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Integration and characterisation of superconducting thin films and high spin orbit coupling InSb 2DEGs

Robinson, Thomas 2023. Integration and characterisation of superconducting thin films and high spin orbit coupling InSb 2DEGs. PhD Thesis, Cardiff University.
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One theoretical route towards fault proof quantum computing is the realisation of Majorana Zero Modes (MZMs) at the ends of InSb nanowires coupled to conventional s-wave superconductors. The high spin-orbit coupling within the InSb is predicted to result in a proximity superconducting state that exhibits elements of p_x+ip_y superconducting pairing, which required to support a topological state hosting MZMs. Nanowire devices initially showed promise, however material concerns and analytical complications have led to several notable retractions. Additionally, direct experimental evidence of the necessary p_x+ip_y superconduting state remains under debate. This thesis explores an alternate material approach towards the same end - high mobility InSb two-dimensional electron gases (2DEGs). InSb 2DEGs with additional electrical confinement via gating are relatively underexplored scalable alternative. The top-down integration of high-quality superconducting films and characterisation of the size, quality, temperature and field dependence of the resulting proximity induced state is requisite for this. A series of 300×600μm Nb contacts on an InSb 2DEG with increasing spacings were fabricated. Gap sizes spanned the range 1.5-8μm, comparable to the ballistic length of the 2DEGs. Measurement with both AC and DC measurement techniques observed a conductance dip, supressed as a function of temperature, with a T_c≈7.5K, in agreement with our Nb. A dip was seen in all gaps up to and including 8μm. These features were found to be consistent with the BTK theory for a SN junction with a high degree of interfacial roughness, and a large proportion of interfacial scattering. Proximity superconductivity extending across such a long junction is indicative of successful induction of superconductivity with the 2DEG layer via top-down films, despite poor interfacial quality. Subsequent magnetic field measurements result in an alteration on the conductance spectra, that persisted after the removal of the field. Thermal cycling to T_room was required to change this alteration. This behaviour, and the required temperature range is believed to be indicative of field induced alteration of trapped charge states at the SN interface. This work presents low energy muon spin relaxation measurements on a superconductor/high spin orbit coupling InSb 2DEG for the first time. Measurements on 50nm Nb film deposited on an InSb 2DEG failed due to defects within the Nb precluding measurement of Meissner screening and showed no evidence of proximity induction. Measurements of a 50nm Pb/InSb with a modified 2DEG structure showed Meissner screening extending deep within the sample, well into the InSb layers. This was indicative of successful proximity induction via a second superconducting film. This screening profile was supressed by the application of a higher field. Secondary measurements and subsequent analysis indicate this suppression was a direct response to the magnitude of the applied field, and distinct from the field induced alteration seen in the electrical measurements. Such a suppression is possible evidence of p_x+ip_y superconducting elements, however this work cannot fully resolve that from other possible elements. Despite this, evidence of successful induction of proximity superconductivity with two different SC films opens many immediate directions for further analysis and material improvements for similar devices.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Physics and Astronomy
Subjects: Q Science > QC Physics
Uncontrolled Keywords: Indium Antimonide, InSb, Quantum Well, Superconductor, Majorana Zero Mode, Muon Spectroscopy, Low energy muons, spin orbit coupling, 2DEG, Quantum Devices, low dimensional physics.
Funders: EPSRC
Date of First Compliant Deposit: 13 February 2024
Last Modified: 13 Feb 2024 16:00

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