Scanning Tunneling Spectroscopy Studies of Metal Capping layers on Nb for Superconducting Qubit Development

ORAL

Abstract

Recent studies have shown significant improvement in the T1 decay time of transmon qubits by using a thin (~10 nm) capping layer of Al, Ta or TiN on top of the Nb capacitor film, thereby preventing the formation of the RF lossy Nb oxide layer. Here we demonstrate that scanning tunneling spectroscopy (STS) can play an important role in identifying optimal capping layers, including metals that have little or no surface oxide layer such as TiN and Re. STS directly reveals the proximity induced gap in normal metal capping layers and probes any sub-gap quasiparticle states that might cause non-TLS losses, down to atomic scale. Inhomogeneities in superconducting gap values can also lead to potential TLS losses. A focus is on a comparison of sub-gap states capped Nb films as compared to bare Nb films where the native oxide has been removed by ion milling. Preliminary data indicate that capping layers can lead to a reduction in sub-gap quasiparticle states.

* This work was supported by the U.S. Department of Energy, Office of Science, National Quantum Information Science Research Centers, Superconducting Quantum Materials and Systems Center (SQMS) under contract No. DE-AC02-07CH11359.

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Presenters

  • Seth J Rice

    Illinois Institute of Technology, Temple University, Illinois Institute of Technology, Chicago, IL 60616, Temple University, Philadelphia, PA, FNAL Superconducting Quantum Materials and Systems Center, Illinois Institute of Technology

Authors

  • Seth J Rice

    Illinois Institute of Technology, Temple University, Illinois Institute of Technology, Chicago, IL 60616, Temple University, Philadelphia, PA, FNAL Superconducting Quantum Materials and Systems Center, Illinois Institute of Technology

  • Junki Makita

    Temple University

  • Giulia Berti

    Temple University

  • Francesco Crisa

    Fermi National Accelerator Laboratory, Illinois Institute of Technology, Fermilab, Illinois Institute of Technology

  • Mustafa Bal

    Fermilab, Fermi National Accelerator Laboratory

  • Jae-Yel Lee

    Fermi National Accelerator Laboratory, Fermilab, FermiLab

  • Akshay A Murthy

    Fermilab, Fermi National Accelerator Laboratory

  • Anna Grassellino

    Fermilab, Fermi National Accelerator Laboratory

  • John F Zasadzinski

    Illinois Institute of Technology, Illinois Institute of Technology, Chicago, IL 60616, FNAL Superconducting Quantum Materials and Systems Center

  • Maria Iavarone

    Temple University, Temple University, Philadelphia, PA, USA, FNAL Superconducting Quantum Materials and Systems Center