Hybridization gap in the heavy-fermion compound UPd$_2$Al$_3$ via quasiparticle scattering spectroscopy

TL;DR

This study uses point-contact spectroscopy to investigate the hybridization gap in UPd$_2$Al$_3$, revealing how magnetic excitations influence its visibility and providing insights into the dual nature of 5f electrons in this heavy-fermion compound.

Contribution

It demonstrates the temperature-dependent behavior of the hybridization gap and conductance enhancement, linking magnetic excitations to electronic structure in UPd$_2$Al$_3$.

Findings

Hybridization gap is 7.2 meV at 4 K.

Conductance enhancement vanishes around 18 K.

Hybridization gap persists up to ~28 K, above the magnetic transition.

Abstract

We present results from point-contact spectroscopy of the antiferromagnetic heavy-fermion superconductor UPd$_2$Al$_3$: conductance spectra are taken from single crystals with two major surface orientations as a function of temperature and magnetic field, and analyzed using a theory of co-tunneling into an Anderson lattice. Spectroscopic signatures are clearly identified including the distinct asymmetric double-peak structure arising from the opening of a hybridization gap when a coherent heavy Fermi liquid is formed. Both the hybridization gap, found to be 7.2 $\pm$ 0.3 meV at 4 K, and the conductance enhancement above a flat background decrease upon increasing temperature. While the hybridization gap is extrapolated to remain finite up to $\sim$28 K, close to the temperature around which the magnetic susceptibility displays a broad peak, the conductance enhancement vanishes at $\sim$18 K, slightly above the antiferromagnetic transition temperature ($T_\textrm{N}$ $\approx$ 14 K). This rapid decrease of the conductance enhancement is understood as a consequence of the junction drifting away from the ballistic regime due to increased scattering off magnons associated with the localized U 5$f$ electrons. This shows that while the hybridization gap opening is not directly associated with the antiferromagnetic ordering, its visibility in the conductance is greatly affected by the temperature-dependent magnetic excitations. Our findings are not only consistent with the 5$f$ dual-nature picture in the literature but also shed new light on the interplay between the itinerant and localized electrons in UPd$_2$Al$_3$.