Scientists from Brown University in the US have finally proved that materials can conduct an electric current without resistance – an ability known as superconductivity – even when exposed to a magnetic field. They do this by entering a superconductive state that was first proposed in 1964.
*“It took 50 years to show that this phenomenon indeed happens,”* said Vesna Mitrovic, the leader of the project, in a press release. *“We have identified the microscopic nature of this exotic quantum state of matter.”*
Superconductors themselves put out a magnetic field, which is so powerful it can levitate objects, such as trains, allowing them to travel extremely fast. Being able to achieve superconductivity within magnetic fields greatly alters our understanding of the phenomenon, and also opens up new opportunities for superconductive technology.
But for a long time, magnetic fields were the enemy of superconductivity. In order for a material to gain superconductivity, the electrons travelling through it need to pair up with electrons of the opposite *“spin”* – these pairings are known as Cooper pairs. Once paired up with a buddy, electrons can easily travel along a material without causing resistance, instead of rattling around as individual electrons do.
But in a magnetic field, it’s extremely difficult to form these Cooper pairs, as the field changes the *“spin”* state of the electrons, leaving more *“spin-down”* than *“spin-up”* electrons. This means less electrons pair together and there are a whole bunch left shaking around causing resistance.
*”The question is what happens when we have more electrons with one spin than the other,”* said Mitrovic in the release. *”What happens with the ones that don’t have pairs? Can we actually form superconducting states that way, and what would that state look like?”*
In 1964, physicists predicted that a magnetic field didn’t necessarily have to ruin all of the superconducting fun – their theory was that the unpaired electrons would gather together in bands across the superconducting material and conduct normally, while the rest of the material would be superconductive. They called this state the FFLO phase (named after the theorists Peter Fulde, Richard Ferrell, Anatoly Larkin and Yuri Ovchinniko, who came up with the idea).
However, no one has been able to prove this actually happens – until now.
In this experiment, the researchers from Brown University used an organic superconducting material, made up of ultra-thin sheets stacked on top of each other, to investigate where the FFLO state is possible. They also increased the temperature – in the past scientists had only tested the phenomenon in extremely cold settings, and had failed to observe the FFLO phase.
Using nuclear magnetic resonance, the team was able to watch what was happening to the spin state of the electrons when they applied a magnetic field to the material.
What they saw was regions across the material where unpaired, spin-up electrons had congregated. They behave: *”like little particles constrained in a box,”* said Mitrovic. This is what is called an Andreev bound state.
*”What is remarkable about these bound states is that they enable transport of supercurrents through non-superconducting regions,”* said Mitrovic. *”Thus, the current can travel without resistance throughout the entire material in this special superconducting state.”* The results are published in Nature Physics.
The breakthrough may not sound too relevant, but seeing as superconductors are being touted as the future of ultra-fast transport, such as maglev trains, and imaging technology, it’s a pretty important find.
*“This really goes beyond the problem of superconductivity,”* said Mitrovic in the release. *“It has implications for explaining many other things in the universe, such as behavior of dense quarks, particles that make up atomic nuclei.”*
Learn more here https://news.brown.edu/articles/2014/10/superconductivity