[seminar] SEMINAR na IF-u u CETVRTAK, 16. 12. 2010.

[Berislav Horvatic]

      Institut za fiziku, Bijenička cesta 46,

      predavaonica u zgradi Mladen Paić

      četvrtak, 16. prosinca 2010, u 15:00 sati

       
     
      Magnetism of Defective Graphenic Nanostructures:

      Transition Metal Impurities and Monovacancies

       
     
      Dr. Daniel Sánchez-Portal
     
       

      Centro de Física de Materiales, CSIC-UPV/EHU and Donostia Internacional Physics Center (DIPC), San Sebastián, Spain

       

      E-mail: sqbsapod at sc.ehu.es

       
     
      We report a theoretical study of substitutional impurities of transition metals in graphene and carbon nanotubes. We pay special attention to the traditional ferrromagnets: Ni, Co and Fe. Of these, only Cosub impuritiy is magnetic with a magnetic moment of ~1?B for the isolated impurity. However, when the Co substitution takes place in more than one site, the total magnetic moment of the system exhibits a strong dependency on the relative position of the Cosub impurities. More precisely the magnetic moment depends on the number of Co substitutions in A and B sublattices. This behaviour is better understood when we realize that the electronic structure of a substitutitonal Co impurity nearby the Fermi energy is equivalent to that of a carbon vacancy in a simple ?-tight-biding model of graphene. Therefore, we can expect the Lieb's[1] theorem to apply to this situation and the total magnetic moment to behave as |CoA-CoB|, where CoA and CoB are, respectively, the number Co substitutions in the A and B graphene sublattices [2].

      In contrast to Co impurities, Nisub defects, which have been recently detected in carbon nanotubes by Ushiro et al.[3] using extended x-ray absorption fine structure (EXAFS) and x-ray absorption near edge structure (XANES) data, show a zero magnetic moment in flat graphene. However, Nisub impurities develop a non-zero magnetic moment in metallic carbon nanotubes [4]. This surprising behavior stems from the peculiar curvature dependence of the electronic structure of Nisub. A similar magnetic/nonmagnetic transition of Nisub can be expected by applying other kinds of anisotropic strain to a flat graphene layer [5].

      In general, we have found that we can draw an analogy between the electronic structure in the neighborhood of the Fermi energy, and its strain dependence, of substitutional Co, Ni and Fe atoms in graphenic systems and that of an unreconstructed carbon vacancy in graphene with different charge states. With this analogy at hand we can easily understand and predict many complex and interesting phenomena for graphenic nanostructures substitutionally doped with different transition metals [6].

      Finally, time permitting, I will comment on our recent results on the influence of the rippling of the graphene layer on the structural, electronic and magnetic properties of a carbon monovacancy. [7]

      [1] E. H. Lieb, Phys. Rev. Lett. 62, 1201 (1989).

      [2] E. J. G. Santos, D. Sánchez-Portal and A. Ayuela, Phys. Rev. B 81, 125433 (2010).

      [3] M. Ushiro, K. Uno, T. Fujikawa, Y. Sato, K. Tohji, F. Watari, W. J. Chun, Y. Koike, and K. Asakura, Phys. Rev. B 73, 144103 (2006).

      [4] E. J. G. Santos, A. Ayuela, S. B. Fagan, J. Mendes Filho, D. L. Azevedo, A. G. Souza Filho and D. Sánchez-Portal, Phys. Rev. B 78, 195420 (2008).

      [5] D. Sánchez-Portal, E. J. G. Santos and A. Ayuela, in preparation.

      [6] E. J. G. Santos, A. Ayuela, D. Sánchez-Portal, New Journal of Physics 12, 053012 (2010)

      [7] E. J. G. Santos, D. Sánchez-Portal and A. Ayuela, submitted to Nano Letters

       

       

      http://dipc.ehu.es/group/index.htm
     
      Voditelji seminara IF-a:  Berislav Horvatić  &  Ivica Živković
     

 
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