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Theory of the magnetic-field-induced insulator in neutral graphene sheets

Speaker: 
Jeil Jung (Graphene Centre, NUS)
Date: 
Wed, 24/04/2013 - 11:00am to 12:00pm
Location: 
S13-M01-11 (Physics Conference Room)
Event Type: 
Seminars

Abstract

In this talk I will introduce basic notions of electron interaction induced broken symmetry phases of graphene in the quantum Hall regime, paying particular attention to the nature of the ground state at the neutrality point. I will show how the four-fold spin/valley degrees of freedom of graphene together with the valley-sublattice resolved wave function structure of the anomalous zero-energy Landau levels play a role in defining different possible ground-states in the system. We explain why in experiments the neutral graphene sheets have an insulating ground state in the presence of an external magnetic field. I will show numerical studies carried out in the π-band tight-binding-model Hartree-Fock approximation which examines the competition between distinct candidate insulating ground states. We conclude that for graphene sheets on substrates the ground state is most likely a field-induced spin-density wave and that a charge-density-wave state is possible for suspended samples. Neither of these density-wave states support gapless edge excitations. We will comment on the relevance of lattice-scale details of the Coulomb interactions in deciding the nature of the ground states, and discuss on the possibility of inducing phase transitions from insulating density wave states to metallic spin-polarized ground-states by enhancing Zeeman coupling through parallel magnetic fields.

Reference: J. Jung and A. H. MacDonald, Phys. Rev. B 80, 235417 (2009).

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