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There is now a CONTENT FREEZE for Mercury while we switch to a new platform. It began on Friday, March 10 at 6pm and will end on Wednesday, March 15 at noon. No new content can be created during this time, but all material in the system as of the beginning of the freeze will be migrated to the new platform, including users and groups. Functionally the new site is identical to the old one. webteam@gatech.edu
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MRSEC SEMINAR SERIES WITH DR JEAN-MARIE POUMIROL
The first MRSEC seminar of the Fall 2011 semester will be held on Monday, August 22, 2011 at 3pm in the Marcus Nanotechnology Building seminar rooms located on the first floor.
We are pleased to have Dr. Jean-Marie Poumirol from the National High Magnetic Field Laboratory, in Tallahassee, Florida as our first speaker in the Fall semester. His talk is titled: Unveiling the Landau Levels Structure of Graphene Nanoribbons.
Below is an abstract:
Carbon-based nano-electronics is, in the actuality, one of the most promising subjects of nanotechnology. The lateral confinement in graphene leads to a series of 1-D electronic sub-bands with a confinement gap. In presence of a large enough magnetic field, the band structure evolves to magneto-electric sub-bands and graphene-like Landau levels are expected to develop. Up to now, Hall quantization in graphene nanoribbons (GNRs) remains puzzling since no experimental evidence has been found for widths smaller than 200 nm [1].
Lithographically patterned GNRs of 100 and 70 nm widths are made using oxygen plasma etching and a PMMA etching mask. These GNRs present a weakly diffusive transport regime. Magnetoresistance (MR) measurement show the first experimental evidence of Hall quantization in GNRs for filling factors ν= 2 and 6. On the other hand, anomalies in the magneto-transport measurements are evidenced: (i) The maxima of MR for all the ribbons, fingerprint of the Landau levels depopulation [2], present an upshift of several Tesla compared to the theoretical value [3], (ii) the narrower ribbons exhibit the expected 6G0 conductance maxima for a two-terminal measurement [2] but the 2G0 plateau is absent and the depopulation of the N=2 Landau level goes along with an unusual double peak of the resistance. To unveil the origin of the singular Landau spectrum we performed numerical simulations of the GNR band structure as a function of the perpendicular magnetic field. We directly compared the oscillatory behaviour of the magneto-resistance and the onset of the magneto-electric sub-bands. The simulations give evidence of magneto-oscillations of the Fermi energy which consistently explains the broadening of the magnetoresistance peaks and their up-shift lo larger magnetic field [4]. The presence of a second peak in the MR spectrum also finds a natural explanation: this is the clear signature of the orbital degeneracy lifting enhanced by the magnetic field and the pinning of the Fermi energy.
References
[1] J. M. Poumirol, et al. PRB, 82 (2010) 041413.
[2] J. R. Williams et al. PRB, 80 (2009) 045408.
[3] N. M. R. Peres et al. PRB, 73 (2006) 241403
[4] R. L. Ribeiro et al. (submitted to PRL).
