Unboxing a New Spin-Triplet Superconductor
Sheng Ran, Physicist, University of Maryland and the National Institute of Standards and Technology
Nature often reveals itself in surprising ways, so scientists, who study nature, have a lot of opportunities to be surprised. Over the past year and a half, I have been working on UTe2, a simple compound made of two parts, uranium and tellurium, which I discovered is a superconductor. That is, it conducts electricity without resistance under certain conditions.
Superconductivity is exciting because it’s almost like a superpower: It can help scientists detect very weak brain activity, accelerate particles to nearly the speed of light, and potentially provide the building blocks for futuristic quantum computers that could solve complex problems our current computers can’t touch.
As our newest discovered superconductor, uranium ditelluride keeps surprising me, our research team, and the scientific community. Following what Forrest Gump says, research “is like a box of chocolates; you never know what you’re gonna get.” The story I am going to share about UTe2 is exactly like a box of chocolates, and I hope you enjoy it.
When I discovered this strange property in uranium ditelluride, I was working on a completely different project. I was trying to create single crystals of U7Te12, a ferromagnetic compound, meaning that it is magnetic in the way iron is magnetic. For some reason, instead of getting single crystals of U7Te12, I kept getting single crystals of UTe2. After spending an hour Googling the properties of UTe2, I quickly realized that there could be an opportunity here.
UTe2 has been studied for decades, and it seemed to be a pretty uninteresting material. Its magnetic properties were first studied in the 1960s. In the 1990s, scientists measured how well single crystals conducted electricity, but it remained a magnetically disordered metal, i.e., not magnetic like iron, which is considered a boring vegetable. However, all the measurements done previously were carried out above the ultracold temperature of 1.8 kelvin (K) (-271.4 C or -456.4 F). Scientists did not measure its behavior at the very lowest temperatures that can be achieved in a lab.
For a compound containing a magnetic element, in this case, uranium, the system will usually disperse its energy by having all the spins of its atoms or electrons organized in an ordered manner, which is called a magnetically ordered state. (“Spin” doesn’t correspond to anything in the macroscopic world we inhabit, but you can think about it just like a small magnet sitting on the electron or atom, either pointing up or pointing down.) Examples are ferromagnetic state, in which all spins align in the same direction, or antiferromagnetic state, in which adjacent spins align in opposite directions.
I felt almost certain that if I could cool down UTe2 a little further, I would find such a magnetically ordered state. This was completely doable! Indeed, the first time I cooled it down below 1.8 K it transitioned to an ordered state. However, it was not a ferromagnetic state, nor antiferromagnetic state, as I would expect. It was a superconducting state! The temperature below which superconductivity happens, the superconducting critical temperature, is 1.6 K (-271.5 C or -456.8 F), only 0.2 K below the temperature at which all the previous measurements were performed for a few decades.
So, the first surprise is simply that UTe2 is a superconductor!
Surprise №1 alone is not earth-shattering. Superconductivity is actually a very common state of matter at low temperatures. Scientists discover superconducting compounds fairly regularly. No big deal. It is surprise №2 that really caught our attention.
One of the fundamental properties of a superconductor is that it loses its superconductivity when it’s exposed to a magnetic field. The stronger the magnetic field applied to a superconductor, the colder you have to make it before it will become a superconductor again. At a certain field strength, called the critical field, superconductivity will be suppressed completely even if you cool the material even further beyond the point at which superconductivity would normally emerge. It’s a good rule of thumb that the critical temperature and critical field are roughly proportional; superconductors that have higher critical temperatures usually have higher critical fields as well.
For example, most recently, scientists pressurized lanthanum hydride and found that it was superconducting at a very high temperature, relatively speaking, specifically 250 K (-23.2 C or -9.7 F). The critical field of this superconductor is about 150 tesla (T), a measure of a magnetic field’s strength (1 tesla is equal to the strength of about 100 average kitchen magnets). On the other hand, the critical field of the first superconducting material discovered, mercury, with a critical temperature of 4 K (-269.1 C or -452.5 F), is less than 0.1 T. Mercury exhibits the more typical behavior for superconductors.
Because the critical temperature of UTe2 is 1.6 K, one would expect the critical field is in the range of a few tesla. UTe2 has an orthorhombic crystal structure, which means the response to the magnetic field in the three directions, x, y and z, are potentially different. When I applied the magnetic field along the first two of the three crystal axes, I found the critical field to be less than 10 T. However, when I applied the magnetic field along the third axis, superconductivity remained even up to 20 T. In fact, the temperature required for superconductivity was suppressed from the original value of 1.6 K to 1 K by applying a magnetic field of 20 T. From these initial measurements, we estimated the critical field to be at least 30 T. A critical field of 30 T for a critical temperature of only 1.6 K is very large. To compare with the pressurized lanthanum hydride I mentioned already, if we scaled the critical temperature of UTe2 to the critical temperature of pressurized lanthanum hydride by multiplying by 250, the critical field would be 5,000 T! This is extremely high!
The extremely high critical field made us think there might be a third surprise waiting for us.
The quantum physics origin of superconductivity is related to a pair of electrons called a Cooper pair. Of the thousands of superconducting compounds discovered so far, almost all of them share one common property: The two electrons forming the Cooper pair have opposite spin direction, and, therefore, a net magnetic moment of zero. These are called spin-singlet superconductors.
Physicists have been speculating that two electrons with the same spin direction can also form a Cooper pair, and this is called spin-triplet pairing. Currently, there are fewer than a dozen superconductors, out of almost 10,000, that are considered candidates for spin-triplet superconductivity. Searching for spin-triplet superconductors is thus a very interesting research topic. In the past decade, it became even more important because it turns out these spin-triplet superconductors might be the best materials to build robust quantum computers due to their ability to resist perturbations that can derail these delicate systems.
Staring at the critical field data, I suspected that UTe2 might be the long-sought-after spin-triplet superconductor. The reason is very simple. If the spins of the electrons in the Cooper pair have opposite directions and we apply a magnetic field, it will flip one of the spins to make it the same direction as the magnetic field. Therefore, the Cooper pair is broken, and superconductivity is suppressed. However, if the spins already have the same direction, they can both remain in the direction in the magnetic field and, therefore, remain paired. This can naturally explain the high critical field of UTe2.
Of course, this is not the only way to explain this phenomenon; we needed more experiments to check this hypothesis. The most direct experiment is “nuclear magnetic resonance,” which probes the spin state of electrons. If it is spin-singlet pairing, then the total spin moment gradually goes to zero. But if it is spin-triplet pairing, the spin moment does not change. When our collaborator did the experiment, the spin moment showed no change. It’s triplet pairing! To a certain extent, this was expected, but we still felt surprised because it’s such a rare phenomenon at the quantum level.
In order to know how high the critical field really is, we performed experiments at the National High Magnetic Field Lab, in Tallahassee, Florida. We found that the critical field of UTe2 is as high as 35 T when the magnetic field is applied exactly along one of the axes of the crystal. When the magnetic field is misaligned by only a few degrees, the critical field quickly decreases to 15 T. However, upon increasing the magnetic field, we saw the resistance drop back to zero again. In other words, the magnetic field first kills superconductivity, as expected, and then helps superconductivity to come back. This really goes against our expectations of superconductivity. There are only a few other examples showing magnetic-field-induced superconductivity, including the ferromagnetic superconductors. However, as mentioned, UTe2 does not have a magnetically ordered state. There is no well-understood mechanism to explain how the magnetic field can induce superconductivity in UTe2.
As we continued our experiments, we found that the superconductivity induced by the magnetic field was again killed by further increasing the magnetic field to 35 T. After rotating the magnetic field further, I saw that, at some arbitrary but very large angle range, the resistance dropped back to zero once again, but at much larger magnetic field, more than 40 T. This time, even the highest magnetic field we had for the experiment, 65 T, was not enough to suppress the superconductivity. As mentioned above, UTe2 is not the only material showing superconductivity induced by a magnetic field, but it is the only compound that shows two different superconducting phases induced by a magnetic field, with the second phase existing in an extremely high magnetic field. We were not just surprised; we were shocked. This behavior greatly challenges the current theoretical understanding of superconductivity.
The discovery of spin-triplet superconductivity in UTe2 is not only interesting in terms of fundamental physics but also has potential applications for quantum computation. Our paper on UTe2 immediately attracted attention and has inspired a lot of ongoing experiments.
This is the end of my story for now, but I think more surprises are on the way.
This post originally appeared on Taking Measure, the official blog of the National Institute of Standards and Technology (NIST) on November 19, 2019.
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About the Author
Sheng Ran is currently a postdoctoral researcher at NIST and the University of Maryland. He obtained his Ph.D. in physics at Iowa State University. Besides doing research in quantum materials, he is also a meditation instructor.