  We propose a new approach of smearing origins of a zero-bias conductance peak
(ZBCP) in high-Tc superconductor tunnel junctions through the analysis based on
the circuit theory for a d-wave pairing symmetry. The circuit theory has been
recently developed from conventional superconductors to unconventional
superconductors. The ZBCP frequently appears in line shapes for this theory, in
which the total resistance was constructed by taking account of the effects
between a d-wave superconductor and a diffusive normal metal (DN) at a junction
interface, including the midgap Andreev resonant states (MARS), the coherent
Andreev reflection (CAR) and the proximity effect. Therefore, we have analyzed
experimental spectra with the ZBCP of Ag-SiO-Bi-2212 planar tunnel junctions
for the {110}-oriented direction by using a simplified formula of the circuit
theory for d-wave superconductors. The fitting results reveal that the spectral
features of the ZBCP are well explained by the circuit theory not only
excluding the Dynes's broadening factor but also considering only the MARS and
the DN resistance. Thus, the ZBCP behaviors are understood to be consistent
with those of recent studies on the circuit theory extended to the systems
containing d-wave superconductor tunnel junctions.
