Symmetry Analysis of Zero-Field Antiferroquadrupole Order in CeB6: Extremely Low-Frequency B-11-NQR Study

The zero-field state in the antiferroquadrupole (AFQ) ordered phase of CeB6 has been investigated by a B-11-nuclear quadrupole resonance (NQR) measurement that is sensitive to changes in local charge distribution. In order to achieve this experiment, we have improved our low-frequency NQR techniques, so that the B-11-NQR spectrum has been successfully observed at an NQR frequency nu(Q) similar to 560 kHz, exceptionally low frequency among direct NQR measurements on strongly correlated electron systems. The temperature dependence of the NQR spectrum reveals that nu(Q) rapidly decreases just below AFQ ordering temperature T-Q = 3.3 K, while the spectral shape does not show any sign of site splitting below T-Q. The latter is incompatible with the theoretical prediction based on the O-xy-type AFQ order, suggesting that the ordered state in the zero-field region differs from that in the nonzero-field region. Some new possible interpretations to explain the obtained results are proposed.