Feedback of superconductivity on the magnetic excitation spectrum of UTe$_2$

International audienceWe investigate the spin dynamics in the superconducting phase of UTe$_2$ by triple-axis inelastic neutron scattering on a single crystal sample. At the wave-vector k1=(0, 0.57, 0), 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 the a-axis polarization of the fluctuations, that characterise the normal state, are essentially maintained below Tsc. The high ratio Ω/kBTsc ≈ 7.2 contrasts with the most common behaviour in heavy fermion superconductors