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Jeudi 06 Juin 2013, 14h
Amphi Holweck, Esc C, 1ème etage
Topological phases in strained graphene
Jérôme Cayssol
LOMA Bordeaux-1 and MPIPKS Dresden
I will first introduce Dirac materials and two-dimensional topological insulators using graphene as a guideline.
Then I will talk about our results related to topological phases arising in strained graphene in the absence of magnetic field [1]. The strain generates nearly uniform pseudo magnetic fields which are opposite for the two valleys of graphene. The non-interacting part of our model describes the zero magnetic field pseudo Landau level (PLL) structure recently proposed [2] and experimentally reported [3] in strained graphene. Since the reported effective magnetic fields [2] range from 60 T up to 300 T, the interaction-driven phases might conceivably be realized with larger energy gaps than in Fractional Quantum Hall states under a real magnetic field. Besides strained graphene, our results also pertain for artificial graphenes such as patterned electron gases and cold atoms in hexagonal lattices.
More specifically, we have investigated the zero energy PLL at 2/3 filling [3]. In presence of the unscreened Coulomb interaction, electrons realize a 2/3 Hall state breaking time-reversal symmetry. Upon tuning the local part of the interaction, this 2/3 state can be destabilized towards a time-reversal symmetric state realizing a 1/3 Laughlin state in each valley. This state has a 9-fold ground state degeneracy and can be seen as a valley fractional topological insulator (FTI). For local attractive interactions, the 1/3+1/3 FTI has a transition towards a superconducting state. On raising the filling to the neutrality point, namely for the half-filled zero energy PLL, we find either a ferromagnet or a valley polarized state depending on the strength of the on-site interactions.
[1] P. Ghaemi, J. Cayssol, D. N. Sheng and A. Vishwanath, Fractional topological phases and broken time reversal symmetry in strained graphene, Phys. Rev. Lett. 108, 266801 (2012).
[2] F. Guinea, M.I. Katsnelson, and A.K. Geim, Energy gaps and a zero-field quantum Hall effect in graphene by strain engineering, Nat. Phys. 6, 30 (2010).
[3] L. Levy et al., Strain-induced pseudomagnetic fields greater than 300 tesla in graphene nanobubbles, Science 329, 544 (2010).