Localized-to-Itinerant Transition Preceding Antiferromagnetic Quantum Critical Point Leads to Odd-Frequency Superconductivity in CeRh_0.5Ir_0.5In₅

Abstract

A fundamental problem posed from the study of correlated electron compounds is the need to understand the physics of states near a quantum critical point (QCP). In the prototype systems of heavy-fermion compounds, the pertinent issue is when the f-electrons become itinerant. The most influential proposal has been the so-called Kondo breakdown scenario where the localized-to-itinerant transition takes place right at the antiferromagnetic QCP. Here, we review a recent work by pressure-dependent ¹¹⁵In nuclear quadrupole resonance (NQR) measurements on heavy-fermion antiferromagnetic superconductor CeRh_0.5Ir_0.5In₅ (T_N=3.0 K, T_c=0.9 K). The experiments reveal an antiferromagnetic (AF) QCP at P_c^AF=1.2 GPa where a dome of superconductivity reaches a maximum transition temperature T_c^max=1.4 K. Preceding P_c^AF, however, the NQR frequency ν_Q undergoes an abrupt increase at P_c*=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. These findings are at odds with the Kondo breakdown scenario. Furthermore, an unusually large fraction of gapless excitations was observed well below T_c even though the superconductivity is optimized there. This implicates spin-singlet, odd-frequency pairing symmetry which can be understood as a direct consequence of a large Fermi surface at the QCP.