Accessing the transport properties of graphene and its multi-layers at high carrier density

We present a comparative study of high carrier density transport in mono-, bi-, and trilayer graphene using electric-double-layer transistors to continuously tune the carrier density up to values exceeding 10^{14} cm^{-2}. Whereas in monolayer the conductivity saturates, in bi- and trilayer flling of the higher energy bands is observed to cause a non-monotonic behavior of the conductivity, and a large increase in the quantum capacitance. These systematic trends not only show how the intrinsic high-density transport properties of graphene can be accessed by field-effect, but also demonstrate the robustness of ion-gated graphene, which is crucial for possible future applications.Comment: 4 figures, 4 page

Quantitative analysis of electronic transport through weakly-coupled metal/organic interfaces

Using single-crystal transistors, we have performed a systematic experimental study of electronic transport through oxidized copper/rubrene interfaces as a function of temperature and bias. We find that the measurements can be reproduced quantitatively in terms of the thermionic emission theory for Schottky diodes, if the effect of the bias-induced barrier lowering is included. Our analysis emphasizes the role of the coupling between metal and molecules, which in our devices is weak due to the presence of an oxide layer at the surface of the copper electrodes.Comment: 4 pages, 3 figure

Magnetic resonance in the antiferromagnetic and normal state of NH_3K_3C_60

We report on the magnetic resonance of NH_3K_3C_60 powders in the frequency range of 9 to 225 GHz. The observation of an antiferromagnetic resonance below the phase transition at 40 K is evidence for an antiferromagnetically ordered ground state. In the normal state, above 40 K, the temperature dependence of the spin-susceptibilty measured by ESR agrees with previous static measurements and is too weak to be explained by interacting localized spins in an insulator. The magnetic resonance line width has an unusual magnetic-field dependence which is large and temperature independent in the magnetically ordered state and decreases rapidly above the transition. These observations agree with the suggestion that NH_3K_3C_60 is a metal in the normal state and undergoes a Mott-Hubbard metal to insulator transition at 40 K.Comment: 4 pages, 5 figures. Submitted to Phys. Rev.

Status of the Standard Solar Model Prediction of Solar Neutrino Fluxes

The Standard Solar Model (BP04) predicts a total 8B neutrino flux that is 17.2% larger than measured in the salt phase of the SNO detector (and if it were significant it will indicate oscillation to sterile neutrinos). Hence it is important to examine in details uncertainties (and values) of inputs to the SSM. Currently, the largest fractional uncertainty is due to the new evaluation of the surface composition of the sun. We examine the nuclear input on the formation of solar 8B [S17(0)] and demonstrate that it is still quite uncertain due to ill known slope of the measured astrophysical cross section factor and thus ill defined extrapolation to zero energy. This yields an additional reasonably estimated uncertainty due to extrapolation of +0.0 -3.0 eV-b (+0% -14%). Since a large discrepancy exists among measured as well as among predicted slopes, the value of S17(0) is dependent on the choice of data and theory used to extrapolate S17(0). This situation must be alleviated by new measurement(s). The "world average" is driven by the Seattle result due to the very small quoted uncertainty, which we however demonstrate it to be an over-estimated accuracy. We propose more realistic error bars for the Seattle results based on the published Seattle data.Comment: Fifth International Conferenceon Non-Accelerator New Physics, Dubna, June 20-25, 2005. Work Supported by USDOE Grant No. DE-FG02-94ER4087

Infrared spectra of one- and two-dimensional fullerene polymer structures: RbC60 and rhombohedral C-60

We compare the infrared spectra of two types of fullerene polymers: the linear-chain RbC60 and the two-dimensional pressure-polymerized rhombohedral C-60. Both the splitting of the F-1u modes and the structure of newly activated Lines are in agreement with fully ordered structures of molecular symmetry D-2h and D-3d, respectively

Global Production Increased by Spatial Heterogeneity in a Population Dynamics Model

Spatial and temporal heterogeneity are often described as important factors having a strong impact on biodiversity. The effect of heterogeneity is in most cases analyzed by the response of biotic interactions such as competition of predation. It may also modify intrinsic population properties such as growth rate. Most of the studies are theoretic since it is often difficult to manipulate spatial heterogeneity in practice. Despite the large number of studies dealing with this topics, it is still difficult to understand how the heterogeneity affects populations dynamics. On the basis of a very simple model, this paper aims to explicitly provide a simple mechanism which can explain why spatial heterogeneity may be a favorable factor for production.We consider a two patch model and a logistic growth is assumed on each patch. A general condition on the migration rates and the local subpopulation growth rates is provided under which the total carrying capacity is higher than the sum of the local carrying capacities, which is not intuitive. As we illustrate, this result is robust under stochastic perturbations

Evidence for Insulating Behavior in the Electric Conduction of (NH3_3)K3_3C60_{60} Systems

Microwave study using cavity perturbation technique revealed that the conductivity of antiferromagnet (NH$_3$)K$_{3-x}$Rb$_x$C$_{60}$ at 200K is already 3-4 orders of magnitude smaller than those of superconductors, K$_3$C$_{60}$ and (NH$_3$)$_x$NaRb$_2$C$_{60}$, and that the antiferromagnetic compounds are {\it insulators} below 250K without metal-insulator transitions. The striking difference in the magnitude of the conductivity between these materials strongly suggests that the Mott-Hubbard transition in the ammoniated alkali fullerides is driven by a reduction of lattice symmetry from face-centered-cubic to face-centered-orthorhombic, rather than by the magnetic ordering.Comment: accepted for publication in PR

Raman Scattering Study of Ba-doped C60 with t1g States

Raman spectra are reported for Ba doped fullerides, BaxC60(x=3,4,and 6). The lowest frequency Hg modes split into five components for Ba4C60 and Ba6C60 even at room temperature, allowing us a quantitative analysis based on the electron-phonon couping theory. For the superconducting Ba4C60, the density of states at the Fermi energy was derived as 7 eV-1, while the total value of electron-phonon coupling \lambda was found to be 1.0, which is comparable to that of K3C60. The tangential Ag(2) mode, which is known as a sensitive probe for the degree of charge transfer on C60 molecule, shows a remarkable shift depending on the Ba concentration, being roughly consistent with the full charge transfer from Ba to C60. An effect of hybridization between Ba and C60 \pi orbitals is also discussed.Comment: 15 pages, 6 figures submitted to Phys. Rev. B (December 1,1998

Low-energy cross section of the 7Be(p,g)8B solar fusion reaction from Coulomb dissociation of 8B

Final results from an exclusive measurement of the Coulomb breakup of 8B into 7Be+p at 254 A MeV are reported. Energy-differential Coulomb-breakup cross sections are analyzed using a potential model of 8B and first-order perturbation theory. The deduced astrophysical S_17 factors are in good agreement with the most recent direct 7Be(p,gamma)8B measurements and follow closely the energy dependence predicted by the cluster-model description of 8B by Descouvemont. We extract a zero-energy S_17 factor of 20.6 +- 0.8 (stat) +- 1.2 (syst) eV b.Comment: 14 pages including 16 figures, LaTeX, accepted for publication in Physical Review C. Minor changes in text and layou

Optical conductivity in the normal state fullerene superconductors

We calculate the optical conductivity, $\sigma(\omega)$, in the normal state fullerene superconductors by self-consistently including the impurity scatterings, the electron-phonon and electron-electron Coulomb interactions. The finite bandwidth of the fullerenes is explicitely considered, and the vertex corection is included $a$ $la$ Nambu in calculating the renormalized Green's function. $\sigma(\omega)$ is obtained by calculating the current-current correlation function with the renormalized Green's function in the Matsubara frequency and then performing analytic continuation to the real frequency at finite temperature. The Drude weight in $\sigma(\omega)$ is strongly suppressed due to the interactions and transfered to the mid-infrared region around and above 0.06 eV which is somewhat less pronounced and much broader compared with the expermental observation by DeGiorgi $et$ $al$.Comment: 6 pages, 4 figures. To be published in Physical Review B, July 1