Tuesday, September 8, 2020

What's the big deal about twisted bilayer graphene?

 Twisted bilayer graphene seems to be the hottest topic in condensed matter physics right now. I tend to not follow fashion, both in clothing and science, for a multitude of reasons. However, I recently tried to catch up and read several of the nice perspectives on the topic at the Journal Club for Condensed Matter Physics.

Electronic bands of twisted graphene layers by Francisco Guinea

New correlated phenomena in magic-angle twisted bilayer graphene/s by Michael Zaletel.

What drives superconductivity in twisted bilayer graphene? by T. Senthil

Here are just a few big picture comments. I welcome feedback. I am just dipping into the subject.

Why is this attracting so much interest?

It is a playground for both experimentalists and theorists. There is some beautiful mathematics, even at the level of Moire patterns, large unit cells for the crystal structure (7204 carbon atoms!), and electronic band structure. For experimentalists, it presents a tuneable system with a rich phase diagram.

The band structure is unique in having topological features (Chern numbers), Wannier orbitals with subtle features, and non-abelian gauge fields.

The discovery of superconductivity and ferromagnetism was unexpected (I think).

There is a subtle competition between many different strongly correlated phases: Mott insulators, ferromagnetism, superconductivity, Dirac metals, ...

The possibility that superconductivity is associated with (i.e. in close proximity in the phase diagram) a Mott insulator suggests some possible similarities to cuprate superconductors.

What are some outstanding issues?

All the theory has a precise and uniform twist angle between the two sheets of graphene. However, there will inevitably be some spatial inhomogeneity in the twist angle across any real laboratory sample. How much does this inhomogeneity matter in the experiments that have been reported so far?

What is the role of the substrate that the twisted bilayer sits on?

Is the superconductivity always "derived" from a Mott insulator?

Is the superconductivity unconventional in being non-phononic and/or having non-s-wave pairing?

Can we achieve consensus on a model effective Hamiltonian and what its phase diagram is?

Will this interest last?

Interest may fade if further and more careful experiments on better samples can never definitely answer the questions above OR if the experiments do find some of the following to be true.

The sample inhomogeneity matters and some of the exciting results reported do not survive in better samples.

The superconductivity is not intimately connected to the Mott insulator.

The superconductivity is conventional.

Some caution and skepticism are in order. Many results published in luxury journals do not stand the test of time. Furthermore, condensed matter physics is a field that rapidly goes through fashions that attract a crowd that quickly moves onto to the next ``big thing,'' i.e. exotic phenomena.

I welcome comments and corrections. I do want to learn more about this fascinating subject.


  1. I went to an online colloquium that covered this recently. The sensitivity to the angle of bilayer graphene was mentioned, and the fact that it was difficult to control. However, they went on to talk about bilayers of WSe2/WS2 moiré superlattices as being much more robust - less sensitivity to angle. It was Kin Fai Mak from this group: https://sites.google.com/site/makshangroup/

  2. I think there is growing consensus that the connection to the Mott insulator in TBLGZ is limited. And there is only a superficial resemblance to cuprate superconductivity. TBLG is really its own beast. The recent work by A. Young from UCSB gives evidence for this.

    I think there might be some exciting practical applications though in terms of single photon detection down in the THz regime due to how low the carrier density is compared to Tc.