Researchers Anton V. Khvalyuk of LPMMC, Université Grenoble Alpes, and Mikhail V. Feigel’man of the Nanocenter CENN and Jozef Stefan Institute report that microwave loss in strongly disordered superconductors isn’t caused by material defects, but by “bulk localized collective modes” arising from the internal structure of the superconducting state. Their new microscopic theory explains why standard models of superconductivity, like the Mattis-Bardeen theory, fail in these materials due to a pseudogap ΔP exhibiting a hard pseudogap both below and above the transition temperature T c. The team’s analysis of the resonator quality factor Q(ω,T) reveals it decreases with frequency and unexpectedly exhibits two-level-system-like growth with temperature when temperatures are significantly below the critical temperature. This understanding, detailed in Physical Review Letters 136, published June 23, volume 136, published June 23, offers strategies to reduce intrinsic microwave losses in superconducting devices built from materials like InO x, titanium nitride, and niobium nitride.
Mattis-Bardeen Failure in Strongly Disordered Superconductors
A fundamental challenge to understanding superconductivity has emerged: the established Mattis-Bardeen theory demonstrably fails when applied to strongly disordered superconductors (SDSCs), due to the presence of a pseudogap (ΔP) that exhibits a hard pseudogap both below and above the transition temperature Tc. This pseudogap, distinct from the superconducting order parameter Δ, fundamentally alters the behavior of these materials and necessitates a revised theoretical framework for predicting their properties. Researchers Mikhail V. Feigel’man of Nanocenter CENN and Jozef Stefan Institute have developed a new microscopic theory addressing this discrepancy, focusing on the origins of microwave dissipation within SDSCs. Their analysis reveals a surprising trend in the resonator quality factor Q(ω,T); it decreases markedly with increasing frequency (ω) while simultaneously exhibiting a two-level-system-like increase with temperature (T) when T is significantly below Tc.
The team’s work provides a crucial microscopic understanding of experiments conducted on thin films of materials like InO x, TiN, and NbN, and is also relevant to granular aluminum films. Khvalyuk and Feigel’man state that their theory provides a microscopic understanding of existing and future experiments, suggesting potential strategies for minimizing microwave losses in SDSC-based quantum devices and improving superconducting technologies.
Localized Collective Modes and Resonator Quality Factor Q(ω,T)
The pursuit of robust superconducting devices faces a persistent challenge: microwave dissipation that limits coherence times, particularly in strongly disordered superconductors (SDSCs). Existing models, like the standard Mattis-Bardeen theory, struggle to explain this loss because they fail to account for the pseudogap (ΔP) exhibited both below and above the transition temperature (Tc). These modes, rather than material defects or boundaries, now appear to dominate low-frequency dissipation within the material itself. Khvalyuk and Mikhail V. Feigel’man’s research, detailed in Physical Review Letters 136, published June 23, volume 136, published June 23, not only illuminates the origins of microwave loss in SDSC-based devices, including those utilizing InO x, but also suggests potential strategies for minimizing these intrinsic losses, which could lead to improved quantum device performance.
