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.


David A. Muller said...

Sounds like a good candidate for an NSF - PIRE proposal if you can find a collaborator at the University of Wakanda.

&y said...

I am so grateful that you wrote this. Finally seeing BP on video, and 30 mins into it, I'm googling "vibranium phonon" to see what my fellow nerds have already said.