HIGH-FIELD NMR UNVEILS FIELD-REINFORCED SUPERCONDUCTIVITY IN UTe₂

Y. Tokunaga, Japan Atomic Energy Agency, Tokai, Japan, M. Horvatić and Steffen Krämer, LNCMI Grenoble.

The uranium-based superconductor UTe₂ provides an attractive platform for studying the novel physics of spin-triplet and topological superconductivity in bulk materials. Resistivity measurements performed in 2019 at LNCMI, both in continuous and pulsed fields, revealed that superconductivity in UTe₂ is significantly enhanced when the magnetic field (H) is applied along the crystallographic b axis (inset of figure). This leads to an increase of the upper critical field up to the field of a metamagnetic transition at H_m = 35 T. It was proposed that an essential ingredient for this field-reinforced superconductivity is the interplay with U-5f-electron spin fluctuations.To confirm the presence and elucidate the nature of spin fluctuations in UTe₂, we conducted ¹²⁵Te-NMR experiments in high magnetic fields using specially prepared, ¹²⁵Te-enriched, high-quality single crystals having T_c = 2.0 K, synthetized using the molten-salt flux method discovered recently by our team. The natural abundance of ¹²⁵Te is only 7 %, so that our enrichment up to 99 % largely enhanced the NMR signal, allowing us to measure the field dependence of the NMR relaxation rates 1/T₁ and 1/T₂ up to 32 T.
The observed magnetic-field dependence of these data for H || b (figure) and the scaling between the two relaxation rates, T₂^-1/T₁^-1 ≈ 36, demonstrates that the dominant fluctuations in this material are longitudinal ones. While 1/T₁ and 1/T₂ are nearly field independent at lower fields, both quantities start to increase above about 15 T and show a tendency to diverge above 32 T, where the NMR spin-echo signal becomes unobservable because T₂ becomes shorter than the dead time of our NMR spectrometer. This confirms the divergence of spin fluctuations near the field-induced metamagnetic transition at H_m. Previous macroscopic studies gave H* ≈ 15 T as the characteristic field above which T_c shows an upturn and a high-field superconducting phase emerges on top of a low-field superconducting phase (inset of figure). Our NMR results show that H* is also the characteristic field above which spin fluctuations begin to develop on approaching the metamagnetic transition, confirming that these fluctuations indeed enhance the pairing interactions.

Figure: Magnetic-field dependence of 1/T₂ and 1/T₁ (scaled by a factor 35.5) in UTe₂ for H || b, compared to the previously published quadratic coefficient A of the low-T resistivity data. The data provide a direct signature of longitudinal spin fluctuations related to the metamagnetic transition at 35 T. Inset: Field-dependent T_c data and corresponding schematic H-T phase diagram, presenting a high-field (HFSC) as well as a low-field (LFSC) superconducting (SC) phase.

Longitudinal Spin Fluctuations Driving Field-Reinforced Superconductivity in UTe₂, T. Y. Tokunaga, H. Sakai, S. Kambe, P. Opletal, Y. Tokiwa, Y. Haga, S. Kitagawa, K. Ishida, D. Aoki, G. Knebel, G. Lapertot, S. Krämer, and M. Horvatić, Phys. Rev. Lett. 131, 226503 (2023).

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.131.226503

Contact: tokunaga.yo@jaea.go.jp, mladen.horvatic@lncmi.cnrs.fr