Institut Néel Grenoble

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  • Superconducting Quantum Circuits
  • QSPINMOTION : coherent transport of a single flying electron spin
  • FLYELEC : Quantum optics with flying electrons
  • SUPERNEMS : Nanomechanics using superconducting diamonds
  • Graphene
  • SEMITOPO : Strained HgTe, a 3D topological insulator
  • MESOGLASS : probing spin glass with mesoscopic physics
  • FREQJO : FREquency-to-CUrrent conversion in JOsephson Crystals
  • GEOMDISS : a topological quantum pump
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Considerable progress has been achieved in the synthesis of two-dimensional graphene. Since the seminal works [1, 2] which used exfoliated graphite flakes transferred onto SiO2 substrates, full wafers of epitaxial graphene can now be grown by high temperature graphitization of Silicon Carbide (SiC) crystals starting either from their Carbon or Silicon face [3]. More recently, MBE growth on SiC substrate [4] and CVD synthesis of large area graphene films have also been achieved on the surface of transition metals in high vacuum [5] or at ambient pressure [6, 7] and their subsequent transfer to a large variety of substrates. These synthesis methods are scalable and offer some real perspectives for micro-electronic applications. A number of characterization techniques are available for the grown layers : STM, AFM, Raman, TEM/SEM and photo-emission have proven their usefulness. On the other hand, the relationship between the growth conditions, the film morphologies and the electronic properties have not yet been systematically investigated. We have used, low field magneto-resistance to correlate the transport properties, the growth conditions and the morphologies of epitaxially-grown graphene films elaborated from the different surfaces of 6H-SiC. The films studied have been grown with different graphene layer numbers, both from the Si and C terminated faces, some in ultra-high vacuum other in inert atmospheres. Depending on the SiC polytypes and on the growth conditions, distinct surface morphologies can be observed which lead to very different magnetoresistance behaviors. Low field magneto-resistance is a sensitive probe for electronic transport as it measures the effect of quantum interferences along closed paths. Depending on the closed loop size, the interferences can be constructive or destructive. For very small loops, it has been demonstrated both theoretically [8, 9] and experimentally [10, 11] that interferences between identical time reversed paths are destructive in graphene leading to a negative magnetoconductance (positive magnetoresistance), which is characteristic of anti-localization of electron waves. For graphene, electron wavefunctions have four components and may be characterized by two additional quantum numbers : the isospin and the pseudospin. The isospin measures the relative wavefunction amplitude on the equivalent sites (A-B) in graphene unit cell, while the pseudospin measures to which band valley K+ or K- the quantum states belong [12]. Antilocalization is a characteristic feature of graphene as the isospin (collinear to momentum) undergoes a full rotation on a closed loop, changing the wavefunction sign and so forbidding the backscattering. As the loop size increases, scattering mechanisms lead to additional rotations of the isospin, as well as to the scattering between different valley states, such that the pseudospin need not be preserved on long paths. The ``overall’’ effect of these processes on the interferences along time-reversed paths is to change their sign back to the ``normal’’ positive magneto-conductance due to coherent backscattering observed in other two-dimensional systems.

[1]Zhang, Y. B. and Tan, Y. W. and Stormer, H. L. and Kim, P., “Experimental observation of the quantum Hall effect and Berry’s phase in graphene”, Nature 438, 201-204 (2005)

[2]Novoselov, K. S. and Geim, A. K. and Morozov, S. V. and Jiang, D. and Katsnelson, M. I. and Grigorieva, I. V. and Dubonos, S. V. and Firsov, A. A., “Two-dimensional gas of massless Dirac fermions in graphene”, Nature 438, 197-200 (2005)

[3]Berger, C. and Song, Z. M. and Li, T. B. and Li, X. B. and Ogbazghi, A. Y. and Feng, R. and Dai, Z. T. and Marchenkov, A. N. and Conrad, E. H. and First, P. N. and de Heer, W. A., “Ultrathin epitaxial graphite : 2D electron gas properties and a route toward graphene-based nanoelectronics”, Journal of Physical Chemistry B 108, 19912-19916 (2004)

[4]Moreau, E. and Ferrer, F. J. and Vignaud, D. and Godey, S. and Wallart, X., Graphene growth by molecular beam epitaxy using a solid carbon source, Physica Status Solidi a-Applications and Materials Science 207, 300-303 (2010)

[5]Coraux, J. and N’Diaye, A. T. and Busse, C. and Michely, T., “Structural coherency of graphene on Ir(111)”, Nano Letters 8, 565-570(2008)

[6]Li, X. S. and Cai, W. W. and An, J. H. and Kim, S. and Nah, J. and Yang, D. X. and Piner, R. and Velamakanni, A. and Jung, I. and Tutuc, E. and Banerjee, S. K. and Colombo, L. and Ruoff, R. S., “Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils”, Science 324, 1312-1314 (2009)

[7]Kim, K. S. and Zhao, Y. and Jang, H. and Lee, S. Y. and Kim, J. M. and Kim, K. S. and Ahn, J. H. and Kim, P. and Choi, J. Y. and Hong, B. H., “Large-scale pattern growth of graphene films for stretchable transparent electrodes”, Nature 457, 706-710 (2009)

[8]McCann, E. and Kechedzhi, K. and Fal’ko, V. I. and Suzuura, H. and Ando, T. and Altshuler, B. L., “Weak-localization magnetoresistance and valley symmetry in graphene”, Physical Review Letters 97, 146805-146809 (2006)

[9]Kechedzhi, K. and McCann, E. and Fal’ko, V. I. and Suzuura, H. and Ando, T. and Altshuler, B. L., “Weak localization in monolayer and bilayer graphene”, European Physical Journal-Special Topics 148, 39-54 (2007)

[10]Wu, X. S. and Li, X. B. and Song, Z. M. and Berger, C. and de Heer, W. A., “Weak antilocalization in epitaxial graphene : Evidence for chiral electrons”, Physical Review Letters 98, 136801-136805 (2007)

[11]Tikhonenko, F. V. and Kozikov, A. A. and Savchenko, A. K. and Gorbachev, R. V., “Transition between Electron Localization and Antilocalization in Graphene”, Physical Review Letters 103, 226801-226805 (2009)

[12]Castro Neto, A. H. and Guinea, F. and Peres, N. M. R. and Novoselov, K. S. and Geim, A. K., “The electronic properties of graphene”, Reviews of Modern Physics 81, 109-162 (2009)

Contact : Cécile Naud, Laurent Lévy