Stoichiometry effect on the superconductivity of YB₆ thin films

Rare earth borides exhibit a rich range of physical properties including superconductivity, metal-metal transitions, mixed-valence chemistry, etc. Rare earth borides display complex phase diagrams with various chemical compounds, and the stoichiometric effect in these materials is considered to play a critical role in altering their physical properties. Yttrium hexaboride (YB₆) is a known superconducting material (T_c ~ 7 K), but its superconducting properties have been reported to be strongly dependent on the composition. To systematically address its stoichiometric effect on the physical properties of YB₆, we fabricated natural compositional spread films of YB₆ by sputtering. Here we show the change in the structural and superconducting characteristics of YB₆ thin films depending on the B/Y ratio. It is found that B deficiency improves the superconducting transition temperature (T_c), but the superconducting coherence length is maximum near stoichiometric YB₆. YB₆ thin films with robust superconductivity and its extremely small lattice misfit with SmB₆ provide a platform to explore exotic superconductivity with topological insulator SmB₆ by fabricating isostructural SmB₆/YB₆ bilayers.