Localized-to-itinerant transition preceding antiferromagnetic quantum critical point and gapless superconductivity in CeRh$_{0.5}$Ir$_{0.5}$In$_5$

TL;DR

This study investigates the pressure-induced transition from localized to itinerant electron behavior and its relation to superconductivity near an antiferromagnetic quantum critical point in CeRh$_{0.5}$Ir$_{0.5}$In$_5$, revealing gapless superconductivity and Fermi surface changes.

Contribution

It provides experimental evidence of a localized-to-itinerant transition preceding the antiferromagnetic QCP and links it to gapless superconductivity in a heavy-fermion compound.

Findings

Antiferromagnetic QCP at 1.2 GPa with maximum $T_c$

Abrupt increase in NQR frequency at 0.8 GPa indicating electron delocalization

Presence of gapless excitations suggesting odd-frequency pairing

Abstract

A fundamental problem posed from the study of correlated electron compounds, of which heavy-fermion systems are prototypes, is the need to understand the physics of states near a quantum critical point (QCP). At a QCP, magnetic order is suppressed continuously to zero temperature and unconventional superconductivity often appears. Here, we report pressure ($P$) -dependent $^{115}$In nuclear quadrupole resonance (NQR) measurements on heavy-fermion antiferromagnet CeRh$_{0.5}$Ir$_{0.5}$In$_5$. These experiments reveal an antiferromagnetic (AF) QCP at $P_{\rm c}^{\rm AF}$ = 1.2 GPa where a dome of superconductivity reaches a maximum transition temperature $T_{\rm c}$. Preceding $P_{\rm c}^{\rm AF}$, however, the NQR frequency $\nu_{\rm Q}$ undergoes an abrupt increase at $P_{\rm c}^{\rm *}$ = 0.8 GPa in the zero-temperature limit, indicating a change from localized to itinerant character of cerium's $f$-electron and associated small-to-large change in the Fermi surface. At $P_{\rm c}^{\rm AF}$ where $T_{\rm c}$ is optimized, there is an unusually large fraction of gapless excitations well below $T_{\rm c}$ that implicates spin-singlet, odd-frequency pairing symmetry.