Choisissez le fuseau horaire
Le fuseau horaire de votre profil:
Description
Graphene is a two-dimensional crystal of carbon atoms in the shape of a honeycomb lattice. The band structure of conduction electrons has two essential peculiarities: first, the energy spectrum is gapless featuring two linear contact points between valence and conduction bands at which the Fermi energy lies; and second, the corresponding spinor eigenstates have quantized vortices in their relative phase endowing the electrons with a pi Berry phase. These two features are captured by a low energy effective description, in the vicinity of the Fermi surface, in terms of a 2+1 massless Dirac (or Weyl) hamiltonian. Hence the name “Dirac fermions” for the quasi-particles in graphene. Deforming the honeycomb lattice, it is possible to make these contact points move in reciprocal space until they meet and merge, which can be seen as both a gap opening in the spectrum and an annihilation of a vortex-antivortex pair for the eigenstates. This scenario, known as a Lifshitz topological phase transition, has not been realized in real graphene due to the un-physically large strain required to reach the transition. However, recent experiments have been performed with artificial graphene made of cold atoms in an optical lattice, the tunability of which allowed the observation of the merging transition.
