High Temperature Superconductivity Insights: The Dance Of Electrons
The long-mysterious phenomenon of high-temperature superconductivity just got less so with the discovery that these materials actively fight against superconduction right up until the critical temperature is reached.
Superconductivity was discovered in 1911 by Dutch physicist Heike Onnes. It is a fascinating phenomenon in which the resistance to the flow of electrical current drops to zero making it 100% efficient meaning there’s no wasteful and expensive heat loss. Conventional electronics produce lots of heat as the flowing electrons collide with the vibrating ions. These collisions rob the electrons of kinetic energy transforming it into the heat you feel as your laptop rests on your lap.
Conventional Superconductivity, on the other hand, occurs at near absolute zero temperatures, far below the temperature of your lap. At these temperatures, zero resistance (and therefore superconductivity itself) is thought to occur primarily because electrons join forces, forming what’s called Cooper Pairs. There is simply not enough energy in the cold, slowly vibrating matter to deflect these paired super electrons, so they flow without any distractions. In fact, this bizarre coupling of electrons is also responsible for the bizarre behavior of helium-3 causing it to enter a superfluid state of zero viscosity.
How can two electrons be bound together? Don’t two negative charges repel each other? Yes but in this case, it has been shown that the cooper pairs are formed through an electron-phonon interaction that can keep them relatively close. To best understand this, let’s consider a somewhat classical interpretation. A moving negative electron can attract the positive ions around it. This causes an unusually high concentration of positive charge to moves along with the electron which, at the same time, attracts another electron to be loosely coupled to the original one. Ultimately then, the interaction of the electron and this moving ripple of positive charge (a phonon) creates the Cooper Pairs.
As cool as it was (ahem), superconductivity was considered mostly a lab curiosity with important but minimal real-world applications since the crazy-low temperatures required (typically 1-30 Kelvin) were expensive and difficult to maintain. Still the lure of a superconducting world was great. Billions would be saved in electric power transmission since waste heat would essentially be eliminated. Electronics could be far smaller and use much less electricity. Imagine a coil of superconductor holding a current for millennia or far longer with no loss.
This future seemed tantalizingly close in 1986 when a new class of superconductors was found. HTS or High Temperature Superconductors shocked the scientific world and captured the public’s (and my) attention. These ceramic based materials were superconductive at 35 Kelvin which was beyond the 30K fundamental limit ascribed to conventional superconductivity and earned the discovers a Nobel Prize. This limit was then quickly blown away when other related compounds brought the maximum temperature to 92K. This was actually a significant advance well beyond being just a little bit less cold. This temperature allowed the use of liquid nitrogen as a refrigerant instead of the much more expensive liquid helium. At this time I often read that liquid nitro was as cheap as milk and could even be produced on site.
Since those heady days, temperatures have been brought up to a respectable 133-138 Kelvin and some progress has been made in understanding what’s going on during HTS. Ultimately though I can’t help but think “where the fuck’s my room temperature superconducting wire?”.
This agonizing delay has occurred ultimately because this is some complicated shit. No shit, right? One reason for this state of affairs is that we can’t use classical superconductivity as much of a guide since it appears that the electron-phonon interaction (critical for classic superconductivity) cannot explain the high temperature version. Condensed matter researchers can’t even agree what’s going on yet. One thing some of them have recently discovered though is that when a material starts to Superconduct at a high temperature, it enters a distinct phase or state called the Pseudogap that may be critical for not only understanding HTS but also for bringing it into room-temperature range (around 295 K).
To learn what’s going on, the scientists and engineers at Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory have been working on this for no less than 20 years. Most recently, they’ve been using a process called ARPES or angle-resolved photoemission spectroscopy. This technique knocks off electrons from a HTS material like copper oxide and then determines the energy and momentum of each one. By using ARPES over a wide range of temperatures and conditions and meticulously counting electrons, the team was able to determine that at the critical temperature just before resistance hits zero, the pseudogap was fighting against superconductivity. Thomas Devereaux, professor at Stanford and co-author of the study had this to say.
“The pseudogap tends to eat away the electrons that want to go into the superconducting state, The electrons are busy doing the dance of the pseudogap, and superconductivity is trying to cut in, but the electrons are not letting that happen. Then, as the material goes into the superconducting state, the pseudogap gives up and spits the electrons back out. That’s really the strongest evidence we have that this competition is occurring.”
I love the dance metaphor. Who said scientists can’t be poetic.
This concept of the pseudogap has been theorized for a while. The real breakthrough here is the discovery that it seems to work against the superconductive state. The other real-big win is that they can now replicate this intricate behavior in their calculations. That means they may be able to change certain variables and determine why the pseudogap prevents superconduction at higher temperatures. Perhaps they will determine that the bad pseudogap phase needs to happen in order for the good stuff to happen as well. On the other hand perhaps they can remove the pseudogap entirely and allow superconductivity to happen at room temperature.
I can almost feel that spool of room-temperature superconducting wire already.
Image Credits: Kazuhiro Fujita, Cornell University
SLAC National Accelerator Laboratory