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).  

Tuesday, February 13, 2018

Rice Cleanroom position

In case someone out there is interested, Rice is hiring a cleanroom research scientist.  The official job listing is here.  To be clear:  This is not a soft money position.

The Cleanroom Facility at Rice University is a shared equipment facility for enabling micro- and nanofabrication research in the Houston metropolitan area. Current equipment includes deposition, lithography, etching and a number of characterization tools. This facility attracts users from the George R. Brown School of Engineering and the Wiess School of Natural Science and regional universities and corporations whose research programs require advanced fabrication and patterning at the micro- and nanoscale. A new state of the art facility is currently being constructed and is expected to be in operation in summer 2018. Additionally, with new initiatives in Molecular Nanotechnology, the Rice University cleanroom is poised to see significant growth in the next 5-10 years. This job announcement seeks a motivated individual who can lead, manage, teach and grow this advanced facility.

The job responsibilities of a Cleanroom Research Scientist include conducting periodic and scheduled maintenance and safety check of equipment and running qualification and calibration recipes. The incumbent will be expected to maintain the highest safety standards, author and update standard operation procedures (SOPs), maintain and calibrate processes for all equipment. The Cleanroom Research Scientist will help facilitate new equipment installation, contact vendors and manufacturers and work in tandem with them to resolve equipment issues in a timely and safe manner. Efficient inventory management of parts, chemicals and supplies will be required. The Cleanroom Scientist will also oversee personal one-to-one training of users. Additionally, the incumbent will help develop cleanroom laboratory short courses that provide lectures to small groups of students. The incumbent will also coordinate with technical staff members in Rice SEA (Shared Equipment Authority).

Saturday, February 10, 2018

This week in the arxiv

Back when my blogging was young, I had a semi-regular posting of papers that caught my eye that week on the condensed matter part of the arxiv.  As I got busy doing many things, I'd let that fall by the wayside, but I'm going to try to restart it at some rate.  I generally haven't had the time to read these in any detail, and my comments should not be taken too seriously, but these jumped out at me.

arxiv:1802.01045 - Sangwan and Hersam; Electronic transport in two-dimensional materials
If you've been paying any attention to condensed matter and materials physics in the last 14 years, you've noticed a huge amount of work on genuinely two-dimensional materials, often exfoliated from the bulk as in the scotch tape method, or grown by chemical vapor deposition.  This looks like a nice review of many of the relevant issues, and contains lots of references for interested students to chase if they want to learn more.

arxiv:1802.01385 - Froelich; Chiral Anomaly, Topological Field Theory, and Novel States of Matter
While quite mathematical (relativistic field theory always has a certain intimidating quality, at least to me), this also looks like a reasonably pedagogical introduction of topological aspects of condensed matter.  This is not for the general reader, but I'm hopeful that if I put in the time and read it carefully, I will gain a better understanding of some of the topological discussions I hear these days about things like axion insulators and chiral anomalies.

arXiv:1802.01339 - Ugeda et al.; Observation of Topologically Protected States at Crystalline Phase Boundaries in Single-layer WSe2
arXiv:1802.02999 - Huang et al.; Emergence of Topologically Protected Helical States in Minimally Twisted Bilayer Graphene
arXiv:1802.02585 - Schindler et al.; Higher-Order Topology in Bismuth
Remember back when people didn't think about topology in the band structure of materials?  Seems like a million years ago, now that a whole lot of systems (often 2d materials or interfaces between materials) seem to show evidence of topologically special edge states.   These are three examples just this week of new measurements (all using scanning tunneling microscopy as part of the tool-set, to image edge states directly) reporting previously unobserved topological states at edges or surface features.

Sunday, February 04, 2018

New readers: What is condensed matter physics? What is special about the nanoscale?

If you're a new reader, perhaps brought here by the mention of this blog in the Washington Post, welcome!   Great to have you here.  Just a couple of quick FAQs to get you oriented:

What is condensed matter physics?  Condensed matter (once known as "solid state) is a branch of physics that deals with the properties of matter consisting of large numbers of particles (usually atoms or (electrons+the rest of the atoms)) in "condensed" states like liquids and solids - basically the materials that make up an awful lot of the stuff you interact with all the time.  New properties can emerge when you bring lots of particles together.  See here for an example involving plastic balls, or here (pdf) for a famous essay about this general point.  Condensed matter physicists are often interested in identifying the different types of states or phases that can arise, and understanding transitions between those states (like how does water boil, or how does magnetism turn on in iron as its temperature is lowered from the melting point, or how does a ceramic copper oxide suddenly start letting electricity flow without resistance below some particular temperature).  Hard condensed matter typically deals with systems where quantum mechanics is directly important (electronic, magnetic, and optical properties of materials, for example), while soft condensed matter describes systems where the main actors (while quantum deep down like all matter) are not acting in a quantum way - examples include the jamming of grains of sand when you build a sand castle, or the spontaneous alignment of rod-like molecules in the liquid crystal display you're using to read this.

While particle physics tries to look at the tiniest bits of stuff, condensed matter hits on some of the same (literally the same concepts and math) deep ideas about symmetry, and often has direct implications for technologies that affect your daily life.   Understanding this stuff has given us things like the entire electronics industry, the telecommunications industry, and soon probably quantum computers.  

A powerful concept in physics in general and condensed matter in particular is universality.  For example, materials built out of all kinds of different ingredients can be mechanically rigid solids; there is something universal about mechanical rigidity that makes it emerge independent of the microscopic details.  Another example:  Lots of very different systems (metallic lead; waxy crystals of buckyball molecules with some alkaline metal atoms in between; ceramic copper oxides; hydrogen sulfide gas under enormous pressure) can conduct electricity without resistance at low temperatures - why and how is superconductivity an emergent property?

What is special about the nanoscale?  Because it's about collective properties, traditional condensed matter physics often uses a lot of nice approximations to describe systems, like assuming they're infinite in extent, or at least larger than lots of physically important scales.   When you get down to the nanoscale (recall that a typical atom is something like 0.3 nanometers in diameter), a lot of the typical approximations can fail.  As the size of the material or system becomes small compared to the length scales associated with various physical processes, new things can happen and the properties of materials can change dramatically.  Tightly confined liquids can act like solids.  Colorless materials can look brilliantly chromatic when structured on small scales.  Two electrical insulators brought together can produce a nanoscale-thick metallic layer.   We now have different techniques for structuring materials on the nanoscale and for seeing what we're doing down there, where the building blocks are often far smaller than the wavelengths of light.  Investigations at the nanoscale are tied to some of the most active topics in condensed matter, and verge into the interdisciplinary boundaries with chemistry, biology, materials science, electrical engineering, and chemical engineering.   That, and it's fun.

Please browse around through the archives, and I hope you find it interesting.

Friday, February 02, 2018

Why science blogging still matters

Nature has a piece up about science blogging.  It's pretty much on target.  I'm a bit surprised that there wasn't more discussion of blogging vs. twitter vs. other social media platforms, or the interactions between blogs and formal journalism.