Re-evaluating the Magnetic "Spin-Triplet" Mystery in Strontium Ruthenate

Strontium ruthenate (SRO214) stands out as a great example of unconventional superconductivity. Its unique superconducting properties were first identified by a research team that included Yoshiteru Maeno, now based at the Toyota Riken–Kyoto University Research Center. The findings were published in Physical Review Letters.

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Understanding superconductors and quantum materials is already a complex task, but unconventional superconductors take that challenge even further. These materials resist explanation within the bounds of traditional superconductivity theory, adding an extra layer of mystery to an already intricate field.

Spin-triplet superconductivity, where electron pairs retain their magnetic (spin) alignment and can transport quantum information without electrical resistance, has long been suspected in SRO214. However, recent nuclear magnetic resonance (NMR) experiments have challenged those earlier conclusions, highlighting the need for independent verification using different experimental approaches.

Since then, numerous studies have followed, yet the fundamental nature of SRO214's superconducting state (specifically, the symmetry of its superconducting order parameter) remains unresolved. This uncertainty prompted a collaborative research effort led by Maeno to pursue a new experimental approach.

The researchers employed magnetic resonance to implant muons (µ, elementary particles closely linked to electrons) into high-quality single crystals of SRO214. The crystals were studied using a recently developed μSR spectrometer at the Paul Scherrer Institute, PSI.

This allowed for accurate measurements of extremely minute changes in the internal magnetic fields in the superconducting state when an external field was applied. These alterations, known as the Knight shift, reveal important information about how electrons form pairs and enter the superconducting state.

In the course of their investigation, the researchers also uncovered a critical flaw in a commonly used experimental technique: stacking multiple small crystals side by side to enhance signal strength. They found that stray magnetic fields from neighboring superconducting crystals can distort μSR (muon spin rotation) data, producing misleading signals that don’t accurately reflect the material’s intrinsic properties.

To circumvent this, the researchers created a novel measurement methodology that combines μSR with complementary measurements using a superconducting quantum interference device, or SQUID. This allowed them to clearly identify a reduction in the Knight shift upon entering the superconducting state.

Using their novel approach, the researchers were able to show conclusively that SRO214's superconductivity is consistently explained by spin-singlet superconductivity. These findings are likely to help enhance the study of superconductors utilizing muon-based magnetic resonance methods, giving complementary insights to those acquired using 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.

Rustem Khasanov, Study Co-Author, Paul Scherrer Institute