Proximitized Topological Insulator Charge Island Fabricated via In Situ Multi-Angle Stencil Lithography

arXiv:2604.18736 · cond-mat.mes-hall, cond-mat.mtrl-sci, cond-mat.supr-con · Submitted 2026-04-20 · Read on arXiv

Listen

Transcript

Introduction to the show: ident: Quantum Radio. Generated commentary on the latest quantum physics and condensed matter papers.

Kai: Today's paper: "Proximitized Topological Insulator Charge Island Fabricated via In Situ Multi-Angle Stencil Lithography".

Mira: The gist: This work introduces a fully in situ, multi-angle stencil lithography technique to fabricate proximitized charge islands in topological insulators,

Kai: First, who's behind it and why it matters.

Paper summary: Kai: So, this paper, "Proximitized Topological Insulator Charge Island Fabricated via In Situ Multi-Angle Stencil Lithography," is all about building a stable charge island in a topological insulator that has superconductivity added to it. The main idea is overcoming the huge hurdle of making clean interfaces between these materials because TI surfaces degrade really fast when exposed to the air.

Mira: Exactly, and what they introduce here is this fully in situ, multi-angle stencil lithography technique. They claim this method lets them fabricate these proximitized charge islands directly within the topological insulator structure. It’s about realizing a system where you can control everything at once during fabrication instead of trying to clean up interfaces later.

Kai: The core thesis seems to be that they've managed to create these setups that show robust Coulomb blockade and a strong suppression of low-energy conductance, which they connect directly to proximity-induced superconductivity in the island itself. They also report seeing this effect even when the magnetic field is applied.

Mira: And what matters for condensed matter is how they achieve those clean interfaces and well-defined tunnel barriers. They managed to create pristine interfaces for that proximity effect and get really well-defined tunnel barriers, which is a big deal because it means the physics they see isn't just some artifact of bad material contacts.

Kai: They also set up a global back gate using the chip carrier's metallic back plate, and this allowed them to measure periodic Coulomb blockade peaks in the differential conductance as you change that voltage. That periodicity is key for understanding charge transport on these islands.

Mira: And they have some interesting results on the charging energy. They found it was lower than what electrostatic simulations predicted, around three hundred sixty microelectron volts, and they attribute that to enhanced screening from the surrounding metallic mask structure which increases the total island capacitance.

Kai: That sounds like a practical detail that matters for scaling up these devices. The paper also highlights that their extracted charging energy is below the induced superconducting gap of about one hundred fifty microelectron volts, which they say satisfies a necessary energetic condition for two-electron periodic Coulomb blockade.

Mira: And they point out that because the charging energy is lower than the gap, there's an expected crossover to one-electron periodicity if the charging energy were higher than the gap, but here it stays in this two-electron regime. This whole setup is designed to probe parity effects in a very specific way.

Kai: So, basically, they built a functional device that demonstrates proximity superconductivity on a TI island using an advanced fabrication method that keeps things clean and controllable during the process. This sets up the stage for what they want to explore next in topological physics.

Conclusion: Kai: Looking at the title, "Proximitized Topological Insulator Charge Island Fabricated via In Situ Multi-Angle Stencil Lithography," it sounds like they’ve really nailed the control over the process from start to finish. The focus is on a specific type of quantum device—a charge island in a TI that has been made superconducting through proximity effects, and they used this unique fabrication method to get there.

Mira: From my perspective, what this implies is that we can move beyond just observing these phenomena in idealized systems or very limited setups. The authors are showing that you can actually build the necessary architecture—the specific interfaces and barriers—right into the device structure while it's being grown.

Kai: It means they’ve opened up a path for building more complex TI-based hybrid quantum devices, not just simple ones where you might have some surface states. The reproducibility of these clean interfaces is what makes this work important for experimental physics.

Mira: And the implications extend into exploring topological superconductivity and maybe even Majorana physics, which are the big goals in this field. If you can reliably create a system where Cooper pairs can tunnel and exhibit these effects, it gets much closer to realizing the exotic states we're hunting for in topological materials.

Kai: So, to put it simply, this paper provides a solid experimental platform. It gives researchers a reliable way to make these TI-superconductor hybrids with good interfaces and predictable charging behavior. It’s a versatile nanofabrication tool that opens up new routes into studying superconductivity in these topological nanostructures.

Mira: And the method itself is quite adaptable, which is crucial for future work. They suggest this fabrication scheme could be readily adapted for tunnel spectroscopy of extended TI nanowires or even more complicated device architectures down the line. It’s about establishing a platform that’s flexible enough for further exploration.

Kai: Right, so they’ve built the island and shown it works with very clean connections, and they've given us a way to make more complex devices later on using this stencil lithography technique. That seems like the main point of this paper.

PGI-9, Forschungszentrum Jülich and JARA Jülich-Aachen Research Alliance · RWTH Aachen University

cond-mat.mes-hall, cond-mat.mtrl-sci, cond-mat.supr-con

Submitted: 2026-04-20

Updated: 2026-10-07

Comments: 7 pages, 4 figures; Supporting Information: 5 pages, 4 figures, 1 table

DOI: 10.1021/acs.nanolett.6c02350

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 83/100

The gist: The gist: This work introduces a fully in situ, multi-angle stencil lithography technique to fabricate proximitized charge islands in topological insulators, revealing robust Coulomb blockade and a

Key concepts

Proximitized Topological Insulator Charge Island: This refers to creating a tiny island made of a topological insulator (TI) material that is brought into contact with a superconductor. The TI's unique electronic properties are 'proximitized' by the superconductor, meaning it gains superconducting characteristics at its interface, allowing for the observation of quantum effects like Coulomb blockade.
Coulomb Blockade: This is a quantum phenomenon observed in nanoscale conductors where adding or removing a single electron requires a discrete amount of energy. It manifests as periodic peaks in conductance when measuring current versus gate voltage, indicating that the island's charge is quantized and resists changes until specific energy thresholds are met.
Proximity-Induced Superconductivity: This occurs when a superconductor influences an adjacent material, like a topological insulator. The superconducting properties 'leak' across the interface into the TI. This induced superconductivity opens up a gap in the material's electronic states, which is responsible for suppressing low-energy electrical conductance.
In Situ Multi-Angle Stencil Lithography: This is a fabrication technique where patterning and material deposition happen simultaneously and at multiple angles within the same setup. It allows for precise, complex geometries, such as creating trenches and patterned layers, directly on the sample without needing to remove it for separate steps.

Terminology

Anyons — 2× in this paper · explained in 2 episodes
Core — 1× in this paper · explained in 2 episodes
Hybrid System — 1× in this paper · explained in 2 episodes

Summary

The gist: This work introduces a fully in situ, multi-angle stencil lithography technique to fabricate proximitized charge islands in topological insulators, revealing robust Coulomb blockade and a pronounced suppression of low-energy conductance consistent with proximity-induced superconductivity > ref:2604.18736

Fabrication Technique

The fabrication process builds on selective area growth (SAG) combined with shadow evaporation, extending the approach introduced in Ref. [12] > ref:2604.18736

A lower mask defines the selective growth region for the TI, while an upper, partially free-standing mask enables shadowed deposition of multiple materials > ref:2604.18736

The trenches are patterned by lithography in Si3N4 followed by wet etching of SiO2 to create the partially free-standing geometry > ref:2604.18736

The TI nanoribbon is grown first under rotation, confined to the rectangular selective area, to a thickness of 18 nm, width of 200 nm and length of 1 µm with a composition of (20 ± 2)% Bi and (80 ± 2)% Sb > ref:2604.18736

To suppress interdiffusion between the TI and the superconductor, a 3 nm Pt diffusion barrier is deposited [29] > ref:2604.18736

A stoichiometric Al2O3 tunnel barrier is subsequently grown under rotation to cap the exposed ends of the TI ribbon > ref:2604.18736

Transport Measurements and Findings

Low-temperature transport measurements reveal robust Coulomb blockade and a pronounced suppression of low-energy conductance which vanishes with magnetic field, consistent with proximityinduced superconductivity in the island > ref:2604.18736

The fabrication process enables the formation of pristine interfaces for proximity-induced superconductivity and the realization of well-defined tunnel barriers > ref:2604.18736

The global back gate is realized using the metallic back plate of the chip carrier > ref:2604.18736

Differential conductance dI/dV as a function of back-gate voltage at zero bias shows periodic Coulomb blockade peaks > ref:2604.18736

Bias spectroscopy at fixed gate voltage reveals suppressed conductance at −0.05 mV ref:2604.18736

The extracted charging energy is lower than expected from electrostatic simulations (≈ 360 µeV), likely due to enhanced screening by the surrounding metallic mask structure, which increases the total island capacitance > ref:2604.18736

The extracted charging energy lies below the induced superconducting gap (∆∗ ≈ 150 µeV), satisfying a necessary energetic condition for 2e-periodic Coulomb blockade, whereas for EC > ∆ a crossover to 1e periodicity is expected [31] > ref:2604.18736

Proximity Effect Analysis

At zero field, the differential conductance outside the blockade regime is strongly suppressed at low bias, consistent with a proximity-induced gap in the density of states > ref:2604.18736

At high field, where superconductivity is suppressed, this additional reduction in conductance disappears > ref:2604.18736

The absence of 2e periodicity is consistent with the presence of subgap quasiparticle states in the proximitized island > ref:2604.18736

The observed soft gap allows single-electron tunneling even at low energies, preventing parity conservation > ref:2604.18736

The broad distribution of spacings is attributed to charge rearrangements in the electrostatic environment, likely dominated by trapped charges in the surrounding oxides > ref:2604.18736

Conclusion and Outlook

In conclusion, we demonstrate a proximitized topological insulator charge island realized using a fully in situ multi-angle fabrication technique > ref:2604.18736

The approach yields clean superconductor–TI interfaces, reproducible tunnel barriers, and scalable device geometries > ref:2604.18736

Overall, this work advances experimental control in TI–superconductor hybrids and supports a more systematic exploration of topological superconductivity and Majorana physics > ref:2604.18736

The fabrication scheme is readily adaptable to tunnel spectroscopy of extended TI nanowires and more complex device architectures > ref:2604.18736

This work establishes a versatile nanofabrication platform that enables access to previously unexplored TI-based hybrid quantum devices and opens new routes for investigating superconductivity in topological nanostructures > ref:2604.18736

Supplemental Measurements

Coulomb diamonds at different magnetic in-plane fields show that with increasing field, the conductance outside of the diamonds increases > ref:2604.18736

Coulomb diamonds at different magnetic out-of-plane fields show that with increasing field, the conductance outside of the diamonds increases > ref:2604.18736

With increasing in-plane field, the conductance outside of the diamonds increases > ref:2604.18736

With increasing out-of-plane field, the conductance outside of the diamonds increases > ref:2604.18736

References

[1] A. Yu. Kitaev, Fault-tolerant quantum computation by anyons, Annals of Physics 303, 2 (2003) > ref:2604.18736

[2] V. Mourik, K. Zuo, S. M. Frolov, S. R. Plissard, E. P. A. M. Bakkers, and L. P. Kouwenhoven, Signatures of Majorana Fermions in Hybrid SuperconductorSemiconductor Nanowire Devices, Science 336, 1003 (2012) > ref:2604.18736

[3] A. P. Higginbotham, S. M. Albrecht, G. Kiršanskas, W. Chang, F. Kuemmeth, P. Krogstrup, T. S. Jespersen, J. Nygård, K Flensberg, and C M Marcus Parity lifetime of bound states in a proximitized semiconductor nanowire Nature Physics 11 1017 (2015) > ref:2604.18736

[4] S. M. Albrecht, A. P. Higginbotham, M. Madsen, F Kuemmeth, T S Jespersen, J Nygård, P Krogstrup, and C M Marcus Exponential protection of zero modes in Majorana islands Nature 531 206 (2016) > ref:2604.18736

[5] J Shen, S Heedt, F Borsoi, B van Heck, S Gazibegovic, R L M Op het Veld, D Car, J A Logan, M Pendharkar Parity transitions in the superconducting ground state of hybrid InSb–Al Coulomb islands Nature Communications 9 4801 (2018) > ref:2604.18736

[6] J Shen, G W Winkler, F Borsoi, S Heedt, V Levajac, J Y Wang, D van Driel, D Bouman S Gazibegovic R L M Op Het Veld D Car J A Logan M Pendharkar C J Palmstrøm E P A M Bakkers L P Kouwenhoven and B van Heck Full parity phase diagram of a proximitized nanowire island Physical Review B 104 045422 (2021) > ref:2604.18736

[7] M Valentini, M Borovkov, E Prada, S Marti-Sanchez, M Botifoll, A Hofmann, J Arbiol, R Aguado P SanJose and G Katsaros Majorana-like Coulomb spectroscopy in the absence of zero bias peaks Nature 612 442 (2022) arXiv:2203.07829 [cond-mat] > ref:2604.18736

[8] A M Cook, M M Vazifeh, and M Franz Stability of Majorana fermions in proximity-coupled topological insulator nanowires Physical Review B 86 155431 (2012) > ref:2604.18736

[9] D Heffels, D Burke, M R Connolly, P Schüffelgen, D Grützmacher, and K Moors Robust and Fragile Majorana Bound States in Proximitized Topological Insulator Nanoribbons Nanomaterials 13 723 (2023) > ref:2604.

Improvements for AI systems

Acceleration of Topological Quantum Computing Simulation Capability: The fabrication process enables scalable fabrication of hybrid nanostructures without post-growth processing, which can be used to create highly controlled, reproducible testbeds for simulating topological phases under tunable parameters like magnetic flux and chemical potential, directly advancing the study of Majorana zero modes.
Development of Novel Charge Transport Diagnostics: The transport measurements reveal a pronounced suppression of low-energy conductance which vanishes with magnetic field, allowing AI systems to be trained on the relationship between magnetic field dependence and proximity-induced superconductivity, enabling better prediction of topological phase transitions in hybrid TI systems.
Design of Materials Discovery Algorithms: By systematically varying deposition parameters (e.g., angle-controlled deposition of diffusion barriers, superconducting Al), AI can rapidly screen material combinations to optimize interface quality, addressing the challenge where rapid surface oxidation degrades interface transparency.
Creation of High-Fidelity Interface Models: The ability to create pristine interfaces for proximity-induced superconductivity allows AI models to generate highly accurate computational representations of the superconducting gap in TI charge islands, improving predictive accuracy for Majorana physics.

Abstract

Hybrid superconductor-topological insulator (TI) nanostructures constitute a promising material platform for exploring proximity-induced superconductivity in systems with topologically protected surface states. A key obstacle has been the realization of clean and well-controlled superconductor-TI interfaces, as TI surfaces rapidly degrade under ambient conditions. Here, we introduce a fully in situ, multi-angle stencil lithography technique that enables the fabrication of proximitized charge islands in TIs. The approach combines selective area growth of (Bi,Sb) 2 Te 3 nanoribbons with angle-controlled deposition of diffusion barriers, superconducting Al, and ultrathin oxide tunnel barriers, allowing scalable fabrication of hybrid nanostructures without post-growth processing. Low-temperature transport measurements reveal robust Coulomb blockade and a pronounced suppression of low-energy conductance, which vanishes with a magnetic field, consistent with proximity-induced superconductivity in the island. These results establish a versatile nanofabrication platform that enables access to previously unexplored TI-based hybrid quantum devices and opens new routes for investigating superconductivity in topological nanostructures.