Constraining symmetry
Most superconductors have only one transition point and, below a certain temperature, their electrical resistance drops to zero. In very rare cases, another superconducting transition appears at a lower temperature. By measuring its specific heat, Hayes et al. reveal that this two-step superconductivity occurs in the compound uranium ditelluride. Complementary optical measurements indicated the breaking of time reversal symmetry, constraining the possible symmetries of the order parameter in this material.
Science, abb0272, this issue p. 797
Abstract
An unconventional superconducting state was recently discovered in uranium┬аditelluride (UTe2), in which spin-triplet superconductivity emerges from the paramagnetic normal state of a heavy-fermion material. The coexistence of magnetic fluctuations and superconductivity, together with the crystal structure of this material, suggests that a distinctive set of symmetries, magnetic properties, and topology underlie the superconducting state. Here, we report observations of a nonzero polar Kerr effect and of two transitions in the specific heat upon entering the superconducting state, which together suggest that the superconductivity in UTe2 is characterized by a two-component order parameter that breaks time-reversal symmetry. These data place constraints on the symmetries of the order parameter and inform the discussion on the presence of topological superconductivity in UTe2.
