Superconductivity and Structural Conversion with Na and K Doping of the Narrow-Gap Semiconductor CsBi₄Te₆

The monoclinic narrow-gap (∼0.08 eV) semiconductor CsBi₄Te₆ is a unique layered system which can be doped to achieve high thermoelectric performance as well as superconductivity. Here, we report superconductivity and structure change induced by alloying CsBi₄Te₆ single crystals with Na and K. Substitution of Na in CsBi₄Te₆ with doping levels ≥0.39 and of K with ≥0.63 transforms the original monoclinic structure (p-type) to the orthorhombic RbBi_3.67Te₆-type structure (n-type). When the K level is ≤0.18, the monoclinic structure type of CsBi₄Te₆ is retained. Transport and magnetic measurements on all as-synthesized doped single crystals demonstrate type-II, bulk superconductivity. A maximal superconducting transition at 5.07 K, which is the highest temperature in bismuth chalcogenide-based superconductors, was obtained in Cs_0.82K_0.18Bi₄Te₆ with a high upper critical field of ∼15 T. These findings suggest superconductivity may be induced by proper doping in narrow-gap semiconductors.