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The electronic state in 1T-TiSe₂, which has attracted attention as an excitonic insulator, can be changed by electronic doping through Cu⁺ intercalation. Indeed, the superconductivity was reported in Cu_xTiSe₂ on 0.04 ≤ x ≤ 0.10 [1]. We succeeded in synthesis of single crystal samples of Cu_xTiSe₂ (x = 0 ~ 0.35) and performed physical property measurements, synchrotron x-ray diffraction experiments, and density-functional-theory calculations. It was found that x = 1/4 and 1/3 were magical numbers in this system and disorder-order transitions were caused by the interaction among Cu atoms. Furthermore, in x = 1/3, a hybrid-type phase transition caused by the commensurate connection between Cu-ordering and a new honeycomb lattice-type charge-density-wave in TiSe₂ layers was realized.
[1] E. Morosan et al., Nat. Phys. 2, 544 (2006).