IQIM Postdoctoral and Graduate Student Seminar

Friday May 30, 2014 4:30 PM

Measuring the pseudo-spin space of hydrogenated graphene

Speaker: Thomas Szkopek, Department of Electrical and Computer Engineering, McGill University
Location: East Bridge 114

Abstract: We have observed the quantum Hall effect (QHE) and Shubnikov-de Haas (SdH) oscillations in millimetre scale hydrogenated graphene. We find that the pseudo-spin structure of graphene is remarkably robust to the sub-lattice symmetry breaking induced by hydrogenation. Hydrogenation of pristine graphene is experimentally observed to increase electrical resistance and introduce neutral point defects (up to ~ 0.1% concentration) as evidenced by Raman spectroscopy. In the highly resistive limit, strongly insulating behaviour is observed with a two-point resistance as high as 250 h/e^2 at low temperature, far above the Ioffe-Regel limit for metallic conduction. Upon application of a magnetic field, colossal negative magnetoresistance is observed with the emergence of a nu = -2 QHE state at 45 T from an insulating state, with the notable absence of SdH oscillations (J. Guillemette et al, Phys. Rev. Lett. 110, 176801 (2013)). The rapid collapse of resistance is observed to occur when the magnetic length is comparable to the mean spacing of neutral point defects. We have observed SdH oscillations in graphene hydrogenated to a lower resistivity at magnetic fields up to 65 T. Analysis of SdH oscillation frequency in 1/B indicates that the Landau level (LL) sequence remains four-fold degenerate. We also observe the nu = -2 QHE state in all samples. We therefore conclude that the topological part of the Berry phase, meaning the pseudo-spin winding number that determines the LL sequence, is preserved upon hydrogenation of large scale graphene. Time permitting, I will discuss other less well understood properties of hydrogenated graphene. Refreshments follow the seminar on the Bridge patio.

Series IQIM Postdoctoral and Graduate Student Seminar Series

Contact: Marcia Brown at 626-395-4013 marciab@caltech.edu