Current transport in two-dimensional HTS NID junctions

Abstract

Motivated by existing difficulties in the experimental observation of the zero bias anomaly (ZBA) in HTS NID and DID structures with thick dielectric barriers we have studied theoretically the effect of scattering of electrons on localized states (LS) within a barrier on width and amplitude of the ZBA. Our model structure consists of thick dielectric barrier with predominantly two-dimensional (2D) character of the electron transport (e.g. PBCO), separated normal metal (N), and an HTS superconductor with d-wave superconducting correlation (D). The barrier is modeled by a 2D rectangular potential with δ-functional cavities describing LS. To study conductance spectra the concept of coherent transport through systems with complex barrier developed by Büttiker and Belogolovskii et al. has been generalized to the case of breaking of the conservation of the quasiparticle momentum component parallel to the interfaces in the tunneling process. We have shown that transport 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 calculations 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 current component, which usually was not taken into account in the description of NIS contacts, suppresses the amplitude of ZBA provided by the direct tunneling process. The resultant ZBA peak should be very narrow and its amplitude is effectively suppressed by any scatters located in the barrier. This makes the experimental ZBA observation in NID structures with thick barriers very questionable.