Collective spin resonance excitation in the gapped itinerant multipole hidden order phase of URu2Si2

An attractive proposal for the hidden order (HO) in the heavy electron compound URu2Si2 is an itinerant multipole order of high rank. It is due to the pairing of electrons and holes centered on the zone center and boundary, respectively, in states that have maximally different total angular momentum components. Due to the pairing with a commensurate zone boundary ordering vector the translational symmetry is broken and a HO quasiparticle gap opens below the transition temperature T-HO. Inelastic neutron scattering (INS) has demonstrated that for T < T-HO the collective magnetic response is dominated by a sharp spin exciton resonance at the ordering vector Q that is reminiscent of spin exciton modes found inside the gap of unconventional superconductors and Kondo insulators. We use an effective two-orbital tight-binding model incorporating the crystalline-electric-field effect to derive closed expressions for quasiparticle bands reconstructed by the multipolar pairing terms. We show that the magnetic response calculated within that model exhibits the salient features of the resonance found in INS. We also use the calculated dynamical susceptibility to explain the low-temperature NMR relaxation rate.