K. Schindler

University of Leipzig , Leipzig, Saxony, Germany

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Publications (14)40.7 Total impact

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    ABSTRACT: In this work we investigated correlations between the internal microstructure and sample size (lateral as well as thickness) of mesoscopic, tens of nanometer thick graphite (multigraphene) samples and the temperature $(T)$ and field $(B)$ dependence of their electrical resistivity $\rho(T,B)$. Low energy transmission electron microscopy reveals that the original highly oriented pyrolytic graphite material -- from which the multigraphene samples were obtained by exfoliation -- is composed of a stack of $\sim 50 $nm thick and micrometer long crystalline regions separated by interfaces running parallel to the graphene planes. We found a qualitative and quantitative change in the behavior of $\rho(T,B)$ upon thickness of the multigraphene samples, indicating that their internal microstructure is important.} {The overall results indicate that the metallic-like behavior of $\rho(T)$ at zero field measured for bulk graphite samples is not intrinsic of ideal graphite. The results suggest that the interfaces between crystalline regions may be responsible for the superconducting-like properties observed in graphite. Our transport measurements also show that reducing the sample lateral size as well as the length between voltage electrodes decreases the magnetoresistance, in agreement with recently published results. The magnetoresistance of the multigraphene samples shows a scaling of the form ($(R(B) - R(0))/R(0))/T^\alpha = f(B/T)$ with a sample dependent exponent $\alpha \sim 1$, which applies in the whole temperature 2 K $\le T \le 270$K and magnetic field range $B \le 8 $T.
    physica status solidi (a) 10/2008; · 1.21 Impact Factor
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    ABSTRACT: High-resolution magnetoresistance data in highly oriented pyrolytic graphite thin samples manifest nonhomogenous superconductivity with critical temperature Tc∼25 K and higher temperature. Our claim is based mainly in the observation of anomalous hysteresis loops of resistance versus magnetic field that cannot be assigned to magnetic irreversibility but indicates the existence of Josephson-coupled superconducting grains. In addition we observe quantum resonances that can be assigned to Andreev reflections and the absence of Schubnikov de Haas oscillations. The results indicate that graphite is a system with nonpercolative superconducting domains immersed in a semiconductinglike matrix. As possible origin of the superconductivity in graphite we discuss interior-gap superconductivity when two very different type of carriers with different masses are present.
    Physical review. B, Condensed matter 10/2008; 78(13).
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    ABSTRACT: Kelvin probe force microscopy measurements on micrometer small magnetic spots produced by proton bombardment on bulk graphite reveal a charge transfer from the center of the spot to an external ring with potential variation on the order of 50 mV. The total charge in the spot is neutral. The results can be well understood in terms of practically unscreened potentials, an insulating property, although the nonbombarded, surrounding graphite region exhibits good conductance. Scanning transmission x-ray microscopy measurements on magnetic spots prepared on graphitic films reveal similar charge distribution. The insulating behavior is fundamental to understand the magnetism in graphite.
    Physical review. B, Condensed matter 01/2008; 78(4).
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    ABSTRACT: Conduction electrons in graphite are expected to have micrometer large de Broglie wavelength as well as mean free path. A direct influence of these lengths in the electric transport properties of finite-size samples was neglected in the past. We provide a direct evidence of this effect through the size dependence of the magnetoresistance, which decreases with the sample size even for samples hundreds of micrometers large. Our findings may explain the absence of magnetoresistance in small few graphene layers samples and ask for a general revision of the experimental and theoretical work on the transport properties of this material.
    Physical Review Letters 12/2007; 99(21):216601. · 7.94 Impact Factor
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    ABSTRACT: In this work we investigate the electrical transport properties and growth conditions of tungsten carbon (WC) and palladium carbon (PdC) nanostructures on Si substrates using a focused ion beam and scanning electron microscope. In situ energy dispersive x-ray (EDX) characterizations reveal that electron-beam-induced WC and PdC nanostructure depositions (EBID) show a lower metal concentration (below 3% atomic percentage) than in ion-beam-induced deposition (IBID) (above 20%). In the case of PdC the growth pattern and the Pd/C content were optimized by adjusting the deposition temperature of the precursor material. In situ measurements of the resistivity of the nanostructures as a function of thickness reveal a minimum at a thickness approximately 200 nm. The lowest resistivity obtained for the PdC and WC structures is two orders of magnitude higher than the corresponding bulk values for pure Pd and W. The EBID samples show a non-metallic behaviour due to the low metal content. The temperature and magnetic field dependence of the IBID structures reveal a behaviour similar to disordered or granular conductors. The upper critical field and critical current density of the WC structures were measured below the superconducting critical temperature of approximately 5 K.
    Nanotechnology 12/2007; 18(49):495202. · 3.84 Impact Factor
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    ABSTRACT: A Reply to the Comment by S. Sadewasser and Th. Glatzel.
    Physical Review Letters 06/2007; 98(26). · 7.94 Impact Factor
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    ABSTRACT: We present a study of the magnetoresistance of highly oriented pyrolytic graphite (HOPG) as a function of the sample size. Our results show unequivocally that the magnetoresistance reduces with the sample size even for samples of hundreds of micrometers size. This sample size effect is due the large mean free path and Fermi wavelength of carriers in graphite and may explain the observed practically absence of magnetoresistance in micrometer confined small graphene samples where quantum effects should be at hand. These were not taken into account in the literature yet and ask for a revision of experimental and theoretical work on graphite. Comment: 4 pages, 3 figures
    06/2007;
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    ABSTRACT: We have studied the field and temperature dependence of the magnetic moment of single crystalline sapphire substrates with different surface orientations. All the substrates show a ferromagnetic behavior that partially changes after surface cleaning. The amount of magnetic impurities in the substrates was determined by particle induced X-ray emission. The overall analysis of the data indicates that the magnetic impurities very likely contribute to the measured ferromagnetic behavior but does not rule out completely intrinsic contributions. Our work stresses the necessity to use other than bulk characterization methods for the study of weak ferromagnetic signals of thin films grown on oxide substrates.
    Journal of Magnetism and Magnetic Materials 01/2007; 317(1):53-60. · 1.83 Impact Factor
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    ABSTRACT: We present measurements of the electric potential fluctuations on the surface of highly oriented pyrolytic graphite using electrostatic force and atomic force microscopy. Micrometric domainlike potential distributions are observed even when the sample is grounded. Such potential distributions are unexpected given the good metallic conductivity of graphite because the surface should be an equipotential. Our results indicate the coexistence of regions with "metalliclike" and "insulatinglike" behaviors showing large potential fluctuations of the order of 0.25 V. In lower quality graphite, this effect is not observed. Experiments are performed in Ar and air atmospheres.
    Physical Review Letters 09/2006; 97(7):076805. · 7.94 Impact Factor
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    ABSTRACT: In this study, ferromagnetic microstructures in highly oriented pyrolytic graphite and superparamagnetic spots in polyimide foils were created by 2.25 MeV proton microbeam irradiation and characterized using atomic and magnetic force microscopy. For this purpose, graphite samples were irradiated with cross-like patterns of 15 μm × 15 μm size using ion fluences in the range of (0.003–2.5) × 1018 cm−2. The irradiated crosses showed strong magnetic signals and a complex domain structure in the magnetic images depending on the geometrical dimensions of the crosses. Furthermore, polyimide foils were irradiated with microspots and fluences in the range of (0.016–3.1) × 1019 cm−2. Magnetic force microscopy shows very strong phase shifts in these irradiated areas.
    Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 06/2006; 250:303-308. · 1.27 Impact Factor
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    ABSTRACT: Proton irradiation is a reproducible method to induce magnetic order in pure carbon. In this contribution recent achievements in this field are presented. (1) Using a proton microbeam one is able to produce magnetic microstructures on pure graphite surfaces in which magnetic domain patterns at 300 K could be resolved using magnetic force microscopy (MFM). (2) Proton irradiation of polyimide films leads to clear magnetic signals at room temperature. (3) The magnetic response at the edges of carbon nanowalls has been studied using MFM, before and after proton irradiation. The results suggest the existence of strong magnetic field gradients at some of the edges indicating ferromagnetic order in part of the grown nanowalls. These studies were complemented by SQUID measurements, which show an increase of the ferromagnetic signal after irradiation.
    Thin Solid Films 06/2006; 505:85-89. · 1.60 Impact Factor
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    ABSTRACT: Carbon–sulfur films were grown by pulsed laser deposition at room temperature using different graphite–sulfur mixtures as targets. The structure of the films was characterized by X-ray diffraction and transmission electron microscopy. The composition and the chemical bonds were analyzed by Rutherford-backscattering spectroscopy, X-ray photoelectron spectroscopy and energy dispersive X-ray analysis. The films were composed of amorphous carbon with sp2-, sp3- and S–C–C–S bonds and textured graphite on the top of the film. The thin graphite layer on top of the carbon–sulfur films is highly oriented, comparable to highly oriented pyrolytic graphite, and free of sulfur in the graphite lattice. The lateral size of the oriented graphite grains in the films was up to 8 μm. Magnetic measurements reveal that the films prepared under the conditions of our study show neither magnetic ordering nor superconductivity in the studied temperature range T > 2 K.
    Carbon 06/2006; 44(14):3064-3072. · 5.87 Impact Factor
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    ABSTRACT: Recent publications demonstrated the generation of magnetic order at room temperature on graphite surfaces after irradiation with protons. In this contribution we show that it is possible to produce magnetic structures in the micrometer range using a proton micro beam by proton irradiation of the surface of highly oriented pyrolytic graphite samples. The experimental results are compared with micromagnetic simulations.
    Acta Physica Polonica Series a 01/2006; 109:249. · 0.53 Impact Factor
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    ABSTRACT: The critical current density and flux-creep activation energy of YBa2C3O7 rings were determined from ac-susceptibility measurements. A novel approach was developed theoretically and experimentally to derive the flux-creep exponent from higher susceptibility harmonics. An alternative method using a scaling approach yielded similar sample-to-sample variations, but qualitatively different values for the flux-creep exponent. This is discussed considering the limits of validity of the logarithmic approximation to the flux-line activation energy.
    Physica C Superconductivity 01/2005; 417(3):141-149. · 0.72 Impact Factor

Publication Stats

60 Citations
128 Downloads
590 Views
40.70 Total Impact Points

Institutions

  • 2005–2008
    • University of Leipzig
      • Institut für Experimentelle Physik
      Leipzig, Saxony, Germany
  • 2006–2007
    • Spanish National Research Council
      Madrid, Madrid, Spain