Precise magnetic control of the supercurrent and fluxoids in nanodevices: Memory effects and diode effects

Song, Xiangyu

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https://hdl.handle.net/2142/127316 Copy

Description

Title

Precise magnetic control of the supercurrent and fluxoids in nanodevices: Memory effects and diode effects

Author(s)

Song, Xiangyu

Issue Date

2024-08-29

Director of Research (if dissertation) or Advisor (if thesis)

Bezryadin, Alexey

Doctoral Committee Chair(s)

Eckstein, James N.

Committee Member(s)

• Peng, Jen-Chieh
• Giannetta, Russell W.

Department of Study

Physics

Discipline

Physics

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• superconducting diode
• superconducting memory
• kinetic inductance
• Josephson junction array
• superconducting nanodevice
• topological insulator
• bulk-insulating topological insulator
• Majorana fermion
• Majorana zero mode

Abstract

Topological insulators (TIs), when proximity-coupled to s-wave superconductors, are predicted to host Majorana Zero Modes (MZMs), which can be used for topologically protected quantum computing, as introduced by Fu and Kane in 2008. These predictions have since attracted significant attention. Various platforms of single Josephson junctions on TIs have been investigated to realize MZMs, but distinguishing the MZM signal is challenging due to the inherently weak signal and the presence of conventional states that coexist with MZMs. To address this issue, we fabricated Josephson junction arrays (JJAs) on TI films. JJAs provide an excellent platform for studying 2D superconductivity and searching for signatures of MZMs, as they allow control over the phase and supercurrent by adjusting the sample geometry and the applied magnetic field. Additionally, the large number of junctions in JJAs could amplify the MZM signal and simplify its detection. In this work, the TI films we used are confirmed to be bulk insulating, and it has been reported that introducing the proximity effect in such materials is challenging. However, we are among the first to demonstrate global superconductivity in our JJAs and achieve a relatively high critical current. This result has been consistently reproduced across different JJAs. Additionally, we observed that when a magnetic field is applied, the critical current of such an array resembles the optical diffraction grating, with the multiple columns of junctions in the JJA producing interference effects. This results in a sequence of sharp critical current peaks measured against the external magnetic field, significantly sharper than those observed in a conventional Superconducting Quantum Interference Device (SQUID). This effect makes JJAs sensitive absolute magnetic field detectors capable of distinguishing magnetic fields at the scale of 10 nT using a simple DC-current measurement approach. In addition, the relatively flat background between the sharp peaks offers a promising area to search for MZMs. In these regions, where interference is destructive and conventional supercurrent is suppressed, MZMs are expected to be more pronounced due to their unique 4π current-phase relationship. Another key finding is the pronounced asymmetry observed at the central sharp peak. This occurs at an extremely weak magnetic field, approximately one vortex per the entire array. The asymmetry indicates that our JJA exhibits a superconducting diode effect, which can be controlled by the magnetic field. We proposed a theoretical model to explain the asymmetry through the high kinetic inductance of the TI film and the inherent inhomogeneity across the array. However, the model predicts a lower level of asymmetry than what was observed experimentally. Interestingly, recent theoretical predictions suggest that the existence of MZMs could introduce a diode effect. Therefore, the asymmetry that could not be fully explained by our model may be related to the contribution of MZMs. We also studied another asymmetrical system with high kinetic inductance, consisting of two nanoconstrictions (Dayem bridges) with slightly different sizes. In this device, we observed that it functions as a fully superconducting kinetic inductance memory (SKIM) element, storing information in the form of single flux quanta. The most important result is that we can change the fluxoid number simply by sending a bias current, without applying a magnetic field. Studying this system enhances our understanding of nanowires, which exhibit fundamentally different critical phase and magnetic field responses compared to Josephson junctions. The device allows for precise control of the number of fluxoids introduced into the superconducting loops and demonstrates reliable operation at temperatures up to 2.8 K, achieving an error rate as low as one in 10e5 operations.

Graduation Semester

2024-12

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/127316

Copyright and License Information

Copyright © Xiangyu Song, 2024. All rights reserved.

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