Spin Gap State of S = 1/2 Heisenberg Antiferromagnet YbAl3C3

YbAl 3 C 3 has been investigated in detail, which was previously reported to show an antiferroquadrupolar (AFQ) ordering at an extremely high ordering temperature of 80 K. The present study has revealed that the specific heat, magnetic susceptibility, and electrical resistivity of the single crystalline YbAl 3 C 3 clearly exhibit the characteristic features of the first order transition around 77 K. In addition, we have discovered that LuAl 3 C 3 also exhibits a sharp peak in the specific heat around 110 K, which most likely corresponds to the 77 K transition of YbAl 3 C 3 . Therefore, the 77 K transition of YbAl 3 C 3 is considered to be irrelevant to the nature of the 4 f -electrons such as the AFQ ordering but is ascribed to a structural one. Low temperature physical properties of YbAl 3 C 3 below 77 K is indicative of a spin gap and a nonmagnetic ground state. A new model is proposed, in which the adjacent spins on the triangular lattice of Yb 3+ ions are antiferromagnetically coupled to form a dimer ...