Unusual Nonmagnetic Ordered State in CeCoSi Revealed by $^{59}$Co-NMR and NQR Measurements

TL;DR

This study reveals a nonmagnetic ordered state in CeCoSi below a phase transition, likely an electric multipolar order of Ce 4f electrons, identified through $^{59}$Co-NMR and NQR measurements under pressure.

Contribution

It provides the first evidence of an electric multipolar ordered state in CeCoSi using NMR and NQR, highlighting pressure-induced Kondo effects and complex electronic ordering.

Findings

Phase transition at T0 is nonmagnetic with no symmetry lowering at Co site.

NMR spectra indicate an induced magnetic field, suggesting complex ordering.

Pressure enhances Kondo effect, stabilizing the multipolar ordered state.

Abstract

We performed $^{59}$Co nuclear magnetic and quadrupole resonance (NMR and NQR) measurements under pressure on a single-crystalline CeCoSi, which undergoes an unresolved phase transition at $T_0$. The NQR spectra clearly showed that the phase transition at $T_0$ is nonmagnetic, but any symmetry lowering at the Co site was not seen irrespective of the feature of second-order phase transition. By contrast, the NMR spectra were split by the induced magnetic field perpendicular to the external magnetic field. These results show that the phase below $T_0$ is not a simple paramagnetic state but is most likely electric multipolar ordered state of Ce $4f$ electrons. The development of the Kondo effect by applying pressure is thought to be crucial to stabilize this state and to show novel features beyond commonality of tetragonal Ce-based systems.