Is Bi(110) on Ge(111) topological or trivial?

In the realm of topological materials, bismuth is regarded as one of the prime candidates owing to its large spin–orbit coupling. Recent studies have revealed that the rectangular Bi(110) phase may host topological edge or higher-order topological hinge states. The topological nature of Bi(110) depends critically on the buckling of the (110) lattice. Only buckling below 0.1 Å may yield topologically non-trivial states. In this work, we examine the Bi(110) phase grown on Ge(111) using scanning tunneling microscopy and spectroscopy. The Bi(110) islands grow in four sets of threefold high-symmetry directions, which are rotated by ±13.7° and ±16.3° with respect to the high-symmetry directions of the (3×3)R30° Bi wetting layer covering the Ge(111) surface, revealing that the diagonals of the Bi(110) and Bi wetting layer unit cells align. Multilayer Bi(110) islands show a large buckling and an increased density of states at step edges near −0.35 eV. We find this state to emerge from the Bi wetting layer surface rather than the edge itself. The buckling of bilayer Bi(110) islands is lower and lies on the cusp of the trivial to topological transition. However, we find no evidence of electronic enhancement at its edges or corners.