Ferroic quadrupolar ordering in CeCoSi revealed using $^{59}$Co-NMR measurements

TL;DR

This study uses $^{59}$Co-NMR measurements to reveal ferroic quadrupole ordering in CeCoSi below 12 K, showing that quadrupole order can occur even with well-separated crystal-electric-field levels.

Contribution

It provides experimental evidence of ferroic quadrupole ordering in CeCoSi, highlighting the occurrence of quadrupole order in systems with well-separated CEF levels.

Findings

Ferroic quadrupole order detected below 12 K in CeCoSi.

External magnetic field induces Ce dipole moments orthogonal to the field.

Quadrupole ordering occurs despite well-separated CEF levels.

Abstract

A nonmagnetic phase transition at $T_0 \sim 12$ K in the tetragonal system CeCoSi with a Kramers doublet ground state is reminiscent of an electric quadrupole ordering, even though its well-separated crystal-electric-field (CEF) levels are unlikely to acquire higher-order multipole degrees of freedom. Here, we report $^{59}$Co nuclear magnetic resonance (NMR) studies that are highly compatible with a ferroic quadrupole ordering below $T_0$. Changes in the NMR spectra below $T_0$ suggest that an external magnetic field induces ferroic Ce dipole moments orthogonal to the field, enabling domain selection in the nonmagnetic phase. Our findings suggest the presence of a ferroic $O_{zx}$-type quadrupole component in CeCoSi and demonstrate that quadrupole ordering may occur under well-separated CEF levels in tetragonal systems.