Localized-to-itinerant transition preceding antiferromagnetic quantum critical point and gapless superconductivity in CeRh0.5Ir0.5In5

A fundamental problem posed from the study of heavy-fermion systems 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 [1]. Here we report pressure ($P$)-dependent $^{115}$In nuclear quadrupole resonance (NQR) measurements on heavy-fermion antiferromagnetic superconductor CeRh$_{0.5}$Ir$_{0.5}$In$_5$ ($T_{\rm N}$ = 3.0 K, $T_{\rm c}$ = 0.9 K) [2]. As shown in Figs. 1, we find an antiferromagnetic (AF) QCP at $P_{\rm c}^{\rm AF}$ = 1.2 GPa where a dome of superconductivity reaches a maximum transition temperature $T_{\rm c}$ [3]. 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 [3]. We also find that the residual value of nuclear-spin lattice relaxation rate divided by temperature (1/$T_1T$) well below $T_{\rm c}$ depends significantly on pressure. The results indicate that, 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 [3]. Details about quantum criticality and unconventional gapless superconductivity there in CeRh$_{0.5}$Ir$_{0.5}$In$_5$ will be discussed.

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[2] P. G. Pagliuso $et$ $al$., Phys. Rev. B ${\bf 64}$, 100503(R) (2001).
[3] S. Kawasaki $et$ $al$., Commun Phys ${\bf 3}$, 148 (2020).