Probing Unconventional Superconductivity in Hybrid Josephson Junctions
Doctoral thesis, 2026
Topological superconductivity represents a promising direction for fault-tolerant quantum computing through the braiding of Majorana bound states. Realising robust experimental platforms, however, remains challenging due to disorder, competing transport channels, and the need for precise symmetry engineering. Superconducting hybrid systems provide a versatile platform for combining superconductivity, spin-orbit coupling, magnetism, and mesoscopic phenomena in tuneable low-dimensional devices.
This thesis investigates superconductivity and transport in Josephson junctions based on ultra-thin Al-AlOx-Al tunnel junctions, hybrid EuS-Al-InAs nanowires, and Bi2Se3 topological-insulator nanoribbons. The objective is to understand how superconducting correlations, symmetry breaking, and mesoscopic transport can be engineered and characterised in regimes relevant for topological superconductivity.
The first part of the thesis focuses on superconducting transport and spectroscopy in ultra-thin Al-AlOx-Al Josephson junctions. By analysing the supercurrent peak within the framework of the Ivanchenko-Zil'berman and resistively and capacitively shunted junction models, intrinsic device parameters governing Josephson transport are extracted. Magnetotransport measurements reveal Fraunhofer-like interference patterns, enabling characterisation of the effective transport cross-sectional area. The results further demonstrate that ultra-thin aluminium junctions can operate as superconducting spectrometers at magnetic fields exceeding 1 T.
The second part investigates non-reciprocal superconducting transport in hybrid EuS-Al-InAs nanowire Josephson junctions. By varying the magnitude and orientation of external magnetic fields, a pronounced superconducting diode effect is observed. Importantly, a finite diode efficiency persists even in the absence of an externally applied magnetic field, demonstrating that engineered exchange fields alone can break the symmetries required for non-reciprocal superconductivity.
The final part explores quantum transport in Josephson junctions based on Bi2Se3 nanoribbons. By systematically varying junction geometry, distinct transport regimes are identified across different length scales. Short junctions are dominated by diffusive bulk transport, while intermediate junction lengths exhibit signatures consistent with superconducting transport with significant surface-state contributions. For longer junctions, the Josephson transport again becomes predominantly diffusive.
Collectively, these studies advance the understanding of superconducting transport and engineered hybrid superconducting systems relevant for topological superconductivity.
This thesis investigates superconductivity and transport in Josephson junctions based on ultra-thin Al-AlOx-Al tunnel junctions, hybrid EuS-Al-InAs nanowires, and Bi2Se3 topological-insulator nanoribbons. The objective is to understand how superconducting correlations, symmetry breaking, and mesoscopic transport can be engineered and characterised in regimes relevant for topological superconductivity.
The first part of the thesis focuses on superconducting transport and spectroscopy in ultra-thin Al-AlOx-Al Josephson junctions. By analysing the supercurrent peak within the framework of the Ivanchenko-Zil'berman and resistively and capacitively shunted junction models, intrinsic device parameters governing Josephson transport are extracted. Magnetotransport measurements reveal Fraunhofer-like interference patterns, enabling characterisation of the effective transport cross-sectional area. The results further demonstrate that ultra-thin aluminium junctions can operate as superconducting spectrometers at magnetic fields exceeding 1 T.
The second part investigates non-reciprocal superconducting transport in hybrid EuS-Al-InAs nanowire Josephson junctions. By varying the magnitude and orientation of external magnetic fields, a pronounced superconducting diode effect is observed. Importantly, a finite diode efficiency persists even in the absence of an externally applied magnetic field, demonstrating that engineered exchange fields alone can break the symmetries required for non-reciprocal superconductivity.
The final part explores quantum transport in Josephson junctions based on Bi2Se3 nanoribbons. By systematically varying junction geometry, distinct transport regimes are identified across different length scales. Short junctions are dominated by diffusive bulk transport, while intermediate junction lengths exhibit signatures consistent with superconducting transport with significant surface-state contributions. For longer junctions, the Josephson transport again becomes predominantly diffusive.
Collectively, these studies advance the understanding of superconducting transport and engineered hybrid superconducting systems relevant for topological superconductivity.
Superconductivity
Topological superconductivity
Proximity effect
Topological insulators
Josephson diode effect
Hybrid superconducting systems
Mesoscopic transport
Josephson junctions
Kollektorn MC2
Opponent: Carmine Attanasio, Perofessor Università Degli Studi Di Salerno, Italy
Author
Nermin Trnjanin
Chalmers, Microtechnology and Nanoscience (MC2), Quantum Device Physics
Magnetotransport properties of thin Josephson junctions for spectroscopic applications in the presence of large magnetic fields
Applied Physics Letters,;Vol. 127(2025)
Journal article
N Trnjanin, I. Cools, V Buccheri, Y Liu, J Nygård, and T Bauch, Magnetic- Field Anisotropy of the Josephson Diode Effect in Ferromagnetic Hybrid Nanowires
K. Niherysh, N. Trnjanin, A. P. Surendran, G. Kunakova, X. Palermo, D. Montemurro, J. Andzane, D. Erts, D. S. Golubev, S. Lara-Avila, et al., Ballistic transport in Al-Bi2Se3-Al nanoribbon Josephson junctions: indi- cation from Fabry–Pérot interference and length dependent IcRn product
Kvantmekanik beskriver materiens beteende på de minsta skalorna och ligger till grund för många moderna teknologier. Ett av dess mest anmärkningsvärda fenomen är supraledning, där elektrisk ström kan flyta utan resistans eftersom elektroner bildar korrelerade par som rör sig kollektivt genom ett material utan energiförlust. Dessa egenskaper gör supraledare attraktiva för framtida kvantteknologier.
En stor utmaning inom kvantdatorer är att kvanttillstånd är mycket känsliga för störningar från sin omgivning. En föreslagen lösning är topologisk kvantberäkning, där information lagras i kvanttillstånd som är intrinsikt mer robusta mot lokalt brus. Att realisera sådana system experimentellt kräver dock nya materialplattformar och en djupare förståelse för kvanttransport i nanoskaliga komponenter.
Denna avhandling undersöker hybrida supraledande system där supraledare kombineras med halvledare, magnetiska material och topologiska material. Målet är att förstå hur supraledning, magnetism och kvanttransport samverkar i lågdimensionella nanostrukturer relevanta för topologiska kvantteknologier.
Den första delen fokuserar på ultratunna Josephsonövergångar av aluminium och visar hur de kan användas som supraledande spektrometrar vid höga magnetfält. Den andra delen undersöker hybrida EuS-Al-InAs-nanotrådar, där en supraledande diodeffekt observeras, vilket visar att konstruerade magnetiska gränsytor kan skapa den symmetribrytning som krävs för okonventionella supraledande tillstånd. Den sista delen studerar transport i Bi2Se3 nanoband av topologiska isolatorer och identifierar hur olika transportmekanismer uppstår när komponenternas geometri förändras.
Tillsammans bidrar dessa studier till ökad förståelse för supraledande transport i hybrida nanoskaliga system relevanta för framtida kvantteknologier.
En stor utmaning inom kvantdatorer är att kvanttillstånd är mycket känsliga för störningar från sin omgivning. En föreslagen lösning är topologisk kvantberäkning, där information lagras i kvanttillstånd som är intrinsikt mer robusta mot lokalt brus. Att realisera sådana system experimentellt kräver dock nya materialplattformar och en djupare förståelse för kvanttransport i nanoskaliga komponenter.
Denna avhandling undersöker hybrida supraledande system där supraledare kombineras med halvledare, magnetiska material och topologiska material. Målet är att förstå hur supraledning, magnetism och kvanttransport samverkar i lågdimensionella nanostrukturer relevanta för topologiska kvantteknologier.
Den första delen fokuserar på ultratunna Josephsonövergångar av aluminium och visar hur de kan användas som supraledande spektrometrar vid höga magnetfält. Den andra delen undersöker hybrida EuS-Al-InAs-nanotrådar, där en supraledande diodeffekt observeras, vilket visar att konstruerade magnetiska gränsytor kan skapa den symmetribrytning som krävs för okonventionella supraledande tillstånd. Den sista delen studerar transport i Bi2Se3 nanoband av topologiska isolatorer och identifierar hur olika transportmekanismer uppstår när komponenternas geometri förändras.
Tillsammans bidrar dessa studier till ökad förståelse för supraledande transport i hybrida nanoskaliga system relevanta för framtida kvantteknologier.
Quantum mechanics governs the behaviour of matter at the smallest scales and underlies many modern technologies. One of its most remarkable phenomena is superconductivity, where electrical current flows without resistance because electrons form correlated pairs that move collectively through a material without energy loss. These properties make superconductors attractive for quantum technologies.
A major challenge in quantum computing is that quantum states are highly sensitive to disturbances from their surroundings. One proposed solution is topological quantum computing, where information is stored in quantum states that are intrinsically more robust against local noise. Realising such systems experimentally, however, requires new material platforms and a deeper understanding of quantum transport in nanoscale devices.
This thesis investigates hybrid superconducting systems in which superconductors are combined with semiconductors, magnetic materials, and topological materials. The aim is to understand how superconductivity, magnetism, and quantum transport interact in low-dimensional nanostructures relevant for topological quantum technologies.
The first part focuses on ultra-thin aluminium Josephson junctions and demonstrates how they can operate as superconducting spectrometers at high magnetic fields. The second part investigates hybrid EuS-Al-InAs nanowires, where a superconducting diode effect is observed, showing that engineered magnetic interfaces can produce the symmetry breaking required for unconventional superconducting states. The final part studies transport in Bi2Se3 topological-insulator nanoribbons, identifying how different transport mechanisms emerge as the device geometry changes.
Together, these studies improve the understanding of superconducting transport in hybrid nanoscale systems relevant for future quantum technologies.
A major challenge in quantum computing is that quantum states are highly sensitive to disturbances from their surroundings. One proposed solution is topological quantum computing, where information is stored in quantum states that are intrinsically more robust against local noise. Realising such systems experimentally, however, requires new material platforms and a deeper understanding of quantum transport in nanoscale devices.
This thesis investigates hybrid superconducting systems in which superconductors are combined with semiconductors, magnetic materials, and topological materials. The aim is to understand how superconductivity, magnetism, and quantum transport interact in low-dimensional nanostructures relevant for topological quantum technologies.
The first part focuses on ultra-thin aluminium Josephson junctions and demonstrates how they can operate as superconducting spectrometers at high magnetic fields. The second part investigates hybrid EuS-Al-InAs nanowires, where a superconducting diode effect is observed, showing that engineered magnetic interfaces can produce the symmetry breaking required for unconventional superconducting states. The final part studies transport in Bi2Se3 topological-insulator nanoribbons, identifying how different transport mechanisms emerge as the device geometry changes.
Together, these studies improve the understanding of superconducting transport in hybrid nanoscale systems relevant for future quantum technologies.
Simulated Majorana states (SiMS)
European Commission (EC) (EC/H2020/804988), 2019-06-01 -- 2024-01-31.
Areas of Advance
Nanoscience and Nanotechnology
Subject Categories (SSIF 2025)
Nano-technology
Condensed Matter Physics
Infrastructure
Myfab (incl. Nanofabrication Laboratory)
DOI
10.63959/chalmers.dt/5899
ISBN
978-91-8103-442-4
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5899
Publisher
Chalmers
Kollektorn MC2
Opponent: Carmine Attanasio, Perofessor Università Degli Studi Di Salerno, Italy