Point Contact Study of the Superconducting Order Parameter in Rusr 2 Gdcu 2 O 8

We have performed a detailed study of the conductance characteristics obtained by point contact junctions realized between a normal Pt/Ir tip and syntered RuSr 2 GdCu 2 O 8 (Ru-1212) samples. Indeed, this compound is subject of great interest due to the coexistence of both magnetic order and bulk superconductivity. In our experiments, the low temperature tunneling spectra reproducibly show a zero bias conductance peak that can be well reproduced by a generalized BTK model in the case of d-wave symmetry of the superconducting order parameter. From the discovery of high T c superconductors several experimental and theoretical analysis have been realized to reveal the electronic structure and to understand the pairing state of these materials. Recently a new compound of this family, the RuSr 2 GdCu 2 O 8 (Ru-1212), has raised great interest. This rutheno-cuprate super-conductor is a triple perovskite material with alternating CuO 2 bilayers and RuO 2 monolayers. In addition to the superconducting transition, the rutheno-cuprate materials show magnetic order at ∼ 135K. The magnetic order of the Ru moments is predominantly antiferromagnetic along the c axis 1 , while a ferromagnetic component has been observed along the a, b planes 2. In this report we study the Point– Contact conductance characteristics obtained on Ru-1212 compound. The majority of the tunneling curves shows a zero-bias conductance peak (ZBCP), which can be modeled by assuming a d − wave pairing symmetry of the superconducting order parameter in this material. The Ru-1212 pellets were melt-textured synthesized samples 3. Resistivity measurements 4 have shown a critical temperature T onset c = 40K and T c (ρ = 0) = 25K. To realize our experiments we used a Pt-Ir tip, chemically etched in a 37% solution of HCl and in an ultrasound bath. We have established the contact between the sample and the tip at T = 4.2K, and we measured the differential conductance dI/dV as a function of the voltage bias V using a conventional four-probe method and lock-in technique at 377Hz. We obtained different conductance