Gate-Tunable Ambipolar Josephson Current in a Topological Insulator

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.

💡 Research Summary

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The paper reports the realization of gate‑tunable ambipolar Josephson supercurrents in lateral Josephson junctions (JJs) fabricated from molecular‑beam‑epitaxy (MBE) grown (Bi,Sb)₂Te₃ thin films. Two film thicknesses were investigated: 5 quintuple layers (QL) (denoted JJ‑1) and 15 QL (JJ‑2). Both devices employ Nb electrodes patterned on an insulating SrTiO₃ (111) substrate, with a global back‑gate formed by the SrTiO₃ crystal to electrostatically tune the carrier density across the Dirac point of the topological‑insulator (TI) surface states.

For the 5 QL device, the normal‑state resistance Rₙ exhibits a sharp peak at the charge‑neutral point (Vg = Vg₀), indicating that the chemical potential crosses the Dirac point where the surface carrier density is minimal. The critical current Ic is strongly suppressed at Vg₀ (≈ 40 nA) but recovers to ≈ 200 nA on both the p‑type (negative gate bias) and n‑type (positive gate bias) sides. Consequently, the product Ic Rₙ shows a pronounced minimum at the Dirac point and remains roughly constant away from it. This behavior demonstrates true ambipolar Josephson transport: the supercurrent can be carried by either electrons or holes, analogous to graphene‑based JJs but now mediated by the Dirac surface states of a TI.

In contrast, the 15 QL device displays a markedly weaker ambipolar response. While the p‑type side shows a sizable increase of Ic (up to ≈ 400 nA) as the gate voltage is made more negative, the n‑type side shows only a modest change, and Ic remains nearly constant. The asymmetry is attributed to the coexistence of bulk conduction channels that become increasingly important in thicker films. Bulk carriers are largely insensitive to the gate voltage, so they dominate transport on the n‑type side, whereas the p‑type side still benefits from a stronger contribution of the top‑surface Dirac states, leading to the observed imbalance.

Both devices were examined under out‑of‑plane magnetic fields. Fraunhofer interference patterns were observed, but they deviate from the ideal sinusoidal envelope: side lobes are non‑monotonic, and the effective junction length inferred from the period (≈ 1 µm) exceeds the lithographic length (20 nm). This discrepancy is ascribed to flux focusing by the Meissner effect in the Nb electrodes and to edge roughness, which cause a spatially varying effective length across the junction width. Near the Dirac point, the Fraunhofer lobes become strongly broadened and higher‑order lobes disappear, indicating reduced magnetic‑field resilience when the proximity‑induced superconducting gap Δ in the TI is suppressed.

To rationalize the experimental findings, the authors performed recursive Green’s‑function simulations of a three‑dimensional TI model that includes structural inversion asymmetry (SIA) between the top and bottom surfaces. The simulations reproduce the ambipolar Ic(Vg) curve for thin films, where transport is dominated by the surface Dirac states, and they also capture the pronounced p‑type/n‑type asymmetry for thicker films, where gate‑insensitive bulk channels coexist with the top‑surface Dirac channel. The calculations show that the asymmetric supercurrent in the 15 QL device is a precursor to ambipolar behavior that is suppressed by bulk conduction.

Additional analysis includes extraction of the induced superconducting gap Δ from multiple Andreev reflection (MAR) spectra. In the 5 QL device, Δ ≈ 12–20 µeV, decreasing near the Dirac point, consistent with the reduced Ic. In the 15 QL device, MAR peaks up to the sixth order are observed in the p‑type regime, indicating relatively ballistic transport (e Ic Rₙ/Δ ≈ 1). The e Ic Rₙ/Δ ratios across gate voltages remain of order unity, suggesting high‑quality Nb/(Bi,Sb)₂Te₃ interfaces with few defects.

Overall, the work demonstrates that high‑quality MBE‑grown (Bi,Sb)₂Te₃ films can host gate‑tunable ambipolar Josephson currents when the film is sufficiently thin so that surface Dirac states dominate transport. The ability to switch the supercurrent on and off by moving the chemical potential through the Dirac point, together with the observed sensitivity of the supercurrent to magnetic fields, provides a versatile platform for electrically controllable topological superconductivity. This platform paves the way toward voltage‑controlled Majorana zero modes and topological qubits, where the superconducting proximity effect can be turned on or off in a deterministic, reversible manner.