超伝導量子素子内で発生する新たなタイプの正孔伝導現象を観測～超伝導体内に侵入した水素の量子性の解明にも期待～

2026-04-16 九州大学

九州大学の研究グループは、超伝導量子素子内に侵入した水素原子核（プロトン）の量子挙動に起因する新たな正孔伝導現象を初めて観測した。ニオブ製ジョセフソン接合に水素を導入し極低温で電流‐電圧特性を解析した結果、プロトンのトンネル運動により多重アンドレエフ伝導が促進され、電気伝導が増加することを確認した。これは超伝導転移に伴いプロトンのトンネル確率が増大する可能性を示すものである。本成果は、量子コンピュータにおけるノイズ源や超伝導特性劣化の原因解明に寄与するとともに、水素の量子性を活用した新規量子デバイス開発への応用が期待される。

プロトンが侵入したニオブ超伝導－常伝導－超伝導ジョセフソン接合。

＜関連情報＞

水素不純物によって誘発される狭窄型超伝導ジョセフソン接合におけるコンダクタンス異常
Conductance anomalies in constriction-type superconducting Josephson junctions induced by hydrogen impurities

Zizhou Tai;Kazuki Miyakawa;Hiroki Takata;Masanobu Shiga;Ken-ichi Hashizume;Tatsuya Kawae
Journal of Applied Physics Published:April 01 2026
DOI:https://doi.org/10.1063/5.0320043

To microscopically investigate the influence of hydrogen on transport properties in superconductors, we studied the current–voltage characteristics of hydrogen-absorbed niobium (Nb) and hydrogen-adsorbed lead (Pb) superconductor–normal metal–superconductor (SNS) Josephson junctions. The junctions were fabricated by stretching Nb and Pb wires using a mechanically controllable break junction technique. In hydrogen-absorbed Nb SNS Josephson junctions, where hydrogen was absorbed into the Nb lattice while a fraction of H₂ molecules remained adsorbed on the junction surface by low-temperature loading at ∼4.2 K, the differential conductance (dI/dV) spectra exhibit peak-shaped anomalies at V ∼ 0.8 mV and above 1 mV, together with multiple spike-like anomalies having an almost uniform spacing of about 0.1 mV within the superconducting gap. In the NbH_0.1 SNS Josephson junction, where hydrogen is preloaded above room temperature and the surface is free of H₂ molecules, the dI/dV spectra exhibit several peak-shaped anomalies at V ∼ 0.8 mV and above 1 mV, whose temperature dependence is consistent with higher-order multiple Andreev reflection (MAR) processes. In contrast, hydrogen-covered Pb SNS Josephson junction showed only multiple spike-like anomalies. These results suggest that proton tunneling assists hole transport in the normal region of the SNS junction, giving rise to peak-shaped anomalies at higher order MAR voltages, whereas the spike-like anomalies originate from excitations of hydrogen molecules adsorbed on the junction surface.