Wednesday, March 14, 2018

Stephen Hawking, science communicator

An enormous amount has already been written by both journalists and scientists (here too) on the passing of Stephen Hawking.  Clearly he was an incredibly influential physicist with powerful scientific ideas.  Perhaps more important in the broad scheme of things, he was a gifted communicator who spread a fascination with science to an enormous audience, through his books and through the careful, clever use of his celebrity (as here, here, here, and here).   

While his illness clearly cost him dearly in many ways, I don't think it's too speculative to argue that it was a contributor to his success as a popularizer of science.  Not only was he a clear, expository writer with a gift for conveying a sense of the beauty of some deep ideas, but he was in some ways a larger-than-life heroic character - struck down physically in the prime of life, but able to pursue exotic, foundational ideas through the sheer force of his intellect.   Despite taking on some almost mythic qualities in the eyes of the public, he also conveyed that science is a human endeavor, pursued by complicated, interesting people (willing to do things like place bets on science, or even reconsider their preconceived ideas).

Hawking showed that both science and scientists can be inspiring to a broad audience.  It is rare that top scientists are able to do that, though a combination of their skill as communicators and their personalities.  In physics, besides Hawking the ones that best spring to mind are Feynman (anyone who can win a Nobel and also have their anecdotes described as the Adventures of a Curious Character is worth reading!) and Einstein.   

Sometimes there's a bias that gifted science communicators who care about public outreach are self-aggrandizing publicity hounds and not necessarily serious intellects (not that the two have to be mutually exclusive).  The outpouring of public sympathy on the occasion of Hawking's passing shows how deep an impact he had on so many.  Informing and inspiring people is a great legacy, and hopefully more scientists will be successful on that path thanks to Hawking.   

Wednesday, March 07, 2018

APS March Meeting, day 3 and summary thoughts

Besides the graphene bilayer excitement, a three other highlights from today:

David Cobden of the University of Washington gave a very nice talk about 2d topological insulator response of 1T'-WTe2.  Many of the main results are in this paper (arxiv link).    This system in the single-layer limit has very clear edge conduction while the bulk of the 2d layer is insulating, as determined by a variety of transport measurements.  There are also new eye-popping scanning microwave impedance microscopy results from Yongtao Cui's group at UC Riverside that show fascinating edge channels, indicating tears and cracks in the monolayer material that are otherwise hard to see. 

Steve Forrest of the University of Michigan gave a great presentation about "How Organic Light Emitting Diodes Revolutionized Displays (and maybe lighting)".  The first electroluminescent organic LED was reported about thirty years ago, and it had an external quantum efficiency of about 1%.  First, when an electron and a hole come together in the device, they only have a 1-in-4 chance of producing a singlet exciton, the kind that can readily decay radiatively.  Second, it isn't trivial to get light out of such a device because of total internal reflection.  Adding in the right kind of strong spin-orbit-coupling molecule, it is possible to convert those triplets to singlets and thus get nearly 100% internal quantum efficiency.  In real devices, there can be losses due to light trapped in waveguided modes, but you can create special substrates to couple that light into the far field.  Similarly, you can create modified substrates to avoid losses due to unintentional plasmon modes.  The net result is that you can have OLEDs with about 70% external quantum efficiencies.   OLED displays are a big deal - the global market was about $20B/yr in 2017, and will likely displace LCD displays.  OLED-based lighting is also on the way.  It's an amazing technology, and the industrial scale-up is very impressive.

Barry Stipe from Western Digital also gave a neat talk about the history and present state of the hard disk drive.  Despite the growth of flash memory, 90% of all storage in cloud data centers remains in magnetic hard disks, for capacity and speed.  The numbers are really remarkable.  If you scale all the parts of a hard drive up by a factor of a million, the disk platter would be 95 km in diameter, a bit would be about the size of your finger, and the read head would be flying above the surface at an altitude of 4 mm, and to get the same data rate as a drive, the head would have to be flying at 0.1 c.  I hadn't realized that they now hermetically seal the drives and fill them with He gas.  The He is an excellent thermal conductor for cooling, and because it has a density 1/7 that of air, the Reynolds number is lower for a given speed, meaning less turbulence, meaning they can squeeze additional, thinner platters into the drive housing.  Again, an amazing amount of science and physics, plus incredible engineering.

Some final thoughts (as I can't stay for the rest of the meeting):

  • In the old days, some physicists seemed to generate an intellectual impression by cultivating resemblance to Einstein.  Now, some physicists try to generate an intellectual impression by cultivating resemblance to Paul McEuen.
  • After many years of trying, the APS WiFi finally works properly and well!  
  • This was the largest March Meeting ever (~ 12000 attendees).  This is a genuine problem, as the meeting is growing by several percent per year, and this isn't sustainable, especially in terms of finding convention centers and hotels that can host.  There are serious discussions about what to do about this in the long term - don't be surprised if a survey is sent to some part of the APS membership about this.

Superconductivity in graphene bilayers - why is this exciting and important

As I mentioned here, the big story of this year's March Meeting is the report, in back-to-back Nature papers this week (arxiv pdf links in this sentence), of both Mott insulator and superconductivity in graphene bilayers.  I will post more here later today after seeing the actual talk on this (See below for some updates), but for now, let me give the FAQ-style report.  Skip to the end for the two big questions:
Moire pattern from twisted bilayer
graphene, image from NIST.

  • What's the deal with graphene?  Graphene is the name for a single sheet of graphite - basically an atomically thin hexagonal chickenwire lattice of carbon atoms.  See here and here.  Graphene is the most popular example of an enormous class of 2d materials.  The 2010 Nobel Prize in physics was awarded for the work that really opened up that whole body of materials for study by the physics community.  Graphene has some special electronic properties:  It can easily support either electrons or holes (effective positively charged "lack of electrons") for conduction (unlike a semiconductor, it has no energy gap, but it's a semimetal rather than a metal), and the relationship between kinetic energy and momentum of the charge carriers looks like what you see for massless relativistic things in free space (like light).
  • What is a bilayer?  Take two sheets of graphene and place one on top of the other.  Voila, you've made a bilayer.  The two layers talk to each other electronically.  In ordinary graphite, the layers are stacked in a certain way (Bernal stacking), and a Bernal bilayer acts like a semiconductor.  If you twist the two layers relative to each other, you end up with a Moire pattern (see image) so that along the plane, the electrons feel some sort of periodic potential.
  • What is gating?  It is possible to add or remove charge from the graphene layers by using an underlying or overlying electrode - this is the same mechanism behind the field effect transistors that underpin all of modern electronics.
  • What is actually being reported? If you have really clean graphene and twist the layers relative to each other just right ("magic angle"), the system becomes very insulating when you have just the right number of charge carriers in there.  If you add or remove charge away from that insulating regime, the system apparently becomes superconducting at a temperature below 1.7 K.
  • Why is the insulating behavior interesting?  It is believed that the insulating response in the special twisted case is because of electron-electron interactions - a Mott insulator.  Think about one of those toys with sliding tiles.  You can't park two tiles in the same location, so if there is no open location, the whole set of tiles locks in place.  Mott insulators usually involve atoms that contain d electrons, like NiO or the parent compounds of the high temperature copper oxide superconductors.  Mott response in an all carbon system would be realllllly interesting.  
  • Why is the superconductivity interesting?  Isn't 1.7 K too cold to be useful?  The idea of superconductivity-near-Mott has been widespread since the discovery of high-Tc in 1987.  If that's what's going on here, it means we have a new, highly tunable system to try to understand how this works.  High-Tc remains one of the great unsolved problems in (condensed matter) physics, and insights gained here have the potential to guide us toward greater understanding and maybe higher temperatures in those systems.  
  • Why is this important?  This is a new, tunable, controllable system to study physics that may be directly relevant to one of the great open problems in condensed matter physics.  This may be generalizable to the whole zoo of other 2d materials as well. 
  • Why should you care?  It has the potential to give us deep understanding of high temperature superconductivity.  That could be a big deal.  It's also just pretty neat.  Take a conductive sheet of graphene, and another conducting sheet of graphene, and if you stack them juuuuuust right, you get an insulator or a superconductor depending on how many charge carriers you stick in there.  Come on, that's just wild.
Update:  A few notes from seeing the actual talk.
  • Pablo painted a picture:  In the cuprates, the temperature (energy) scale is hundreds of Kelvin, and the size scale associated with the Mott insulating lattice is fractions of a nm (the spacing between Cu ions in the CuO2 planes).  In ultracold atom optical lattice attempts to look at Mott physics, the temperature scale is nK (and cooling is a real problem), while the spatial scale between sites is more like a micron.  In the twisted graphene bilayers, the temperature scale is a few K, and the spatial scale is about 13.4 nm (for the particular magic angle they use).
  • The way to think about what the twist does:  In real space, it creates a triangular lattice of roughly Bernal-stacked regions (the lighter parts of the Moire pattern above).  In reciprocal space, the Dirac cones at the K and K' points of the two lattices become separated by an amount given by \(k_{\theta} \approx K \theta\), where \(\theta\) is the twist angle, and we've used the small angle approximation.  When you do that and turn on interlayer coupling, you hybridize the bands from the upper and lower layers.  This splits off the parts of the bands that are close in energy to the dirac point, and at the magic angles those bands can be very very flat (like bandwidths of ~ 10 meV, as opposed to multiple eV of the full untwisted bands).  Flat bands = tendency to localize.   The Mott phase then happens if you park exactly one carrier (one hole, for the superconducting states in the paper) per Bernal-patch-site.  
  • Most persuasive reasons they think it's really a Mott insulating state and not something else, besides the fact that it happens right at half-filling of the twist-created triangular lattice:  Changing the angle by a fraction of a degree gets rid of the insulating state, and applying a magnetic field (in plane or perpendicular) makes the system become metallic, which is the opposite of what tends to happen in other insulating situations.  (Generally magnetic fields tend to favor localization.)
  • They see spectroscopic evidence that the important number of effective carriers is determined not by the total density, but by how far away they gate the system from half-filling.
  • At the Mott/superconducting border, they see what looks like Josephson-junction response, as if the system breaks up into superconducting regions separated by weak links.  
  • The ratio of superconducting Tc to the Fermi temperature is about 0.5, which makes this about as strongly coupled (and therefore likely to be some weird unconventional superconductor) as you ever see.
  • Pablo makes the point that this could be very general - for any combo of van der Waals layered materials, there are likely to be magic angles.  Increasing the interlayer coupling increases the magic angle, and could then increase the transition temperature.
Comments by me:
  • This is very exciting, and has great potential.  Really nice work.
  • I wonder what would happen if they used graphite as a gate material rather than a metal layer, given what I wrote here.   It should knock the disorder effects down a lot, and given how flat the bands are, that could really improve things.
  • There are still plenty of unanswered questions.  Why does the superconducting state seem more robust on the hole side of charge neutrality as well as on the hole side of half-filling?  This system is effectively a triangular lattice - that's a very different beast than the square lattice of the cuprates or the pnictides.  That has to matter somehow.  Twisting other 2d materials (square lattice MXenes?) could be very interesting.
  • I predict there will be dozens of theory papers in the next two months trying to predict magic twist angles for a whole zoo of systems.

APS March Meeting 2018, day 2

Day 2 of the meeting was even more scattered than usual for me, because several of my students were giving talks, all in different sessions spread around.  That meant I didn't have a chance to stay too long on any one topic.   A few highlights:

Jeff Urban from LBL gave an interesting talk about different aspects of the connection between electronic transport and thermal transport.  The Wiedemann-Franz relationship is a remarkably general expression based on a simple idea - when charge carriers move, they transport some (thermal) energy as well as charge, so thermal conductivity and electrical conductivity should be proportional to each other.  There are a bunch of assumptions that go into the serious derivation, though, and you could imagine scenarios when you'd expect large deviations from W-F response, particularly if scattering rates of carriers have some complicated energy dependence.  Urban spoke about hybrid materials (e.g., mixtures of inorganic components and conducting polymers).  He then pointed out a paper I'd somehow missed last year about apparent W-F violation in the metallic state of vanadium dioxide.  VO2 is a "bad metal", with an anomalously low electrical conductivity.  Makes me wonder how W-F fairs in other badly metallic systems.

Ali Hussain of the Abbamonte group at Illinois gave a nice talk about (charge) density fluctuations in the strange metal phase (and through the superconducting transition) of the copper oxide superconductor BSSCO.  The paper is here.  They use a particular technique (momentum-resolved electron energy loss spectroscopy) and find that it is peculiarly easy to create particle-hole excitations over a certain low energy range in the material, almost regardless of the momentum of those excitations.  There are also systematics with how this works as a function of doping (carrier concentration in the material), with optimally doped material having particularly temperature-independent response. 

Albert Fert spoke about spin-Hall physics, and the conversion of spin currents in to charge currents and vice versa.  One approach is the inverse Edelstein effect (IEE).  You have a stack of materials, where a ferromagnetic layer is on the top.  Driving ferromagnetic layer into FMR, you can pump a spin current vertically downward (say) into the stack.  Then, because of Rashba spin-orbit coupling, that vertical spin current can drive a lateral charge current (leading to the buildup of a lateral voltage) in a two-dimensional electron gas living at an interface in the stack.  One can use the interface between Bi and Ag (see here).  One can get better results if there is some insulating spacer to keep free conduction electrons not at the interface from interfering, as in LAO/STO structures.  Neat stuff, and it helped clarify for me the differences between the inverse spin Hall effect (3d charge current from 3d spin current) and the IEE (2d charge current from 3d spin current). 

Alexander Govorov of Ohio also gave a nice presentation about the generation of "hot" electrons from excitation of plasmons.  Non-thermally distributed electrons and holes can be extremely useful for a variety of processes (energy harvesting, photocatalysis, etc.). At issue is, what does the electronic distribution really look like.  Relevant papers are here and here.  There was a nice short talk similar in spirit by Yonatan Dubi earlier in the day.

Monday, March 05, 2018

APS March Meeting 2018, day 1

As I explained yesterday, my trip to the APS is even more scattered than in past years, but I'll try to give some key points.  Because of meetings and discussions with some collaborators and old friends, I didn't really sit down and watch entire sessions, but I definitely saw and heard some interesting things.

Markus Raschke of Colorado gave a nice talk about the kinds of ultrafast and nonlinear spectroscopy you can do if you use a very sharp gold tip as a plasmonic waveguide.  The tip has a grating milled onto it a few microns away from the sharp end, so that hitting the grating with a pulsed IR laser excites a propagating surface plasmon mode that is guided down to the really sharp point.  One way to think about this:  When you use the plasmon mode to confine light down to a length scale \(\ell\) comparable to the radius of curvature of the sharp tip, then you effectively probe a wavevector \(k_{\mathrm{eff}} \sim \2\pi/\ell\).  If \(\ell\) is a couple of nm, then you're dealing with \(k_{\mathrm{eff}}\) values associated in free space with x-rays (!).  This lets you do some pretty wild optical spectroscopies.  Because the waveguiding is actually effective over a pretty broad frequency range, that means that you can get very short pulses down there, and the intense electric field can lead to electron emission, generating the shortest electron pulses in the world.  

Andrea Young of UCSB gave a very pretty talk about looking at even-denominator fractional quantum Hall physics in extremely high quality bilayer graphene.  Using ordinary metal electrodes apparently limits how nice the effects can be in the bilayer, because the metal is polycrystalline and that disorder in local work function can actually matter.   By using graphite as both the bottom gate and the top gate (that is, a vertical stack of graphite/boron nitride/bilayer graphene/boron nitride/graphite), it is possible to tune both the filling fraction (ratio of carrier density to magnetic field) in the bilayer and the vertical electric field across the bilayer (which can polarize the states to sit more in one layer or the other).  Capacitance measurements (e.g., between the top gate and the bottom gate, or between either gate and the bilayer) can show extremely clean quantum hall data.

Sankar Das Sarma of Maryland spoke about the current status of trying to use Majorana fermions in semiconductor wire/superconductor electrode structures for topological quantum computing.  For a review of the topic overall, see here.   This is the approach to quantum computing that Microsoft is backing.  The talk was vintage Das Sarma, which is to say, full of amusing quotes, like "Physicists' record at predicting technological breakthroughs is dismal!" and "Just because something is obvious doesn't mean that you should not take it seriously."  The short version:  There has been great progress in the last 8 years, from the initial report of possible signatures of effective Majorana fermions in individual InSb nanowires contacted by NbTiN superconductors, to very clean looking data involving InAs nanowires with single-crystal, epitaxial Al contacts.  However, it remains very challenging to prove definitively that one has Majoranas rather than nearly-look-alike Andreev bound states.

In case you are interested in advanced (beyond-first-year) undergraduate labs and how to do them well, you should check out the University of Minnesota's site, as well as the ALPhA group from the AAPT.   There is also an analogous group working on projects to integrate computation into the undergraduate physics curriculum.

One potentially very big physics news story that I heard about during the day, but won't be here to see the relevant talk: [Update:  Hat tip to a colleague who pointed out that there is a talk tomorrow morning that will cover this!]  There are back-to-back brand new papers in Nature today by Yuan Cao et al. from the Jarillo-Herrero group at MIT.  (The URLs don't work yet for the articles, but I'll paste in what Nature has anyway.)  The first paper apparently shows that when you take two graphene layers and rotationally offset them from graphite-like stacking by 1.05 degrees (!), the resulting bilayer is alleged to be a Mott insulator.  The idea appears to be that the lateral Moire superlattice that results from the rotational offset gives you very flat minibands, so that electron-electron interactions are enough to lock the carriers into place when the number density of carriers is tuned correctly.  The second paper apparently (since I can't read it yet) shows that as the carrier density is tuned away from the Mott insulator filling, the system becomes a superconductor (!!), with a critical temperature of 1.7 K.  This isn't particularly high, but the idea of tuning carrier density away from a Mott state and getting superconductivity is basically the heart of our (incomplete) understanding of the copper oxide high temperature superconductors.  This is very exciting, as summarized in this News and Views commentary and this news report.  

Sunday, March 04, 2018

APS March Meeting 2018

It's that time of year again:  The running of the physicists annual APS March Meeting, a gathering of thousands of (mostly condensed matter) physicists.  These are (sarcasm mode on) famously rowdy conferences (/sarcasm).  This year the meeting is in Los Angeles.  I came to the 1998 March Meeting in LA, having just accepted a fall '98 postdoctoral fellow position at Bell Labs, and shortly after the LA convention center had been renovated.   At the time, the area around the convention center was really a bit of a pit - very few restaurants, few close hotels, and quite a bit of vacant and/or low-end commercial property.  Fast forward 20 years, and now the area around the meeting looks a lot more like a sanitized Times Square, with big video advertisements and tons of high end flashy stores.

Anyway, I will try again to write up some of what I see until I have to leave on Thursday morning, though this year between DCMP business, department chair constraints, and other deadlines, I might be more concise or abbreviated.  (As I wrote last year, if you're at the meeting and you don't already have a copy, now is the perfect time to swing by the Cambridge University Press exhibit at the trade show and pick up my book :-) ).

Thursday, February 22, 2018

Vibranium and its properties

Fictional materials can be a fun starting point for thinking about and maybe teaching about material properties.  Back in 2015 I touched on this here, when I mentioned a few of my favorite science fictional materials (more here, here, and here).  

With the release of Black Panther (BP), we now have much more information about the apparent properties of vibranium in the Marvel Cinematic Universe.   

Vibranium is pretty amazing stuff - like many fictional materials, it sometimes seems to have whatever properties are necessary to the story.  As a physicist I'm not qualified to talk about its putative medicinal properties mentioned in BP, but its physical properties are fun to consider.  Vibranium appears to be a strong, light, silvery metal (see here), and it also has some remarkable abilities in terms of taking macroscopic kinetic energy (e.g., of a projectile) and either dissipating it (look at the spent bullets in the previously linked video) or, according to BP, storing that energy for later release.  At the same time, Captain America's vibranium shield is able to bounce around with incredibly little dissipation of energy, prompting the Spider-Man quote at right.

In the spirit of handwaving physics, I think I've got this figured out.  

In all solids, there is some coupling between the deformation of the atomic lattice and the electronic states of the material (here is a nice set of slides about this).  When we talk about lattice vibrations, this is the electron-phonon coupling, and it is responsible for the transfer of energy from the electrons to the lattice (that is, this is why the actual lattice of atoms in a wire gets warm when you drive electrical current through the material).  The e-ph coupling is also responsible for the interaction that pairs up electrons in conventional superconductors.  If the electron-phonon coupling is really strong, the deformation of the lattice can basically trap the electron - this is polaron physics.  In some insulating materials, where charge is distributed asymmetrically within the unit cell of the crystal, deformation of the material can lead to big displacements of charge, with a corresponding buildup of a voltage across the system - this is piezoelectricity.  

The ability of vibranium to absorb kinetic energy, store it, and then later discharge it with a flash, suggests to me that lattice deformation ends up pumping energy into the electrons somehow.  Moreover, that electronically excited state must somehow be metastable for tens of seconds.  Ordinary electronic excitations in metals are very short-lived (e.g., tens of femtoseconds for individual excited quasiparticles to lose their energy to other electrons).  Gapped-off collective electronic states (like the superconducting condensate) can last very long times.  We have no evidence that vibranium is superconducting (though there are some interesting maglev trains in Wakanda).  That makes me think that what's really going in involves some topologically protected electronic states.  Clearly we need to run experiments (such as scanning SQUID, scanning NV center, or microwave impedance microscopy) to search for the presence of edge currents in percussively excited vibranium films to test this idea.

Thursday, February 15, 2018

Physics in the kitchen: Jamming

Last weekend while making dinner, I came across a great example of emergent physics.  What you see here are a few hundred grams of vacuum-packed arborio rice:
The rice consists of a few thousand oblong grains whose only important interactions here are a mutual "hard core" repulsion.  A chemist would say they are "sterically hindered".  An average person would say that the grains can't overlap.  The vacuum packing means that the whole ensemble of grains is being squeezed by the pressure of the surrounding air, a pressure of around 101,000 N/m2 or 14.7 pounds per in2.  The result is readily seen in the right hand image:  The ensemble of rice forms a mechanically rigid rectangular block.  Take my word for it, it was hard as a rock. 

However, as soon as I cut a little hole in the plastic packaging and thus removed the external pressure on the rice, the ensemble of rice grains lost all of its rigidity and integrity, and was soft and deformable as a beanbag, as shown here. 

So, what is going on here?  How come this collection of little hard objects acts as a single mechanically integral block when squeezed under pressure?  How much pressure does it take to get this kind of emergent rigidity?  Does that pressure depend on the size and shape of the grains, and whether they are deformable? 

This onset of collective resistance to deformation is called jamming.  This situation is entirely classical, and yet the physics is very rich.  This problem is clearly one of classical statistical physics, since it is only well defined in the aggregate and quantum mechanics is unimportant.  At the same time, it's very challenging, because systems like this are inherently not in thermal equilibrium.  When jammed, the particles are mechanically hindered and therefore can't explore lots of possible configurations.   It is possible to map out a kind of phase diagram of how rigid or jammed a system is, as a function of free volume, mechanical load from the outside, and temperature (or average kinetic energy of the particles).   For good discussions of this, try here (pdf), or more technically here and here.   Control over jamming can be very useful, as in this kind of gripping manipulator (see here for video).