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 RbBi3.67Te6-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 Cs0.82K0.18Bi4Te6 with a high upper critical field of ∼15 T. These findings suggest superconductivity may be induced by proper doping in narrow-gap semiconductors.