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arxiv: 2509.05587 · v1 · submitted 2025-09-06 · ❄️ cond-mat.mes-hall · cond-mat.supr-con

Gate-Tunable Ambipolar Josephson Current in a Topological Insulator

Bomin Zhang , Xiaoda Liu , Junjie Qi , Ling-Jie Zhou , Deyi Zhuo , Han Tay , Hongtao Rong , Annie G. Wang
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Zhiyuan Xi Chao-Xing Liu Chui-Zhen Chen Cui-Zu Chang
This is my paper

Pith reviewed 2026-05-18 18:48 UTC · model grok-4.3

classification ❄️ cond-mat.mes-hall cond-mat.supr-con
keywords topological insulatorJosephson junctionambipolar supercurrentDirac pointgate tunablesurface statesproximity superconductivity
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The pith

Gate voltage tunes ambipolar Josephson current in topological insulator junctions that persists across the Dirac point.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper demonstrates gate-tunable ambipolar Josephson current in lateral junction devices made from molecular beam epitaxy grown bulk-insulating (Bi,Sb)2Te3 thin films. The supercurrent decreases markedly as the chemical potential approaches the Dirac point yet continues through it, with a clearer ambipolar gate response in thinner films. Near the Dirac point the current also loses resilience to external magnetic fields. Numerical simulations reproduce the ambipolar transport and attribute weaker ambipolarity plus asymmetry in thicker films to the added presence of bulk conduction channels alongside Dirac surface states. The work aims to create a controllable platform for proximity-induced topological superconductivity.

Core claim

In lateral Josephson junction devices on bulk-insulating (Bi,Sb)2Te3 thin films, a gate-tunable ambipolar supercurrent is observed that is suppressed as the chemical potential approaches the Dirac point but persists across it. Thinner films exhibit pronounced ambipolar behavior while thicker films show weaker response. The supercurrent becomes significantly less resilient to magnetic fields when tuned near the Dirac point. Simulations reproduce the ambipolar characteristics and link the asymmetry in thicker films to coexistence of Dirac surface states and bulk conduction channels.

What carries the argument

Gate-tunable lateral Josephson junctions on MBE-grown bulk-insulating topological insulator thin films, where supercurrent is carried by proximity-induced superconductivity in Dirac surface states.

If this is right

  • Electrical gating switches the supercurrent between electron and hole regimes for studies of topological superconductivity.
  • Tuning through the Dirac point isolates a regime where surface-state contributions dominate the Josephson effect.
  • Reduced magnetic-field resilience near the Dirac point signals changes in pairing or coherence length in that regime.
  • Simulations confirm that bulk channels weaken ambipolarity, so thinner films provide cleaner surface-state transport.
  • The platform supports future electrically tunable Majorana modes in topological insulator systems.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Further reduction in film thickness or improved insulation could isolate pure surface-state supercurrent for cleaner Majorana searches.
  • Combining gate control with other parameters such as magnetic flux or material strain may stabilize desired topological phases.
  • The demonstrated thickness dependence suggests a general strategy for minimizing bulk shunting in other Dirac or topological materials with induced superconductivity.

Load-bearing premise

The observed ambipolar supercurrent, its gate dependence, and its reduced magnetic resilience near the Dirac point are dominated by Dirac surface states rather than residual bulk conduction channels.

What would settle it

Fabricating the same junctions on still thinner films with fully eliminated bulk conduction and checking whether supercurrent vanishes exactly at the Dirac point or retains its ambipolar gate dependence and magnetic-field fragility.

read the original abstract

Dirac surface states in a topological insulator (TI) with proximity-induced superconductivity offer a promising platform for realizing topological superconductivity and Majorana physics. However, in TIs, the Josephson effect is usually observed in regimes where transport is dominated by either substantial bulk conduction channels or unipolar surface states. In this work, we demonstrate gate-tunable ambipolar Josephson current in lateral Josephson junction (JJ) devices based on bulk-insulating (Bi,Sb)2Te3 thin films grown by molecular beam epitaxy (MBE). For thinner films, the supercurrent exhibits pronounced gate-tunable ambipolar behavior and is significantly suppressed as the chemical potential approaches the Dirac point, yet persists across it. In contrast, thicker films exhibit a much weaker ambipolar response. Moreover, we find that the supercurrent becomes significantly less resilient to external magnetic fields when the chemical potential is tuned near the Dirac point in both thickness regimes. Our numerical simulations demonstrate the ambipolar behavior of these TI JJ devices and attribute the asymmetric supercurrent observed in thicker TI films to the coexistence of Dirac surface states and bulk conduction channels. The demonstration of gate-tunable ambipolar Josephson transport in MBE-grown TI films paves the way for realizing Dirac-surface-state-mediated topological superconductivity and establishes a foundation for future exploration of electrically tunable Majorana modes.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 2 minor

Summary. The paper claims to demonstrate gate-tunable ambipolar Josephson current in lateral Josephson junction devices based on MBE-grown bulk-insulating (Bi,Sb)2Te3 thin films. For thinner films the supercurrent exhibits pronounced ambipolar behavior, suppressed as the chemical potential approaches the Dirac point yet remaining finite across it; thicker films show weaker ambipolar response. The supercurrent is markedly less resilient to external magnetic fields near the Dirac point in both regimes. Numerical simulations reproduce the ambipolar transport and attribute the asymmetry seen in thicker films to the coexistence of Dirac surface states and bulk conduction channels.

Significance. If the central interpretation holds, the work supplies direct experimental evidence for gate-controlled supercurrent carried across the Dirac point in a topological insulator, strengthening the platform for proximity-induced topological superconductivity and tunable Majorana modes. The combination of thickness-dependent transport data, magnetic-field fragility measurements, and qualitative simulations provides a coherent picture; the absence of fitted parameters in the primary ambipolar observation is a positive feature.

major comments (1)
  1. [Discussion (thinner-film regime)] Discussion section on thinner-film regime: the claim that the finite supercurrent persisting at the Dirac point is dominated by proximity-induced Dirac surface states (rather than residual bulk or MBE-induced defect channels) is load-bearing for the headline interpretation, yet the simulations address asymmetry only in the thicker-film case; no independent quantitative bound (e.g., Hall density or activated bulk resistivity measured on the same junctions) is provided to limit possible parallel bulk conduction at the Dirac point.
minor comments (2)
  1. [Results / Figures] Figure 3 (or equivalent magnetic-field dependence panels): the scaling of the critical-current suppression factor near the Dirac point should be shown with explicit error bars or shaded regions to allow direct comparison with the thicker-film data.
  2. [Abstract] The abstract states 'significantly suppressed' without quoting the numerical factor; adding the approximate suppression ratio (e.g., I_c at Dirac point / I_c at maximum) would improve precision.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and the constructive comment. We address the major point below and have revised the manuscript accordingly.

read point-by-point responses

  1. Referee: Discussion section on thinner-film regime: the claim that the finite supercurrent persisting at the Dirac point is dominated by proximity-induced Dirac surface states (rather than residual bulk or MBE-induced defect channels) is load-bearing for the headline interpretation, yet the simulations address asymmetry only in the thicker-film case; no independent quantitative bound (e.g., Hall density or activated bulk resistivity measured on the same junctions) is provided to limit possible parallel bulk conduction at the Dirac point.

    Authors: We agree that a direct quantitative bound on residual bulk conduction at the Dirac point, obtained on the same junctions, would strengthen the interpretation. Our numerical simulations focus on reproducing the weaker ambipolar response and asymmetry in thicker films, where bulk channels coexist with surface states. For the thinner-film regime, the headline claim rests on the experimental observation of pronounced gate-tunable ambipolar supercurrent that is strongly suppressed near the Dirac point yet remains finite across it, together with the reduced magnetic-field resilience near the Dirac point. These features are difficult to reconcile with dominant parallel bulk or defect channels, which would not produce such clear ambipolar behavior. We have revised the Discussion section to explicitly acknowledge the absence of a same-junction Hall or activated-resistivity bound, to clarify the supporting role of the thickness-dependent data and magnetic-field measurements, and to moderate the language around surface-state dominance while retaining the central interpretation supported by the ambipolar transport. revision: partial

Circularity Check

0 steps flagged

No significant circularity; claims rest on direct measurements and standard models

full rationale

The paper's core result is an experimental observation of gate-tunable ambipolar supercurrent in MBE-grown TI films, documented via transport data across multiple devices and thicknesses. Numerical simulations are invoked only to interpret the observed asymmetry in thicker films as arising from known coexistence of surface and bulk channels; these simulations employ standard Dirac-surface and bulk conduction models without fitting parameters that define or presuppose the ambipolar persistence itself. No self-citations supply load-bearing uniqueness theorems, no ansatzes are smuggled via prior work, and no quantities are renamed or predicted by construction from the input data. The derivation chain therefore remains self-contained against external benchmarks of TI Josephson physics.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The paper relies on established models of proximity-induced superconductivity in TI surface states and standard two-channel (surface + bulk) transport; no new free parameters, axioms beyond domain standards, or invented entities are introduced.

axioms (1)
  • domain assumption Dirac surface states in a topological insulator with proximity-induced superconductivity can mediate Josephson current
    Invoked throughout the interpretation of ambipolar behavior and magnetic-field response.

pith-pipeline@v0.9.0 · 5816 in / 1363 out tokens · 50944 ms · 2026-05-18T18:48:30.513156+00:00 · methodology

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Reference graph

Works this paper leans on

3 extracted references · 3 canonical work pages

  1. [1]

    To date, several studies have claimed that JJ devices are realized based on bulk -insulating TIs (Ref. 21,22,24,27-33), but none have demonstrated ambipolar Josephson current , a supercurrent in a single JJ device that can be carried by either electrons or holes, as observed in graphene 34. The key impediment is the inability to cleanly separate the proxi...

    work page

  2. [2]

    using the same recipe and perform transport measurements on this device (Fig. 2). For -80 V ≤ (Vg - Vg0) ≤ 0 V , i.e., the p-type regime, the value Rn increases monotonically as (Vg - Vg0) increases. Ic reaches a maximum of ~400 nA at (Vg - Vg0) = -10 V , where the IcRn product also attains its highest value of ~35 V . For 0 V ≤ (Vg - Vg0) ≤ 40 V , i.e.,...

    work page

  3. [3]

    At (Vg - Vg0) = 0 V , i.e., near the Dirac point, the supercurrent I similarly exhibits reduced resilience to Bz (Fig

    Figures 3d to 3f show the Fraunhofer patterns of the JJ-2 device at different ( Vg - Vg0), where anomalous Fraunhofer patterns are also observed. At (Vg - Vg0) = 0 V , i.e., near the Dirac point, the supercurrent I similarly exhibits reduced resilience to Bz (Fig. 3f). To support our experimental results, we perform numerical simulations of the critical J...

    work page arXiv 2003