59th Annual Report on Research 2014

Figure 1.  Electronic structure of commensurate SLG-TI and BLG-TI heterostructures. The grey line show the band structure of isolated SLG  (BLG).

We then formulated the theory for the case in which the graphene layer and the TI are in a non-commensurate stacking configuration due to the lattice mismatch and the presence of a twist angle between the two materials. Our results show that by changing the relative twist angle the electronic structure of the TI-graphene system can be considerably tuned. In addition, we found that the Fermi surface exhibit very non-trivial spin textures that can be qualitatively modified by changing the twist angle.
This work resulted in a paper published in Physical Review Letters, and one contributed talk at the 2014 APS March meeting.

Figure 2. Example of spin structure on the Fermi surface of a TI-SLG heterostructure.

In a related project we have started to investigate how the relative twist angle affects the electronic properties of heterostructures formed by two one-atom thick layers of metal dichalcogenides such as MoS₂. This work resulted in a talk at the 2014 APS March meeting and a manuscript in preparation.

Figure 3. Sketch of a TI-Ferromagnet-Superconductor heterostructure.

We then considered more complex heterostructures, formed by three materials with very different properties: a TI, a ferromagnet, and a superconductor, see Fig. 3. In this case we find that the coupling between the electronic degrees of freedom of these three different class of materials can give rise to very unusual properties, such as the appearance of " odd-frequency" superconductivity. This work has been done in collaboration with Alexander Balatski at the Los Alamos National Laboratory (LANL) and has resulted in a paper published in Physical Review B, and one contributed talk at the 2014 APS March meeting.

In another project we undertook a systematic study of the effect of disorder on the electronic structure of graphene heterostructures formed by two sheets of SLG (or BLG) separated by a very thin insulating layer. In Dirac materials like graphene charge impurities, whose presence is unavoidable in realistic conditions, induces a long-range disorder potential. Such a potential in turn causes the formation of strong, long-range, carrier density inhomogeneities (electron/hole puddles) that strongly affect the electronic structure and the properties of the system. In the past year we have developed the theory, and computational techniques, to characterize the disorder-induced electron/hole puddles in graphene heterostructures. In particular, our results  show how the size and "depth" of these electron-hole puddles in graphene heterostructures depend on the graphenic sheets forming the structure, the disorder strength, and the doping level of each graphenic layer. This work resulted in a paper published in Physical Review B.

Figure 4. Example of electron/hole puddles induced by disorder in a graphene-graphene heterostucture. Panels (a) and (b) show the carrier density profile in bottom, top, layer respectively. Panels (c), (d) show the profile of the screened disorder potential.

In the past six months we have started to develop the theory to describe the electronic structure of graphene nanoribbons placed on a graphene sheet. We have obtained preliminary results for the case of long ribbons placed on graphene in a commensurate stacking, see Fig. 5. Our goal is to have a theory that is able to describe the electronic structure of finite size ribbons placed with an arbitrary twist angle on a graphene layer (or graphene bilayer).  

Figure 5. Examples systems formed by graphene nanoribbons (red) placed on a sheet of graphene (blue).

The research supported by the grant in the past year has had a very strong impact on the research and career advancement of the students involved and of the PI. Under the guidance of the PI Jonathan Fischer completed his senior thesis and coauthored his first paper. He is now a PhD student in statistics at UC Berkeley. The results for the TI-graphene heterostructure attracted the interest of experimental groups working on these systems. This prompted the start of a collaboration between our group and the experimental groups of Brian LeRoy (University of Arizona) and Pablo Jarillo-Herrero (MIT) aimed to understand the quasiparticle interference patterns that these groups observe using scanning-tunneling-microscopy. The project on the TI-superconductor heterostructures catalyized the collaboration with the group of Alexander Balatsky at LANL. As a result of such collaboration graduate student Chris Triola this summer has been employed by LANL to conduct further research on novel 2D electronic materials. All the students involved in the research have had the opportunity to present their work at the 2014 APS March meeting, one of the largest conferences in physics.