Superconducting-Spin Reorientation in Spin-Triplet Multiple Superconducting Phases of UTe2

TL;DR

This paper investigates the spin reorientation phenomena in the multiple superconducting phases of UTe2, revealing spin-triplet pairing and coexistence of order parameters through NMR and susceptibility measurements.

Contribution

It provides the first detailed experimental evidence of spin-direction rotation and coexistence of multiple SC order parameters in UTe2.

Findings

Spin susceptibility remains unchanged in SC2 phase.

Spin modulation occurs below the SC2-SC3 transition.

Spin-triplet superconductivity with spin parallel to b-axis confirmed.

Abstract

Superconducting (SC) state has spin and orbital degrees of freedom, and spin-triplet superconductivity shows multiple SC phases due to the presence of these degrees of freedom. However, the observation of spin-direction rotation occurring inside the SC state (SC spin rotation) has hardly been reported. UTe2, a recently discovered topological superconductor, exhibits various SC phases under pressure: SC state at ambient pressure (SC1), high-temperature SC state above 0.5 GPa (SC2), and low-temperature SC state above 0.5 GPa (SC3). We performed nuclear magnetic resonance and AC susceptibility measurements on single-crystal UTe2. The b-axis spin susceptibility remains unchanged in SC2, unlike in SC1, and decreases below the SC2-SC3 transition with spin modulation. These unique properties in SC3 arise from the coexistence of two SC order parameters. Our NMR results confirm the spin-triplet superconductivity with SC spin parallel to b in SC2, and unveil the remaining of spin degrees of freedom in superconducting UTe2.