Public defence, Engineering Physics, MSc Weijun Zeng

Title of the thesis: Low frequency noise in quantum devices based on graphene and superconducting cobalt disilicide

Thesis defender: Weijun Zeng 
Opponent: Professor Dan Fleetwood, Vanderbilt University, USA
Custos: Professor Pertti Hakonen, Aalto University School of Science

Understanding 1/f noise is critical for enhancing the performance of next-generation quantum electronic devices. This thesis systematically investigates the mechanisms of 1/f noise in quantum systems based on graphene and superconducting cobalt disilicide CoSi₂ utilizing graphene Corbino disks, kinetic-inductance superconducting cavities, and Josephson-inductance coupled systems. 

First, 1/f noise in graphene was probed by introducing neon adatoms, applying perpendicular magnetic fields, and performing second-spectrum analysis. The results reveal that the diffusion and clustering of neon atoms significantly modulate the noise profile. Crucially, the data indicates that mobility fluctuations, rather than carrier density fluctuations, dominate 1/f noise in graphene, while second-spectrum analysis confirms long-time correlations among microstructural fluctuators. 

Second, kinetic inductance in superconducting CoSi₂ was investigated using microwave cavities patterned from thin films with thicknesses of 10, 25, 60, and 105 nm. The extracted kinetic inductance exhibits a non-BCS power-law temperature dependence. Furthermore, 1/f frequency fluctuations in the 25-nm cavity display a distinct temperature-dependent pivoting feature and decay with increasing probing power. 

Finally, frequency fluctuations were measured in niobium superconducting cavities coupled with a graphene Josephson junction (GJJ). The integration of the GJJ increased the overall device noise compared to bare niobium cavities. Notably, the observed noise exhibited a lack of gate-voltage dependence, suggesting that mechanisms other than critical current fluctuations dominate 1/f noise in the investigated hybrid system. 

These findings provide vital insights into the fundamental material and physical factors limiting the coherence and performance of quantum devices.

Thesis available for public display 7 days prior to the defence at Aalto University's public display page. 

Contact Information:
Email: weijun.zeng@mail.sdu.edu.cn 
Linkedin: https://www.linkedin.com/in/weijun-zeng-27169822b