Andreev tunneling through a ferromagnet/quantum-dot/superconductor system

We study Andreev tunneling through a ferromagnet/quantum-dot (QD)/superconductor system. By using nonequilibrum Green function method, the averaged occupation of electrons in QD and the Andreev tunneling current are studied. Comparing to the norma-metal/quantum-dot/supercondiictor, the system shows significant changes: (i) The averaged occupations of spin-up and spin-down electrons are not equal. (ii) With the increase of the polarization of ferromagnetic lead, the Andreev reflection current decreases. (iii) However, even the ferromagnetic lead reaches full polarization, the averaged occupation of spin-down electrons is not zero. The physics of these changes is discussed.