Sara Ghotb

twisted bilayer Graphene

As my PhD general exam topic I worked on the twisted bilayer Graphene.

It was a long time that 3-dimensional (Diamond and graphite), 1-dimensional (nanotubes) and 0-dimensional (fullerenes) of carbons were known and finally at 2004 it became possible to fill the missing place for two-dimensional carbon in the list. Samples of graphene one atom thick were produced in 2004 by K. S. Novoselov and A. K. Geim [1]. Producing the 2-dimensional graphene from graphite by micromechanical cleavage method was the starting point for producing two-dimensional crystals of other materials. This finding was very important due to the fact that two-dimensional crystals exist, and they are stable at ambient condition [2]. Graphene is very interesting material due to its unique electrical properties. Its most important electronic property is the linear dispersion relation near the charge neutrality. This material can stack on top of each other which exhibit very different and interesting results as it is seen in bilayer graphene. Layers of graphene are separated by van der Waals forces and one of the important aspects of these van der Waals material is that they can be rotated on top of each other. The mismatch between these two lattice points is called Moiré pattern which modulate the hybridization between the two layers of graphene. The new platform for investigate strongly correlated material were suggested recently in the twisted bilayer graphene (TBG). There are some theories which predicted that for certain twist angles which are called “magic angle” the interlayer coupling is tuned in a way that it ends up having flat bands near Fermi energy and the velocity around Dirac points vanishes completely. Appearance of unconventional superconductivity in twisted bilayer graphene at this magic angle is suggested to arise from the doping at half-filled insulating state which is very similar to the high Tc cuprates. Superconducting state in TBG at magic angle is attributed to electron correlations rather than weak electron- phonon coupling. The fabrication of high-quality samples with precise and controlled twist angles is very important for investigating this twisted bilayer graphene which is achieved by this new technique of “tear and stack”. The important aspect of this technique is that single piece of graphene is teared apart and two halves with same original orientation are stack together which leads to the controlled twist angle in this system.