SCL hosted IPB-wide seminar by Mark Oliver Goerbig from the Laboratoire de Physique des Solides, CNRS, Université Paris-Sud, France on 21 January 2010, entitled "Graphene under uni-axial strain and 2D layered organic material α-(BEDT-TTF)₂I₃ - the physics of tilted Dirac cones".

Abstract of the talk:
Dirac-fermion physics in condensed matter physics has become a major issue since the discovery of graphene, where the low-energy electronic properties are governed by the relativistic Dirac rather than a Schrödinger equation. It has recently been argued that Dirac fermions would also be responsible for the particular electronic properties in the layered organic material α-(BEDT-TTF)₂I₃, whose temperature-dependent transport hints at a narrow or zero semiconductor under pressure.

An interesting difference as compared to graphene arises from the tilt of the (anisotropic) Dirac cones in this material, which has been obtained in a tight-binding calculation. The talk will discuss the origins of this tilt as well as its consequence for the Landau-level quantisation in a strong magnetic field. Surprisingly, the effect of the tilt yields the same modification of the Landau levels as an in-plane electric field for the case of non-tilted cones, which lowers indeed the Landau-level spacing as a consequence of an underlying Lorentz invariance. Whereas neither the tilt of the Dirac cones nor the in-plane electric field may lift, on their own, the twofold valley degeneracy in these systems, the combination of the two achieves such as degeneracy lifting. This scenario may eventually be tested with the help of infrared spectroscopy that has proven to be a powerful tool in the determination of the particular relativistic Landau-level structure in graphene.