Spin‐Mixing Enhanced Proximity Effect in Aluminum‐Based Superconductor–Semiconductor Hybrids

Abstract

Narrow-gap semiconductors with a large g-factor and low carrier density (such as InAs and InSb) are most commonly used, either as 1D nanowires [3] or 2D electron gases. [4] The first generation of semiconductor-superconductor hybrids was made using Nb [5] and NbTiN [6] as the superconductor. While these materials offer a large superconducting gap and resilience to high magnetic fields, the hybrids suffered from a finite in-gap conductance (often described as "soft-gap"). In addition, Nb-based hybrids have not been shown to host parity-conserving transport-a key ingredient for the development of topological qubits. [7] These drawbacks remained even after substantial improvements of the fabrication, such as epitaxial growth of the superconductor. [8] In the meantime, aluminum has emerged as the material of choice. Thin shells made of this metal combined with an oxide-free interface result in clean electronic transport. [9,10] This includes suppressed sub-gap tunneling conductance (hard induced gap) and parity-conserving transport, [11] which enables the search for topological superconductivity. For a topological phase to emerge, the minimal condition states that the Zeeman energy V g B Z B

Subgap spectroscopy along hybrid nanowires by nm-thick tunnel barriers

“…4 b top) suggests that the subgap state is localized close to the semiconducting junction. Such subgap states are commonly detected in tunneling spectroscopy with semiconducting tunnel barriers in two-terminal 31 and three-terminal 16 , 32 – 35 hybrid nanowire devices. Interestingly, the conductance dependences in the bottom panels of Fig.…”

Zero-frequency supercurrent susceptibility signatures of trivial and topological zero-energy states in nanowire junctions

“…We discretize the continuum model of equation ( 1) into a tight-binding lattice with lattice constant a = 10 nm and set t = 25.4 meV to be the nearest neighbor hopping, which corresponds to an effective mass m = 0.015m e , in accordance with experimental values for InSb [2,77]. With this, we then model an SNS junction with N (S) regions of finite length L N(S) , whose values are defined when presenting the respective junctions below.…”

Impact of Junction Length on Supercurrent Resilience against Magnetic Field in InSb-Al Nanowire Josephson Junctions

“…The junction length has been found to be an essential parameter that determines the supercurrent evolution in a parallel magnetic field. In the long devices (L ≈ 160 nm), the supercurrent is suppressed quickly in a magnetic field and fully vanishes at parallel fields of ∼0.7 T. In contrast, the supercurrent in short devices (L ≈ 30 nm) persists up to parallel fields of ∼1.3 T, approaching the critical in-plane magnetic field of the Al film (∼1.5 T 26,27,34 ). Despite the influence of the electrochemical potential in the juntions, the resilient supercurrent is present only in the short devices (L ≈ 30 nm).…”

“…Preserving the supercurrent in hybrid nanowire JJs at high magnetic fields thus becomes critically important. Selecting high critical field superconductors, such as NbTiN, 23 Pb, 24 Sn, 25 or Al doped by Pt, 26 seems to be an option for improving the magnetic field compatibility of the supercurrent. However, none of these material platforms have yielded a supercurrent at high magnetic fields.…”

“…The superconducting Al shell has a mean free path l e of ∼0.9 nm according to a recent work, 26 which uses the same machine for the Al growth. The extraordinarily short l e in the thin Al shell is most likely due to massive surface scatterings and moderate nonuniformities.…”