Quasiparticle spectroscopy, transport, and magnetic properties of Nb films used in superconducting transmon qubits

TL;DR

This paper provides a comprehensive analysis of niobium thin films used in superconducting qubits, highlighting their superconducting properties, magnetic behavior, and potential challenges for quantum coherence.

Contribution

It offers a detailed characterization of niobium films, combining multiple analytical techniques to inform improvements in superconducting qubit performance.

Findings

High superconducting transition temperature of 9.35 K

Clean superconducting gap and enhanced superfluid density

Magnetic response shows irreversibility and thermo-magnetic instabilities

Abstract

Niobium thin films on silicon substrate used in the fabrication of superconducting qubits have been characterized using scanning and transmission electron microscopy, electrical transport, magnetization, quasiparticle spectroscopy, and real-space real-time magneto-optical imaging. We study niobium films to provide an example of a comprehensive analytical set that may benefit superconducting circuits such as those used in quantum computers. The films show outstanding superconducting transition temperature of $T_{c}=9.35$ K and a fairly clean superconducting gap, along with superfluid density enhanced at intermediate temperatures. These observations are consistent with the recent theory of anisotropic strong-coupling superconductivity in Nb. However, the response to the magnetic field is complicated, exhibiting significantly irreversible behavior and insufficient heat conductance leading to thermo-magnetic instabilities. These may present an issue for further improvement of transmon quantum coherence. Possible mitigation strategies are discussed.