Negative spin Δ T noise induced by spin-flip scattering and Andreev reflection

We study chargeΔTnoise, followed by an examination of spinΔTnoise, in the normal metal-spin flipper-normal metal-insulator-superconductor junction. Our analysis reveals a key contrast: while chargeΔTnoise remains strictly positive, spinΔTnoise 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 remain positive and sign-definite, which is valid for spin quantum shot noise also. The emergence of negative spinΔTnoise has two major implications. First, it establishes a clear distinction between spin resolvedΔTnoise 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 spinΔTnoise 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 negativeΔTnoise 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 inΔTnoise and highlights their role as distinctive fingerprints of spin-dependent scattering in superconducting hybrid devices.