Spectroscopic Visualization of a Robust Electronic Response of Semiconducting Nanowires to Deposition of Superconducting Islands

Abstract

Following significant progress in the visualization and characterization of Majorana end modes in hybrid systems of semiconducting nanowires and superconducting islands, much attention is devoted to the investigation of the electronic structure at the buried interface between the semiconductor and the superconductor. The properties of that interface and the structure of the electronic wavefunctions that occupy it determine the functionality and the topological nature of the superconducting state induced therein. Here we study this buried interface by performing spectroscopic mappings of superconducting aluminum islands epitaxially grown in-situ on indium arsenide nanowires. We find unexpected robustness of the hybrid system as the direct contact with the aluminum islands does not lead to any change in the chemical potential of the nanowires, nor does it induce a significant band bending in their vicinity. We attribute this to the presence of surface states bound to the facets of the nanowire. Such surface states, that are present also in bare nanowires prior to aluminum deposition, pin the Fermi-level thus rendering the nanowires resilient to surface perturbations. The aluminum islands further display Coulomb blockade gaps and peaks that signify the formation of a resistive tunneling barrier at the InAs-Al interface. The extracted interface resistivity, ρ ≈ 1.3 × 10 −6 Ω cm 2 , will allow to proximity-induce superconductivity with negligible Coulomb blockade effects by islands with interface area as small as 0.01 µm 2 . At low energies we identify a potential energy barrier that further suppresses the transmittance through the interface. A corresponding barrier exists in bare semiconductors between surface states and the accumulation layer, induced to maintain charge neutrality. Our observations elucidate the delicate interplay between the resistive nature of the InAs-Al interface and the ability to proximitize superconductivity and tune the chemical potential in semiconductor-superconductor hybrid nanowires.

Improved effective equation for the Rashba spin-orbit coupling in semiconductor nanowires

“…This is because, at this gate voltage value, the electric field inside the wire is basically zero. It occurs at V gate = 0 due to the surface charge present at the nanowire facets, which introduces a small pinned electrostatic field [57]. As was noticed in previous works [44], the SO coupling is larger for the zinc-blende crystal than for the wurtzite one, approximately a factor of four.…”

“…This surface charge cannot be removed using gates and it thus gives an intrinsic contribution to the electrostatic potential. In this work, we choose ρ surf = 5 × 10 −3 ( e nm 3 ), similarly to previous theoretical works [53,54,56], and in agreement with experimental evidence [39,57]. This value however does not play any fundamental role but simply results in a small particular contribution to the intrinsic doping of the wire and its SO coupling [54].…”

Coulomb blockade effects in a topological insulator grown on a high-Tc cuprate superconductor

“…8), similarly to what had been observed in quantum dots 37 . Another explanation for these peaks may be due to the capacitive coupling between the tip and the island 36 , which could be…”

“…Coulomb blockade origins of the spectral gap Given the additional barrier at the interface we have discovered and the finite size of the system, a natural explanation for the gap could be understood in terms of a Coulomb blockade (CB) gap 30 which arises due to single electrons exchanging energy with the environment as they tunnel through a barrier 31,32 . The CB effect has been widely reported in tunneling measurements of finite-size heterostructures when there is an extra barrier at the interface of the two materials sandwiched together [33][34][35][36] . It can be modeled by a double tunnel junction, one being the tip-sample junction and the other one being the sample-substrate junction, each consisting of a capacitor and a resistor connected in parallel (Fig.…”

Sub-Band Spectrum Engineering via Structural Order in Tapered Nanowires

“…We harvested the NWs onto a Au substrate and transferred them in a designated ultrahigh vacuum suitcase, as developed in our previous studies. 19 , 20 The results of the STM measurements showing the topography and demonstrating the impact of tapering on its energy spectrum are presented in Figure 3 .…”