Y. Kopelevich

University of Campinas, Conceição de Campinas, São Paulo, Brazil

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Publications (118)408.12 Total impact

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    ABSTRACT: The carrier density in tens of nanometers thick graphite samples (multi-layer-graphene, MLG) has been modified by applying a gate voltage Vg perpendicular to the graphene planes. Surface potential microscopy shows inhomogeneities in the carrier density (n) in the sample near surface region and under different values of Vg at room temperature. Transport measurements on different MLG samples reveal that under a large enough applied electric field these regions undergo a superconducting-like transition at T < 17 K. A magnetic field applied parallel or normal to the graphene layers suppresses the transition without changing appreciably the transition temperature.
    Carbon 06/2014; 72:312-320. · 6.16 Impact Factor
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    ABSTRACT: The structural, optical, and transport properties of graphene grown by chemical vapor deposition (CVD) of propane under hydrogen on the Si face of SiC substrates have been investigated. We show that little changes in temperature during the growth can trigger the passivation of the SiC surface by hydrogen. Depending on the growth condition, hole or electron doping can be achieved, down to a few 1011 cm−2. When the growth temperature is high (T≈1500–1550∘C), we obtain electron-doped graphene monolayers lying on a buffer layer. When the growth temperature is slightly lowered (T≈1450–1500∘C), hole-doped graphene layers are obtained, lying on a hydrogen-passivated SiC surface, as confirmed by the enhancement of the mobility (of the order of 4500 cm2/Vs) and the persistence of weak localization almost up to room temperature (250 K). The high homogeneity of this graphene allows the observation of the half-integer quantum Hall effect, typical of graphene, at the centimeter scale in the best cases. The influence of the SiC steps on the transport properties is discussed.
    Physical Review B 02/2014; 89:085422. · 3.66 Impact Factor
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    ABSTRACT: We report a highly anisotropic in-plane magnetoresistance (MR) in graphite that possesses in-plane parallel line-like structural defects. In a current direction perpendicular to the line defects (LD), MR is negative and linear in low fields with a crossover to a positive MR at higher fields, while in a current direction parallel to LD, we observed a giant super-linear positive MR. These extraordinary MRs are respectively explained by a hopping magnetoresistance via non-zero angular momentum orbitals, and by the magnetoresistance of inhomogeneous media. The linear negative orbital MR is a unique signature of the broken time-reversal symmetry (TRS). We discuss the origin of the disorder-induced TRS-breaking in graphite.
    Journal of Physics Condensed Matter 10/2013; 25(46):466004. · 2.22 Impact Factor
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    ABSTRACT: We studied the in-plane magnetoresistance R(B,T) anisotropy in epitaxial multilayer graphene films grown on the Si face of a 6H-SiC substrate that originates from steplike morphology of the SiC substrate. To enhance the anisotropy, a combination of argon atmosphere with graphite capping was used during the film growth. The obtained micro-Raman spectra demonstrated a complex multilayer graphene structure with the smaller film thickness on terraces as compared to the step edges. Several Hall bars with different current/steps mutual orientations have been measured. A clear anisotropy in the magnetoresistance has been observed, and attributed to variations in electron mobility governed by the steplike structure. Our data also revealed that (i) the graphene-layer stacking is mostly Bernal type, (ii) the carriers are massive, and (iii) the carriers are confined to the first 2–4 graphene layers following the buffer layer.
    Physical review. B, Condensed matter 09/2013; 82(8):5438. · 3.66 Impact Factor
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    ABSTRACT: Highly-anisotropic in-plane magneto-resistance (MR) in graphite (HOPG) samples has been recently observed (Y. Kopelevich et al., arXiv:1202.5642) which is negative and linear in low fields in some current direction while it is giant, super-linear and positive in the perpendicular direction. In the framework of the hopping conductance theory via non-zero angular momentum orbitals we link extraordinary MRs in graphite and in organic insulators (OMAR) observed in about the same magnetic fields. The theory predicts quadratic negative MR (NMR) when there is a time-reversal symmetry (TRS), and linear NMR if TRS is broken. We argue that the observed linear NMR could be a unique signature of the broken TRS both in graphite and organic compounds. While some local paramagnetic centers are responsible for the broken TRS in organic insulators, a large diamagnetism of our HOPG samples may involve a more intriguing scenario of TRS breaking.
    08/2012;
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    ABSTRACT: This comment addresses several issues in the paper by Sepioni et al., where it is stated that the ferromagnetism in pristine highly oriented pyrolytic graphite (HOPG) reported by several groups in the previous years is most likely due to impurity contamination. In this comment, clear arguments are given why this statement is not justified. Furthermore, it is pointed out, that there are already measurements using element-sensitive microscopic techniques, e.g. X-ray Magnetic Circular Dichroism (XMCD) that directly proved the intrinsic origin of the ferromagnetism in graphite, also in pristine HOPG.
    EPL (Europhysics Letters) 06/2012; 98(5). · 2.27 Impact Factor
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    ABSTRACT: This comment addresses several issues in the paper by Sepioni et al., where it is stated that the ferromagnetism in pristine highly oriented pyrolytic graphite (HOPG) reported by several groups in the previous years is most likely due to impurity contamination. In this comment, clear arguments are given why this statement is not justified. Furthermore, it is pointed out, that there are already measurements using element-sensitive microscopic techniques, e.g. X-ray Magnetic Circular Dichroism (XMCD) that directly proved the intrinsic origin of the ferromagnetism in graphite, also in pristine HOPG.
    EPL (Europhysics Letters) 06/2012; 98(5):57006-. · 2.27 Impact Factor
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    ABSTRACT: We report on magnetization, magnetoresistance, and Shubnikov-de Haas oscillations experiments in Na-implanted samples of highly oriented pyrolitic graphite (HOPG). Different ion fluences were applied so that samples with Na contents of 0.5, 1.0, 1.5, and 2.0 at. % were obtained in the implanted region. Ferromagnetic-like hysteresis was observed in magnetization experiments where the field was applied parallel to the graphene planes. The observed saturation moment increases systematically as a function of the implanted ion concentration up to Na 1 at. %, where it goes through a maximum before decreasing slightly towards Na 2 at. %. The planar magnetoresistance amplitude at fixed field and temperature closely correlates with the saturation magnetization data. This result suggests that the strong planar magnetoresistance in graphite is at least partially related to a spin dependent mechanism. The magnetoresistance experiments also reveal the occurrence of Shubnikov-de Haas oscillations. The characteristic frequencies and the effective masses could be estimated and do not depend on the Na concentration. The reported experiments show that the expressive enhancement observed in ferromagnetic-like response in Na-implanted HOPG is primarily due to point defects produced by the implantation process.
    Journal of Applied Physics 05/2012; 111(9). · 2.19 Impact Factor
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    ABSTRACT: We have studied the temperature and magnetic field dependence of the electrical resistance of mesoscopic, tens of nanometers thick multigraphene samples as a function of a bias voltage applied perpendicular to the graphene planes. We found that the resistance changes asymmetrically with the bias voltage sign. For large and negative bias voltages the resistance shows a non-percolative superconducting-like transition at $T \sim 15 ... 20 $K. A large enough magnetic field suppresses the transition.
    02/2012;
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    ABSTRACT: Magnetization measurements were performed on CeCoIn5 at temperatures down to 20 mK and magnetic fields up to 17 T applied along different crystallographic orientations. For field configurations nearly parallel to the ab plane (θ≲40∘ and T≤50 mK), we have found an intriguing vortex dynamics regime revealed by a hysteretic and metastable anomalous peak effect (APE), which gives evidence of surface barrier effects enhanced by antiferromagnetic fluctuations in the mixed state of CeCoIn5. Furthermore, we have observed crossover features in the torque and magnetization traces at fields below Hc2, which are consistent with vortices lattice phase transitions and with the anomalies speculated to be the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) superconducting state in CeCoIn5. All of the above features were found to be dramatically perturbed in Ce0.98Gd0.02CoIn5.
    Physical review. B, Condensed matter 02/2012; 85(5). · 3.66 Impact Factor
  • Y.kopelevich, P.esquinazi, M.ziese
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    ABSTRACT: Different experimental techniques reveal a nearly magnetic-field-independent depinning of the flux line lattice (FLL) between 20 and 30 K in highly anisotropic high temperature superconductors like Bi 2:2:1:2 and similar compounds. This anomalous behavior of the FLL has been partially unnoticed in the literature and is the subject of this experimental review. We compare data from literature obtained with different experimental techniques and discuss a possible common origin based on a “softening” of the interplane Josephson coupling.
    Modern Physics Letters B 11/2011; 08(25). · 0.69 Impact Factor
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    Luis A. Baring, Robson R. da Silva, Yakov Kopelevich
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    ABSTRACT: We performed magnetization M(H,T) and magnetoresistance R(T,H) measurements on powdered (grain size ~ 149 micrometers) as well as highly oriented rhombohedral (A7) bismuth (Bi) samples consisting of single crystalline blocks of size ~ 1x1 mm2 in the plane perpendicular to the trigonal c-axis. The obtained results revealed the occurrence of (1) local superconductivity in powdered samples with Tc(0) = 8.75 \pm 0.05 K, and (2) global superconductivity at Tc(0) = 7.3 \pm 0.1 K in polycrystalline Bi triggered by low-resistance Ohmic contacts with silver (Ag) normal metal. The results provide evidence that the superconductivity in Bi is localized in a tiny volume fraction, probably at intergrain or Ag/Bi interfaces. On the other hand, the occurrence of global superconductivity observed for polycrystalline Bi can be accounted for by enhancement of the superconducting order parameter phase stiffness induced by the normal metal contacts, the scenario proposed in the context of "pseudogap regime" in cuprates [E. Berg et al., PRB 78, 094509 (2008)].
    Low Temperature Physics 09/2011; · 0.88 Impact Factor
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    ABSTRACT: Both experimental and theoretical studies of the magnetic properties of micrographite and nanographite indicate a crucial role of the partial oxidation of graphitic zigzag edges in ferromagnetism. In contrast to total and partial hydrogenation, the oxidation of half of the carbon atoms on the graphite edges transforms the antiferromagnetic exchange interaction between graphite planes and over graphite ribbons to the ferromagnetic interaction. The stability of the ferromagnetism is discussed.
    Physical Review B 06/2011; 83:233408. · 3.66 Impact Factor
  • Ivan Naumov, Yakov Kopelevich, Alexander Bratkovsky
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    ABSTRACT: We study the origins of high-temperature ferromagnetic behavior in graphite by means of unbiased ab-initio calculations and compare them with our data. The experimental results show that oxygen/sulfur-induced edges of graphitic fragments (via unzipping effect) play an essential role in this phenomenon, and that the finite magnetic moment appears if edges in a graphitic ribbon are occupied asymmetrically by either oxygen or sulphur. In particular, our ab-initio calculations performed within the LDA and GGA approximations showed that in the case of pure graphene ribbon, its zig-zag edge carbon atoms carry large magnetic moment (˜1 muB/C). In an oxidized or sulfurized graphene, however, the magnetic moment at the edge with absorbed atoms gets considerably reduced, leading to effective ferromagnetic (more precisely, ferri-magnetic) behavior of the sample.
    03/2011;
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    Yakov Kopelevich, Igor A. Luk'yanchuk
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    ABSTRACT: Nobel Prize in Physics 2010 was given for "groundbreaking experiments regarding the two-dimensional material graphene." In fact, before graphene has been extracted from graphite and measured, some of its fundamental physical properties have already been experimentally uncovered in bulk graphite. In this Letter to the Nobel Committee we propose to include those findings in the Scientific Background
    11/2010;
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    ABSTRACT: The electronic Raman scattering of bulk graphite at zero magnetic field reveals a structureless signal characteristic of a metal. For B≳2T , several peaks at energies scaling linearly with magnetic field were observed and ascribed to transitions from the lowest energy Landau level(s) [LL(s)] to excited states belonging to the same ladder. The LLs are equally (unequally) spaced for high (low) quantum numbers, being consistent with the LL sequence from massive chiral fermions [m∗=0.033(2)me] with Berry’s phase 2π found in graphene bilayers. These results provide spectroscopic evidence that some of the physics recently revealed by graphene multilayers is also shared by bulk graphite.
    Physical review. B, Condensed matter 06/2010; 81(24). · 3.66 Impact Factor
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    ABSTRACT: We have studied the c-axis interlayer magnetoresistance (ILMR), R_c(B) in graphite. The measurements have been performed on strongly anisotropic highly oriented pyrolytic graphite (HOPG) samples in magnetic field up to B = 9 T applied both parallel and perpendicular to the sample c-axis in the temperature interval 2 K < T < 300 K. We have observed negative magnetoresistance, dR_c/dB < 0, for B || c-axis above a certain field B_m(T) that reaches its minimum value B_m = 5.4 T at T = 150 K. The results can be consistently understood assuming that ILMR is related to a tunneling between zero-energy Landau levels of quasi-two-dimensional Dirac fermions, in a close analogy with the behavior reported for alpha-(BEDT-TTF)2I3 [N. Tajima et al., Phys. Rev. Lett. 102, 176403 (2009)], another multilayer Dirac electron system. Comment: 14 pages, including 4 figures
    Physics Letters A 04/2010; · 1.63 Impact Factor
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    ABSTRACT: In the present work, we demonstrate both experimentally and by ab-initio simulations that room-temperature ferromagnetism can be induced in graphitic materials by means of adsorbed oxygen or sulfur. The results show that oxygen/sulfur-induced edges of graphitic fragments (via unzipping effect) play an essential role in this phenomenon, and that the finite magnetic moment takes place if zig-zag edges in a graphitic ribbon are terminated asymmetrically, i.e. with a different oxygen/sulfur occupation at opposite edges. In particular, our ab-initio calculations performed within the local spin density approximation showed that in the case of pure graphene ribbon, its edge carbon atoms carry large magnetic moment (˜0.72 muB/C). In an oxidized graphene, however, the magnetic moment at the edge with absorbed oxygen atoms vanishes, leading to effective ferro(ferri)-magnetic behavior of the sample.
    03/2010;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We study the origins of high-temperature ferromagnetic behavior in graphite by means of unbiased ab-initio calculations and compare them with our data. The experimental results show that oxygen/sulfur-induced edges of graphitic fragments (via unzipping effect) play an essential role in this phenomenon, and that the finite magnetic moment appears if edges in a graphitic ribbon are occupied asymmetrically by either oxygen or sulphur. In particular, our ab-initio calculations performed within the local spin density approximation showed that in the case of pure graphene ribbon, its zig-zag edge carbon atoms carry large magnetic moment ( 0.7 muB/C). In an oxidized graphene, however, the magnetic moment at the edge with absorbed oxygen atoms gets considerably reduced, leading to effective ferromagnetic (more precisely, ferri-magnetic) behavior of the sample. The estimates show that the Curie temperature may be very high owing to peculiar band structure of the edge states. The results suggest ways of controlling magnetic behavior of graphitic systems at room temperature.
    03/2010;
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    ABSTRACT: Four-probe current-voltage (I-V) characteristics were measured on 80-250 nm thick graphite samples, obtained by cleaving and placed on Si substrate, in the temperature interval 2 K
    03/2010;

Publication Stats

2k Citations
408.12 Total Impact Points

Institutions

  • 1993–2014
    • University of Campinas
      • Instituto de Física "Gleb Wataghin" (IFGW)
      Conceição de Campinas, São Paulo, Brazil
  • 2007
    • École Supérieure de Physique et de Chimie Industrielles
      Lutetia Parisorum, Île-de-France, France
  • 2004–2007
    • Université de Picardie Jules Verne
      • LPMC - Laboratoire de physique de la matière condensée
      Amiens, Picardie, France
  • 1998–2006
    • University of Leipzig
      • Institut für Experimentelle Physik
      Leipzig, Saxony, Germany
  • 1993–1998
    • University of Bayreuth
      • Institute of Physics
      Bayreuth, Bavaria, Germany