Point-Contact Spectroscopy on RuSr2GdCu2O8

We present Point-Contact experiments on polycrystalline RuSr$_2$GdCu$_2$O$_8$ samples. The majority of tunneling curves shows a zero-bias conductance peak, which is modeled by assuming a d-wave pairing symmetry of the superconducting order parameter.The magnetic field dependence of the conductance spectra has been measured in very stable junctions. In some cases, due to the granularity of the samples, clusters of grains in series introduce peculiar features in the conductance spectra.Comment: 4 pages, 4 figures, published on Journal of Physics and Chemistry of Solid

Point Contact Study of the Superconducting Order Parameter in RuSr2GdCu2O8

We have performed a detailed study of the conductance characteristics obtained by point contact junctions realized between a normal Pt/Ir tip and syntered RuSr2GdCu2O8 (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.Comment: 3 pages, 2 figures, IJMPB style. Presented at New3SC5 Conference, China, June 200

Subharmonic gap structures and Josephson effect in MgB2/Nb micro-constrictions

Superconducting micro-constrictions between Nb tips and high quality MgB$_{2}$ pellets have been realized by means of a point-contact inset, driven by a micrometric screw. Measurements of the current-voltage characteristics and of the dynamical conductance versus bias have been performed in the temperature range between 4.2 K and 500 K. Above the Nb critical temperature T$_{C}^{Nb}$, the conductance of the MgB$_2$/normal-metal constrictions behaves as predicted by the BTK model for low resistance contacts while high resistance junctions show quasiparticle tunneling characteristics. Consistently, from the whole set of data we infer the value $\Delta_{\pi} = 2.5 \pm 0.2$ meV for the three-dimensional gap of MgB$_2$. Below T$_{C}^{Nb}$, low resistance contacts show Josephson current and subharmonic gap structures (SGS), due to multiple Andreev reflections. Simultaneous observations of both features, unambiguously indicate coupling of the 3D band of MgB$_2$ with the Nb superconducting order parameter. We found that the temperature dependence of the Josephson critical current follows the classical Ambegaokar-Baratoff behavior with a value $I_CR_N=(2.1 \pm 0.1)$ meV at low temperatures.Comment: 8 pages, 5 figures. Replaced with published versio

Pairing state in the rutheno-cuprate superconductor RuSr2GdCu2O8: A point contact Andreev Reflection Spectroscopy study

The results of Point Contact Andreev Reflection Spectroscopy on polycrystalline RuSr$_2$GdCu$_2$O$_8$ pellets are presented. The wide variety of the measured spectra are all explained in terms of a modified BTK model considering a \emph{d-wave} symmetry of the superconducting order parameter. Remarkably low values of the energy gap $\Delta=(2.8\pm 0.2)meV$ and of the $2\Delta/k_BT_c\simeq 2$ ratio are inferred. From the temperature evolution of the $dI/dV$ vs $V$ characteristics we extract a sublinear temperature dependence of the superconducting energy gap. The magnetic field dependence of the conductance spectra at low temperatures is also reported. From the $\Delta$ vs $H$ evolution, a critical magnetic field $H_{c_2}\simeq 30 T$ is inferred. To properly explain the curves showing gap-like features at higher voltages, we consider the formation of a Josephson junction in series with the Point Contact junction, as a consequence of the granularity of the sample.Comment: 8 pages, 7 EPS figures. Accepted in Phys. Rev.

Point Contact Spectra on YBa2_2Cu3_3O7−x_{7-x}/La0.7_{0.7}Ca0.3_{0.3}MnO3_3 bilayers

We present conductance characteristics of point contact junctions realized between a normal Pt-Ir tip and YBa$_2$Cu$_3$O$_{7-x}$/La$_{0.7}$Ca$_{0.3}$MnO$_3$ (YBCO/LCMO) bilayers. The point contact characteristics show a zero bias conductance peak, as a consequence of the formation of Andreev bound states at the YBCO Fermi level. The temperature evolution of the spectra reveals a depressed zero bias peak and a reduced superconducting energy gap, both explainable in terms of spin polarization effects due to the LCMO layer.Comment: 4 pages, 4 EPS figures. Proceedings of EUCAS 2005. Accepted in Journal of Physics: Conference Serie

Generalized Blonder-Tinkham-Klapwijk theory and conductance spectra with particle-hole mixing interface potential

We extend the Blonder-Tinkham-Klapwijk treatment including particle-hole mixing boundary conditions in the Bogoliubov-de Gennes scattering problem to describe anomalous conductance features often reported in normal-metal/superconductor junctions. We calculate the differential conductance spectra and show that conductance dips, not expected in the standard formulation, can be explained in terms of a phase π-shift between the bulk and the interface order parameter. A tight-binding model is also introduced to give a quantitative description of the phase-shift in terms of the transparency and polarization of the interface. We characterize the physics arising from particle-hole mixing boundary conditions at the interface and its effects on the conductance anomalies in superconductor-normal heterostructures

Charge transfer and trapping as origin of a double dip in the transfer characteristics of graphene based field-effect transistors

We discuss the origin of an additional dip other than the charge neutrality point observed in transfer characteristics of graphene-based field-effect transistors. The double-dip is proved to arise from charge transfer between graphene and metal electrodes, while charge storage at the graphene/SiO2 interface enhances it. Considering different Fermi energy from the neutrality point along the channel and partial charge pinning at the contacts, we propose a model which explains all features in gate voltage loops.Comment: 14 pages, 5 figure

Andreev reflection in ferrimagnetic CoFe2O4/SrRuO3 spin filters

We have performed point contact spectroscopy measurements on a sample constituted by a metallic ferromagnetic oxide (SrRuO_3) bottom electrode and a tunnel ferrimagnetic (CoFe_2O_4) barrier. Andreev reflection is observed across the tunnel barrier. From the comparison of Andreev reflection in SrRuO3 and across the CoFe_2O_4 barrier we infer that the ferrimagnetic barrier has a spin filter efficiency not larger than +13%. The observation of a moderate and positive spin filtering is discussed in the context of the microstructure of the barriers and symmetry-related spin filtering effects.Comment: 13 pages, 5 figures. To appear in Phys. Rev. B