Ferroic Quadrupolar Ordering in CeCoSi Revealed Using ^59Co-Nmr Measurements

A nonmagnetic phase transition at T_0 ∼ 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 ^59Co 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.