Saturday, November 24, 2012

Topological insulators get more interesting

Topological insulators (TIs) are certainly a hot topic. However, there are two things that might make one nervous about all the excitement.

1. All the materials being studied as TIs [e.g. Bi2Se3] actually aren't TIs.
What!? A TI is by definition a bulk insulator with surface metallic states that are topologically protected. However, the actual materials turn out not to be bulk insulators. On a practical level this makes separating out bulk and surface contributions, particularly in transport measurements, tricky. But, also presents an ideological problem: one is not actually studying the phase of matter one wishes one was studying.

2. One could argue that TIs are "just a band structure effect", i.e., they do not involve any quantum many-body physics.

However, these objections are put to rest by a preprint
Discovery of the First True Three-Dimensional Topological Insulator: Samarium Hexaboride
Steven Wolgast, Cagliyan Kurdak, Kai Sun, J. W. Allen, Dae-Jeong Kim, Zachary Fisk

They report electrical transport measurements that show that SmB6 is a bulk insulator with surface metallic states.
This is of particular interest for several reasons

a. The material really is a true topological insulator.
b. The material is a Kondo insulator. [Although strictly the material is in the mixed valence rather than the local moment regime.] The insulating state emerges from strong electronic correlations.
c.  This resolves long standing puzzles about previous transport measurements on this material which did not show activated conductivity at low temperatures. This can now be explained as a sample dependent contribution from metallic surface states.
d. This material was predicted to be a topological Kondo insulator by Dzero, Sun, Coleman, and Galitski.

I also note a recent paper Actinide Topological Insulator Materials with Strong Interaction.

I thank Tony Wright for bringing the preprint to my attention.


  1. I would also like to notify you of our preprint on the same system - reporting tunneling spectroscopy and the observation of Fano lineshapes due to the Kondo hybridization. The preprint is available on arXiv:

    We are hesitant to call this a "true topological insulator" quite yet, since there is really no direct evidence of the topological nature of the surface states. However the consistency is very interesting, and will promote future work.

  2. I'm not sure about objection (2). There are plenty of very interesting things that are not many-body physics.

  3. Also... Strained HgTe is a pretty good "true" 3D TI, so I think the title of the work is a little misplaced. This doesn't mean it and the Maryland work is not very interesting though...

  4. Hi Prof Mckenzie,

    I am a grad student at NYU and silent reader of this blog. I came across a paper that measured SmB6 in 2001 when topological insulators were not the craze, consequently their explanation was different and possibly wrong.

    It is interesting that the current spurt of papers doesn't seem to mention it. The 2001 paper also measured thermopower, and the thermopower seems to show the surface metal shorting out the bulk most beautifully. fig1.C in this paper


    Since the conductivity of the surface channel is much larger than the bulk insulator, and thermopower of a two channel system is the conductivity weighted average, thermopower should show a large reduction. And what a reduction it shows! From 800 microvolts per K to negligible.

    I might be missing something, but I think the data was there all along.