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It will likely look like a duck if it swims and quacks like one.
Scientists searching for an unusual type of superconductor have provided the most substantial evidence yet that they have. Researchers at the University of Maryland’s Quantum Materials Center (UMD) and their colleagues showed that uranium dioxide (or UTe2) has many characteristics of a topological superconductor. This material could be used to create quantum computers or other futuristic devices.
Johnpierre Paglione is a professor of Physics at UMD and the director of QMC. He was also the senior author of one paper. There could be other reasons why we see all this weird stuff. But honestly, it’s the first time I’ve ever seen anything like it in my entire career.
All superconductors can carry high-voltage electrical currents with no resistance. It’s part of their job. This feat is impossible to match, and it’s one reason why large coils made of superconducting wires are used in MRI machines and other scientific equipment.
Superconductors can achieve superconductivity in different ways. Scientists have been searching for a superconductor since the early 2000s. This type relies on a complex choreography of subatomic particles that carry the current.
The surprising director of this choreography is topology, a branch of mathematics. Topology can be described as grouping shapes easily transformed by pushing and pulling. A ball of dough, for example, can be made into a loaf of bread or a pie-shaped pizza, but it can’t be turned into a donut by poking holes in it. Topologically speaking, a pie and a loaf are the same, but a donut is different. Topological superconductor electrons dance around one another while circling something similar to the hole at the center of a muffin.
There is no way to cut a superconductor open so you can see these electronic dance moves. The best way to determine whether electrons are dancing on an abstract donut is to study a material’s behavior in experiments. Until now, there has not been any conclusive evidence that a superconductor is topological. However, the new papers demonstrate that UTe2 looks and swims like a topological duck.
Paglione’s group and Aharon Kapitulnik from Stanford University have done a study showing two types of superconductivity in UTe2. This result and previously published experimental evidence helped them narrow down the superconductivity present in UTe2. Their findings were published in the journal Science on July 15, 2021.
Steven Anlage, a UMD professor of physics and QMC member, conducted another study that revealed strange behavior on the surface of the same material. These findings support the long-cherished phenomenon of topologically protected Majorana mode behavior. Majorana modes are exotic particles that behave like half an electron and will likely be found on topological superconductors. Scientists are particularly interested in these particles because they could be the foundation for powerful quantum computers. Anlage and his colleagues published their findings in a paper published in Nature Communications on May 21, 2021.
The unique properties of superconductors are only revealed below a specific temperature. This is similar to water freezing below zero Celsius. Ordinary superconductors have electrons that pair up in a conga line of two, following each other through metal. In rare cases, electron couples may perform a circular dance around one another, similar to a waltz. Topological circumstances are even more unusual. The circular dance of electrons includes a vortex. It is like an eye in the swirling winds of a hurricane. The vortex forms when electrons are paired up in this manner. This distinguishes a topological superconductor from one that is simply a fair-weather electron dance.
In 2018, Paglione’s group and Nicholas Butch (an adjunct professor of physics at UMD and a physicist from the National Institute of Standards and Technology, NIST) discovered that UTe2 is a superconductor. It was immediately apparent that this was different from your typical superconductor. It seemed unphased even by large magnetic fields that commonly disrupt superconductivity by splitting the electron couples. This was the first indication that the electron couples in UTe2 held onto each other tighter than usual. This is likely due to their paired circular dance. This resulted in a lot more research and interest from other researchers.
Anlage says it’s like a perfect thunderstorm superconductor. It combines many different things that no one has ever seen before.
Paglione and his colleagues reported in the new Science paper two unique measurements that revealed the internal structure of the UTe2. The UMD team measured specific heat. This is the energy required to heat the material by one degree. The specific heat was measured at various temperatures, and the team watched the temperature change as the sample became superconducting.
Paglione says that there is an average jump in heat at superconducting transitions. There are two jumps. This is evidence that there are two superconducting transformations and not one. This is highly unusual.
These two jumps suggest that electrons in UTe2 may pair up to create one of two dance patterns.
The Stanford team measured again by shining laser light onto a piece of UTe2 and found that the light reflecting was slightly twisted. The reflected light from the Stanford team bobbed up and down when they sent it in but also moved slightly left and right. This indicated that something in the superconductor was twisting the light up and not untwisting its path out.
The Stanford team of Kapitulnik also discovered that a magnetic field could coerce UTe2 to twist light in one direction or another. The light from the sample would tilt to the left if they used a magnetic field pointed up. The light would lean to the right if they pointed the magnetic force down. Researchers discovered that the crystal’s up-and-down directions were particular for electrons dancing within it.
The researchers sought the advice of Daniel F. Agterberg (a theoretical physicist at the University of Wisconsin-Milwaukee and co-author of the Science paper) to help them understand what this all meant for electrons dancing in a superconductor. The theory states that the arrangement of uranium atoms and tellurium atoms within the UTe2 crystal allows electron pairs to form eight different dance combinations. Agterberg listed all possible combinations of these eight dances, as Agterberg’s heat measurement clearly shows two dancing at once. Because of the nature of the reflected sunlight and the coercive power exerted by a magnetic field along its up-down axis, there are only four options. Previous research that showed UTe2’s superiority under large magnetic fields constrained it to just two dance pairs. These dance pairs form a vortex indicating a topological, stormy dance.
Paglione says, “What’s fascinating is that given the constraints of the experimental evidence, our best theory points towards a certainty that superconducting states are topological.”
If superconductivity is topological in a material, resistance will remain at zero in bulk. Still, something special will occur on the surface: Particles (known as Majorana modes) will form a fluid that is not a superconductor. These particles can also be found on the surface, despite defects or minor environmental disruptions. These particles might provide a foundation for quantum computers, according to researchers. By encoding quantum information in several Majoranas, which are located far apart, the data is virtually immune to local disturbances, which have so far been the bane for quantum computers.
The team of Anlage wanted to examine the surface of UTe2 closer to finding signatures of the Majorana sea. They used microwaves to send microwaves at a chunk of UTe2 and measured the microwaves coming out the other side. They could compare the output with and without the sample, which allowed them both to test the properties of the bulk and the surface.
The surface affects the microwaves’ strength, resulting in a slightly subdued output. The bulk, which is a superconductor and offers no resistance to microwaves, doesn’t alter their strength. It slows them down, which causes delays that cause output to bob up and back out of sync. The researchers determined how many electrons in the material participated in the paired dance at different temperatures by looking at the out-of-sync parts of their response. The behavior was in line with Paglione’s circular dances.
Even more critical, the microwave response’s in-sync section showed that UTe2’s surface isn’t superconducting. This is unusual as superconductivity can be contagious. Superconductivity spreads to metals when it is placed close to superconductors. The superconductivity transmitted from the bulk to the surface of UTe2 wasn’t captured by the latter, as expected for a topological superconductor. Instead, the feeling of UTe2 uniquely responded to microwaves.
Anlage states that the surface behaves differently from any superconductor they’ve ever seen. “Then the question becomes, ‘What is the interpretation of this anomalous result?’ One interpretation consistent with all other data is that we have a topologically protected state that acts like a wrapper around a superconductor that is impossible to eliminate.
It is tempting to think that UTe2’s surface is covered by a sea of Majorana mode and declare victory. Extraordinary claims require extraordinary evidence. Anlage and his team tried to find every possible explanation for their observations. They systematically ruled out all possibilities, from oxidization at the surface to light-hitting edges of the sample. A surprising alternative answer may exist.
“In the back of your mind, you’re always thinking, Oh, maybe it’s cosmic rays’ or Maybe it was something else,” says Anlage. “You cannot eliminate all other possibilities.”
Paglione claims that surface Majorana mode quantum computations are the only way to find out. Although the surface of UTe2 may contain many Majorana modes, there currently needs to be a way to isolate or manipulate them quickly. This might be possible with a thin film instead of the crystals (easier) that were used in these experiments.
Paglione states that there are some ideas to make thin films. It’s radioactive and uranium, so it needs new equipment. Next, we need to grow films. The next task is to create devices. It would take several years to make devices, but it is possible.
Whether UTe2 is the topological superconductor that everyone has been waiting for or a pigeon who learned to swim and quack like a duck and everything in between, Paglione and Anlage are eager to see what the material holds.
Anlage states, “it’s quite clear though that there is a lot of cool physics within the material.” “Whether it’s Majoranas or not on the surface is a significant issue, but exploring novel physics is the most exciting stuff.”
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