Real-axis solution of Eliashberg equations in various order-parameter symmetries and tunneling conductance of optimally-doped HTSC

In the present work we calculate the theoretical tunneling conductance curv es of SIN junctions involving high-T-c superconductors, for different possi ble symmetries of the order parameter (s, d, s+id, sf d, anisotropic s and extended s). To do so, we solve the real-axis Eliashberg equations in the c ase of an half-filled infinite band. We show that some of the peculiar char acteristics of HTSC tunneling curves (dip and hump at eV > Delta, broadenin g of the gap peak, zero bias and so on) can be explained in the framework o f the Migdal-Eliashberg theory. The theoretical dI/dV curves calculated for the different symmetries at T = 4 K are then compared to various experimen tal tunneling data obtained in optimally-doped BSCCO, TBCO, HBCO, LSCO and YBCO single crystals. To best fit the experimental data, the scattering by non-magnetic impurities has to be taken into account, thus limiting the sen sitivity of this procedure in determining the exact gap symmetry of these m aterials. Finally, the effect of the temperature on the theoretical tunneli ng conductance is also discussed and the curves obtained at T = 2 K are com pared to those given by the analytical continuation of the imaginary-axis s olution.