed to confirm the magnetic nature of the (1/2,1/2,1/2)±satellites.

Spontaneous Magnetic Ordering in the Fullerene Charge-Transfer Salt (TDAE)C60.

The zero-field muon spin relaxation technique has been used in the direct observation of spontaneous magnetic order below a Curie temperature (T(c)) of approximately 16.1 kelvin in the fullerene charge-transfer salt (tetrakisdimethylaminoethylene)C(60) [(TDAE)C(60)]. Coherent ordering of the electronic magnetic moments leads to a local field of 68(1) gauss at the muon site at 3.2 kelvin (parentheses indicate the error in the last digit). Substantial spatially inhomogeneous effects are manifested in the distribution of the local fields, whose width amounts to 48(2) gauss at the same temperature. The temperature evolution of the internal magnetic field below the freezing temperature mirrors that of the saturation magnetization, closely following the behavior expected for collective spin wave (magnon) excitations. The transition to a ferromagnetic state with a T(c) higher than that of any other organic material is now authenticated.

Orientational Disorder of C60 in Li2CsC60.

The x-ray diffraction of the nonsuperconducting ternary fulleride Li(2)CsC(60) reveals at room temperature a face-centered-cubic (Fm3m) disordered structure that persists to a temperature of 13 Kelvin. The crystal structure is best modeled as containing quasispherical [radius of 3.556(4) angstroms] C(60)(3-) ions, in sharp contrast to their orientational state in superconducting face-centered-cubic K(3)C(60) (merohedral disorder) and primitive cubic Na(2)CsC(60) (orientational order). The orientational disorder of the carbon atoms on the C(60)(3-) sphere was analyzed with symmetry-adapted spherical-harmonic functions. Excess atomic density is evident in the <111> directions, indicating strong bonding Li(+)-C interactions, not encountered before in any of the superconducting alkali fullerides. The intercalate-carbon interactions and the orientational state of the fullerenes have evidently affected the superconducting pair-binding mechanism in this material.

Crystal Structure, Bonding, and Phase Transition of the Superconducting Na2CsC60 Fulleride.

The crystal structure of superconducting Na(2)CsC(60) was studied by high-resolution powder neutron diffraction between 1.6 and 425 K. Contrary to the literature, the structure at low temperatures is primitive cubic [See equation in the PDF file], isostructural with pristine C(60). Anticlockwise rotation of the C(60) units by 98 degrees about [111] allows simultaneous optimization of C(60)-C(60) and alkali-fulleride interactions. Optimal Na(+)-C(60)(3-) coordination is achieved with each sodium ion located above one hexagon face and three hexagon-hexagon fusions of neighboring fulleride ions (coordination number 12). Reduction of the C(60) molecule lengthens the hexagon-hexagon fusions and shortens the pentagon-hexagon fusions (to approximately 1.43 angstroms). On heating, Na(2)CsC(60) undergoes a phase transition to a face-centered-cubic [See equation in the PDF file] phase, best modeled as containing quasi-spherical C(60)(3-) ions. The modified structure and intermolecular potential provide an additional dimension to the behavior of superconducting fullerides and should sensitively affect their electronic and conducting properties.