Long-range superharmonic Josephson current and spin-triplet pairing   correlations in a junction with ferromagnetic bilayers
  The long-range spin-triplet supercurrent transport is an interesting phenomenon in the superconductor/ferromagnet ($S/F$) heterostructure containing noncollinear magnetic domains. Here we study the long-range superharmonic Josephson current in asymmetric $S/F_1/F_2/S$ junctions. It is demonstrated that this current is induced by spin-triplet pairs $\mid\uparrow\uparrow\rangle-$$\mid\downarrow\downarrow\rangle$ or $\mid\uparrow\uparrow\rangle+$$\mid\downarrow\downarrow\rangle$ in the thick $F_1$ layer. The magnetic rotation of the particularly thin $F_2$ layer will not only modulate the amplitude of the superharmonic current but also realise the conversion between $\mid\uparrow\uparrow\rangle-$$\mid\downarrow\downarrow\rangle$ and $\mid\uparrow\uparrow\rangle+$$\mid\downarrow\downarrow\rangle$. Moreover, the critical current shows an oscillatory dependence on thickness and exchange field in the $F_2$ layer. These effect can be used for engineering cryoelectronic devices manipulating the superharmonic current. In contrast, the critical current declines monotonically with increasing exchange field of the $F_1$ layer, and if the $F_1$ layer is converted into half-metal, the long-range supercurrent is prohibited but $\mid\uparrow\uparrow\rangle$ still exists within the entire $F_1$ region. This phenomenon contradicts the conventional wisdom and indicates the occurrence of spin and charge separation in present junction, which could lead to useful spintronics devices. 
Hao Meng, Jiansheng Wu, Xiuqiang Wu, Mengyuan Ren and Yajie Ren
cond-mat.supr-con