Twisted bilayers

a-b) Illustration of a moiré pattern arising from a rotation angle of 9.6° and the emergence of van Hove singularities as 
a consequence of the reciprocal space rotation. c) STM images of several MP with different rotation angle. d)Measure 
of the of van Hove singularities on the real graphene rotated layers

Twisted graphene bilayers have emerged recently as a promising graphene based material whose properties can be controlled by a single geometrical parameter which is the interlayer rotation angle. Electronic structure calculations indicate that this angle should lead to a reduction of the Fermi velocity (even to zero), induce low energy van Hove singularities and tend to localize the density of states in real space. We demonstrated experimentally that the expected novel properties for twisted layers show up in the electronic structure in a wide range of rotation angles

Physical Review Letters 109, 196802 (2012) [article]

Quasiparticle pseudospin

  
Many of the tantalizing electronic properties of graphene can be understood as due to the conservation of pseudospin and quasiparticle chirality, two entities that have no equivalence in any other two-dimensional system. They are responsible, for example, of the new ‘chiral’ quantum Hall effects (QHE) observed for monolayer and bilayer exfoliated graphene, which are the most direct evidence for Dirac fermions in graphene. Our work shows how pseudospin and quasiparticle chirality can be experimentally probed at the nanoscale by means of STM, since they are reflected in the quasiparticle interference processes that take place in graphene. Our STM data, complemented by theoretical calculations, demonstrate that the quasiparticles in epitaxial graphene on SiC(0001) have the chirality predicted for ideal (free standing) graphene, which proves the Dirac character of the quasiparticles in this system.

Physical Review Letters 101, 206802 (2008) [article]
Physical Review B 086, 45444, (2012) [article]