Unusual Nonmagnetic Ordered State in CeCoSi Revealed by 59Co-NMR and NQR Measurements

We performed 59Co nuclear magnetic and quadrupole resonance (NMR and NQR) measurements under pressure on a single-crystalline CeCoSi, which undergoes an unresolved phase transition at T0. The NQR spectra clearly showed that the phase transition at T0 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 T0 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.