Observation of the Superconducting Proximity Effect in the Surface State of ${\mathrm{SmB}}_6$ Thin Films

The proximity effect at the interface between a topological insulator and a superconductor is predicted to give rise to chiral topological superconductivity and Majorana fermion excitations. In most topological insulators studied to date, however, the conducting bulk states have overwhelmed the transport properties and precluded the investigation of the interplay of the topological surface state and Cooper pairs. Here, we demonstrate the superconducting proximity effect in the surface state of ${\mathrm{SmB}}_6$ thin films which display bulk insulation at low temperatures. The Fermi velocity in the surface state deduced from the proximity effect is found to be as large as ${10}^5\mathrm{m}/\mathrm{s}$, in good agreement with the value obtained from a separate transport measurement. We show that high transparency between the topological insulator and a superconductor is crucial for the proximity effect. The finding here opens the door to investigation of exotic quantum phenomena using all-thin-film multilayers with high-transparency interfaces.

Popular Summary

Superconductivity in the topologically protected surface states of a three-dimensional topological insulator has been predicted to be a promising platform for exploring exotic quantum states such as Majorana fermion excitations. The superconducting proximity effect is a phenomenon occurring at a superconductor–normal-metal interface in which Cooper pairs diffuse into the normal metal, resulting in induced surface or local superconductivity in the normal metal. Although there have been previous experimental efforts focused on studying the superconducting proximity effect in bilayer structures between a superconductor and a chalcogenide topological insulator, suppressing the conducting bulk contribution and securing high interfacial transparency between a superconductor and a topological insulator have been major experimental bottlenecks to demonstrating superconductivity. Here, we demonstrate the superconducting proximity in the surface state of a topological insulator realized by a samarium hexaboride (${\mathrm{SmB}}_6$) thin film with in situ deposition of a superconducting layer.

Known as a prototypical Kondo insulator, ${\mathrm{SmB}}_6$ has recently been reported to possess a topologically protected surface surrounding its insulting core. We ensure high interfacial quality between a superconductor and ${\mathrm{SmB}}_6$ by growing a superconducting layer immediately after fabrication of the ${\mathrm{SmB}}_6$ films, without breaking the vacuum in the deposition chamber. We extract physical parameters of the surface state of ${\mathrm{SmB}}_6$, including the thickness of the surface state (approximately 6 nm), the normal coherence length, and the Fermi velocity; these data provide unambiguous evidence of the superconducting proximity effect induced in the surface of ${\mathrm{SmB}}_6$.

Our findings provide unique insight into the surface state of ${\mathrm{SmB}}_6$. The strong proximity effect we observe at the superconductor-${\mathrm{SmB}}_6$ interface is an important stepping stone for pursuing novel quantum phenomena using thin-film topological insulator devices.

Figure 1

Sheet conductance ($G_{\square }$) of ${\mathrm{SmB}}_6$ thin films at 2 and 300 K as a function of film thickness. Dashed lines are linear fits to the experimental results. The slope of 300 K data represents 3D bulk conductivity at 300 K in ${\mathrm{SmB}}_6$ thin films, and the fit to the data (black dot line) extrapolates to zero at zero thickness, consistent with the fact that the conduction in ${\mathrm{SmB}}_6$ thin films is dominated by a 3D bulk transport at 300 K. At 2 K, $G_{\square }$ is independent of the thickness, and the guide line (red dotted line) is the fit to the data, which indicates the average value of $G_{\square }$ at 2 K. The inset shows resistance versus temperature curve for a 25-nm ${\mathrm{SmB}}_6$ thin film.

Figure 2

Estimating the thickness of the surface conduction channel in ${\mathrm{SmB}}_6$ thin films. (a) Conductance versus temperature of a 25-nm ${\mathrm{SmB}}_6$ thin film. The red line is a best fit to the experimental data using the parallel conductance model. (b) Activation energy ($E_a$) and the thickness of the surface conduction channel ($t_{\text{surface}}$) extracted from the fit as a function of the film thickness for ${\mathrm{SmB}}_6$.

Figure 3

Model for the proximity effect for superconducting-TI system. Given the surface conducting channel of the ${\mathrm{SmB}}_6$ as discussed in the text, we adopt the model of the proximity effect for the superconducting layer and the surface conduction channel, the surface state of ${\mathrm{SmB}}_6$. The plot in the schematic represents the evolution of the superconducting pair potential [$\mathrm{\Delta }(\mathrm{z})$]. In this model, we consider the case where the normal coherence length of ${\mathrm{SmB}}_6$ (${\xi }_{{\mathrm{SmB}}_6}$) is larger than the thickness of the surface state of ${\mathrm{SmB}}_6$ ($t_{\text{surface}}$).

Figure 4

Proximity effect in ${\mathrm{Nb}/\mathrm{SmB}}_6$ bilayer. Resistance versus temperature curves of (a) single Nb layers and (b) $\mathrm{Nb}/{\mathrm{SmB}}_6$ bilayers for different Nb layer thicknesses ($d_{\mathrm{Nb}}$) where the ${\mathrm{SmB}}_6$ layer thickness is fixed to be 50 nm. Each Nb layer is deposited on ${\mathrm{SmB}}_6$ in situ. The resistance values are normalized by values obtained at temperatures slightly above each transition temperature ($R_{\text{residual}}$). (c) $T_{cb}/T_{cs}$ (i.e., ratio of $T_c$ of $\mathrm{Nb}/{\mathrm{SmB}}_6$ bilayer to $T_c$ of single Nb layer) as a function of $d_{\mathrm{Nb}}$. The data points from bilayers with different ${\mathrm{SmB}}_6$ layer thicknesses (10 and 25 nm) are also included. The solid line is a fit using the Usadel equation. The star indicates $T_{cb}/T_{cs}$ of the $\mathrm{Nb}/{\mathrm{SmB}}_6$ bilayer where 20-nm Nb is deposited on ${\mathrm{SmB}}_6$ (50 nm) after its surface is first exposed to air, forming an ex situ interface.