Monday, April 24, 2017

Quantum conduction in bad metals, and jphys+

I've written previously about bad metals.  We recently published a result (also here) looking at what happens to conduction in an example of such a material at low temperatures, when quantum corrections to conduction (like these) should become increasingly important.   If you're interested, please take a look at a blog post I wrote about this that is appearing on jphys+, the very nice blogging and news/views site run by the Institute of Physics.

Sunday, April 23, 2017

Thoughts after the March for Science

About 10000 people turned out (according to the Houston Chronicle) for our local version of the March for Science.   Observations:

  • While there were some overtly partisan participants and signs, the overarching messages that came through were "We're all in this together!", "Science has made the world a better place, with much less disease and famine, a much higher standard of living for billions, and a greater understanding of the amazingness of the universe.", "Science does actually provide factual answers to properly formulated scientific questions", and "Facts are not opinions, and should feed into policy decisions, rather than policy positions altering what people claim are facts."
  • For a bunch of people often stereotyped as humorless, scientists had some pretty funny, creative signs.  A personal favorite:  "The last time scientists were silenced, Krypton exploded!"  One I saw online:  "I can't believe I have to march for facts."
  • Based on what I saw, it's hard for me to believe that this would have the negative backlash that some were worrying about before the event.  It simply wasn't done in a sufficiently controversial or antagonistic way.  Anyone who would have found the messages in the first point above to be offensive and polarizing likely already had negative perceptions of scientists, and (for good or ill) most of the population wasn't paying much attention anyway.
So what now?

  • Hopefully this will actually get more people who support the main messages above to engage, both with the larger community and with their political representatives.  
  • It would be great to see some more scientists and engineers actually run for office.  
  • It would also be great if more of the media would get on board with the concept that there really are facts.  Policy-making is complicated and must take into account many factors about which people can have legitimate disagreements, but that does not mean that every statement has two sides.  "Teach the controversy" is not a legitimate response to questions of testable fact.  In other words, Science is Real
  • Try to stay positive and keep the humor and creativity flowing.  We are never going to persuade a skeptical, very-busy-with-their-lives public if all we do is sound like doomsayers.

Thursday, April 20, 2017

Ready-to-peel semiconductors!

Every now and then there is an article that makes you sit up and say "Wow!"  

Epitaxy is the growth of crystalline material on top of a substrate with a matching (or very close to it) crystal structure.  For example, it is possible to grow InAs epitaxially on top of GaSb, or SiGe epitaxially on top of Si.  The idea is that the lattice of the underlying material guides the growth of the new layers of atoms, and if the lattice mismatch isn't too bad and the conditions are right, you can get extremely high quality growth (that is, with nearly perfect structure).  The ability to grow semiconductor films epitaxially has given us a ton of electronic devices that are everywhere around us, including light emitting diodes, diode lasers, photodiodes, high mobility transistors, etc.   Note that when you grow, say, AlGaAs epitaxially on a GaAs substrate, you end up with one big crystal, all covalently bonded.  You can't readily split off just the newly grown material mechanically.  If you did homoepitaxy, growing GaAs on GaAs, you likely would not even be able to figure out where the substrate ended and the overgrown film began.

In this paper (sorry about the Nature paywall - I couldn't find another source), a group from MIT has done something very interesting.  They have shown that a monolayer of graphene on top of a substrate does not screw up overgrowth of material that is epitaxially registered with the underlying substrate.  That is, if you have an atomically flat, clean GaAs substrate ("epiready"), and cover it with a single atomic layer of graphene, you can grow new GaAs on top of the graphene (!), and despite the intervening carbon atoms (with their own hexagonal lattice in the way), the overgrown GaAs will have registry (crystallographic alignment and orientation) with the underlying substrate.  Somehow the short-ranged potentials that guide the overgrowth are able to penetrate through the graphene.  Moreover, after you've done the overgrowth, you can actually peel off the epitaxial film (!!), since it's only weakly van der Waals bound to the graphene.  They demonstrate this with a variety of overgrown materials, including a III-V semiconductor stack that functions as a LED.  

I found this pretty amazing.  It suggests that there may be real opportunities for using layered van der Waals materials to grow new and unusual systems, perhaps helping with epitaxy even when lattice mismatch would otherwise be a problem.  I suspect the physics at work here (chemical interactions from the substrate "passing through" overlying graphene) is closely related to this work from several years ago.

Wednesday, April 19, 2017

March for Science, April 22

There has been a great deal written by many (e.g., 1 2 3 4 5 6) about the upcoming March for Science.  I'm going to the Houston satellite event.  I respect the concern that such a march risks casting scientists as "just another special interest group", or framing scientists as a group as leftists who are reflexively opposed to the present US administration.  Certainly some of the comments from the march's nominal twitter feed are (1) overtly political, despite claims that the event is not partisan; and (2) not just political, but rather extremely so.

On balance, though, I think that the stated core messages (science is not inherently partisan; science is critical for the future of the country and society; policy making about relevant issues should be informed by science) are important and should be heard by a large audience.   If the argument is that scientists should just stay quiet and keep their heads down, because silence is the responsible way to convey objectivity, I am not persuaded.  

Friday, April 14, 2017

"Barocalorics", or making a refrigerator from rubber

People have spent a lot of time and effort in trying to control the flow and transfer of heat.  Heat is energy transferred in a disorganized way among many little degrees of freedom, like the vibrations of atoms in a solid or the motion of molecules in a gas.  One over-simplified way of stating how heat likes to flow:  Energy tends to be distributed among as many degrees of freedom as possible.  The reason heat flows from hot things to cold things is that tendency.  Manipulating the flow of heat then really all comes down to manipulating ways for energy to be distributed.

Refrigerators are systems that, with the help of some externally supplied work, take heat from a "cold" side, and dump that heat (usually plus some additional heat) to a "hot" side.  For example, in your household refrigerator, heat goes from your food + the refrigerator inner walls (the cold side) into a working fluid, some relative of freon, which boils.  That freon vapor gets pumped through coils; a fan blows across those coils and (some of) the heat is transferred from the freon vapor to the air in your kitchen.   The now-cooler freon vapor is condensed and pumped (via a compressor) and sent back around again.  

There are other ways to cool things, though, than by running a cycle using a working fluid like freon. For example, I've written before about magnetic cooling.  There, instead of using the motion of liquid and gas molecules as the means to do cooling, heat is made to flow in the desired directions by manipulating the spins of either electrons or nuclei.  Basically, you can use a magnetic field to arrange those spins such that it is vastly more likely for thermal energy to come out of the jiggling motion of your material of interest, and instead end up going into rearranging those spins.

Stretching a polymer tends to heat it, due to the barocaloric
effect.  Adapted from Chauhan et al., doi:10.1557/mre.2015.17 
It turns out, you can do something rather similar using rubber.  The key is something called the elasto-caloric or barocaloric effect - see here (pdf!) for a really nice review.  The effect is shown in the figure, adapted from that paper.   An elastomer in its relaxed state is sitting there at some temperature and with some entropy - the entropy has contributions due to the jiggling around of the atoms, as well as the structural arrangement of the polymer chains.  There are lots of ways for the chains to be bunched up, so there is quite a bit of entropy associated with that arrangement.  Roughly speaking, when the rubber is stretched out quickly (so that there is no time for heat to flow in or out of the rubber) those chains straighten, and the structural piece of the entropy goes down.  To make up for that, the kinetic contribution to the entropy goes up, showing up as an elevated temperature.  Quickly stretch rubber and it gets warmer.  A similar thing happens when rubber is compressed instead of stretched.  So, you could imagine running a refrigeration cycle based on this!  Stretch a piece of rubber quickly; it gets warmer (\(T \rightarrow T + \Delta T\)).  Allow that heat to leave while in the stretched state (\(T + \Delta T \rightarrow T\)).  Now release the rubber quickly so no heat can flow.  The rubber will get colder now than the initial \(T\); energy will tend to rearrange itself out the kinetic motion of the atoms and into crumpling up the polymer chains.  The now-cold rubber can be used to cool something.  Repeat the cycle as desired.  It's a pretty neat idea.  Very recently, this preprint showed up on the arxiv, showing that a common silicone rubber, PDMS, is great for this sort of thing.  Imagine making a refrigerator out of the same stuff used for soft contact lenses!  These effects tend to have rather limited useful temperature ranges in most elastomers, but it's still funky.

Monday, April 10, 2017

Shrinkage - the physics of shrink rays

It's a trope that's appeared repeatedly in science fiction:  the shrink ray, a device that somehow takes ordinary matter and reduces it dramatically in physical size.  Famous examples include Fantastic Voyage, Honey I Shrunk the Kids, Innerspace, and Ant Man.  This particular post was inspired partly by my old friend Rob Kutner's comic series Shrinkage, where tiny nanotech-using aliens take over the mind of the (fictitious) President, with the aim of turning the world into a radioactive garden spot for themselves.  (Hey Rob - your critters thrive on radioactivity, yet if they're super small, they're probably really inefficient at capturing that radiation.  Whoops.)  Coincidentally, this week there was an announcement about a film option for Michael Crichton's last book, in which some exotic (that is to say, mumbo jumbo) "tensor field" is used to shrink people.

It's easy to enumerate many problematic issues that should arise in these kinds of stories:
  • Do the actual atoms of the objects/people shrink?  
  • If so, even apart from how that's supposed to work, what do these people breathe?  (At least Ant Man has a helmet that could be hand-waved to shrink air molecules....)  Or eat/drink?
  • What about biological scaling laws?
  • If shrunken objects keep their mass, that means a lot of these movies don't work.  Think about that tank that Hank Pym carries on his keychain....  If they don't keep their mass, where does that leave the huge amounts of energy (\(mc^2\)) that would have to be accounted for?
  • How can these people see if their eyes and all their cones/rods become much smaller than the wavelength of light?
  • The dynamics of interacting with a surrounding fluid medium (air or water) are completely different for very small objects - a subject explored at length by Purcell in "Life at Low Reynolds Number". 
The only attempt I've ever seen in science fiction to discuss some kind of real physics that would have to be at work in a shrink ray was in Isaac Asimov's novel Fantastic Voyage II.   One way to think about this is that the size of atoms is set by a competition between the electrostatic attraction between the electrons and the nucleus, and the puffiness forced by the uncertainty principle.  The typical size scale of an atom is given by the Bohr radius, \( a_{0} \equiv (4 \pi \epsilon_{0} \hbar^{2})/(m_{\mathrm{e}}e^{2}) \), where \(m_{\mathrm{e}} \) is the mass of the electron, and e is the electronic charge.   Shrinking actual atoms would require rejiggering some fundamental natural constants.  For example, you could imagine shrinking atoms by cranking up the electronic charge (and hence the attractive force between the electron and the nucleus).  That would have all kids of other consequences, however - such as screwing up chemistry in a big way.  

Of course, if we want to keep the appearances that we see in movies and TV, then somehow the colors of shrunken objects have to remain what they were at full size.   That would require the typical energy scale for optical transitions in atoms, for example, to remain unchanged.  That is, the Rydberg \( \equiv  m_{\mathrm{e}}e^4/(8 \epsilon_{0}^2 h^3 c) \) would have to stay constant.  Satisfying these constraints is very tough.  Asimov's book takes the idea that the shrink ray messes with Plank's constant, and I vaguely recall some discussion about altering c as well.  

While shrinking rays (and their complement) are great fun in story-telling, they're much more in the realm of science fantasy than true science fiction....