D. Quintavalle

Budapest University of Technology and Economics, Budapeŝto, Budapest, Hungary

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Publications (15)30.77 Total impact

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    ABSTRACT: The low temperature microwave absorption anomaly reported by Corzilius et al. [Phys. Rev. B 75, 235416 (2007)] in single-wall carbon nanotubes (SWCNTs) is revisited. It was originally reported that the microwave absorption of CVD grown SWCNTs shows an unexpected increase below ∼20 K (using flow cryostats) which depends on the microwave power. The original observation was made using the microwave cavity perturbation method while sweeping the microwave frequency. We reproduced this effect on arc-discharge based SWCNTs, using static cryogenic conditions with cooled microwave cavities, and employing a stable frequency source locked to the cavity resonance. Our observation shows that the microwave absorption anomaly is robust against the tube type and the experimental conditions.
    physica status solidi (b) 12/2012; 249(12):2487-2490. · 1.49 Impact Factor
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    ABSTRACT: We describe a quasi-optical multifrequency ESR spectrometer operating in the 75-225 GHz range and optimized at 210 GHz for general use in condensed matter physics, chemistry and biology. The quasi-optical bridge detects the change of mm wave polarization at the ESR. A controllable reference arm maintains a mm wave bias at the detector. The attained sensitivity of 2x10^10 spin/G/(Hz)1/2, measured on a dilute Mn:MgO sample in a non-resonant probe head at 222.4 GHz and 300 K, is comparable to commercial high sensitive X band spectrometers. The spectrometer has a Fabry-Perot resonator based probe head to measure aqueous solutions, and a probe head to measure magnetic field angular dependence of single crystals. The spectrometer is robust and easy to use and may be operated by undergraduate students. Its performance is demonstrated by examples from various fields of condensed matter physics.
    10/2009;
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    ABSTRACT: C60 and C70 form with cubane (C8H8) molecules rotor-stator phases in which the fullerenes are rotating and the cubanes are static. Heating the rotor-stator phase up to 470 K a fulleride copolymer, poly (C60C8H8), is obtained. Here, we study using microwave conductivity measurements, electron-spin resonance, and superconducting quantum interference device magnetometry the effect of potassium intercalation on the electric and magnetic properties of poly (C60C8H8). The resulting K2C60C8H8 copolymer is metallic for T>200 K in contrast to the insulating pristine poly (C60C8H8) copolymer. At lower temperatures, a disorder-driven Anderson localization of the electrons takes place and the ground state is a magnetic insulator.
    Physical review. B, Condensed matter 07/2009; 80(3). · 3.77 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: Electron spin resonance (ESR) measurements on SWCNTs doped with alkali atoms (potassium) using a solid state reaction is reported. We find the emergence of the ESR signal of itinerant electrons upon doping with a signal intensity that is comparable to that expected from band structure calculations.The ESR line-width and microwave conductivity weakly depend on the temperature indicating that doped SWCNTs are bad metals. It is argued to result from the lack of crystalline order in the tubes and the large impurity concentration. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (b) 10/2008; 245(10). · 1.49 Impact Factor
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    ABSTRACT: We report on the temperature stability of N@C-60 peapods using electron spin resonance. The atomic nitrogen escapes at higher temperatures in the peapod than in the crystalline form. We also report on the synthesis of novel magnetic fullerene peapod, a fullerene with an attached pyridine-nitroxide spin-label. This molecule can be the ultimate inner spin label for the SWCNTs as it is available in microscopic amounts and is not air sensitive. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    physica status solidi (b) 09/2008; 245(10):2034. · 1.49 Impact Factor
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    ABSTRACT: We studied the temperature stability of the endohedral fullerene molecule, N@C60, inside single-wall carbon nanotubes using electron spin resonance spectroscopy. We found that the nitrogen escapes at higher temperatures in the encapsulated material as compared to its pristine, crystalline form. The temperature dependent spin-lattice relaxation time, T_1, of the encapsulated molecule is significantly shorter than that of the crystalline material, which is explained by the interaction of the nitrogen spin with the conduction electrons of the nanotubes. Comment: 5 pages, 4 figures, 1 table
    Physical Review B 06/2008; 77:214409. · 3.66 Impact Factor
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    ABSTRACT: We present a study on the structural, spectroscopic, conducting and magnetic properties of Mg5C60, a two dimensional (2D) fulleride polymer. The polymer phase is stable up to the exceptionally high temperature of 823 K. Infrared and Raman studies suggest the formation of single bonds between fulleride ions and possibly Mg - C60 covalent bonds. Mg5C60 is a metal at ambient temperature as shown by electron spin resonance and microwave conductivity measurements. The smooth transition from a metallic to a paramagnetic insulator state below 200 K is attributed to Anderson localization driven by structural disorder.
    Physical Review B 04/2008; 77:155431. · 3.66 Impact Factor
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    ABSTRACT: We present a study on the structural, spectroscopic, conducting and magnetic properties of Mg5C60, a two dimensional (2D) fulleride polymer. The polymer phase is stable up to the exceptionally high temperature of 823 K. Infrared and Raman studies suggest the formation of single bonds between fulleride ions and possibly Mg - C60 covalent bonds. Mg5C60 is a metal at ambient temperature as shown by electron spin resonance and microwave conductivity measurements. The smooth transition from a metallic to a paramagnetic insulator state below 200 K is attributed to Anderson localization driven by structural disorder.
    01/2008;
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    ABSTRACT: We synthesized a series of MgxC60 samples with nominal composition of x between 1 and 6 via solid state reaction between C60 and Mg powder. For x = 5 a two dimensional structure was found. The polymer phase is unusually stable up to high temperatures, it remains unchanged at 823 K for 20 minutes as confirmed by ESR. The mid-infrared absorption spectrum suggests that the polymer sheets contain single bonded fulleride ions. Both the spin susceptibility and microwave conductivity measurements show that Mg5C60 is metallic for T > 200 K. The transition from metallic to insulating states around 200 K may be attributed to Anderson localization.
    physica status solidi (b) 09/2007; 244:3853-3856. · 1.49 Impact Factor
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    ABSTRACT: C59N magnetic fullerenes inside single-wall carbon nanotubes (SWCNTs) are used to probe the density of states (DOS) on the host tubes using electron spin resonance (ESR). The C59N radicals are separated by C60 fullerenes to prevent dimerization and C59N-C60 heterodimers are formed at low temperatures. The electron spin-lattice relaxation time, T1, of the heterodimers is deduced from the homogeneous ESR line-width. The analysis is supported by saturation ESR studies. The inverse of the heterodimer T1 follows a linear behavior in the 20-300 K temperature range, the so-called Korringa law, evidencing a metallic DOS on all tubes in a bundle.
    physica status solidi (b) 01/2007; · 1.49 Impact Factor
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    ABSTRACT: Na2C60 is believed to be an electron-hole counterpart of the Mott-Jahn--Teller insulator A4C60 salts. We present a study of infrared, ESR, NMR spectroscopy, X-ray diffraction, chemical composition and neutron scattering on this compound. Our spectroscopic results at room temperature can be reconciled in a picture of segregated, separate regions of the size 3--10 nm. We observe a significant insulating C60 phase and at least two more phases, one of which we assign to metallic Na3C60. The separation disappears on heating by jump diffusion of the sodium ions, which we followed by neutron scattering. Above ~460 K we see infrared spectroscopic evidence of a Jahn--Teller distorted (C60)2- anion.
    Physical Review B 08/2006; 74:195402. · 3.66 Impact Factor
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    ABSTRACT: There is continuous interest in the nature of alkali metal fullerides containing C604- and C602-, because these compounds are believed to be nonmagnetic Mott-Jahn-Teller insulators. This idea could be verified in the case of A4C60, but Na2C60 is more controversial. By comparing the results of infrared spectroscopy and X-ray diffraction, we found that Na2C60 is segregated into 3-10 nm large regions. The two main phases of the material are insulating C60 and metallic Na3C60. We found by neutron scattering that the diffusion of sodium ions becomes faster on heating. Above 470 K Na2C60 is homogeneous and we show IR spectroscopic evidence of a Jahn-Teller distorted C602- anion.
    AIP Conference Proceedings 09/2005; 786:17-20.
  • Katalin Kamarás, Dario Quintavalle, Norbert M. Nemes
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    ABSTRACT: Na2C60 is the only known solid fulleride salt containing the divalent fulleride ion C60^2-. Calculations predict a Jahn-Teller distortion of this ion, similar to the A4C60 compounds, to which they are related by electron-hole symmetry. However, by combining various experimental methods, we found that divalent ions exist only above 450 K in solids with composition Na2C60; at room temperature and below, methods sensitive to molecular symmetry and charge (infrared absorption, ESR, NMR) detect at least two phases, most probably C60 and Na3C60. We explain our data by a model where nanosegregated regions of the size 3-30 nm with different Na concentration coexist. The concentration gradient disappears at higher temperature by diffusion of sodium, observed by neutron scattering. High temperature infrared spectra show evidence of a uniaxial (D3d/D5d) distortion of the fullerene balls.
    03/2005;
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    ABSTRACT: We describe a multifrequency quasi-optical electron spin resonance (ESR) spectrometer operating in the 75–225GHz range and optimized at 222.4GHz for general use in condensed matter physics and chemistry. The quasi-optical bridge detects the change of millimeter wave polarization at the ESR condition. A controllable reference arm maintains a millimeter wave bias at the detector. The sensitivity of 2×1010spin/(GHz0.5), measured on a dilute Mn:MgO sample in a non-resonant probe head at 222.4GHz and 300K, is comparable to that of commercial high-sensitive X-band spectrometers. The spectrometer is robust, easy to use and may be operated by undergraduate students. Its performance is demonstrated by examples from various fields of condensed matter physics.
    Applied Magnetic Resonance 40(1):47-63. · 0.83 Impact Factor