Monday, August 24, 2015

News items: Feynman, superconductors, faculty shuffle

A few brief news items - our first week of classes this term is a busy time.

  • Here is a video of Richard Feynman, explaining why he can't readily explain permanent magnets to the interviewer.   This gets right to the heart of why explaining science in a popular, accessible way can be very difficult.  Sure, he could come up with really stretched and tortured analogies, but truly getting at the deeper science behind the permanent magnets and their interactions would require laying a ton of groundwork, way more than what an average person would want to hear.
  • Here is a freely available news article from Nature about superconductivity in H2S at very high pressures.   I was going to write at some length about this but haven't found the time.  The short version:  There have been predictions for a long time that hydrogen, at very high pressures like in the interior of Jupiter, should be metallic and possibly a relatively high temperature superconductor.  There are later predictions that hydrogen-rich alloys and compounds could also superconduct at pretty high temperatures.  Now it seems that hydrogen sulfide does just this.  Crank up the pressure to 1.5 million atmospheres, and that stinky gas becomes what seems to be a relatively conventional (!) superconductor, with a transition temperature close to 200 K.  The temperature is comparatively high because of a combination of an effectively high speed of sound (the material gets pretty stiff at those pressures), a large density of electrons available to participate, and a strong coupling between the electrons and those vibrations (so that the vibrations can provide an effective attractive interaction between the electrons that leads to pairing).    The important thing about this work is that it shows that there is no obvious reason why superconductivity at or near room temperature should be ruled out.
  • Congratulations to Prof. Laura Greene, incoming APS president, who has been named the new chief scientist of the National High Magnetic Field Lab.  
  • Likewise, congratulations to Prof. Meigan Aronson, who has been named Texas A&M University's new Dean of Science.  


Anonymous said...

Layperson here, is the following wikipedia explanation that references [5]

A. Leggett (2006). "What DO we know about high Tc?". Nature Physics

still accurate in 2015?

"The question of how superconductivity arises in high-temperature superconductors is one of the major unsolved problems of theoretical condensed matter physics. The mechanism that causes the electrons in these crystals to form pairs is not known.[5] Despite intensive research and many promising leads, an explanation has so far eluded scientists. One reason for this is that the materials in question are generally very complex, multi-layered crystals (for example, BSCCO), making theoretical modelling difficult."

Anonymous said...

The H2S story is about conventional superconductivity, where the pairing mechanism is well known.

The pairing in other "unconventional" superconductors, such as the complex, multilayered crystals mentioned, are not yet (completely) understood.

What may need redefining is "high temperature superconductors", which used to be a synonym for unconventional superconductors - because it was thought (though not by all) that the pairing in conventional superconductors would not be able to induce superconductivity at "high temperatures".
Now there is a conventional high temperature superconductor in H2S.

Douglas Natelson said...

Anon@7:20, thanks. I should have been more clear in my brief description.

I'm going to write more about this soon if I have the time. The bottom line is that superconductivity turns out to be one of a very small number of ground states for many-electrons-in-a-solid. Because superconductivity is in some ways so generic, it can be tough to figure out the underlying interactions that drive it. In conventional superconductors, pairing of the electrons is driven by the electron-phonon interaction. In "unconventional" superconductors, other interactions are thought to be important (often spin fluctuations are put forward). The "unconventional" = "high Tc" equivalency is not necessarily true.