Hybrid Symmetry Epitaxy of the Superconducting Fe(Te,Se) Film on a Topological Insulator.

It is challenging to grow an epitaxial 4-fold compound superconductor (SC) on a 6-fold topological insulator (TI) platform due to the stringent lattice-matching requirement. Here, we demonstrate that Fe(Te,Se) can grow epitaxially on a TI (BiTe) layer due to accidental, uniaxial lattice match, which is dubbed as "hybrid symmetry epitaxy". This new growth mode is critical to stabilizing robust superconductivity with as high as 13 K. Furthermore, the superconductivity in this FeTeSe/BiTe system survives in the Te-rich phase with Se content as low as = 0.03 but vanishes at Se content above = 0.56, exhibiting a phase diagram that is quite different from that of the conventional Fe(Te,Se) systems. This unique heterostructure platform that can be formed in both TI-on-SC and SC-on-TI sequences opens a route to unprecedented topological heterostructures.