Studies On The Transport Properties Of Electron In The Ferromagnetic Superconductor Tunnel Junctions

With the development of spintronics and the latest development and utilization of new-style device products, in recent years, the academic circle has an increasing more interest in the research of transmission characters of spin-polarized quasiparticles in ferromagnet/superconductor tunnel junctions. Because the system of tunnel junctions is a basic device for memory store and has a wide application in many areas, and because there exist many interesting physical features in the system, it has become an active theme for research. Recently, the research on the coexistence between ferromagnetism and superconductivity arouses great interest in the academic circle of condensed matter physics, among which two kinds of the coexistence between ferromagnetism and superconductivity attract the academic circle's attention. The first kind is that, in the interface of tunnel junctions of ferromagnet/superconductor, the proximity effect can lead to the coexistence between ferromagnetism and superconductivity. The second kind of coexistence between ferromagnetism and superconductivity is the superconductivity recently discovered by experts in large bulk ferromagnetic materials such as CeCoIn5, UGe2, ZrZn2 and URhGe. The research shows CeCoIn5 has the character of d-wave superconductor of spin singlet state, while regarding UGe2, ZrZn2 and URhGe, the symmetry of the electrons pairing is still in argument. Although there are many research reports about transportation property of spin-polarized quasiparticles in tunnel junctions of ferromagnet/superconductor, magnetic semiconductor/superconductor, superconductor/ferromagnet/superconductor etc, little research covers the field of shot noise in the tunnel junctions of semiconductor/ ferromagnetic d-wave superconductor and Josephson Effect in ferromagnetic superconductor/ferromagnet/ferromagnetic superconductor junctions. This paper, based on some newly-discovered new-style ferromagnetic superconducting materials such as CeCoIn5, UGe2, ZrZn2 and URhGe, makes a research on transportation property in ferromagnetic superconducting tunnel junctions. Its main research and result are as follows:In chapter one, some fundamental conceptions are introduced, including the development of the study for superconductors'properties, Andreev reflection properties and Josephson current effect in the superconductor tunnel junctions. Also, we give an introduction for theoretical methods in studying Josephson current.In the second chapter, we give an introduction for BdG equation and BTK theory, study the proximity effect and the coexistence for superconductivity and ferromagnetism in ferromagnet/superconductor junction. In the mean-field approximation, we solve a self consistent equation for the d-wave superconducting gap and the effective exchange field in ferromagnetic d-wave superconductor, and study the zeeman effect on the d-wave superconducting gap and thermodynamic potential, discuss the condition for the coexistence of superconductivity and ferromagnetism.In chapter three, considering the dependence for the gap and exchange energy in ferromagnetic d-wave superconductor, we apply an extended Blonder-Tinkham-Klapwijk approach to investigate the shot noise in semiconductor/ferromagnetic d-wave superconductor junction by solving the Bogoliubov-de Gennes equation. It is shown that (1) the exchange energy h0 in the ferromagnetic d-wave superconductor may induce a split of shot noise for the double-peak at the zero-bias and energy gap peak, and the width between the two splitted peaks is equal to 2h0 ; (2) with increasing the exchange energy h0 , the shot noise and the ratio of shot noise power to average current are restrained.In the last chapter, we study the Josephson effect in ferromagnetic d-wave superconductor/ferromagnet/ferromagnetic d-wave superconductor double tunnel junctions. There are two oscillation components with different periods in the Josephson current, in which the long period corresponds crossover from 0 andπstate with increasing ferromagnet layer thickness. It is found that the short period component can be separated from the long period one by increasing the exchange splitting in ferromagnet and the barrier strength at interface, and vice versa, indicating that the long and short period components arise from quantum interference effect, respectively, due to the Andreev and normal reflection at interface. Also, it is found that the Josephson critical current depend on the relative orientation of the effective exchange field and the crystal orientation of two ferromagnetic d-wave superconductors. The exchange field under certain condition of ferromagnet thickness can enhance the Josephson critical current.