Negative Spin Cross-Correlation in a Cooper Pair Splitter

The control and measurement of the spin degree of freedom of solid-state electrons has a wide range of prospective applications, be it in spintronics logic devices or in fundamental research, for example to demonstrate spin correlations in quantum mechanical systems. To this end, we have introduced ferromagnetic split-gates (FSGs) to individually polarize the electron spins in semiconductor quantum dots (QDs) [1]. We first demonstrate the working principle of such electronic spin filters in a double QD spin valve [2], consisting of two individually polarized, weakly coupled QDs in series, showing spin polarization up to 80%. We then implement such spin filters in a Cooper pair splitting (CPS) device [3] with two FSG/QD elements coupled in parallel to a superconducting reservoir to demonstrate a negative correlation between the spin currents emitted from the splitting of spin-singlet Cooper pairs. We measure a strong negative spin correlation of -1/3, which deviates from the ideal value of -1 mainly due to the finite spin polarization of the QD states [4]. Such QD spin filters are suitable for various applications, for example in spin projection experiments investigating spin structures in Rashba nanowires [5], or, in equal spin Andreev reflection [6] at Majorana type bound states.