High Field Superconducting Phases of Ultra Clean Single Crystal UTe2

TL;DR

This study investigates the high-field superconducting phases of ultra-clean UTe2 crystals, revealing complex reentrant superconductivity linked to magnetic fluctuations and Fermi surface properties, with implications for understanding unconventional superconductivity.

Contribution

It provides detailed angular dependence of the upper critical field and identifies multiple field-reentrant superconducting phases in UTe2, highlighting the role of anisotropic magnetic fluctuations.

Findings

Reentrant superconductivity extends up to 24° from b to c-axis.

Superconductivity persists when field is perpendicular to magnetization easy axis.

Quantum oscillations confirm quasi-two-dimensional Fermi surfaces.

Abstract

We report the magnetoresistance of high-quality single crystals of UTe2 with Tc=2.1K in high magnetic fields up to 36T, with the field direction between the b and c-axes. From the angular dependence of the upper critical field Hc2, we found that the field-reentrant superconducting phase near H // b-axis extends up to a field angle (24 deg) from the b to c-axis, where another field-reentrant superconducting phase begins to appear above the metamagnetic transition field, Hm. Our results suggest that the field-reentrant superconductivity below Hm near the b-axis is closely related to the superconductivity above Hm when the field is tilted toward the c-axis. Superconductivity appears to be robust when the field direction is maintained perpendicular to the magnetization easy axis, implying that fluctuations boosting superconductivity may persist. At first glance, these findings resemble the field-reentrant (reinforced) superconductivity observed in ferromagnetic superconductors URhGe and UCoGe, where Ising-type ferromagnetic fluctuations play a crucial role. However, in UTe2, the fluctuations are more complex. The angular dependence of the upper critical field Hc2 contrasts with that of the initial slope of Hc2 near Tc, revealing the anisotropic field response of fluctuations. Thanks to the high-quality samples, quantum oscillations were detected for field directions close to the c-axis using magnetoresistance (Shubnikov-de Haas effect) and torque (de Haas-van Alphen effect) measurements. The angular dependence of frequencies is in good agreement with those observed previously using the field-modulation technique, confirming quasi-two-dimensional Fermi surfaces.