Important role of alkali atoms in A4C60

We show that hopping via the alkali atoms plays an important role for the t1u band of A4C60 (A=K, Rb), in strong contrast to A3C60. Thus the t1u band is broadened by more than 40 % by the presence of the alkali atoms. The difference between A4C60 and A3C60 is in particular due to the less symmetric location of the alkali atoms in A4C60.Comment: 5 pages, revtex, 2 figures, submitted to Phys. Rev. B more information at http://www.mpi-stuttgart.mpg.de/dokumente/andersen/fullerene

Electronic Structure of Superconducting Ba6c60

We report the results of first-principles electronic-structure calculations for superconducting Ba6C60. Unlike the A3C60 superconductors, this new compound shows strong Ba-C hybridization in the valence and conduction regions, mixed covalent/ionic bonding character, partial charge transfer, and insulating zero-gap band structure.Comment: 11 pages + 4 figures (1 appended, others on request), LaTeX with REVTE

Three-dimensional electronic instabilities in polymerized solid A1C60

The low-temperature structure of A1C60 (A=K, Rb) is an ordered array of polymerized C60 chains, with magnetic properties that suggest a non-metallic ground state. We study the paramagnetic state of this phase using first-principles electronic-structure methods, and examine the magnetic fluctuations around this state using a model Hamiltonian. The electronic and magnetic properties of even this polymerized phase remain strongly three dimensional, and the magnetic fluctuations favor an unusual three-dimensional antiferromagnetically ordered structure with a semi-metallic electronic spectrum.Comment: REVTeX 3.0, 10 pages, 4 figures available on request from [email protected]

Commensurate and modulated magnetic phases in orthorhombic A1C60

Competing magnetically ordered structures in polymerized orthorhombic A1C60 are studied. A mean-field theory for the equilibrium phases is developed using an Ising model and a classical Heisenberg model to describe the competition between inter- and intra-chain magnetic order in the solid. In the Ising model, the limiting commensurate one-dimensional and three-dimensional phases are separated by a commensurate three-sublattice state and by two sectors containing higher-order commensurate phases. For the Heisenberg model the quasi-1D phase is never the equilibrium state; instead the 3D commensurate phases exhibits a transition to a continuum of coplanar spiral magnetic phases.Comment: 11 pages REVTeX 3.0 plus 4 figures appende

Mott Transition in Degenerate Hubbard Models: Application to Doped Fullerenes

The Mott-Hubbard transition is studied for a Hubbard model with orbital degeneracy N, using a diffusion Monte-Carlo method. Based on general arguments, we conjecture that the Mott-Hubbard transition takes place for U/W \propto \sqrt{N}, where U is the Coulomb interaction and W is the band width. This is supported by exact diagonalization and Monte-Carlo calculations. Realistic parameters for the doped fullerenes lead to the conclusion that stoichiometric A_3 C_60 (A=K, Rb) are near the Mott-Hubbard transition, in a correlated metallic state.Comment: 4 pages, revtex, 1 eps figure included, to be published in Phys.Rev.B Rapid Com

The vertical distribution and biological transport of marine microplastics across the epipelagic and mesopelagic water column.

Plastic waste has been documented in nearly all types of marine environments and has been found in species spanning all levels of marine food webs. Within these marine environments, deep pelagic waters encompass the largest ecosystems on Earth. We lack a comprehensive understanding of the concentrations, cycling, and fate of plastic waste in sub-surface waters, constraining our ability to implement effective, large-scale policy and conservation strategies. We used remotely operated vehicles and engineered purpose-built samplers to collect and examine the distribution of microplastics in the Monterey Bay pelagic ecosystem at water column depths ranging from 5 to 1000 m. Laser Raman spectroscopy was used to identify microplastic particles collected from throughout the deep pelagic water column, with the highest concentrations present at depths between 200 and 600 m. Examination of two abundant particle feeders in this ecosystem, pelagic red crabs (Pleuroncodes planipes) and giant larvaceans (Bathochordaeus stygius), showed that microplastic particles readily flow from the environment into coupled water column and seafloor food webs. Our findings suggest that one of the largest and currently underappreciated reservoirs of marine microplastics may be contained within the water column and animal communities of the deep sea

Sourcing Substitution and Related Price Index Biases

intermediate input. We define a class of bias problems that arise when purchasers shift their expenditures among sellers charging different prices for units the purchasers view as the same product but that are not regarded as being the same for the purposes of price measurement. For businesses purchasing from other businesses, these sorts of shifts can cause sourcing substitution bias in the Producer Price Index (PPI) and the Import Price Index (MPI), as well as potentially in the proposed new true Input Price Index (IPI). Similarly, when consumers shift their expenditures for the same products temporally to take advantage of promotional sales or among retailers charging different per unit prices, this can cause a promotions bias problem in the Consumer Price Index (CPI) or a CPI outlet substitution bias. We provide a common framework for these bias problems. Ideal target indexes are defined and discussed that could greatly reduce these biases. We also address the challenges national statistics agencies must surmount to produce price index measures more like the specified target ones. 1

Theory of Superconducting TcT_{c} of doped fullerenes

We develop the nonadiabatic polaron theory of superconductivity of $M_{x}C_{60}$ taking into account the polaron band narrowing and realistic electron-phonon and Coulomb interactions. We argue that the crossover from the BCS weak-coupling superconductivity to the strong-coupling polaronic and bipolaronic superconductivity occurs at the BCS coupling constant $\lambda\sim 1$ independent of the adiabatic ratio, and there is nothing ``beyond'' Migdal's theorem except small polarons for any realistic electron-phonon interaction. By the use of the polaronic-type function and the ``exact'' diagonalization in the truncated Hilbert space of vibrons (``phonons'') we calculate the ground state energy and the electron spectral density of the $C_{60}^{-}$ molecule. This allows us to describe the photoemission spectrum of $C_{60}^{-}$ in a wide energy region and determine the electron-phonon interaction. The strongest coupling is found with the high-frequency pinch $A_{g2}$ mode and with the Frenkel exciton. We clarify the crucial role of high-frequency bosonic excitations in doped fullerenes which reduce the bare bandwidth and the Coulomb repulsion allowing the intermediate and low-frequency phonons to couple two small polarons in a Cooper pair. The Eliashberg-type equations are solved for low-frequency phonons. The value of the superconducting $T_{c}$, its pressure dependence and the isotope effect are found to be in a remarkable agreement with the available experimental data.Comment: 20 pages, Latex, 4 figures available upon reques

Covalency effects on the magnetism of EuRh2P2

In experiments, the ternary Eu pnictide EuRh2P2 shows an unusual coexistence of a non-integral Eu valence of about 2.2 and a rather high Neel temperature of 50 K. In this paper, we present a model which explains the non-integral Eu valence via covalent bonding of the Eu 4f-orbitals to P2 molecular orbitals. In contrast to intermediate valence models where the hybridization with delocalized conduction band electrons is known to suppress magnetic ordering temperatures to at most a few Kelvin, covalent hybridization to the localized P2 orbitals avoids this suppression. Using perturbation theory we calculate the valence, the high temperature susceptibility, the Eu single-ion anisotropy and the superexchange couplings of nearest and next-nearest neighbouring Eu ions. The model predicts a tetragonal anisotropy of the Curie constants. We suggest an experimental investigation of this anisotropy using single crystals. From experimental values of the valence and the two Curie constants, the three free parameters of our model can be determined.Comment: 9 pages, 5 figures, submitted to J. Phys.: Condens. Matte