Uranium Ditelluride Could be Spin Triplet Superconductor and Is a New Wonder Material

Uranium di-telluride could be a wonder material for insanely powerful magnets and it could become the basis of next generation quantum computers.

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. Experiments show 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.

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.

, 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!

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.

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.

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