Zeeman effects on Yu-Shiba-Rusinov states

A magnetic impurity in a superconductor locally disrupts the superconductivity due to the exchange interaction (J) between the spins of the impurity and an itinerant electron, forming so-called Yu-Shiba-Rusinov (YSR) bound states. Upon increasing J, the YSR state undergoes a quantum phase transition from the free-spin to the screened-spin ground states. Although vigorous STM experiments have been done to distinguish these two states [1,2], an unambiguous way has not yet been established. Here we demonstrate that these two ground states exhibit distinct Zeeman effects. By using ultra-low temperature STM [3], we investigated a tunneling junction between Cu(111) surface and a superconducting niobium tip decorated by a single Fe atom. Depending on the condition of the Fe adsorbate, the YSR state either splits or shifts, signifying the screened-spin or free-spin ground states, respectively [4]. We also confirmed the complete spin polarization of these YSR states by comparing the spectra on a bare Cu(111) surface and on a single Fe adatom on Cu(111). These findings make it possible to discriminate the two different YSR states and pave the way for high-resolution spin-polarized spectroscopy [5].



[1] K. J. Franke, et al, Science 332, 940 (2011)

[2] Y.-H. Zhang, et al., Nat. Commun. 4, 2110 (2013)

[3] T. Machida, et al, Rev. Sci. Instrum. 89, 093707 (2018).

[4] T. Machida, et al, Phys. Rev. Research 4, 033182 (2022).

[5] L. Schneider, et al, Sci. Adv. 7, eabd7302 (2021).