Giant Shot Noise in Superconductor/Ferromagnet Junctions with Orbital-Symmetry-Controlled Spin-Orbit Coupling

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A common goal in science and technology is to suppress noise and increase the signal-to-noise ratio1. However,

noise also enables elucidating subtle phenomena, hidden from other experimental probes2. A striking example

is shot noise, which can be used to characterize strongly-correlated systems, strange metals, quantum

entanglement, or fractionally charged quasiparticles3. Since shot noise stems from charge quantization, the

fractional quantum Hall effect will be characterized by the reduced shot noise, while in superconductors, where

the charge is added in Cooper pairs, even doubling of the normal-state shot noise is possible. By measuring the

shot noise, in a superconductor/insulator/ferromagnet (V/MgO/Fe) junctions4, grown in the group of Dr. Lu, we

discover a giant increase, orders of magnitude larger than expected5. The origin of this giant noise is a peculiar

realization of a superconducting proximity effect6, where a simple superconductor influences its neighbors. Our

measurements reveal largely unexplored implications of orbital-symmetry-controlled proximity effects. The

importance of orbital symmetries and the accompanying spin-orbit coupling is manifested by an unexpected

emergence of another superconducting region strikingly different from the parent superconductor. Unlike

vanadium’s common spin-singlet superconductivity, the broken inversion symmetry in V/MgO/Fe junctions and

the resulting interfacial spin-orbit coupling leads to the formation of spin-triplet superconductivity across the

ferromagnetic iron5,6. In contrast to the reports of an enhanced shot noise in Josephson junctions with two

superconductors, we measure such a phenomenon with a single superconductor and attribute it to transport

being determined by the spin-orbit coupling, which provides a large effective scattering. Our findings motivate

further studies for superconducting spintronics and for emulating properties of widely used Josephson junctions

but with only a single superconductor.

References:

1. R. Kononchuk et al., Nature 607, 697 (2022).

2. S. A. Crooker et al., Nature 431, 49 (2004).

3. C. Beenakker and C. Schonenberger, Phys. Today 56, 5, 37 (2003).

4. I. Martínez, P. Högl, C. González-Ruano, J. P. Cascales, C. Tiusan, Y. Lu, M. Hehn, A.

Matos-Abiague, J. Fabian, I. Žutić, F. G. Aliev, Phys. Rev. Applied 13, 014030 (2020).

5. C. González-Ruano, C. Shen, P. Tuero, C. Tiusan, Y. Lu, J. E. Han, I. Žutić, F. G. Aliev, Nat.

Commun., under review.

6. M. Amundsen, J. Linder, J. W. A., Robinson, I. Žutić, N. Banerjee, Rev. Mod. Phys. 96,

021003 (2024).

 

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