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

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YbAl₃C₃ 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 YbAl₃C₃ clearly exhibit the characteristic features of the first order transition around 77 K. In addition, we have discovered that LuAl₃C₃ also exhibits a sharp peak in the specific heat around 110 K, which most likely corresponds to the 77 K transition of YbAl₃C₃. Therefore, the 77 K transition of YbAl₃C₃ is considered to be irrelevant to the nature of the 4f-electrons such as the AFQ ordering but is ascribed to a structural one. Low temperature physical properties of YbAl₃C₃ 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 Yb³⁺ ions are antiferromagnetically coupled to form a dimer by the structural transition at 77 K. The spin gap behavior at low temperatures can be well explained by an isolated dimer model with a singlet–triplet energy gap of about 15 K.