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

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.

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Cited by in corpus (4)

• Superconducting flip-chip devices using indium microspheres on Au-passivated Nb or NbN as under-bump metallization layer
• The nontrivial effects of annealing on superconducting properties of Nb single crystals
• Quasiparticle spectroscopy in technologically-relevant niobium using London penetration depth measurements
• Directional driving of vortex lines with oscillating magnetic field