[Paper Review] Gate-controlled Supercurrent in Ballistic InSb Nanoflag Josephson Junctions

Sedighe Salimian, Matteo Carrega+6 more|arXiv (Cornell University)|Nov 2, 2021

Topological Materials and PhenomenaPhysics and Astronomy91 references18 citations

TL;DR

This study demonstrates gate-tunable supercurrent in ballistic InSb nanoflag Josephson junctions with Ti/Nb contacts, achieving a critical supercurrent of ~50 nA at 250 mK. The devices exhibit clear subharmonic gap structures and a sizable excess current, indicating phase-coherent, highly transparent interfaces, establishing InSb nanoflags as a promising platform for topological quantum devices.

ABSTRACT

High-quality III-V narrow band gap semiconductor materials with strong spin-orbit coupling and large Lande g-factor provide a promising platform for next-generation applications in the field of high-speed electronics, spintronics, and quantum computing. Indium Antimonide (InSb) offers a narrow band gap, high carrier mobility, and a small effective mass, and thus is very appealing in this context. In fact, this material has attracted tremendous attention in recent years for the implementation of topological superconducting states supporting Majorana zero modes. However, high-quality heteroepitaxial two-dimensional (2D) InSb layers are very diffcult to realize owing to the large lattice mismatch with all commonly available semiconductor substrates. An alternative pathway is the growth of free-standing single-crystalline 2D InSb nanostructures, the so-called nanoflags. Here we demonstrate fabrication of ballistic Josephson-junction devices based on InSb nanoflags with Ti/Nb contacts that show gate-tunable proximity-induced supercurrent up to 50 nA at 250 mK and a sizable excess current. The devices show clear signatures of subharmonic gap structures, indicating phase-coherent transport in the junction and a high transparency of the interfaces. This places InSb nanoflags in the spotlight as a versatile and convenient 2D platform for advanced quantum technologies.

Motivation & Objective

• To develop high-quality, free-standing two-dimensional InSb nanoflags (NFs) as a defect-free platform for quantum devices.
• To fabricate ballistic Josephson junctions using InSb NFs with Ti/Nb superconducting contacts.
• To demonstrate gate-tunable proximity-induced supercurrent and probe transport coherence in the junction.
• To investigate the transparency and coherence of the interfaces via multiple Andreev reflection (MAR) features.
• To establish the suitability of InSb NFs for future topological superconductivity and Majorana zero-mode research.

Proposed method

• Growth of single-crystalline, free-standing InSb nanoflags on tapered InP nanowire stems via chemical beam epitaxy (CBE).
• Mechanical transfer of 100 nm thick InSb NFs onto SiO2/Si substrates for device fabrication.
• Fabrication of Josephson junctions with 200 nm interelectrode spacing using 10/150 nm Ti/Nb contacts.
• Electrical characterization via DC I-V measurements and lock-in differential resistance (dV/dI) at 250 mK.
• Numerical derivation of 2D dV/dI maps from measured I-V curves to identify MAR features and excess current.
• Analysis of MAR traces and I-V characteristics using BCS theory and Aminov et al. interface transparency model (γB parameter).

Experimental results

Research questions

• RQ1 Can gate-voltage tuning modulate the supercurrent in ballistic InSb nanoflag Josephson junctions?
• RQ2 What is the degree of phase coherence and interface transparency in the junction, as indicated by subharmonic gap structures?
• RQ3 How does the induced superconducting gap in the InSb NF compare to the BCS gap of Nb?
• RQ4 To what extent does the excess current and normal resistance vary with back-gate voltage?
• RQ5 Can the observed transport behavior confirm ballistic, proximity-induced superconductivity in the InSb NF?

Key findings

• A gate-tunable supercurrent of ~50 nA was observed at 250 mK, with critical current persisting up to 50 nA under back-gate tuning.
• Subharmonic gap structures (MAR features) were clearly resolved, indicating phase-coherent transport and high interface transparency.
• An excess current of 265 ± 12 nA was measured at Vbg = 40 V, with Ie·Rn = 127 ± 7 µV, consistent with theoretical expectations for ballistic junctions.
• The product Ie·Rn remained approximately constant at ~137 ± 19 µV across Vbg from 5 V to 40 V, indicating robustness of the Andreev reflection process.
• The induced gap ∆* was estimated to be significantly smaller than the BCS gap of Nb (1.28 meV), consistent with proximity effect through a Ti interlayer.
• The interface transparency parameter γB ≈ 10 was derived, indicating a moderately transparent interface, while MAR features suggest high transparency of the vertical interface between covered and uncovered InSb regions.