Theory of magnetic excitons in the heavy-fermion superconductor $UPd_{2}Al_{3}$

TL;DR

This paper investigates how unconventional superconductivity affects magnetic excitations in UPd₂Al₃, revealing a resonance peak below 2Δ₀ that can be used to probe the superconducting order parameter via neutron scattering.

Contribution

It provides a microscopic theory linking magnetic exciton behavior to the unconventional superconducting state in UPd₂Al₃, highlighting the resonance peak as a diagnostic tool.

Findings

Resonance peak appears below 2Δ₀ at wave vector Q

Antiferromagnetic order enhances the resonance intensity

Resonance dispersion is dominated by magnetic exciton behavior

Abstract

We analyze the influence of unconventional superconductivity on the magnetic excitations in the heavy fermion compound UPd$_2$Al$_3$. We show that it leads to the formation of a bound state at energies well below 2$\Delta_0$ at the antiferromagnetic wave vector {\textbf Q}=$(0,0,\pi/c)$. Its signature is a resonance peak in the spectrum of magnetic excitations in good agreement with results from inelastic neutron scattering. Furthermore we investigate the influence of antiferromagnetic order on the formation of the resonance peak. We find that its intensity is enhanced due to intraband transitions induced by the reconstruction of Fermi surface sheets. We determine the dispersion of the resonance peak near {\textbf Q} and show that it is dominated by the magnetic exciton dispersion associated with local moments. We demonstrate by a microscopic calculation that UPd$_2$Al$_3$ is another example in which the unconventional nature of the superconducting order parameter can be probed by means of inelastic neutron scattering and determined unambiguously.