Antonio Lombardi, Gaurav Shukla, Giada Bucci, Sedighe Salimian, Valentina Zannier, Simone Traverso, Samuele Fracassi, Niccolo Traverso Ziani, Maura Sassetti, Matteo Carrega, Fabio Beltram, Lucia Sorba, and Stefan Heun
In the last decades, strong interest has revolved around hybrid semiconductor/superconductor (SC) devices, where the proximity effect can give rise to a wide variety of phenomena, including the possible emergence of topological states. In particular, thanks to their peculiar material properties, InSb-based planar superconductor–normal–superconductor (SNS) Josephson junctions have taken center stage in the search for topological superconductivity.
Interestingly, in novel planar junction designs, superconductors can be contacted laterally, leaving the semiconducting surface of the device uncovered and thus accessible to spatially resolved techniques such as Scanning Gate Microscopy (SGM). Over the past two decades, SGM has been successfully employed to image current flow across semiconductor-based devices by using a charged Atomic Force Microscope tip to induce local depletion in the active region underneath. Our recent work demonstrates the applicability of the same technique to locally modulate the supercurrent flow in the normal region of Josephson junctions.
Fig: (a) Graphical representation of a Scanning Gate Microscopy experiment. The light blue section corresponds to the exposed semiconducting region of the nanoflag. (b) SGM map measurement: 20 pixel × 20 pixel critical current map as a function of the position of the tip. Vtip = −10V, Vbg = 9.5V. Yellow outlines correspond to the profile of the device. (c) Numerical simulation of tip-induced critical current modulation map.
Measurements were performed at 300 mK on Nb-contacted InSb nanoflag–based Josephson junctions approximately 200 nm long. When the tip is placed in proximity to the active region of the junctions (about 300 nm above in the vertical direction), tip-induced depletion locally suppresses the superconducting flow, thereby affecting the junction’s critical current. By analysing the reduction in critical current as a function of the tip position, we detect a clear spatial dependence of the signal, indicating the local character of the tip–junction interaction, in agreement with theoretical simulations.
Once higher spatial resolution is achieved—e.g., by protecting the device surface with a dielectric layer to allow the tip to approach more closely—this technique will enable imaging and addressing of individual features in the supercurrent distribution of hybrid devices, such as Josephson vortices. Furthermore, the presence of the polarized tip could be exploited to locally deplete a topological device, inducing a local topological phase transition and potentially leading to the formation of Majorana zero modes.
Publications:
Antonio Lombardi: Supercurrent manipulation in InSb nanoflag-based
Josephson Junctions by Scanning Gate Microscopy, Master Thesis, University of Pisa, Italy, 2024.
Antonio Lombardi, Gaurav Shukla, Giada Bucci, Sedighe Salimian, Valentina
Zannier, Simone Traverso, Samuele Fracassi, Niccolo Traverso Ziani, Maura
Sassetti, Matteo Carrega, Fabio Beltram, Lucia Sorba, and Stefan Heun: Supercurrent modulation in InSb nanoflag-based Josephson junctions by scanning gate microscopy, arXiv:2506.15342v1 [cond-mat.supr-con].
Antonio Lombardi, Gaurav Shukla, Giada Bucci, Sedighe Salimian, Valentina Zannier, Simone Traverso, Samuele Fracassi, Niccolò Traverso Ziani, Maura Sassetti, Matteo Carrega, Fabio Beltram, Lucia Sorba, and Stefan Heun:
Supercurrent modulation in InSb nanoflag-based Josephson junctions by scanning gate microscopy, Communications Materials 6 (2025) 272.
Antonio Lombardi: Supercurrent modulation in Josephson junctions by scanning gate microscopy, Springer Nature Research Communities.
Presented at:
Antonio Lombardi: Towards imaging of supercurrent flow by Scanning Gate Microscopy in InSb Josephson Junctions, TU Delft, The Netherlands, 31 May 2024. [Abstract] [Talk]
Antonio Lombardi: Towards supercurrent flow imaging by Scanning Gate Microscopy in InSb Josephson Junctions, PRIN Meeting, Pisa, Italy, 20 June 2024. [Talk]
Antonio Lombardi: Supercurrent manipulation by Scanning Gate Microscopy in InSb nanoflag-based Josephson Junctions, Master Thesis Defense, University of Pisa, Italy, 28 November 2024. [Talk]
Antonio Lombardi: Supercurrent manipulation by Scanning Gate Microscopy in InSb nanoflag-based Josephson Junctions, PRIN Meeting, University of Genoa, Italy, 16 December 2024. [Talk]
Antonio Lombardi: Supercurrent manipulation by Scanning Gate Microscopy in InSb nanoflag-based Josephson Junctions, Master Thesis Defense (Licenza), Scuola Normale Superiore, Pisa, Italy, 15 July 2025. [Talk]