Fig. 1. STM/STS characterization of Bi₂Te₃/FeTe_0.55Se_0.45 heterostructure.

(A and B) Atomically resolved topography (with bias voltage V_bias = 10 mV and tunneling current I_t = 200 pA) of the (A) FeTe_0.55Se_0.45 substrate and (B) 2-QL Bi₂Te₃ thin film. The measured lattice constants of the top Te/Se surface on FeTe_0.55Se_0.45 with square lattice and Te surface on Bi₂Te₃ with hexagonal lattice are 3.80 and 4.38 Å, respectively. Using a deposition rate of about 0.5 QL/min, we obtain the Bi₂Te₃ thin film with very good connectivity. The thickness changes from place to place. (C) Topographic image (V_bias = 2 V, I_t = 10 pA, 150 × 150 nm²) of Bi₂Te₃ film with different thicknesses. The height difference for each step along the dashed yellow arrowed line is about 1.0 nm, which equals a thickness of 1 QL. (D) Typical differential conductance spectra (set-point bias voltage V_set = 0.45 V, I_set = 300 pA) measured on FeTe_0.55Se_0.45 substrate and Bi₂Te₃ thin films with different thicknesses. The black arrows show the positions of the typical kink features probably from the Dirac point of the Bi₂Te₃ films of different thicknesses. (E) Tunneling spectra with SC feature (V_set = 10 mV, I_set = 50 pA) measured on FeTe_0.55Se_0.45 substrate and 1- to 4-QL Bi₂Te₃/FeTe_0.55Se_0.45. The short horizontal bars with the same color mark the zero differential conductance for each curve. The typical feature of the spectra on the FeTe_0.55Se_0.45 substrate is that one or two pairs of coherence peaks can be observed with peak energies varying from 1.1 to 2.1 meV. The tunneling spectrum measured on 1-QL Bi₂Te₃ also has the trace of coherent peaks of FeTe_0.55Se_0.45. This feature is absent on the 2-QL or thicker Bi₂Te₃ films. a.u., arbitrary units.