Ferromagnetism and superconductivity are usually considered as antagonistic phenomena. Some compounds like UCoGe are exceptions. Below 3 K, UCoGe becomes ferromagnetic; below 600 mK, it also becomes superconducting. It is also a material known as a "heavy fermion" as this alloy contains one atom of uranium, some electrons of which (in incomplete 5f orbitals) interact with conduction electrons, forming "quasi-particles" with high mass.

A team from INAC and LNCMI (French National High Magnetic Field Laboratory at CNRS) has performed measurements of electronic transport in high-quality UCoGe monocrystals, at very low temperature, applying a magnetic field along the "easy" magnetization axis of the crystal. They observed several anomalies concerning the Hall effect and the thermoelectric power of UCoGe, in a range of magnetic fields going up to 35 teslas.

These anomalies have been interpreted as abrupt changes in the topology of the Fermi surface of UCoGe, delimiting the electron states that are occupied. Such topological transitions, known as Lifshitz transitions, are quantum phase transitions that have no clear thermodynamic signature. 

Surprisingly, the scientists demonstrated that weak magnetic fields were sufficient to induce these topological changes in heavy-fermion materials.