Wednesday, October 14, 2020

Room temperature superconductivity!

As many readers know, the quest for a practical room temperature superconductor has been going on basically ever since Kamerlingh Onnes discovered superconductivity over 100 years ago.  If one could have superconductivity with high critical currents and high critical fields in a material that could readily be made into wires, for example, it would be absolutely transformative to the world.  (Just one example:  we lose 5-10% of generated electricity just in transmission lines due to resistive heating.)  

One exotic possibility suggested over 50 years ago by Neil Ashcroft (of textbook fame in addition to his scientific prestige) was that highly compressed metallic hydrogen could be a room temperature superconductor.  The basic ingredients for traditional superconductivity would be a high electronic density of states, light atoms (and hence a high soundspeed for phonon-based pairing), and a strong electron-phonon coupling.  

In recent years, there have been striking advances in hydrogen-rich compounds with steadily increasing superconducting transition temperatures, including H2S (here and here) and LaH10 (here and here), all requiring very high (200+ GPa) pressures obtained in diamond anvil cells.  In those cool gadgets, tiny sample volumes are squeezed between the facets of cut gemstone-quality diamonds, and there is a great art in making electronic, optical, and magnetic measurements of samples under extreme pressures. 

Today, a new milestone has been reached and published.  Using these tools, the investigators (largely at Rochester) put some carbon, sulphur, and hydrogen containing compounds in the cell, zapped them with a laser to do some in situ chemistry, and measured superconductivity with a transition temperature up to 287.7 K (!) at a pressure of 267 GPa (!!).  The evidence for superconductivity is both a resistive transition to (as near as can be seen) zero resistance, and an onset of diamagnetism (as seen through ac susceptibility).  

This is exciting, and a milestone, though of course there are many questions:  What is the actual chemical compound at work here?  How does superconductivity work - is it conventional or more exotic? Is there any pathway to keeping these properties without enormous externally applied pressure?  At the very least, this shows experimentally what people have been saying for a long time, that there is no reason in principle why there couldn't be room temperature (or above) superconductivity.


dlb said...

"Is there any pathway to keeping these properties without enormous externally applied pressure?"

Isn't it an intrinsic requirement of this technology? It seems to me we would likely be more successful looking for superconductivity using different technology, than trying to find practical applications (outside labs) to these diamond anvils.

Anonymous said...

I think the argument is that there might be a metastable phase out there which can be quenched, like diamond. Or perhaps some sort of composite/inclusion compound?

Douglas Natelson said...

Anon, yes, that's the basic idea. One way to view this: It's now established that some configuration of atoms can give the right pairing strength and coherence properties to stabilize superconductivity at room temperature. Is there a way to achieve those parameters without squeezing the ever-loving heck out of something? Your metastability point is also right on target. Ashcroft's original work suggested that metallic H could be metastable at ambient pressure, though later Salpeter showed that the situation is bleak at room temperature.

Anonymous said...

Any thoughts on the comment by Hirsch and Marsiglio (arXiv:2010.10307). I personally hadn't realised that the transitions were anomalously sharp, and that this was an issue.

Douglas Natelson said...

Anon, I just saw that myself. That's a very interesting observation about the data, and it absolutely requires further scrutiny. In previous discussions about purported observations of zero resistance states in various granular things (e.g. here), the question of artifacts due to inhomogeneous current has been discussed. The diamagnetism would be a heck of a coincidence. I need to read more carefully, and it would be great to hear from the authors on this.