Coherent and incoherent tunneling into YSR states revealed by atomic scale shot-noise spectroscopy

TL;DR

This study uses shot-noise spectroscopy to investigate electron tunneling into Yu-Shiba-Rusinov states in superconductors, revealing the coexistence of coherent and incoherent processes and providing insights into their intrinsic lifetime.

Contribution

The paper demonstrates that shot-noise spectroscopy can probe the energy and time scales of YSR states beyond thermal limits, revealing the coexistence of different tunneling processes.

Findings

Electron tunneling into YSR states shows ordered, reduced noise.

Both single-electron and Andreev processes occur simultaneously.

Intrinsic lifetime of YSR states is measured as 1 μeV, much less than thermal energy.

Abstract

The pair breaking potential of individual magnetic impurities in s-wave superconductors generates localized states inside the superconducting gap commonly referred to as Yu- Shiba-Rusinov (YSR) states whose isolated nature makes them ideal building blocks for artificial structures that may host Majorana fermions. One of the challenges in this endeavor is to understand their intrinsic lifetime, $\hbar/\Lambda$, which is expected to be limited by the inelastic coupling with the continuum thus leading to decoherence. Here we use shot-noise scanning tunneling microscopy to reveal that electron tunnelling into superconducting 2H-NbSe$_2$ mediated by YSR states is ordered as function of time, as evidenced by a reduction of the noise. Moreover, our data show the concomitant transfer of charges e and 2e, indicating that incoherent single particle and coherent Andreev processes operate simultaneously. From the quantitative agreement between experiment and theory we obtain $\Lambda$ = 1 $\mu$eV $\ll$ $k_BT$ demonstrating that shot-noise can probe energy- and time scales inaccessible by conventional spectroscopy whose resolution is thermally limited.