Proximity-induced superconductivity in topological Bi${}_2$Te${}_2$Se and Bi${}_2$Se${}_3$ films: Robust zero-energy bound state possibly due to Majorana fermions

Point contact conductance measurements on topological Bi${}_2$Te${}_2$Se and Bi${}_2$Se${}_3$ films reveal a signature of superconductivity below 2–3 K. In particular, critical current dips and a robust zero-bias conductance peak are observed. The latter suggests the presence of zero-energy bound states that could be assigned to Majorana fermions in an unconventional topological superconductor. We attribute these observations to proximity-induced local superconductivity in the films by small amounts of superconducting Bi inclusions or segregation to the surface, and provide supportive evidence for these effects.

Figure 1

X-ray diffraction results of a 400-nm-thick Bi${}_2$Se${}_3$ film deposited on (111) SrTiO${}_3$ wafer. The insets show AFM images of the surface morphology of this film.

Figure 2

X-ray diffraction results of a 400-nm-thick Bi${}_2$Se${}_3$ film deposited on (100) MgO wafer.

Figure 3

X-ray diffraction results of a 100-nm-thick Bi layer on top of a 700-nm-thick Bi${}_2$Te${}_2$Se (BTS) film deposited on (100) LaAlO${}_3$ wafer.

Figure 4

EDS results in atomic % vs the accelerating voltage of SEM of two Bi${}_2$Se${}_3$ films on (111) SrTiO${}_3$ and (100) MgO, and of the Bi-Bi${}_2$Te${}_2$Se bilayer on (100) LaAlO${}_3$.

Figure 5

A depth profile of a contact hole in a 400-nm-thick Bi${}_2$Se${}_3$ film on (111) SrTiO${}_3$ along the line shown in the AFM image of the inset (same film as in Fig. 1). A schematic drawing of the gold coated tip is also outlined.

Figure 6

Conductance spectra at different temperatures of a point contact junction of gold in contact with a 20-nm-thick Bi film deposited on (100) LaAlO${}_3$ wafer. No conductance dips were observed above about 3 K, and a typical background conductance is shown while heating between 6 and 9 K.

Figure 7

Conductance spectra at 1.85 K under different magnetic fields of the junction of Fig. 6.

Figure 8

Resistance vs temperature of two films. One with traces of Bi on top of 700-nm-thick layer of Bi${}_2$Te${}_2$Se and the other of a bilayer of 100-nm Bi on 700-nm Bi${}_2$Te${}_2$Se, together with zoom in on low temperatures in the insets.

Figure 9

Normalized point contact junction conductance spectrum on the bilayer of Fig. 8. The inset shows a schematic drawing of the contact configuration with the proximity induced superconducting islands close to the interface of the Bi and Bi${}_2$Te${}_2$Se layers.

Figure 10

Normalized conductance spectra at different temperatures of another point contact junction on the Bi-Bi${}_2$Te${}_2$Se bilayer of Fig. 8. For clarity, the spectra are shifted down compared to one another by 0.01 conductance units.

Figure 11

Normalized conductance spectra under different magnetic fields normal to the wafer of the same point contact junction of Fig. 10.

Figure 12

Normalized conductance spectra at different temperatures of a point contact junction on a Bi${}_2$Se${}_3$ film on (111) SrTiO${}_3$ wafer.

Figure 13

Normalized conductance spectra of the same point contact junction of Fig. 12 at $\sim$1.9 K under different magnetic fields normal to the wafer.

Figure 14

Conductance spectra of a point contact junction of gold and a Bi${}_2$Se${}_3$ film on (100) MgO at zero field and different temperatures.

Figure 15

Conductance spectra of the same point contact junction of Fig. 14 at $\sim$1.85 K under different magnetic fields.