Artificial Topological Superconductor by the Proximity Effect

Topological superconductors (TSCs), featuring fully gapped bulk and gapless surface states as well as Majorana fermions, have potential applications in fault-tolerant topological quantum computing. Because TSCs are very rare in nature, an alternative way to study the TSC is to artificially introduce superconductivity into the surface states of a topological insulator through the proximity effect [X. L. Qi, T. L. Hughse, S. Raghu, and S. C. Zhang, Phys. Rev. Lett. 102, 187001 (2009); L. Fu and C. L. Kane, Phys. Rev. Lett. 100, 096407 (2008); J. Alicea, Rep. Prog. Phys. 75, 076501 (2012); C. W. J. Beenakker, Annu. Rev. Condens. Matter Phys. 4, 113 (2013)]. Here we report the experimental realization of the proximity effect–induced TSC in ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ thin films grown on a ${\mathrm{NbSe}}_2$ substrate, as demonstrated by the density of states probed using scanning tunneling spectroscopy. We observed Abrikosov vortices and Andreev lower energy bound states on the surface of the topological insulator, with the superconducting coherence length depending on the film thickness and the magnetic field. These results also indicate that the topological surface states of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ thin films are superconducting and, thus, that the ${\mathrm{Bi}}_2{\mathrm{Te}}_3/{\mathrm{NbSe}}_2$ is an artificial TSC. The feasibility of fabricating a TSC with an individual Majorana fermion bound to a superconducting vortex for topological quantum computing is discussed.

Figure 1

Morphology and electronic density of states of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ thin films of different coverages, grown on a ${\mathrm{NbSe}}_2$ substrate. (a) and (b) Large-scale STM images of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ thin films. The inset in (a) shows the atomic resolution of the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ surface taken on a 3QL terrace. (c) $dI/dV$ spectra measured at 4.2 K on 2QL, 3QL, and 5QL ${\mathrm{Bi}}_2{\mathrm{Te}}_3/{\mathrm{NbSe}}_2$ heterostructures and on a 20QL ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ film grown on a Si(111) surface. Blue and red arrows indicate the energy position of VBM and CBM, respectively.

Figure 2

Superconducting energy gap observed on ${\mathrm{Bi}}_2{\mathrm{Te}}_3/{\mathrm{NbSe}}_2$. (a) A series of $dI/dV$ spectra taken on different thicknesses of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ thin films at 0.4 K. (b) $dI/dV$ spectra measured at 0.4 K of bare ${\mathrm{NbSe}}_2$, as well as 2QL and 3QL of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ thin films. All spectra are superimposed with standard BCS-like fitting results. The latter two spectra are shifted upward by 2 and 4, respectively. (c) Thickness dependence of the energy gap obtained from the BCS-like fitting. The dashed line indicates an exponential decay of the gap. The $dI/dV$ spectra measured at 0.4 K on 3QL of a ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ thin film on ${\mathrm{NbSe}}_2$ superimposed with standard BCS-like fitting result is shown in the inset.

Figure 3

(a) Dependence of $dI/dV$ spectra on the magnetic field, measured between vortices on a 3QL ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ thin film at 0.4 K. (b) and (c) Zero-bias $dI/dV$ maps measured at 0.4 K and 0.75 T for ${\mathrm{NbSe}}_2$ and 3QL ${\mathrm{Bi}}_2{\mathrm{Te}}_3/{\mathrm{NbSe}}_2$ heterostructures. (d) and (e) Zero-bias $dI/dV$ maps for a single vortex measured at 0.4 K and 0.1 T on ${\mathrm{NbSe}}_2$ and 5QL ${\mathrm{Bi}}_2{\mathrm{Te}}_3/{\mathrm{NbSe}}_2$. The superimposed hexagons in dashed lines indicate the shape of the vortices. The profile along the dashed line through the center in (d) is fitted and shown in Fig. 4. The black dots at the center of vortex indicate the positions where the zero-bias conductance peaks [shown in Fig. 4] are measured.

Figure 4

(a) Normalized ZBC profiles crossing through the centers of vortices at 0.4 K and 0.1 T on ${\mathrm{NbSe}}_2$ and 3QL ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, respectively. The superimposed lines are fitted results using Eq. (1). The 3QL data are shifted upward by 1.5 for a better view. The obtained coherence length as a function of thickness is summarized in (b). (c) The coherence length as a function of the magnetic field measured on 5QL ${\mathrm{Bi}}_2{\mathrm{Te}}_3/{\mathrm{NbSe}}_2$. (d) Thickness dependence of $dI/dV$ spectra measured at the centers of vortices on ${\mathrm{NbSe}}_2$ and 1QL–6QL ${\mathrm{Bi}}_2{\mathrm{Te}}_3/{\mathrm{NbSe}}_2$ at 0.4 K.