Spin-dependent tunneling between individual superconducting bound states

Magnetic impurities on superconductors induce discrete bound levels inside the superconducting gap, known as Yu-Shiba-Rusinov (YSR) states. YSR levels are fully spin polarized such that the tunneling between YSR states depends on their relative spin orientation. Here, we use scanning tunneling spectroscopy to resolve the spin dynamics in the tunneling process between two YSR states by experimentally extracting the angle between the spins. To this end, we exploit the ratio of thermally activated and direct spectral features in the measurement to directly extract the relative spin orientation between the two YSR states. We find freely rotating spins down to 7 mK, indicating a purely paramagnetic nature of the impurities. Such a noncollinear spin alignment is essential not only for producing Majorana bound states but also as an outlook manipulating and moving the Majorana state onto the tip.

Automated summary

Magnetic impurities on superconductors induce discrete bound levels called Yu-Shiba-Rusinov (YSR) states, which are fully spin polarized. By using scanning tunneling spectroscopy, the spin dynamics in the tunneling process between two YSR states were resolved, with experimentally extracted angle between spins down to 7 mK, revealing purely paramagnetic nature of the impurities. Noncollinear spin alignment is crucial for producing Majorana bound states and manipulating the Majorana state onto the tip.