Easy-plane anisotropy of magnetic fluctuations in UTe2

Zehao Wang; Ewan Scott; David W. Tam; Philippe Bourges; Keke Feng; Tyler W. Wannamaker; Paul Steffens; Arno Hiess; Ryan E. Baumbach; M. Brian Maple; Michal P. Kwasigroch; Pengcheng Dai

arXiv · 2609.26869

Easy-plane anisotropy of magnetic fluctuations in UTe2

Zehao Wang·Ewan Scott·David W. Tam·Philippe Bourges·Keke Feng·Tyler W. Wannamaker·Paul Steffens·Arno Hiess·Ryan E. Baumbach·M. Brian Maple·Michal P. Kwasigroch·Pengcheng Dai

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Abstract

The large directional anisotropy of superconducting UTe2 along the lattice a, b, c directions in magnetic susceptibility, upper critical fields, and Knight shift suggests spin-triplet Cooper pairing. If spin fluctuations bind the Cooper pair, one needs to determine their anisotropy in spin space to characterize the superconducting state. Here we use polarised and unpolarised inelastic neutron scattering (INS) experiments to show that antiferromagnetic (AFM) spin fluctuations at the Y point are highly isotropic within the ac-plane but have no observable magnitude along the b-axis. Our results rule out the assumed Ising anisotropy and constrain the possible pairing channels. AFM exchange is no longer pair forming in the triplet channel and the d vector along b is strongly favoured. We capture the experimental observations within a microscopic framework that encompasses the dual nature of 5f electrons. We include itinerant and localized 5f-moments in a tight-binding model fitted to quantum oscillations data. In congruence with INS, the exchange between localized moments generically consists of a strong FM intra-dimer interaction, maxima close to the Y and T points, mediated by 5p_y and 6d_{3z^2-r^2} orbitals respectively, and magnetically decoupled ab-planes. We find that anisotropy and dimerization of the moments can lead to gapped excitations that, in combination with Landau damping, can produce the observed overdamped response at the Y point. We suggest that enhancement of the response and its observed near-O(2)-symmetry could both be driven by proximity to a quantum critical point where the dimer excitation gap closes and gives place to incommensurate helical order. Analysis of the linearized gap equation for a nearest-neighbour pairing interaction favours the B1u triplet with a dominant d-vector component along b, although the possibility of spin singlet pairing cannot be ruled out.

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Zehao Wang, Ewan Scott, David W. Tam, Philippe Bourges, Keke Feng, Tyler W. Wannamaker, Paul Steffens, Arno Hiess, Ryan E. Baumbach, M. Brian Maple, Michal P. Kwasigroch, Pengcheng Dai. 2026-09-22. Easy-plane anisotropy of magnetic fluctuations in UTe2. https://arxiv.org/abs/2609.26869

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