2026/02/19 by Takayuki Kubo
Engineering · Physics and Astronomy · #Physics of Superconductivity and Magnetism #Superconducting and THz Device Technology #Terahertz technology and applications #cond-mat.supr-con #physics.acc-ph #physics.ins-det #quant-ph
paper · pdf · doi:10.1088/1361-6668/ae8b33
published as Superconductor Science and Technology 39, 075036 (2026) · 16 pages, 12 figures
arxiv created 2026/02/19 · openalex publication_date 2026/07/01 · openalex created_date 2026/07/16 · arxiv updated 2026/07/30 · openalex updated_date 2026/07/30
Superconducting waveguides are a promising platform for ultralow-loss transmission in the millimeter-wave to terahertz band under cryogenic conditions, with potential applications in astronomical instrumentation and emerging quantum technologies. We develop a framework, based on microscopic superconductivity theory, to evaluate the power-flow attenuation constant α of superconducting rectangular waveguides in the 100~GHz--THz range, applicable to arbitrary electronic mean free paths ℓ from the dirty limit ℓ≪ξ0 to the clean limit ℓ≫ξ0. We also derive an analytical expression for two-level-system (TLS)-induced attenuation α\rm TLS in thin native oxide layers within the standard TLS model. Using this framework, we perform numerical evaluations of α for representative materials over standard waveguide sizes from WR15 to WR1. In the high-frequency regime f \gtrsim 0.5 Δ/h, low attenuation favors the clean regime ℓ\gtrsimξ0, indicating that high-purity materials can achieve very low attenuation below their gap frequency. For the TLS contribution, using parameter values representative of native Nb oxides, we find that α\rm TLS can become relevant at sufficiently low temperatures T/Tc\lesssim 0.1-0.2, where quasiparticle dissipation is exponentially suppressed. Finally, we extend the discussion to the strong-excitation regime using a recently developed nonlinear-response theory within the Keldysh--Usadel framework of nonequilibrium superconductivity and show that nonlinear dissipation produces a Higgs-mode peak in α near f≃ Δ/h via a Kerr-type nonlinearity of the dissipative conductivity. This peak provides a distinct hallmark of the Higgs mode that has been largely overlooked so far.