Near-field localization of the boson peak on tantalum films for superconducting quantum devices

TL;DR

This paper uses near-field terahertz microscopy to localize boson peaks in tantalum films, revealing amorphous loss centers that impact superconducting quantum device performance.

Contribution

It introduces a method to spatially localize vibrational loss centers in tantalum films using terahertz s-SNOM and nanospectroscopy, linking amorphous oxides to device losses.

Findings

Localized boson peak at 0.5 THz identified

Amorphous oxides observed on fresh samples

Crystalline phases develop after air aging

Abstract

Superconducting circuits are among the most advanced quantum computing technologies, however their performance is currently limited by losses found in surface oxides and disordered materials. Here, we identify and spatially localize a near-field signature of loss centers on tantalum films using terahertz scattering-type scanning near-field optical microscopy (s-SNOM). Making use of 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 (GIWAXS) shows that oxides on freshly solvent-cleaned samples are amorphous, whereas crystalline phases emerge after aging in air. By localizing defect centers at the nanoscale, our characterization techniques and results will inform the optimization of fabrication procedures for new low-loss superconducting circuits.