Conan Weiland

Revealing the Origin and Nature of the Buried Metal-Substrate Interface Layer in Ta/Sapphire Superconducting Films

  1. Aswin kumar Anbalagan,
  2. Rebecca Cummings,
  3. Chenyu Zhou,
  4. Junsik Mun,
  5. Vesna Stanic,
  6. Jean Jordan-Sweet,
  7. Juntao Yao,
  8. Kim Kisslinger,
  9. Conan Weiland,
  10. Dmytro Nykypanchuk,
  11. Steven L. Hulbert,
  12. Qiang Li,
  13. Yimei Zhu,
  14. Mingzhao Liu,
  15. Peter V. Sushko,
  16. Andrew L. Walter,
  17. and Andi M. Barbour
Despite constituting a smaller fraction of the qubits electromagnetic mode, surfaces and interfaces can exert significant influence as sources of high-loss tangents, which brings forward
the need to reveal properties of these extended defects and identify routes to their control. Here, we examine the structure and composition of the metal-substrate interfacial layer that exists in Ta/sapphire-based superconducting films. Synchrotron-based X-ray reflectivity measurements of Ta films, commonly used in these qubits, reveal an unexplored interface layer at the metal-substrate interface. Scanning transmission electron microscopy and core-level electron energy loss spectroscopy identified an approximately 0.65 \ \text{nm} \pm 0.05 \ \text{nm} thick intermixing layer at the metal-substrate interface containing Al, O, and Ta atoms. Density functional theory (DFT) modeling reveals that the structure and properties of the Ta/sapphire heterojunctions are determined by the oxygen content on the sapphire surface prior to Ta deposition, as discussed for the limiting cases of Ta films on the O-rich versus Al-rich Al2O3 (0001) surface. By using a multimodal approach, integrating various material characterization techniques and DFT modeling, we have gained deeper insights into the interface layer between the metal and substrate. This intermixing at the metal-substrate interface influences their thermodynamic stability and electronic behavior, which may affect qubit performance.
arxiv pdf

Microscopic Relaxation Channels in Materials for Superconducting Qubits

  1. Anjali Premkumar,
  2. Conan Weiland,
  3. Sooyeon Hwang,
  4. Berthold Jäck,
  5. Alexander P.M. Place,
  6. Iradwikanari Waluyo,
  7. Adrian Hunt,
  8. Valentina Bisogni,
  9. Jonathan Pelliciari,
  10. Andi Barbour,
  11. Mike S. Miller,
  12. Paola Russo,
  13. Fernando Camino,
  14. Kim Kisslinger,
  15. Xiao Tong,
  16. Mark S. Hybertsen,
  17. Andrew A. Houck,
  18. and Ignace Jarrige
Despite mounting evidence that materials imperfections are a major obstacle to practical applications of superconducting qubits, connections between microscopic material properties
and qubit coherence are poorly understood. Here, we perform measurements of transmon qubit relaxation times T1 in parallel with spectroscopy and microscopy of the thin polycrystalline niobium films used in qubit fabrication. By comparing results for films deposited using three techniques, we reveal correlations between T1 and grain size, enhanced oxygen diffusion along grain boundaries, and the concentration of suboxides near the surface. Physical mechanisms connect these microscopic properties to residual surface resistance and T1 through losses arising from the grain boundaries and from defects in the suboxides. Further, experiments show that the residual resistance ratio can be used as a figure of merit for qubit lifetime. This comprehensive approach to understanding qubit decoherence charts a pathway for materials-driven improvements of superconducting qubit performance.
arxiv pdf