M. Belli

Consorzio Nazionale Interuniversitario per le Scienze del Mare, Roma, Latium, Italy

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Publications (12)35.22 Total impact

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    ABSTRACT: The superconducting properties of the graphite intercalation compound CaC6, available as high-quality c-axis-oriented polycrystals, were investigated through muon-spin spectroscopy measurements. An unconventional TF-μSR procedure, applicable to superconductors characterized by close-lying critical fields, was successfully adopted for measuring the muon-spin relaxation rates. Field-dependent measurements provide a value λab(0)=62(4) nm for the in-plane magnetic field penetration depth, κ=1.4(2) for the GL parameter, and hint at the presence of a slight gap anisotropy in the ab plane. The dependence of relaxation on temperature confirms the s-wave character of CaC6 superconductivity, with an average zero-temperature superconducting gap of 2.1(1) meV.
    New Journal of Physics 01/2011; 13(1):013038. · 4.06 Impact Factor
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    ABSTRACT: We report on the extraordinary superionic conductivity in the fulleride polymer Li4C60, a crystalline material with no disorder. 7Li, NMR, and dc frequency dependent conductivity show uncorrelated ionic hopping across small energy barriers (DeltaE_{a} approximately 200 meV) and an ionic conductivity of 10;{-2} S/cm at room temperature, higher than in "standard" ionic conductors. Ab initio calculations of the molecular structure find intrinsic unoccupied interstitial sites that can be filled by Li+ cations in stoichiometric Li4C60 even at low temperatures. The low energy required for the occupation of these sites allows a sizable Li+ diffusion above 130 K. The results suggest novel application of lithium intercalated fullerides as electrodes in Li ions batteries.
    Physical Review Letters 05/2009; 102(14):145901. · 7.73 Impact Factor
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    ABSTRACT: The structural relaxation of nanographite ball-milled in argon atmosphere and stored in evacuated ampoules, has been studied by monitoring the evolution of the Raman spectra. Reduction of some Raman bands associated with “in-plane” structural defects, such as vacancies, dislocations or grain boundaries, and those associated with “out-of-plane” defects, such as stacking faults, is accompanied by the rise of new bands around 1180 and 2000 cm–1. The development of these bands is suppressed in nanographite samples exposed to a flux of pure anhydrous hydrogen. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (b) 08/2008; 245(10):2082 - 2085. · 1.49 Impact Factor
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    ABSTRACT: In this paper, we combine synchrotron powder x-ray diffraction, 7Li nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) experiments to study the structural evolution of Li4C60 and how its electronic ground state depends on the crystal symmetry. The compound in the two-dimensional polymer phase with mixed interfullerene bonding motifs is a band gap insulator. EPR, however, reveals the presence of intrinsic centers originating from broken C60–C60 bonds and local Li off-stoichiometry that create states in the band gap and account for the complex temperature dependence of the spin susceptibility as well as the residual temperature dependence of the 7Li NMR shift. At low temperatures, the Li+ ions are statically disordered on the 7Li NMR timescale. The observed 7Li NMR line narrowing at T>200 K is ascribed to the Li+ diffusion dynamics and above room temperature the polymer phase is already a good ionic conductor. Heating the sample to temperatures above ~470 K results in gradual depolymerization to the metallic monomer fcc high temperature structure. The transformation is first order and polymer as well as monomer phases coexist over a broad temperature interval (130 K).
    New Journal of Physics 03/2008; 10(3):033021. · 4.06 Impact Factor
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    ABSTRACT: Although widely accepted, the phonon-mediated, BCS-like theory of superconducting A3C60 fullerides (A=K, Rb, Cs) cannot reproduce correctly all their parameters, even in its strong-coupling, Migdal-Eliashberg limit. The fundamental difficulty, ascribed to intrinsically close phonon and electron energy scales (respectively at 0.2 and 0.25 eV), has been overcome by dynamical mean-field theories (DMFT), which, unlike ME, consider electron-phonon and electron-electron interactions on an equal footing. The unconventional phenomena predicted in the new framework include, among others, isotope effects on spin susceptibility, totally absent from standard theories. We have tested these predictions, finding a significant dependence on the isotopic mass in both Tc and, more importantly, in the normal-state Pauli susceptibility χP. The comparative measurement of χP in two different K3C60 samples (85%13C-enriched vs. natural abundance), both by SQUID magnetometry as well as by 13C NMR Knight shift, definitely confirms the presence of an isotope effect on susceptibility, although a quantitative agreement with theory is still missing.
    EPL (Europhysics Letters) 01/2008; 81(5):57002. · 2.26 Impact Factor
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    ABSTRACT: The restoration of metallicity in the high-temperature, cubic phase of Li4C60 represents a remarkable feature for a member of the A4C60 family (A = alkali metal), invariably found to be insulators. Structural and resonance technique investigations on Li4C60 at T > 600 K, show that its fcc structure is associated with a complete (4e) charge transfer to C60 and a sparsely populated Fermi level. These findings not only emphasize the crucial role played by lattice symmetry in fulleride transport properties, but also re-dimension the role of Jahn-Teller effects in band structure determination. Moreover, they suggest the present system as a potential precursor to a new class of superconducting fullerides. Comment: 4 pages, 3 figures
    01/2007;
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    ABSTRACT: Despite the theoretical prediction of an interesting insulator-to-metal transition in rhombohedral C60 polymers (Rh-C60) upon electron doping, very few attempts have been performed up to now to actually intercalate it with donors. In this Letter we show that Rh-C60 converts to the monomer fcc C60 around 450K; this explains the failure of previous doping attempts which required treatments at higher temperatures. Here we performed the Li doping of Rh-C60 in solution of liquid ammonia, a low temperature technique which avoids thermal depolymerisation. However, the rhombohedral structure is not retained after intercalation, yielding a monomer alkali-metal doped fulleride.
    Chemical Physics Letters - CHEM PHYS LETT. 01/2007; 446(1):56-58.
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    ABSTRACT: The ammoniation of lithium-doped C60 gives rise to (NH3)6LixC60NH3)6LixC60, a system which is chemically expanded while maintaining C60 original cubic symmetry and hence the threefold degeneracy of the C60x- frontier orbitals. Nevertheless, the on-site repulsion U dominates the bandwidth W and, by overcoming the effect of orbital degeneracy, brings to electron localisation. Muon-spin relaxation (μSR)(μSR) and SQUID magnetometry were used to confirm the presence of a magnetically ordered phase at low temperatures, already reported by Durand and co-workers [Nat. Mater. 2 (2003) 605]. Surprisingly, neither the expected antiferromagnetism nor the even-odd effect on electron band filling were observed down to 2 K.
    Physica B Condensed Matter 01/2006; s 374–375:255–258. · 1.28 Impact Factor
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    ABSTRACT: The low‐doped LixC60 compounds (x≤6) were investigated using laboratory X‐ray and synchrotron radiation diffraction, C NMR and Raman spectroscopy. Li4C60 shows an unusual 2D polymerisation in which the C60 units are connected both by [2+2] cycloaddition and by single carbon–carbon bonds, a unique feature among the known polymerised fullerene compounds. This picture is fully supported also by static NMR and Raman measurements. The charge transfer to C60 in the polymeric phase was evaluated from the shift of the Ag(2) mode. The depolymerisation process was investigated as well; despite the presence of two different bonds, the polymer‐to‐monomer transition induced by thermal treatments is a single‐step phenomenon.
    Fullerenes Nanotubes and Carbon Nanostructures - FULLER NANOTUB CARBON NANOSTR. 01/2006; 14:391-400.
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    ABSTRACT: Li4C60 , one of the best representatives of lithium intercalated fullerides, features an unusual type of two-dimensional polymerization. Extensive investigations, including laboratory x-ray and synchrotron radiation diffraction, C13 NMR, magic angle spinning, and Raman spectroscopy, show a monoclinic I2/m structure, characterized by chains of [2+2] -cycloaddicted fullerenes, sideways connected by single C-C bonds. This leads to the formation of polymeric layers, whose insulating nature, deduced from the NMR and Raman spectra, denotes the complete localization of the electrons involved in the covalent bonds.
    Physical Review B 01/2005; 72(15). · 3.66 Impact Factor
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    ABSTRACT: All known fullerene polymers have interfullerene connections via either [2 + 2] cycloaddition or single C-C bonds. The high-resolution synchrotron X-ray powder diffraction technique was employed here to determine the crystal structure of the Li4C60 fulleride. We find that the ground state of Li4C60 is a two-dimensional polymer with monoclinic crystal symmetry and an unprecedented architecture, combining both the [2 + 2] cycloaddition and the single C-C bridging motifs. The small size of the Li+ cations is crucial in stabilizing the resulting tightly packed polymeric structure.
    Journal of the American Chemical Society 12/2004; 126(46):15032-3. · 10.68 Impact Factor
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    ABSTRACT: In this report we combine synchrotron powder X-ray diffraction, 7Li nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) experiments to study the structural evolution of Li4C60 and how its electronic ground state depends on the crystal symmetry. The compound in the two-dimensional polymer phase with mixed interfullerene bonding motifs is a band gap insulator. EPR however reveals the presence of intrinsic centers originating from broken C60-C60 bonds and local Li off-stoichiometry that create states in the band gap and account for the complex temperature dependence of the spin susceptibility as well as the residual temperature dependence of the 7Li NMR shift. Li+ ions are at low temperatures on the 7Li NMR timescale statically disordered. The observed 7Li NMR line narrowing T > 200 K is ascribed to the Li+ diffusion dynamics and above room temperature the polymer phase is already a good ionic conductor. Heating the sample to temperatures above ∼470 K results in the gradual depolymerisation to the metallic monomer fcc high temperature structure. The transformation is first order and polymer as well as monomer phases coexist over broad temperature interval (130 K).