Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.
When we study superconductors, thus far they are all of a cold variety. Very cold. We’re talking about cold enough to make gases into liquids. This is a deep issue because generating these cooled materials is not easy and limits the applications of the superconductor. We want to be able to have mobility and scale with any new technology, and the current superconductors don’t allow for that. Advancements in making warmer superconductors have been slow. In 1986, Georg Bednorz and K. Alex Muller found superconductors that work at over 100 degrees Celsius below room temperature, but that is still way too cold for our purposes. What we want are high-temperature superconductors, but they present their own unique challenges (Wolchover “Breakthrough”).
Most high-temperature superconductors are cuprates, a “brittle ceramic” that has alternating layers of copper and oxygen with some material between them. For the record, the electron structures in oxygen and copper repel each other. Heavily. Their structures do not line up well. However, once cooled to a certain temperature those electrons suddenly stop fighting each other and start to pair together and act like a boson, facilitating the right conditions to conduct electricity easily. Pressure waves encourage the electrons to follow a path that facilitates a parade of them, if you will. So long as it stays cool, a current going through it will go on forever (Ibid).
But for cuprates, this behavior can go on up to -113o Celsius which should be well-beyond the scope of the pressure waves. Some force(s) besides the pressure waves must be encouraging the superconducting properties. In 2002, scientists from the University of California at Berkley found that “charge density waves” were riding through the superconductor as they examined the currents riding through the cuprate. Having them decreases superconductivity, because they cause a de-coherence that inhibits that electron flow. The charge density waves are prone to magnetic fields, so scientists reasoned that given the right magnetic fields the superconductivity could possible increase by lowering those waves. But why were the waves forming in the first place? (Ibid)
The answer is surprisingly complex, involving the geometry of the cuprate. One can view the structure of a cuprate as a copper atom with oxygen atoms surrounding it on the + y axis and the + x axis. The electron charges are not distributed evenly in these groupings but can be clustered at + y axis and sometimes at the + x axis. As an overall structure goes, this causes different densities (with places that lack electrons known as holes) and forms a “d-wave” pattern that results in the charge density waves scientists were seeing (Ibid).
A similar d-wave pattern arises from a quantum property called antiferromagnetism. This involves spin orientation of the electrons going in a vertical orientation but never in a diagonal one. Pairings ensue because of the complementary spins, and as it turns out the antiferromagnetic d-waves can be correlated to the charge d-waves. It’s already known to help encourage the superconductivity we see, so this antiferromagnetism is tied to both promoting superconductivity and inhibiting it (Ibid).
Physics is just so freakin’ amazing.
But high temperature superconductors are also differentiated from their colder counterparts by the level of quantum entanglement they experience. It’s very high in the hotter ones, making discerning properties challenging. It’s so extreme that it has been labeled as a quantum phase change, a somewhat similar idea to matter phase changes. Quantumly, some phases include metals and insulators. And now, high temperature superconductors are distinguished enough from the other phases to warrant their own label. Fully understanding the entanglement behind the phase is challenging because of the number of electrons in the system – trillions. But a place that might help with that is the boundary point where the temperature gets too high for the superconductive properties to take place. This boundary point, the quantum critical point, forms a strange metal, a poorly understood material itself because it fails many quasiparticle models used to explain the other phases. For Subir Sachdev, he looked at the state of strange metals and found a connection to string theory, that amazing but low-result physics theory. He used its description of string-fed quantum entanglement with particles, and the number of connections in it is limitless. It offers a framework to describe the entanglement problem and thus help define the boundary point of the strange metal (Harnett).
Finding the Quantum Critical Point
This concept of a region where quantumly some phase change occurs inspired Nicolas Doiron-Leyraud, Louis Taillefer, and Sven Badoux (all at University of Cherbrooke in Canada) to investigate where this would be with the cuprates. In their cuprate phase diagram, “pure, unaltered cuprate crystals” are placed on the left side and have insulating properties. The cuprates that have different electron structures on the right, acting like metals. Most diagrams have temperature in Kelvin plotted against the hole configuration of electrons in the cuprate. As it turns out, features of algebra come into play when we want to interpret the graph. It is clear that a linear, negative line seems to divide the two sides. Extending this line to the x-axis gives us a root that theorists predict will be our quantum critical point in the superconductor region, around absolute zero. Investigating this point has been challenging because the materials used to get to that temperature exhibit superconductive activity, for both phases. Scientists needed to somehow quiet down the electrons so they could extend the different phases further down the line (Wolchover “The”).
As mentioned earlier, magnetic fields can disrupt the electron pairs in a superconductor. With a large enough one, the property can decrease tremendously, and that is what the team from Cherbrooke did. They used a 90-tesla magnet from the LNCMI located in Toulouse, which uses 600 capacitors to dump a huge magnetic wave into a small coil made of copper and Zylon fiber (a rather strong material) for about 10 milliseconds. The material tested was a special cuprate known as yttrium barium copper oxide that had four different electron hole configurations spanning around the critical point. They cooled it down to minus 223 Celsius then sent in the magnetic waves, suspending the superconductive properties and looking at the hole behavior. Scientists saw an interesting phenomena happen: The cuprate started to fluctuate as if the electrons were unstable – ready to change their configuration at will. But if one approached the point from a different way, the fluctuations died down quickly. And the location of this rapid shifting? Near the expected quantum critical point. This supports antiferromagnetism being a driving force, because the decreasing fluctuations point to the spins lining up as one approaches that point. If we approach the point from a different way, those spins don’t line up and stack up in increasing fluctuations (Ibid).
© 2019 Leonard Kelley