An unconventional superconductor undergoes two transitions

Strontium ruthenate is not your typical superconductor. It defies the conventional picture of superconductivity in which phonons drive the pairing of opposite-spin, or singlet, s-wave electrons. And, despite structural similarities, it is fundamentally distinct from the high-temperature superconducting cuprates whose d-wave electrons also form singlet pairs. In Sr₂RuO₄, p-wave electrons form equal-spin pairs known as triplets. Unlike singlets, those triplets have a net orbital angular momentum that generates a so-called chiral superconducting state with dissipationless, unidirectional surface currents—and, with them, magnetic fields that are usually anathema to superconductors. (For more about superconducting Sr₂RuO₄, see the article by Yoshiteru Maeno, Maurice Rice, and Manfred Sigrist, Physics Today, January 2001, page 42 .)

However, recent NMR experiments call that picture into question. Spin singlets gradually lose their polarizability below the superconducting transition temperature T_c, whereas triplets do not. The NMR data showed a substantial loss of polarizability in Sr₂RuO₄, thereby suggesting that perhaps the evidence for chirality was just an artifact caused by the onset of superconductivity.

Now Vadim Grinenko and Shreenanda Ghosh at Dresden University of Technology in Germany and their collaborators provide clear evidence for chirality in superconducting Sr₂RuO₄. Using thin, single-crystalline samples whose symmetry was broken by a uniaxial stress, they observe that lowering the temperature produces two distinct transitions—first to a normal superconducting state, then to a chiral one.

The researchers looked for the onset of chirality using zero-field muon spin relaxation. In the technique, muons are implanted in the sample as magnetic-field detectors. The chiral superconducting state generates currents and thus a magnetic field; once it arises, the muons’ spins relax at a rate λ that is determined by their local field.

In unstressed Sr₂RuO₄ samples, λ began to increase at the onset of superconductivity—established by observing a jump in the specific heat—so the superconducting state was always chiral. However, when the material was compressed, the jump in specific heat occurred at a higher temperature: T_c was 3.5 K for a stress of 0.70 GPa, whereas λ remained constant until the temperature was just under 1.5 K, as shown in the figure. The muon precession therefore can’t be attributed to the onset of superconductivity.

Although such transition splitting has been seen in a few other materials, such as the superconductor UPt₃, the new results in Sr₂RuO₄ represent the clearest and most substantial evidence for it. The data will constrain attempts to theoretically explain the mechanisms behind unconventional superconductivity (see Physics Today, December 2006, page 23 ). (V. Grinenko et al., Nat. Phys., 2021, doi:10.1038/s41567-021-01182-7 .)