## Sunday, March 28, 2021

### Brief items

Catching up after the APS meeting, here are a couple of links of interest:

• This video has been making the rounds, and it's fun to watch.  It's an updated take on one of those powers-of-ten videos, though in this case it's really powers-of-two.  Nicely done, though I think the discussion of the Planck Length is not really correct.  As far as I know, the Planck Length is a characteristic scale where quantum gravity effects cannot be neglected - that doesn't mean that the structure of the universe is discrete on that scale.
• There have also been a lot of articles like this one implying that new (non-Standard Model) physics has been seen at the LHC.  As is usually the case, it's premature to get too excited.  At the 3$\sigma$ level, there is an asymmetry in decay channels (electrons vs muons) seen by the LHCb experiment when none is expected.  As the always reliable Tommaso Dorigo writes here, everyone should just take a breath before getting too excited.  At least when the LHC starts back up next year, there should be a lot of new data coming in, and either this effect will grow, or it will fade away.  Anyone want to bet on the over/under for the number of theory papers about leptoquarks that are going to show up on the arxiv in the next month?
• We were fortunate enough to have Pablo Jarillo-Herrero give our colloquium this past Wednesday, talking about some really exciting recent results (here, here) in twisted trilayer graphene.
I'll hopefully write more soon, also touching on a recent paper of ours.

Anonymous said...

Just watched the Moire 3.0 talk and wow, this twistronics business is really a treasure trove!

Question: why is the displacement field considered a separate variable from the filling factor? Isn't the filling dictated by the voltage/E-field?

Douglas Natelson said...

Anon, cool, right? My understanding is, if you have both a top gate and a back gate, you have the freedom to dial around the electronic chemical potential (filling factor) and the z-directed electric field independently. Imagine equal capacitances between the channel and both gates, and imagine that (relative to some drain electrode) you have the top gate at +2V and the back gate at -2V. Then consider having the top gate and +1V and the back gate at -1V. Same potential at the channel, but different vertical E fields.

Anonymous said...

Aha! That makes sense.
But it's not clear to me why the E field itself changes so much of the system's physics. Is it like a Stark effect that splits the upper and lower layers?

Douglas Natelson said...

I think that's exactly it. See, e.g., https://doi.org/10.1103/PhysRevB.84.195421.