Theoretical Study Of Superconducting Junctions

Tunnel spectrum is very important to study the superconducting mechanism. BTK theory is the useful method to study the tunnel spectrum. Nowadays, proximity effect in superconducting junctions and the coexistence of superconductivity and ferromagnetism near the interface of ferromagnet/superconductor structures have attracted much attention. Recently, the study on the ferromagnet/untranditional superconductor junctions has been an active subject. In this dissertation, we investigate the superconducting junctions aiming at the present arguments about the study of the superconducting structures.In chapter two, we introduce some fundamental concepts, including superconducting pairing, Andreev reflection in tunnel junctions, BTK theory and BdG equation, superconducting proximity effect, spin-polarized transport and spin-filtering effect, et al. At the same time, we also introduce the development and status of the theoretical study on the superconductor, superconducting junctions, and proximity effect in tunnel junctions.In chapter three, we extend the BTK theory to calculate the tunneling conductance for the normal metal/ferromagnet/triplet p-wave superconductor junctions. Three kinds of pairings for the p-wave superconductor are chosen: waves. We get the zero-bias conductance peak, dip, and double dips structures observed in the experiments. We obtain the following results: (1) Superconducting proximity effect and p-wave superconductor mid-bound state can coexist in the system. The competition between the two mechanisms decides the shape of the tunneling spectrum. (2) The tunneling spectrum exhibits the transition from"0"state to"π"state for different thickness of the ferromagnet and exchange p x ,py,px+ipy energy in the ferromagnetic layer. (3) The damped oscillation behavior of the normalized conductance suggests the possibility of the coexistence of ferromagnetism and superconductivity near the interface of the ferromagnet/p-wave superconductor for the weak exchange energy.In chapter four, we investigate the proximity effect in the normal metal/insulator/ferromagnet/d-wave superconductor junctions. We use the BTK theory to calculate the normalized conductance and obtain the following conclusions: (1) There are two mechanisms in the system: superconducting proximity effect and d-wave superconducting Andreev bound state. (2) The tunneling spectrum exhibits the transition from"0"state to"π"state for different thickness of the ferromagnet and exchange energy in the ferromagnetic layer. (3) Proximity effect strongly depends on the angle between the a-axis of the superconductor and interface normal, as the angle increases the proximity effect is enhanced: for d-wave superconductor, being strongest forα/π= 0.25and for p-wave superconductor, being strongest forα/π= 0.5. (4) The damped oscillation behavior of the normalized conductance suggests the possibility of the coexistence of ferromagnetism and superconductivity near the interface of the ferromagnet/d-wave superconductor for the weak exchange energy.In chapter five, taking into account the thickness of the ferromagnetic insulator and using the square-potential, we study the spin-filter effect, Zeeman effect, and proximity effect in the normal metal/ferromagnetic insulator/normal metal/superconductor tunnel junctions. We find that: (1) Spin-filter effect of the ferromagnetic insulator induces the splitting of the subgap resonance peaks. The spin- polarization due to the spin-filter effect of the ferromagnetic insulator causes an imbalance of the peaks heights. (2) The amplitude of the splitting becomes larger as the barrier height and width increase and the effect of the barrier height on the conductance is larger than that of the barrier width. (3) The applied magnetic field causes the splitting of the gap conductance peaks and the spin-filter effect can enhance the splitting induced by the external field. (4)The spin-filter effect has no contribution to the proximity–effect-induced superconductivity in the normal metal interlayer so that the normalized conductance can't exhibit the transition from"0"to"π"state.In chapter six, we study the effect of the ferromagnetic insulator on the tunneling conductance in normal metal/ferromagnetic insulator/normal metal/superconductor structures taking into account the ordering and coherent tunneling. We obtain: (1) Differential conductance dependent on the ferromagnetic insulator thickness is decided by the barrier strength and the exchange energy. (2) The rough interface scattering can depress the coherence between the electrons and the Andreev reflected holes in the normal metal interlayer and prevent the minigap structure in the tunneling spectrum. (3) The insulator thickness and barrier height can influence the height of the minigap peak and the density of the state near the Fermi surface. (4) The exchange energy in the ferromagnetic insulator can change the width of the minigap structures.