Proximity-effect-induced superconducting phase in the topological insulator Bi${}_2$Se${}_3$

We have studied the electron transport properties of topological insulator-related material Bi${}_2$Se${}_3$ near the superconducting Pb-Bi${}_2$Se${}_3$ interface, and found that a superconducting state is induced over an extended volume in Bi${}_2$Se${}_3$. This state can carry a Josephson supercurrent, and demonstrates a gaplike structure in the conductance spectra as probed by a normal-metal electrode. The establishment of the gap is not by confining the electrons into a narrow space close to the superconductor-normal metal interface, as previously observed in other systems, but presumably via electron-electron attractive interaction in Bi${}_2$Se${}_3$.

Figure 1

(a) and (b) SEM images of the device. The red dotted rectangular area in (a) is magnified in (b). The red and blue rectangles in (b) illustrate the possible areas responsible for the periods of the Fraunhofer diffraction patterns in Figs. 2a and 2b, respectively. (c) Illustration of the measurement configuration for probing the resistance of the PE-affected region. The sections measured by lock-in amplifiers V1 and V2 are denoted as S1 and S2, respectively. The areas in light blue near the Pb film indicate the PE-affected regions. (d) Illustration of a three-terminal measurement configuration for probing the conductance spectrum of a Pd-Bi${}_2$Se${}_3$ contact in the PE-affected region.

Figure 2

(a) 2D plot of the $dV/dI$ data for S1 measured as a function of magnetic field and dc bias current. The white area represents a zero-resistance state. The grid lines and the arrows show the minima of the two superimposed Fraunhofer diffraction patterns. The possible effective areas corresponding to the two periods of the patterns are indicated in Fig. 1b by the red rectangles. The red dotted line is a fitting curve discussed in the text. Left inset: $dV/dI$ versus bias current in zero field. The two down arrows indicate the echo of the superconducting transition of S2. Right inset: zero-bias $dV/dI$ versus magnetic field. (b) 2D plot of the $dV/dI$ data for S2. Besides the main Fraunhofer pattern, echo from the superconducting transition of S1 can be clearly seen as a thin white curve. Left inset: $dV/dI$ versus bias current in zero field. Right inset: zero-bias $dV/dI$ versus magnetic field.

Figure 3

2D plots of $dV/dI$ of S1 as a function of magnetic field and dc bias current at (a) 240 mK, (b) 480 mK, (c) 800 mK, and (d) 1 K. The inset shows the temperature dependence of the zero-field peak height of the Fraunhofer patterns. Black squares: using $dV/dI$ maximum as the criteria. Red squares: using zero resistance as the criteria.

Figure 4

(a) Conductance spectra of a Pd contact 95 nm away from the Pb film, measured in a three-terminal configuration shown in Fig. 1d. The curves other than the 15 mK one are shifted vertically for clarity. (b) Conductance spectra of the same contact measured at 15 mK and in different magnetic fields. Again, the curves other than the one taken in zero field are shifted vertically for clarity. (c) Peak position plotted as functions of temperature (black squares) and magnetic field (red squares). The black line shows the gap-temperature relation expected from the BCS theory. (d) Zero-bias conductance at different temperatures (black squares) and in different fields (red squares).