High Field Superconducting Phases of Ultra Clean Single Crystal UTe₂

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