Constraints on a split superconducting transition under uniaxial strain in Sr$_2$RuO$_4$ from scanning SQUID microscopy

TL;DR

This study uses scanning SQUID microscopy to investigate the superconducting order parameter in Sr$_2$RuO$_4$ under uniaxial strain, finding no evidence of a second phase transition and constraining theoretical models.

Contribution

It provides the first direct experimental test for a second superconducting transition under strain in Sr$_2$RuO$_4$, challenging two-component order parameter theories.

Findings

Only one superconducting transition observed under strain.

Placed a tight upper bound of less than 1% on superfluid density change.

Results suggest the absence of a second transition in the studied conditions.

Abstract

More than two decades after the discovery of superconductivity in Sr$_2$RuO$_4$, it is still unclear whether the order parameter has a single component or two degenerate components. For any two-component scenario, application of uniaxial strain is expected to lift the degeneracy, generating two distinct phase transitions. The presence of a second (lower-temperature) transition may be observable by probes that are sensitive to changes in the London penetration depth, $\lambda$, as a function of temperature, $T$. Here, we use scanning SQUID microscopy combined with a uniaxial strain device to test for a second transition under strain. We only observe a single transition. Within the temperature range where a second transition has been suggested by $\mu$SR measurements, we further place a tight upper bound of less than 1% on the change in the zero temperature superfluid density $n_s\propto\lambda^{-2}(0)$ due to a second transition, suggesting that such a transition does not occur. These results constrain theories of the order parameter in Sr$_2$RuO$_4$.