Kyoto, Japan -- Quantum materials and superconductors are difficult enough to understand on their own. Unconventional superconductors, which cannot be explained within the framework of standard theory, take the enigma to an entirely new level.
A typical example of unconventional superconductivity is strontium ruthenate, SRO214, the superconductive properties of which were discovered by a research team that included Yoshiteru Maeno, who is currently at the Toyota Riken - Kyoto University Research Center.
It has long been believed that this material exhibits spin-triplet superconductivity, in which electron pairs retain magnet-like properties and can transport quantum information without electrical resistance. However, results from recent nuclear magnetic resonance -- NMR -- experiments have overturned previous conclusions, prompting the need for independent verification using other techniques.
Since then there have been a variety of new studies, but the fundamental nature of SRO214's superconducting state -- namely, the symmetry of the superconducting order parameter -- has not yet been clarified. This motivated a collaborative team of researchers led by Maeno to test a new approach.
The researchers used magnetic resonance based on muons -- µ, elementary particles closely related to electrons -- implanting them into high-quality single crystals of SRO214. They then investigated the crystals using a recently improved μSR spectrometer at the Paul Scherrer Institute, PSI, which enabled precise measurements of extremely small changes in the internal magnetic fields in the superconducting state when some external field is applied. These changes are known as the Knight shift and provide crucial information on how electrons form pairs and enter the superconducting state.
During their investigations, the team also uncovered a serious pitfall in a commonly used experimental practice, in which multiple small crystals are arranged side by side to enhance signal intensity. Stray magnetic fields originating from the Meissner effect in neighboring superconducting crystals can generate spurious signals that appear in the μSR data despite not reflecting the material's intrinsic properties. To avoid this, the team introduced a new measurement protocol that combines μSR with complementary measurements using a superconducting quantum interference device, or SQUID, which allowed them to clearly observe a reduction of the Knight shift upon entering the superconducting state.
Using their new method, the researchers were able to conclusively demonstrate that the superconductivity of SRO214 can be consistently explained by spin-singlet superconductivity. These results are expected to further advance the study of superconductors using muon-based magnetic resonance techniques, providing insights that are complementary to those obtained from NMR.
"Our work demonstrates that, thanks to recent instrumental and methodological advances at PSI, μSR has now reached a level of sensitivity that allows us to directly and precisely probe even extremely subtle magnetic signatures," says co-author Rustem Khasanov.
