Current transport in two-dimensional HTS NID junctions

Motivated by existing difficulties in the experimental observation of the z ero bias anomaly (ZBA) in HTS NID and DID structures with thick dielectric barriers we have studied theoretically the effect of scattering of electron s on localized states (LS) within a barrier on width and amplitude of the Z BA. Our model structure consists of thick dielectric barrier with predomina ntly two-dimensional (2D) character of the electron transport (e.g. PBCO), separated normal metal (N), and an HTS superconductor with d-wave supercond ucting correlation (D). The barrier is modeled by a 2D rectangular potentia l with delta -functional cavities describing LS. To study conductance spect ra the concept of coherent transport through systems with complex barrier d eveloped by Buttiker and Belogolovskii et al. has been generalized to the c ase of breaking of the conservation of the quasiparticle momentum component parallel to the interfaces in the tunneling process. We have shown that tr ansport properties of NID structures essentially differ from the transport in contacts with conventional superconductors (NIS) and principally cannot be described in a one-dimensional approximation. Direct analytical calculat ions give that tunneling via LS does not contribute to ZBA independently on their space position in the barrier and on the relation between the metals Fermi energy and LS energy level. It is also shown that the interference c urrent component, which usually was not taken into account in the descripti on of NIS contacts, suppresses the amplitude of ZBA provided by the direct tunneling process. The resultant ZBA peak should be very narrow and its amp litude is effectively suppressed by any scatters located in the barrier. Th is makes the experimental ZBA observation in NID structures with thick barr iers very questionable. (C) 2001 Elsevier Science B.V. All rights reserved.