Destabilization of Excitonic Phase by Elemental Substitution in (Ta_1−xM_x)₂NiSe₅ (M = V, Nb) and Ta₂(Ni_1−yT_y)Se₅ (T = Fe, Co)

We investigated the effect of elemental substitution on an excitonic insulator (EI) candidate Ta2NiSe5, by measuring the electrical resistivity and magnetic susceptibility using single crystalline samples of (Ta1−xMx)2NiSe5 (M = V, Nb) and Ta2(Ni1−yTy)Se5 (T = Fe, Co). These elemental substitutions destabilize the EI state. The reduction rates of the EI transition temperature Tc and energy gap Eg, which characterizes the preformed exciton state, depend on substituting elements and appear to be prominent for V and Co substitution. On the other hand, regardless of substituting elements, Tc shows a universal relation to Eg, suggesting that the BCS–BEC crossover occurs at Eg ∼ 750 K.