Temperature dependence of energy gap in the superconducting state in URu₂Si₂

The excitation spectrum of URu₂Si₂ in the superconducting state has been investigated by a precise neutron-scattering experiment. A weak but clear positive energy shift of ∼40 μeV has been detected in the superconducting phase (at T = 400 mK) for the resonance at Q₀ = (1,0,0). For the resonance at Q₁ = (0.6,0,0), a two times smaller shift is observed but with an incertitude of the same order than this shift. This result is in agreement with our previous results indicating that the wave-vector Q₀ is connected with the order parameters of the hidden order and of superconductivity.