Dynamics in the quantum Hall effect and the phase diagram of graphene (original) (raw)

Toward a theory of the quantum Hall effect in graphene

Low Temperature Physics, 2008

We analyze a gap equation for the propagator of Dirac quasiparticles and conclude that in graphene in a magnetic field, the order parameters connected with the quantum Hall ferromagnetism dynamics and those connected with the magnetic catalysis dynamics necessarily coexist (the latter have the form of Dirac masses and correspond to excitonic condensates). This feature of graphene could lead to important consequences, in particular, for the existence of gapless edge states. Solutions of the gap equation corresponding to recently experimentally discovered novel plateaus in graphene in strong magnetic fields are described.

Coulomb interaction and magnetic catalysis in the quantum Hall effect in graphene

Physica Scripta, 2012

The dynamics of symmetry breaking responsible for lifting the degeneracy of the Landau levels (LLs) in the integer quantum Hall (QH) effect in graphene is studied in a low-energy model with the Coulomb interaction. The gap equation for Dirac quasiparticles is analyzed for both the lowest and higher LLs, taking into account the LLs mixing. It is shown that the characteristic feature of the long-range Coulomb interaction is the dependence of the gap parameters on the LL index n ("running" gaps). The renormalization (running) of the Fermi velocity as a function of n is also studied. The solutions of the gap equation reproduce correctly the experimentally observed integer QH plateaus in graphene in strong magnetic fields.

The Quantum Hall Effect in Graphene

Modern Physics Letters B, 2012

We investigate the quantum Hall effect in graphene. We argue that in graphene in presence of an external magnetic field there is dynamical generation of mass by a rearrangement of the Dirac sea. We show that the mechanism breaks the lattice valley degeneracy only for the n = 0 Landau levels and leads to the new observed ν = ±1 quantum Hall plateaus. We suggest that our result can be tested by means of numerical simulations of planar Quantum Electro Dynamics with dynamical fermions in an external magnetic fields on the lattice.

Experimental observation of the quantum Hall effect and Berry's phase in graphene

Nature, 2005

When electrons are confined in two-dimensional materials, quantum-mechanically enhanced transport phenomena such as the quantum Hall effect can be observed. Graphene, consisting of an isolated single atomic layer of graphite, is an ideal realization of such a two-dimensional system. However, its behaviour is expected to differ markedly from the well-studied case of quantum wells in conventional semiconductor interfaces. This difference arises from the unique electronic properties of graphene, which exhibits electron-hole degeneracy and vanishing carrier mass near the point of charge neutrality 1,2 . Indeed, a distinctive half-integer quantum Hall effect has been predicted 3-5 theoretically, as has the existence of a non-zero Berry's phase (a geometric quantum phase) of the electron wavefunction-a consequence of the exceptional topology of the graphene band structure 6,7 . Recent advances in micromechanical extraction and fabrication techniques for graphite structures 8-12 now permit such exotic two-dimensional electron systems to be probed experimentally. Here we report an experimental investigation of magneto-transport in a high-mobility single layer of graphene. Adjusting the chemical potential with the use of the electric field effect, we observe an unusual halfinteger quantum Hall effect for both electron and hole carriers in graphene. The relevance of Berry's phase to these experiments is confirmed by magneto-oscillations. In addition to their purely scientific interest, these unusual quantum transport phenomena may lead to new applications in carbon-based electronic and magneto-electronic devices.

Quantum Hall Ferromagnetism in Graphene

Physical Review Letters, 2006

Graphene is a two-dimensional carbon material with a honeycomb lattice and Dirac-like lowenergy excitations. When Zeeman and spin-orbit interactions are neglected its Landau levels are four-fold degenerate, explaining the 4e 2 /h separation between quantized Hall conductivity values seen in recent experiments. In this paper we derive a criterion for the occurrence of interactiondriven quantum Hall effects near intermediate integer values of e 2 /h due to charge gaps in broken symmetry states.

Edge states in quantum Hall effect in graphene (Review Article)

Low Temperature Physics, 2008

We review recent results concerning the spectrum of edge states in the quantum Hall effect in graphene. In particular, a special attention is payed to the derivation of the conditions under which gapless edge states exist in the spectrum of graphene with zigzag and armchair edges. We find that in the case of a half-plane or a ribbon with a zigzag edges, there are gapless edge states only when a spin gap dominates over a Dirac mass gap. In the case of a half-plane with an armchair edge, the existence of the gapless edge states depends on the specific type of Dirac mass gaps. The implications of these results for the dynamics in the quantum Hall effect in graphene are discussed.

Excitonic gap, phase transition, and quantum Hall effect in graphene

Physical Review B, 2006

We suggest that physics underlying the recently observed removal of sublattice and spin degeneracies in graphene in a strong magnetic field describes a phase transition connected with the generation of excitonic and spin gaps. The strong-coupling regime is described using a phenomenological model with enhanced Zeeman splitting (spin gap) and excitonic gaps. The experimental form of the Hall conductivity σxy with the additional ν = 0, ±1 plateaus is reproduced. The form of σxy in the case of a strong-coupling regime with no enhanced Zeeman splitting is also discussed.

Global phase diagram of charge-neutral graphene in the quantum Hall regime for generic interactions

Physical Review B

Monolayer graphene at charge neutrality in a quantizing magnetic field is a quantum Hall ferromagnet. Due to the spin and valley (near) degeneracies, there is a plethora of possible ground states. Previous theoretical work, based on a stringent ultra short-range assumption on the symmetryallowed interactions, predicts a phase diagram with distinct regions of spin-polarized, canted antiferromagnetic, inter-valley coherent, and charge density wave order. While early experiments suggested that the system was in the canted antiferromagnetic phase at a perpendicular field, recent scanning tunneling studies universally find Kekulé bond order, and sometimes also charge density wave order. Recently, it was found that if one relaxes the stringent assumption mentioned above, a phase with coexisting canted antiferromagnetic and Kekulé order exists in the region of the phase diagram believed to correspond to real samples. In this work, starting from the continuum limit appropriate for experiments, we present the complete phase diagram of ν = 0 graphene in the Hartree-Fock approximation, using generic symmetry-allowed interactions, assuming translation invariant ground states up to an intervalley coherence. Allowing for a sublattice potential (valley Zeeman coupling), we find numerous phases with different types of coexisting order. We conclude with a discussion of the physical signatures of the various states.

Excitonic gap, phase transition, and quantum Hall effect in graphene: strong-coupling regime

2007

Dynamics in the quantum Hall effect and the phase diagram of graphene (original) (raw)

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