Negative Spin $\Delta_T$ noise Induced by Spin-Flip Scattering and Andreev Reflection

TL;DR

This paper investigates charge and spin $ ext{Delta}_T$ noise in a superconducting hybrid junction, revealing that spin noise can become negative due to spin-flip scattering and Andreev reflection, unlike charge noise.

Contribution

It demonstrates that negative spin $ ext{Delta}_T$ noise uniquely indicates spin-dependent scattering effects not captured by quantum shot noise.

Findings

Charge $ ext{Delta}_T$ noise remains positive.

Spin $ ext{Delta}_T$ noise can reverse sign from positive to negative.

Negative spin $ ext{Delta}_T$ noise highlights spin-flip and Andreev reflection interplay.

Abstract

We study charge $\Delta_T$ noise, followed by an examination of spin $\Delta_T$ noise, in the normal metal-spin flipper-normal metal-insulator-superconductor (N-sf-N-I-S) junction. Our analysis reveals a key contrast: while charge $\Delta_T$ noise remains strictly positive, spin $\Delta_T$ noise undergoes a sign reversal from positive to negative, driven by the interplay between spin-flip scattering as well as Andreev reflection. In contrast, charge quantum shot noise remains positive and sign-definite, which is also valid for spin quantum shot noise. The emergence of negative spin $\Delta_T$ noise has two major implications. First, it establishes a clear distinction between spin-resolved $\Delta_T$ noise and quantum shot noise: the former is dominated by opposite-spin correlations, whereas the latter is led by same-spin correlations. Second, it provides access to scattering mechanisms that are not captured by quantum shot noise alone. Thus, negative spin $\Delta_T$ noise serves as a unique probe of the cooperative effects of Andreev reflection and spin flipping. We further place our results in context by comparing them with earlier reports of negative $\Delta_T$ noise in strongly correlated systems, such as fractional quantum Hall states, and in multiterminal hybrid superconducting junctions. Overall, this work offers new insights into the mechanisms governing sign reversals in $\Delta_T$ noise and highlights their role as distinctive fingerprints of spin-dependent scattering in superconducting hybrid devices.