3. We have calculated the effect of different gap symmetries on the tunneling density of states by solving the real-axis Eliashberg equations for a half-filled band. The resulting tunneling curves have been compared to those obtained by analytical continuation of the imaginary-axis solution (a much simpler approximate procedure). We have shown that, for certain symmetries, the latter procedure does not give satisfactory results. In addition to this, we have studied the effect of small amounts of non-magnetic impurities on the tunneling curves. As an example, we have applied our results to the case of high-temperature superconductors.

G.A. Ummarino, R.S. Gonnelli and D. Daghero
“Solution of real-axis Eliashberg equations with different pair symmetries and tunneling density of states”, Physica C 341-348, 299 (2000).

G. A. Ummarino, R. S. Gonnelli and D. Daghero
“Real-axis solution of Eliashberg equations in various order-parameter symmetries and tunneling conductance of optimally-doped HTSC”, Int. J. Mod. Phys. B 14, 2944 (2000).

G.A. Ummarino, R.S. Gonnelli and D. Daghero
“Tunneling conductance of SIN junctions with different gap symmetries and non-magnetic impurities by direct solution of real-axis Eliashberg equations”, Physica C 377, 292 (2002).

Finally, we have used point-contact measurements on polycrystalline LSCO (see structure) to extract information about the possible gap symmetries in this compound. In addition to the s+d-wave symmetry we have found that also a d+id-wave symmetry is compatible with our experimental data. This latter symmetry has the interesting property of breaking the time-reversal symmetry and should give rise to spontaneous current flow.

D. Daghero, R.S. Gonnelli, G.A. Ummarino and V.A. Stepanov
"Possible evidence of a d+id scenario in La_2-xSr_xCuO₄ by Andreev reflection measurements" Int. J. Mod. Phys. B 17, 649 (2003).