Development of in-gap state and related collapse of two-step energy gap structure by nonmagnetic ion substitution in Kondo insulator YbB₁₂

YbB₁₂, which has attracted attention as an unconventional strongly correlated insulator, has a two-step energy gap structure (ΔE₁, ΔE₂) with an in-gap state. These characteristics cannot be explained by a simple c-f hybridization gap model. To clarify the difference between the ground states in the Kondo insulator phase and in the dilute Kondo metal, we have studied the nonmagnetic ion substitution effect on electrical conductivity σ, magnetic susceptibility χ, and specific heat divided by temperature C/T measurements of the Kondo insulator Yb_1−xR_xB₁₂ (R = Ca²⁺, Lu³⁺, Y³⁺, and Zr⁴⁺). It is found that there are a sensitive substitution effect in the ΔE₂-dominant temperature region (T < 15 K) and an insensitive one in the ΔE₁-dominant region (15 K < T < 100 K). Here, on the basis of Schlottmann's model we have proposed a novel density of states (DOS) model constructed from the x dependences of σ, χ, and C/T with a boundary of x₀ ≃ 0.1. We conclude that our DOS model can also be applied to YbB₁₂ by taking into account a small amount of defects. (author)