Robust Transport Signatures of Topological Superconductivity in Topological Insulator Nanowires

Finding a clear signature of topological superconductivity in transport experiments remains an outstanding challenge. In this work, we propose exploiting the unique properties of three-dimensional topological insulator nanowires to generate a normal-superconductor junction in the single-mode regime where an exactly quantized $2e^2/h$ zero-bias conductance can be observed over a wide range of realistic system parameters. This is achieved by inducing superconductivity in half of the wire, which can be tuned at will from trivial to topological with a parallel magnetic field, while a perpendicular field is used to gap out the normal part, except for two spatially separated chiral channels. The combination of chiral mode transport and perfect Andreev reflection makes the measurement robust to moderate disorder, and the quantization of conductance survives to much higher temperatures than in tunnel junction experiments. Our proposal may be understood as a variant of a Majorana interferometer which is easily realizable in experiments.

Figure 1

(a) A NS junction formed with a TI nanowire. (b) Schematic representation of the modes involved in transport: a chiral mode splits into two Majorana modes at the interface, recombines, and exits as a chiral mode again. (c) The Majorana interferometer proposed in Refs. [36, 37]. $S$ and $D$ denote source and drain, respectively. (d) An unfolded representation of the setup in (b).

Figure 2

(a) The spectrum of a wire of dimensions $h=40\mathrm{nm}$ and $w=160\mathrm{nm}$ for $B_{\perp }=0$ and $\eta =0$ (left), $B_{\perp }=0$ and $\eta =1/2$ (center), and $B_{\perp }=2T$ (right). Note that in the last case the spectrum is independent of $\eta$. (b) NS conductance for $B_{\perp }=0$ and ${\mathrm{\Delta }}_0=0.25\mathrm{meV}$ as a function of $\mu$, for $\eta =0$, $n_v=0$ (dashed line) and $\eta =1/2$, $n_v=1$ (full line) (c) The same for $B_{\perp }=2T$.

Figure 3

Disorder averaged conductance for a finite wire of dimensions $L=400\mathrm{nm}$, $h=40\mathrm{nm}$, and $w=160\mathrm{nm}$ for different values of the disorder strength $g$. (a) Conductance as a function of chemical potential with fixed magnetic field $B_{\perp }=2$ T. Inset: Cross section of the rectangular wire, with the coordinate $s$ depicted as a dashed arrow. (b) Conductance as a function of $B_{\perp }$ at fixed chemical potential $\mu =10\mathrm{meV}$. Inset: The vector potential $A_x$ as given by Eq. (8), and its associated magnetic field profile.