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

TL;DR

This paper investigates the collective spin resonance in the hidden order phase of URu2Si2, revealing how multipolar pairing leads to a quasiparticle gap and a spin exciton resonance consistent with neutron scattering and NMR data.

Contribution

It introduces an effective two-orbital model incorporating crystal field effects to explain the magnetic resonance and quasiparticle gap in URu2Si2's hidden order phase.

Findings

Reproduces the spin exciton resonance observed in INS

Explains the quasiparticle gap formation below T_HO

Accounts for low-temperature NMR relaxation rates

Abstract

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 zone center and boundary, respectively in states that have maximally different total angular momentum components. Due to the pairing with 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.