75As NQR/NMR study of successive phase transitions and energy gap formation in Kondo semiconductor CeRhAs

As-75 NQR/NMR studies were performed to investigate the successive phase transitions found recently, the gap formation and their interplay in a Kondo semiconductor CeRhAs. NQR/NMR spectra in their respective phases change, reflecting lattice modulation modes, q(1) = (0,1/2,1/2), q(2) = (0,1/3,1/3) and q(3) = (1/3,0,0). In particular for well-resolved three NQR lines corresponding to the q(3) mode in the lowest temperature phase, the nuclear spin-lattice relaxation rate (T1T)(-1) shows an activation type T-dependence, suggesting a gap opening over the entire Fermi surface, in contrast to the V-shaped gap in isostructural CeNiSn and CeRhSb. The evaluated gap of 272 K and the bandwidth of about 4000 K are one order of magnitude larger than those in CeNiSn and CeRhSb. A lattice modulation forms a gap different from the V-shaped gap.