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The mystery of the UBe13 superconductivity is resolved


In collaboration with researchers from the Japanese Tohoku University, we have shown that the final superconducting state of UBe13 would be a mixture of two distinct types of superconductivity (or order parameters), whose respective weights are modulated by the the magnetic field and the pressure. This makes UBe13 an ideal candidate for the "topological" superconductivity.

Published on 18 March 2019
Superconducting materials conduct electricity with zero resistance below a certain temperature, called the critical temperature. Some of these materials are already widely used for the production of intense magnetic fields (magnetic resonance imaging, CERN particle accelerator, ToreSupra Tokamaks or ITER ...). The lossless electricity transmission could also make a significant contribution in the field of energy. For applications it is obviously desirable that the critical temperature be high, and research on new materials continue, with the hope of one day finding a superconductor working at room temperature. Many teams are looking in this direction, on different families of compounds like cuprates, pnictides iron, or light atoms compounds under very strong pressure. In parallel to this race towards high temperatures, superconductivity is also a major axis of fundamental research.


Thus, in our laboratory, we are interested in the superconductivity of a very curious family. They are metallic systems, with strong electronic correlations, which show surprising properties at low temperature, where the electrons behave as if they were up to 1000 times more massive than a free electron. UBe13 is one of the most mysterious superconductors. For more than 30 years scientists have not been able to accurately explain its behavior in an applied magnetic field. Indeed, although its critical temperature is low, about 1K, the superconductivity survives in a very intense magnetic field, of more than 12 Tesla. This far exceeds a theoretical limit which would be about 2 Tesla. Moreover, the dependence of this critical field with the temperature has a very abnormal form.

In collaboration with researchers from the Japanese University of Tohoku, we measured the effect of the magnetic field on the superconductivity of UBe13 at very low temperatures and under very high applied pressure, up to 6 GPa - 60000 bar (Figure). We have discovered that the phase diagram evolves strongly with the pressure, and that the superconductivity resists even better to the magnetic field at the highest pressures! All these results are perfectly explained by a very particular model of superconductivity. It is a superconductivity "triplet" (called "p-wave"), in which the electrons of the pair can also have parallel spins. The final superconducting state would actually be a mixture of two distinct types of superconductivity (or order parameters) whose respective weights are modulated by the magnetic field and the pressure. This makes UBe13 one of the most exotic superconductors, ideal candidate for the "topological" superconductivity currently very much sought after in the context of quantum information.


Sample (UBe13) in a diamond anvil cell for a measurement of electrical resistivity under pressure.

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