Transverse magnetic susceptibility reveals gigantic magnetic anisotropy in UTe₂ at high fields

In the past few years, UTe₂, a heavy fermion superconductor, has been proposed as a potential spin-triplet superconductor [1], with an anisotropic and rich phase diagram. A prominent feature of the phase diagram, where much of the scientific effort has been focused, is a re-entrant superconducting phase up to 35 tesla for magnetic fields aligned near the b-axis [2]. This superconducting phase ends when a metamagnetic transition appears [3], suggesting that magnetism and superconductivity are interconnected.

We set out to understand the high field magnetic anisotropy surrounding this superconducting phase by exploring the normal state of UTe₂ with resonant torsion magnetometry [4] in pulsed magnetic fields. High-quality crystals with Tc = 2 K were measured up to 60 T in the a-c and b-c planes at T = 4 K. In both cases, we found a sharp decrease in the magnetotropic susceptibility that sets in around 20 T when the field is aligned near the c-axis. At high fields, our measurements uniquely access the magnetic susceptibility transverse to the applied magnetic field [5]. The drop in the magnetotropic susceptibility for fields along the c-axis thus suggests a significant increase in χaa and χbb -- or an easy plane anisotropy -- at high fields. This behavior is likely related to anisotropic fluctuations of the U-dimers in strong magnetic fields.

[1] S. Ran et al., Science 365, 684 (2019).

[2] S. Ran et al., Nature Phys. 15, 1250 (2019).

[3] A. Miyake et al., J. Phys. Soc. Jpn. 88, 063706 (2019). [4] K. A. Modic et al., Nat. Commun. 9, 3975 (2018).

[5] A. Shekhter et al., Phys. Rev. B 108, 035111 (2023).