Theory of enhanced proximity effect by midgap Andreev resonant state in diffusive normal-metal/triplet superconductor junctions

Enhanced proximity effect by the midgap Andreev resonant state (MARS) in a diffusive normal-metal/insulator/triplet superconductor (DN/TS) junction is studied based on the Keldysh-Nambu quasiclassical Green's-function formalism. By choosing a p-wave superconductor as a typical example of the TS, conductance of the junction and the spatial variation of the quasiparticle local density of states (LDOS) in the DN are calculated as the function of the magnitudes of the resistance R-d, Thouless energy in the DN, and the transparency of the insulating barrier. The resulting conductance spectrum has a zero-bias conductance peak (ZBCP) and the LDOS has a zero energy peak (ZEP) except for alpha=pi/2 (0 <= alpha <= pi/2),where alpha denotes the angle between the lobe direction of the p-wave pair potential and the normal to the interface. The widths of the ZBCP and the ZEP are reduced with the increase of Rd while their heights are drastically enhanced. These peaks are revealed to be suppressed by applying a magnetic field. When the magnitude of R-d/R-0 is sufficiently large, the total zero voltage resistance of the junction is almost independent of the R-d for alpha not equal pi/2. The extreme case is alpha=0, where total zero voltage resistance is always R-0/2. We also studied the charge transport in p(x) +ip(y)-wave junctions, where only the quasiparticles with perpendicular injection feel the MARS. Even in this case, the resulting LDOS in the DN has a ZEP Thus the existence of the ZEP in the LDOS of the DN region is a remarkable feature for DN/TS junctions which have never been expected for the DN/singlet superconductor junctions where the MARS and proximity effect compete with each other. Based on these results, a crucial test to identify triplet pairing superconductors based on tunneling experiments is proposed.