Feedback of superconductivity on the magnetic excitation spectrum of UTe₂

We investigate the spin dynamics in the superconducting phase of UTe₂ by triple-axis inelastic neutron scattering on a single crystal. At the wave-vector k₁ ¼ ð0; 0:57; 0Þ, where the normal state antiferromagnetic correlations are peaked, a modification of the excitation spectrum is evidenced, on crossing the superconducting transition, with a reduction of the relaxation rate together with the development of an inelastic peak at Ω ≈ 1 meV. The low dimensional nature and the a-axis polarization of the fluctuations, that characterise the normal state, are essentially maintained below the superconducting transition. The high ratio Ω=k_BT_sc ≈ 7.2 contrasts with the most common behaviour in heavy fermion superconductors.