For many years SRO214 was widely regarded as a candidate spin-triplet superconductor, in which paired electrons retain magnet-like characteristics and can potentially carry quantum information without electrical resistance. That picture was recently questioned when nuclear magnetic resonance (NMR) experiments reported behavior inconsistent with spin-triplet pairing, creating an urgent need for independent tests using different experimental techniques.
Motivated by this controversy, a collaborative team led by Maeno turned to muon spin rotation and relaxation, a magnetic resonance method based on muons, elementary particles closely related to electrons. The researchers implanted muons into high-quality single crystals of SRO214 and probed them with an upgraded muon spin rotation (muSR) spectrometer at the Paul Scherrer Institute (PSI), which offers the sensitivity required to detect extremely small changes in internal magnetic fields in the superconducting state under applied external fields.
These field-dependent changes, characterized as the Knight shift, reveal how electron spins behave when they form Cooper pairs. By tracking the Knight shift across the superconducting transition, the team could infer whether the pairs preserve or lose their spin polarization. The improved muSR setup at PSI made it possible to resolve subtle magnetic signatures that had previously been difficult to access.
During the study, the team identified a serious pitfall in a common experimental practice: mounting many small crystals side by side to boost signal intensity. They showed that stray magnetic fields generated by the Meissner effect in neighboring superconducting crystals can produce misleading signals in muSR measurements, masquerading as intrinsic features of the material rather than artifacts of the sample configuration.
To address this issue, the researchers established a new protocol that marries muSR with complementary measurements using a superconducting quantum interference device (SQUID). This combined approach allowed them to monitor the sample magnetization and separate genuine Knight-shift changes from spurious contributions caused by stray fields.
With the refined methodology, the team observed a clear reduction of the Knight shift when SRO214 entered the superconducting state. This behavior is consistent with spin-singlet superconductivity, in which electron spins pair in opposite directions and lose their net magnetization, contradicting the earlier spin-triplet interpretation for this material.
The findings demonstrate that the superconductivity of SRO214 can be reconciled with a spin-singlet order parameter, reshaping understanding of this long-studied unconventional superconductor. The work also highlights how methodological subtleties, such as crystal arrangement and magnetic screening, can strongly influence the interpretation of precision measurements in quantum materials.
According to co-author Rustem Khasanov, recent advances at PSI have pushed muSR to a level where it can directly and reliably probe exceptionally subtle magnetic phenomena in superconductors. The researchers expect that their approach will spur further muon-based investigations of superconducting states, providing a powerful complement to established techniques like NMR.
Research Report:Muon Knight Shift as a Precise Probe of the Superconducting Symmetry of Sr2RuO4
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