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

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 $P_{\mathrm{c}}^{\mathrm{AF}}$ = 1.2 GPa where a dome of superconductivity reaches a maximum transition temperature $T_{\mathrm{c}}$. Preceding $P_{\mathrm{c}}^{\mathrm{AF}}$, however, the NQR frequency $\hat{I}{½}_{\mathrm{Q}}$ undergoes an abrupt increase at $P_{\mathrm{c}}^{\ast }$ = 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 $P_{\mathrm{c}}^{\mathrm{AF}}$ where $T_{\mathrm{c}}$ is optimized, there is an unusually large fraction of gapless excitations well below $T_{\mathrm{c}}$ that implicates spin-singlet, odd-frequency pairing symmetry.

12 pages, 8 figures