Discussion on interaction effects in bilayer graphene

Date: January 19, 2012
Venue: Small Seminar Room + Tallahassee airport (Jeanie Lau, by Skype)
Participants: about 30 people

Notes by the moderator (Fogler)

Questions proposed by the moderator:

Theory:
- What are main competing states? Which interactions stabilize which?
- Any "simpler" approach? (Mean-field? Strong coupling?)
- Long-range Coulomb interactions

Experiments:
- Gapped vs. gapless?
- Inhomogeneities?
- Strain, shear?

Main speakers:

MacDonald, Vafek, Levitov, Guinea, Geim, Falko

MacDonald: RG equations and their solutions depend on whether one uses original or anti-symmetrized vertices.

Vafek: The RG equations should be supplemented by further analysis of the response functions. Nematic state is dominant for both spinful and spinless case.

Levitov: The I-V characteristic measured by Lau et al. is indicative of a tunneling contact in the sample, possibly a stacking fault

Geim: Real systems are highly doped near the contacts, and so the "Dirac point" in the experiments of other groups may be a state with a p-n junction, which can act as the tunneling contact. This p-n junction is gapped by the transverse field of the gate. As for Manchester's experiment (which apparently does not have a p-n junction), the observed T-dependence of the resistivity is most consistent with the nematic state.

Guinea: Intermediate states made of domains may be possible. They occur at the boundaries of the phase transitions between different proposed states.

Falko: We carried out the full RG analysis, including all 9 possible couplings and long-range part of the interactions. The nematic and the anti-ferromagnetic states are the phases that occur most often. We also considered effects of the strain and shear. They produce features similar to nematic states but the magnitude depends on the strain vs lattice orientation, which is not the case in Manchester's experiment.