String theory and black holes show a possible path to practical superconductors

A leading candidate for room temperature superconductors is the copper compound cuprate, but no one knew how cuprates facilitated superconductivity...until some brave souls looked inside a black hole and broke out the string theory to explain how they work.

Superconductors that can transmit massive amounts of electricity with zero resistance at room temperature are pretty much the holy grail of applied physics (with good reason), but we're still a long way away from actually building one. Indeed, even figuring out the theoretical underpinnings of a room temperature superconductor has proven tremendously difficult, although a team of MIT physicists may have found an unlikely - and brilliant - way to learn more about how they would work. But first, a little backstory.


Currently, there are two types of superconductors. One group is the low temperature superconductors, which can only work at temperatures near absolute zero and thus require gigantically impractical amounts of coolants. The other set is the high temperature superconductors, which still have to be kept more than a hundred degrees Celsius below zero. They require slightly less impractical but still pretty damn impractical amounts of coolants (that's a technical term). Researchers focus on the second set to see if they can boost the working temperature another hundred or so degrees.

Cuprates are compounds that include copper anions, or copper atoms with more electrons than protons and, as a result, a negative charge. The physicists Georg Bednorz and Karl Müller discovered a cuprate compound, specifically lanthanum barium copper oxide, was a superconductor at the relatively high temperature of 135 degrees Celsius above absolute zero. They won a Nobel Prize for their efforts, and a bunch of other cuprate compounds have since been discovered that also have superconductivity properties.


Although we know a lot about what cuprates do, physicists have struggled to explain how they do it. Cuprates are what's called a "many-body system", which basically means they're made up of huge groups of electrons that interact with each other in ways that are difficult to model mathematically. Quantum mechanics can usually help with many-body systems, but cuprates behave so differently than other such systems that little headway has been made in understanding their workings. That's kind of a problem, because physicists are fairly sure it's precisely that peculiar behavior that gives cuprates their superconductivity, and understanding it might allow us to move towards superconductors at much higher temperatures.

Part of the problem is that cuprates don't follow Fermi's laws, a subset of quantum mechanics that describe the actions of most - but apparently not all - microscopic objects at temperatures near absolute zero. In regular Fermi liquids, electrical resistivity is proportional to the temperature squared, while in materials like cuprates - known as strange metals - the resistivity is proportional to just the temperature. That exponential reduction in resistance is the key to superconductivity. But physicists have absolutely no idea how to explain that fact on a theoretical level.


Here's where the MIT physicists and their offbeat ideas enter the picture. They figured out there was another system that shared the same properties as these superconducting strange metals, and best of all this system could be explained using gravitational mechanics and relativity instead of quantum mechanics. That system is a black hole. At low energy levels, the black hole model is a good match for the traits and behaviors that cuprates exhibit. Most importantly, electrical resistance in a black hole is directly proportional to temperature, not the temperature squared, which is the crucial match for superconducting cuprates.

None of these revelations would be particularly useful if it wasn't possible to correlate the features of the black hole model with those of the strange metals, and that's a tricky task because black holes are described by relativistic features while the cuprates are governed by quantum mechanics. String theory solves that problem, providing a bridge between quantum and gravitational mechanics called gauge/gravity duality.


The physicists simply used general relativity to figure out various key values of the black hole model, then used the duality to translate them to the quantum world of the strange metals. At that point, it was just a matter of connecting the right values, such as the fact that the strength of the electromagnetic field in the black hole corresponds to electron density in the cuprates.

These connections represent a theoretical breakthrough in the study of high temperature superconductors, and will hopefully illuminate a path towards the really high temperature superconductors that would work at room temperature. If nothing else, this represents the first time the gauge/gravity duality has been used to describe anything that existed after the very, very beginning of the universe, and the physicists hope to apply the duality to other modern forms of matter.



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