Effects of Alkali Substitution and Pressure on the Charge-Density Wave Transitions of Two-Dimensional Metals K₃Cu₈S₆ and Rb₃Cu₈S₆

抄録

K₃Cu₈S₆ is a two-dimensional metal which has two phase transitions at T₁ (153 K) and T₂ (55 K), accompanied by an incommensurate and a commensurate superstructure, respectively. In order to investigate these phase transitions, the mixed crystals (K_1−xRb_x)₃Cu₈S₆ have been synthesized and the effect of the alkali substitution and that of pressure have been studied by the measurements of their transport properties. With increasing x, T₁ significantly decreases, reaching T₁ of Rb₃Cu₈S₆ (=110 K). Pressure also reduces T₁ of K₃Cu₈S₆ and Rb₃Cu₈S₆. On the other hand, the T₂ transition becomes obscure with increasing x, and disappears thoroughly for x≥0.50. In Rb₃Cu₈S₆, the T₂ transition is absent at ambient pressure but induced by the pressure of at most 0.5 GPa. Around x=0.06, a remarkable rise in the resistivity appears below T₂. Its mechanism is discussed.