Speaker
Description
Low-energy excess (LEE) is a common phenomenon in various solid-state detectors, acting as a significant limiting factor for rare event searches [1]. These anomalous signals create an unwanted noise floor that masks weak interactions, complicating the distinction between true dark matter signals and its backgrounds.
Graphene Josephson Junction (GJJ) based detectors have recently emerged as a highly promising candidate for extending detection sensitivity into the sub-meV regime, due to graphene’s minimal electronic heat capacity and $\pi$-bond electron interactions.[2] However, like other superconducting platforms, GJJs are significantly hampered by LEE. The microscopic origin of LEE within superconducting circuits is still not fully understood, necessitating more fundamental investigation into its underlying mechanisms.
In this work, we systematically investigate origins of LEE in GJJ-based detectors. We report a correlation between the magnitude of LEE and the superconducting electrode area. Our results indicate that smaller superconductor volumes significantly suppress LEE, suggesting that excess quasiparticles generated within the electrodes are the primary source of the observed background.
Based on these findings, we propose the implementation of normal metal electrodes to serve as quasiparticle reservoirs, providing an efficient dissipation pathway to further mitigate LEE. Our findings offer a practical roadmap for optimizing device geometry and quasiparticle engineering in diverse superconducting platforms, which is essential for reaching the operational thresholds required for super-light dark matter discovery.
[1] Baxter et al., Annu. Rev. Nucl. Part. Sci. 75, 301-326 (2025).
[2] G.-H. Lee, Nature 586, 42-46 (2020); D. Kim et al., Phys. Rev. D 112, 015002 (2025).