Josephson and noise spectroscopy in single-atom junctions

• Applicant: Professorin Dr. Katharina Franke
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 548080365

Josephson junctions including single magnetic adatoms have recently been shown to exhibit non-reciprocal behaviour in the retrapping current, which marks the transition from the normal, i.e. resistive state, to the dissipationless state. This property reveals similarities to a diode, therefore termed Josephson diode. This diode effect originates from asymmetric Yu-Shiba-Rusinov (YSR) states induced by exchange coupling of a magnetic atom to the superconductor. YSR states carry a quasi-particle current and, thereby, dominate the damping, which in turn influences the retrapping current. In general, the dynamics in such junctions is largely determined by fluctuations and damping. The goal of this project is to gain a deeper understanding of the fluctuations in the junction and how they quantitatively affect the dynamics of the Josephson junction. We will create Josephson junctions by approaching with a superconducting tip of a scanning tunnelling microscope to a (magnetic) adatom on a superconducting substrate. We will measure the switching and retrapping current of these junctions. These characteristic currents carry information on fluctuations and frequency-dependent damping. To quantify the effect of noise, we follow two strategies. On the one hand, we deliberately introduce high-frequency radiation into the junction and measure its response in the switching and retrapping current. On the other hand, we will directly measure the current noise in the junction. We will further resolve the effect of magnetic adatoms in the junction on the fluctuation properties. Our results will allow us to draw a comprehensive picture of the response of Josephson junctions to external noise sources as well as the relevance of internal noise sources. This knowledge may prove essential for optimizing the performance of Josephson-junction based devices.

• DFG Programme: Research Grants