Successive phase transitions and energy-gap formation in CeRhAs

Magnetic, transport, and x-ray diffraction measurements on single-crystal CeRhAs, the so-called Kondo insulator, have revealed successive transitions at $T_1=370,$ $T_2=235,$ and $T_3=165\mathrm{}\mathrm{K}.$ Below $T_1,$ the unit cell is doubled along the b and c axes. Thereby, the resistivity jumps upwards and magnetic susceptibility in all the directions drops. Superlattice reflections at (0,1/3,1/3) and (1/3,0,0) appear at $T<\mathrm{}{T}_2.$ The latter suddenly increases below $T_3$ where a transport gap is enhanced. These observations indicate that the gap formation is intimately related to lattice modulations.