## Thursday, March 07, 2019

### APS March Meeting wrapup

I spent the lion's share of today talking w/ my collaborators.  This was great scientifically, but meant that I only went to a couple of talks.

• This one was pretty slick.  If you look at the conduction properties of a Josephson junction as a function of magnetic field through it, you see a Frauenhofer pattern as a function of the enclosed flux (see this pdf, fig 2).  In principle, taking the inverse Fourier transform of this should reveal the real-space current distribution as a function of the distance along the width of the junction.  This group made Josephson junctions using oriented thin pieces of WTe2.  When the current flowed along one direction, they found that the Josephson current was mostly flowing near the edges of the strip of material.  When current flowed along a different direction in the plane, the current distribution was much more uniform.
• Similarly evocative, this talk presented work using magnetic focusing plus scanning gate microscopy plus collimating contacts to look at the real-space paths of electrons in a graphene-hBN bilayer w/ a Moire superlattice.  They could then infer the shape of the Fermi surface in momentum-space, confirming that the Moire superlattice results in a roughly triangular (miniband) Fermi surface.  Cooler than my jargon-heavy description sounds.
• I greatly regret that I was unable to attend the invited session in honor of Millie Dresselhaus.  If one of my readers who did make it could please describe it in a comment, I'd appreciate it.
• One other random note:  I did actually speak to the APS person who was in charge of the trade show, and I asked what the heck was up with the two weird "pain relief" booths, which seemed borderline late-night-infomercial/much more like something you'd see at a cheesy shopping mall.  This was apparently an experiment in allowing local vendors in, and it sounds very unlikely that it'll ever happen again.
If I missed a big story from the meeting, please let me know in the comments.

## Wednesday, March 06, 2019

### APS March Meeting Day 3

A handful of semi-random highlights (broken up by my conversations w/ colleagues and catching up on work-related issues):

• Laura Heydermann from ETH spoke about "artificial" magnetic systems, where mesoscopic, lithographically patterned arrays of magnetic islands can yield rich response.  A couple of representative papers are here and here, and recently they've been moving into 3D fabrication and magnetically sensitive imaging.  Very neat stuff.
• Christian Glatti from Saclay showed a very interesting result, analogous to the ac Josephson effect, but in fractional quantum Hall edge-state tunneling.  The relevant paper, just out in Science, is here.  This idea is, measure electronic shot noise as a function of bias voltage.  Ordinarily this has a minimum at zero bias, and the noise sits at the Johnson-Nyquist level there.  Now shine microwaves of frequency f on the device.  With photon-assisted tunneling, the net result is a change in the noise that has kinks at voltages of +/- hf/e*, where h is Planck's constant, and e* is the effective charge of the low-energy excitations.  Do this in the fractional quantum Hall regime, and you see fractional charge.
• On a related topic, Michael Pepper from Cambridge showed a very recent result.  In quantum point contacts at very low charge carrier densities, they see quantized conductance at some very surprising rational fractions of the usual conductance quantum 2e2/h.  I still need to digest this.
• I spent much of the afternoon at the big Kavli Symposium, on topics spanning from unit cell all the way to biological cells.  All excellent speakers.  I won't try to summarize this - rather, when the talks become available streaming, I will put the link here.  (Claudia Felser did bring donuts for the audience to talk about topology, always a crowd-pleaser.)

### APS March Meeting, Day 2

A random selection from Day 2:

• Thomas Silva at NIST gave a fun talk about some experiments using the linac coherent light source.  Using pump/probe time-resolved x-ray diffraction, they discovered some surprising acoustic modes in thin, polycrystalline metal films, with systematics suggesting that they might be seeing localization of phonons due to scattering off grain boundaries.
• Along those lines, Gang Chen of MIT spoke about seeing reductions in thermal conductivity due to phonon localization.  His group was working with semiconductor superlattices, with little ErAs nanodots embedded in a disordered way at the superlattice interfaces.  They see systematics in the thermal conductivity that suggest that they are seeing Anderson localization of the heat-carrying phonons.
• I stopped by the session on conveying physics to a popular audience, and caught most of the talk by Allison Eck chock full of advice for would-be science writers, and a skyped-in talk by Sean Carroll about podcasting.  The depressing truth: If I really want to expand my audience, I should probably join twitter.  (The problem is, that's a conversational medium and I don't see how I could do it well given everything else.)
• Abe Nitzan gave a prize talk that was a nice overview of the last decade's work on understanding electrons, photons, and phonons in molecular junctions.
• I spent much of the afternoon at this session about the copper oxide superconductors.  Dan Dessau's talk primarily about this paper showed the capabilities of a new technique in analyzing angle-resolved photoemission data, to figure out the actual spatial shape of Cooper pairs in these systems. My collaborator Ivan Bozovic spoke (similar to this), showing the power of his tremendous MBE growth approach, able to create epitaxially perfect materials smoothly and systematically spanning the whole doping range.  The other talks in the session were also very interesting.

## Monday, March 04, 2019

### APS March Meeting, Day 1

A few things I saw at the APS Meeting today, besides 10 inches of fresh, wet snow on the ground this morning (disclaimer:  for various reasons I was session-hopping quite a bit, so this is rather disjointed):
• Ignacio Franco at Rochester spoke about some experiments (here) that I'd not remembered, where carefully controlled, intense femtosecond light pulses were used to turn on a transient current in SiO2, normally one of the best insulators out there.  The theory is interesting, and made me start thinking about possible opportunities in this area.
• A focus topic session on 2D magnetic materials was extremely crowded - so much so that I literally couldn't get in the room for the first talk.  Interesting talks, including Yujun Deng from Fudan presenting this workMasaki Nakano from the University of Tokyo spoke about growing epitaxial films of V5Se8, a cousin of a material with which we've worked; and Boyi Zhou at Washington University in St. Louis presented this work, which seems to show nontrivial electronic conduction in (ordinarily Mott insulating) monolayer RuCl3 layered on graphene.  Lots of interesting activity going on here, many fun ideas.
• Naomi Ginsberg at Berkeley talked about some impressive imaging techniques used to follow energy flow in complex materials.  Combining super-resolution methods, interferometry, and time-resolved techniques is a heck of an enabling technique!
• Peter Abbamonte at Illinois presented some remarkable measurements using an angle-resolved electron energy loss technique (M-EELS) to look at the strange metal state of a cuprate superconductor.  The main result is that this material seems to support a very broad plasmon mode with a lot of properties that are inconsistent with what you'd expect in a Fermi liquid, and may make connection with more exotic pictures of strange metals.
• Wojciech Zurek's talk about the foundations of quantum mechanics (based on this article) was very engaging (and apparently in a superposition of all possible fonts), though again the room was so full that people were sitting on the floor in the aisles and lining three walls.  The session also was running about 10 minutes ahead of schedule, which definitely was not great for people who ended up missing the beginning of Zurek's talk or Rovelli's before it.
The unwieldy size of the meeting is increasingly clear, with lines in the restrooms, and local fastfood places unable to handle the lunch crowd.

## Sunday, March 03, 2019

### APS March Meeting, Day 0

A brief summary of topics/reading material/things I learned today during DCMP and joint DCMP/DMP executive committee meetings:
• As usual, this will be the biggest March Meeting ever, with 11500 registrants ahead of time.  This is still increasingly problematic in terms of organization and availability of sites.
• New APS Strategic Plan
• New APS report on the Impact of Industrial Physics on the US Economy
• DOE Basic Energy Sciences report (pdf) on the impact of the BES at its 40th anniversary
• The upcoming privatization of the US Strategic Helium Reserve looks depressingly unavoidable.  Sounds like changing this is a non-starter in Congress.

### Michelle Simmons and Si-based quantum computing

A last tidbit before the March Meeting.

Earlier this week, Prof. Michelle Simmons came to Rice for our Chapman Lecture series and gave a great talk about her team's project to develop quantum computing in a silicon platform, with individual phosphorus donor atoms as the qubits.   This idea goes back more than twenty years to this proposal by Bruce Kane.   Actually implementing this approach requires overcoming many technical challenges, including positioning individual phosphorus atoms inside single-crystal Si with nearly atomic precision, and similarly fabricating control and read-out electrodes in registry with those.

Prof. Simmons' group has made truly remarkable progress in this direction.  The key enabling technique is using a scanning tunneling microscope (STM) as a lithography tool.  Single-crystal Si surfaces are prepared in ultrahigh vacuum and terminated with a hydrogen.  The STM tip is then used to strip off the hydrogen atoms with atomic precision.  (This is a serial technique, and so scaling up to the production numbers of the present-day Si industry would require something different, but for now it's fine.)  Phosphine gas decomposes in a particular way when exposed to the dangling bonds left behind by stripping the hydrogen, placing P atoms in particular locations.  This approach can also be used to make highly conductive wires and gates by doping, enabling transport measurements through single dopant atoms.   Growing more single-crystal Si on top of the dopants without having the dopants move around is another success story, making possible 3D fabrication schemes.  With isotopically pure Si, encapsulating the donors can give long coherence times.

There are many competing platforms for possible quantum computer implementations, and this approach is undoubtedly difficult.  In terms of technical achievement, though, this effort has shown the power of sustained support - progress has been truly impressive.

## Thursday, February 28, 2019

### APS March Meeting 2019

Once more, it is that time of year, when (mostly) condensed matter physicists gather in ever-increasing numbers to give and watch talks, network, try to get cool swag at the tradeshow, and generally grouse about the poor quality and high price of convention center coffee.  The March Meeting this year is in Boston for the first time since I've been going 2012.  (Strangely, I was unable to find a list of all the March Meeting sites online.  Perhaps a reader knows the last time the meeting was in Boston.)  It's my third and last year as a DCMP member-at-large, so it will be interesting to hear what comes up at the business meetings this time.   As I have done in past years, I'll do my best to write up some of what I see and give my impressions of the conference, though I may be more concise compared to previous years.

## Tuesday, February 19, 2019

### Why twisting materials is interesting

Twisted bilayer graphene is a hot topic, with 32 preprints on the arxiv using those keywords just since the beginning of the year.  It's worth explaining for non-experts, why this system, comprising two atomic layers of graphene twisted relative to each other by some angle, is so interesting.

Let's start w/ the basics.  In the (non-relativistic) quantum world, we talk about the wavefunction $\psi(\mathbf{r},t)$ of a system.  The Schroedinger equation describes how the wavefunction evolves with time, and by solving it we can find the particular energy levels ("stationary states") for a given problem.  The magnitude-squared of the spatial wavefunction, $|\psi(\mathbf{r},t)|^2$ gives the probability of finding the particle in a particular place at a particular time.

The wavefunction a free particle with a well-defined momentum $\mathbf{p}$ can be treated as a wave with a wavevector $\mathbf{p}/\hbar \equiv \mathbf{k}$, and therefore a wavelength $2 \pi \hbar/|\mathbf{p}|$.   Higher momentum = shorter wavelength = the wavefunction has more closely spaced wiggles.  The kinetic energy goes like $p^{2}/2m$, as in classical nonrelativistic mechanics.   (Note that the magnitude-squared of such a wave is constant as a function of spatial position.  That is consistent with the uncertainty principle:  Knowing the momentum precisely means that the position could be anything.)

Take a particle and stick it in an environment where the local potential energy varies periodically in space - ideally in a system so large that we can neglect boundary effects for now.  The classic example of this is an electron in a crystalline solid.   I've talked about this kind of spatial periodicity before.  There are a couple of ways to think about this situation.  We have replaced "continuous translational symmetry" (the environment of the particle is unchanged if we consider moving the particle anywhere) with "discrete translational symmetry" (now we have to move an integer number of lattice spacings to get back to the same environment for the particle).  Mathematically, the single-particle stationary states can still be labeled by a parameter $\mathbf{k}$, but they're Bloch waves rather than plane waves, and the energy $E(\mathbf{k})$ is no longer necessarily proportional to $(\hbar k)^{2}$ all the time.   Physically, when the naive spatial periodicity of the single-particle state matches up with the spatial periodicity (or some harmonic) of the lattice, it makes sense that there should be deviations from what we'd see with a free particle.  The result is "band structure", ranges of energy densely filled with allowed single-particle states, separated by "band gaps", ranges of energy in which there is no way to make a single-particle state and still satisfy the Schroedinger equation with the spatially periodic potential energy.

The particular spatial periodicity of the lattice determines the form of $E(\mathbf{k})$.  For a hexagonal lattice like single-layer graphene, it turns out that there are two "Dirac points", and that near those special values of $\mathbf{k}$, the form of $E(k)$ looks like what is obeyed by photons in free space (!), with energy linearly dependent on $k$.

The key point here:  if we can tune the spatial periodicity of the potential arbitrarily, we can create interesting forms of $E(\mathbf{k})$.  That's really a neat idea.  Carefully growing stacked multilayers of semiconductors along one direction has been used to create "minibands" for optoelectronic devices.  Starting from a 2D surface state in copper, people have been able to put down patterns of CO molecules to create spatial periodicities in 2D, creating structures that look and act like graphene, or very recently even fractals.  People have also tried doing this by patterning semiconductor structures, but it's very hard to get sufficient uniformity so that disorder isn't a problem.

Stacking graphene layers with some relative twist angle is a great way to create a 2D modulation with excellent uniformity over large areas (many many lattice spacings).  This 2D modulation shows up because of the Moire pattern, which gives a spatially periodic coupling between the bands in each of the layers.  By tweaking the relative angle, the spatial pattern can be tuned.  By squishing on the bilayer, in principle the strength of the coupling can be tuned.  This kind of 2D modulation should be possible in principle in twisted bilayers of all kinds of stackable materials.

The situation is even more interesting once we start thinking about electron-electron interactions.

Another way to think of bands:  Start from the atomic energy levels of the individual, isolated constituent atoms.  The electronic levels of each atom are sharply defined.  All of the 4s orbitals, say, have the same energy.  If you think about possible electronic states, the "band" made out of isolated (localized to individual atoms) 4s orbitals is very narrow in energy.  If you built up some linear combination of those 4s orbitals that had a parameter $\mathbf{k}$, the energy $E(\mathbf{k})$ of that state would basically be independent of $\mathbf{k}$.  That is, the band would be "flat".   Turn on hopping between atoms, and band broadens out in energy.

If we throw in a bunch of electrons and ask what is the lowest energy state of the many-electron system, we can often get away with mostly neglecting electron-electron interactions.  Because of the Pauli Principle, we fill up the bands from the bottom up, and very often the (single-particle kinetic + lattice interactions) energy grows very rapidly, so much so that any electron-electron interactions are not very important.   (That's what happens in the periodic table - as you go to atoms containing more and more electrons, the kinetic energy grows fast enough that e-e interactions don't really disrupt the basic hydrogen-like s-p-d-f orbital structure of energy levels.)

In the twisted bilayers, it is possible to end up with some bands that are very flat - so flat that the typical electron-electron interaction energy is comparable or large compared to the bandwidth.  In these flat band situations, electron-electron interactions can end up being very important in determining the collective many-body state of the electrons.  That appears to be what people are seeing in the experiments mentioned previously.

The bottom line:  Twisted stacking is a great, robust way to create a lateral spatially modulated potential, and therefore (within particular geometric limits) a "designer" band structure.  The resulting bands can be very flat, so that electron-electron interaction effects (apparently) can lead to remarkable many-body responses, like the onset of superconductivity or magnetism.