Near-Field Localization of the Boson Peak on Tantalum Films for Superconducting Quantum Devices

Xiao Guo, Zachary Degnan, Julian A. Steele, Eduardo Solano, Bogdan C. Donose, Karl Bertling, Arkady Fedorov, Aleksandar D. Rakić and Peter Jacobson

The Journal of Physical Chemistry Letters 14, 4892 (2023)
Superconducting circuits are among the most advanced quantum computing technologies; however, their performance is limited by losses found in surface oxides and disordered materials. In this work, we demonstrate the identification and spatial localization of a near-field signature of loss centers on tantalum films using terahertz scattering-type scanning near-field optical microscopy. By utilizing terahertz nanospectroscopy, we observe a localized excess vibrational mode around 0.5 THz and identify this resonance as the boson peak, a signature of amorphous materials. Grazing-incidence wide-angle X-ray scattering reveals that oxides on freshly solvent-cleaned samples are amorphous, whereas crystalline phases emerge after aging in air. Through nanoscale localization of defect centers, our findings provide valuable insights for the optimization of fabrication procedures for new low-loss superconducting circuits.