Tuesday, February 21, 2017

Metallic hydrogen?

There has been a flurry of news lately about the possibility of achieving metallic hydrogen in the lab.  The quest for metallic hydrogen is a fun story with interesting characters and gadgets - it would be a great topic for an episode of Nova or Scientific American Frontiers.   In brief faq form (because real life is very demanding right now):

Why would this be a big deal?  Apart from the fact that it's been sought for a long time, there are predictions that metallic hydrogen could be a room temperature superconductor (!) and possibly even metastable once the pressure needed to get there is removed.

Isn't hydrogen a gas, and therefore an insulator?  Sure, at ambient conditions.  However, there is very good reason to believe that if you took hydrogen and cranked up the density sufficiently (by squeezing it), it would actually become a metal.

What do you mean by a metal?  Do you mean a ductile, electrically conductive solid?  Yes on the electrically conductive part, at least.  From the chemistry/materials perspective, a metal often described a system where the electrons are delocalized - shared between many many ions/nuclei.  From the physics perspective (see here), a metal is a system where the electrons have "gapless excitations" - it's possible to create excitations of the electrons (moving an electron from a filled state to an empty state of different energy and momentum) down to arbitrarily low energies.  That's why the electrons in a metal can respond to an applied voltage by flowing as a current.

What is the evidence that hydrogen can become a metal at high densities?  Apart from recent experiments and strong theoretical arguments, the observation that Jupiter (for example) has a whopping magnetic field is very suggestive.

How do you get from a diatomic, insulating gas to a metal?  You squeeze.  While it was originally hoped that you would only need around 250000 atmospheres of pressure to get there, it now seems like around 5 million atmospheres is more likely.  As the atoms are forced to be close together, it is easier for electrons to hop between the atoms (for experts, a larger tight-binding hopping matrix element and broader bands), and because of the Pauli principle the electrons are squeezed to higher and higher kinetic energies.  Both trends push toward metal formation.

Yeah, but how do you squeeze that hard?  Well, you could use a light gas gun to ram a piston into a cylinder full of liquid hydrogen like these folks back when I was in grad school.  You could use a whopping pulsed magnetic field like a z-pinch to compress a cylinder filled with hydrogen, as suggested here (pdf) and reported here.  Or, you could put hydrogen in a small, gasketed volume between two diamond facets, and very carefully turn a screw that squeezes the diamonds together.  That's the approach taken by Dias and Silvera, which prompted the recent kerfuffle.  

How can you tell it's become a metal?  Ideally you'd like to measure the electrical conductivity by, say, applying a voltage and measuring the resulting current, but it can be very difficult to get wires into any of these approaches for such measurements.  Instead, a common approach is to use optical techniques, which can be very fast.  You know from looking at a (silvered or aluminized) mirror that metals are highly reflective.  The ability of electrons in a metal to flow in response to an electric field is responsible for this, and the reflectivity can be analyzed to understand the conductivity.

So, did they do it?  Maybe.  The recent result by Dias and Silvera has generated controversy - see here for example.   Reproducing the result would be a big step forward.  Stay tuned.

1 comment:

Ross H. McKenzie said...

The controversy is one more data point showing how luxury journals such as Science and Nature are problematic for the advance of science.