Nanocomposites and nanomaterials

Resonant Andreev reflection in a ferromagnetic normal metal - magnetic quantum dot - superconductor structure 
in magnetic field
Koshina E. A., Krivoruchko V. N.
Donetsk Institute for Physics and Engineering, the NAS of Ukraine, Prospect Nauki, 46, Kyiv-03028, Ukraine. 
E-mail: elenakoshina1@gmail.com

Spin-dependent resonant tunneling through a quantum dot (QD), a small system characterized by discrete electronic states, coupled with a ferromagnetic normal-metal (F) and s-wave superconductor (S), F-QD-S system, has been among interesting subject of experimental and theoretical investigations. In addition to the basic interest, such hybrid meso-nano-structured systems boast potential applications in future electronic devices which employ both the charge and spin degree of freedom of electrons (see, e.g., [1]). 
In this work, we investigate the electron tunneling through a F-mQD-S system. Different from Ref. [2] where discrete levels of the QD are spin independent, here we assume that the QD has spin-splitted discrete levels (magnetic QD, mQD) and this splitting can be varied by external magnetic field. The Keldysh nonequilibrium-Green's-function method is used to derive the current and, particularly, a probability of Andreev reflection (AR), TA(ω), for the F-mQD-S structure. The sensitivity of the TA(ω) on an external magnetic field (i.e., on the mQD level's spin-splitting) has been studied in detail. For this simple model system, we found interesting results. In addition to the resonant behavior of the Andreev tunneling as obtained in previous works, we find that the current I versus the gate voltage V exhibits different kinds of peaks, depending on: (i) the bias voltage, (ii) the mQD’s level spacing, and (iii) the energy gap of the S electrode. Besides, in the I-V characteristics extra peaks superimposed on the conventional current plateaus emerge, which stem from the resonant ARs. The results open a way for exploring such systems in tunable spintronic devices.
The study was carried out within the Target program fundamental research of the NAS of Ukraine (Project No. 17/20-H)
1.	Sohn L. L., Kouwenhoven L. P., Schön G., Mesoscopic Electronic Transport // Vol. 345 of NATO Advanced Studies Institute, Series E: Appl. Sci. (Kluwer, Dordrecht, 1997). 
2.	Sun Q.F., Wang J., Lin T.H., Resonant Andreev reflection in a normal- metal--quantum-dot--superconductor system // Phys. Rev. B 59, 3831 (1999).