Superconductivity in 3R-Ta_1-xM_xSe₂ (M = W, Mo)

The 3-layer rhombohedral (3R) polytype of TaSe_2-xTe_x is known to display a superconducting transition temperature that is between 6 and 17 times higher than that of the two-layer hexagonal (2H) polytype. The remarkable difference in T _c, although clearly associated with a difference in polytype, could have been due to an electronic effect specific to the Te-Se substitution. Here we report that small amounts of Mo or W doping lead to a 2H to 3R polytype transition in Ta_1-xMo_xSe₂ and Ta_1-xW_xSe₂. The 3R polytype materials are again found to have substantially higher T _c (∼2 K for Ta_0.9W_0.1Se₂ and Ta_0.9Mo_0.1Se₂) than the 2H material (0.15 K), eliminating the possibility that any special characteristics of the Te/Se substitution are responsible for the dramatic difference in T _c. We infer that a three-layer stacking sequence is strongly preferred for superconductivity over a two-layer stacking sequence in the TaSe₂ system.