93 research outputs found
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
Point-Contact Spectroscopy on RuSr2GdCu2O8
We present Point-Contact experiments on polycrystalline RuSrGdCuO
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
Subharmonic gap structures and Josephson effect in MgB2/Nb micro-constrictions
Superconducting micro-constrictions between Nb tips and high quality
MgB 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,
the conductance of the MgB/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 meV for the
three-dimensional gap of MgB. Below T, 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 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
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 RuSrGdCuO 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 and of the ratio are inferred. From the
temperature evolution of the vs 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 vs evolution, a critical magnetic field
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 YBaCuO/LaCaMnO bilayers
We present conductance characteristics of point contact junctions realized
between a normal Pt-Ir tip and
YBaCuO/LaCaMnO (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
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
Piezoelectricity and charge trapping in ZnO and Co-doped ZnO thin films
Piezoelectricity and charge storage of undoped and Co-doped ZnO thin films were investigated by means of PiezoResponse Force Microscopy and Kelvin Probe Force Microscopy. We found that Co-doped ZnO exhibits a large piezoelectric response, with the mean value of piezoelectric matrix element d33 slightly lower than in the undoped sample. Moreover, we demonstrate that Co-doping affects the homogeneity of the piezoelectric response, probably as a consequence of the lower crystalline degree exhibited by the doped samples. We also investigate the nature of the interface between a metal electrode, made up of the PtIr AFM tip, and the films as well as the phenomenon of charge storage. We find Schottky contacts in both cases, with a barrier value higher in PtIr/ZnO than in PtIr/Co-doped ZnO, indicating an increase in the work function due to Co-doping
Imaging the spontaneous formation of vortex-antivortex pairs in planar superconductor/ferromagnet hybrid structures
Low-temperature magnetic force microscopy has been used to visualize spontaneous formation of vortex-antivortex pairs in hybrid ferromagnet/superconductor systems. Vortex-antivortex pairs are induced by the periodic stray field of the ferromagnet. We find general equilibrium conditions for which spontaneous vortex-antivortex pairs are formed during zero-field cooling of the hybrid ferromagnet/superconductor bilayers. Vortices can be generated by the ferromagnet domains in the absence of an external field and they are thermodynamically stable for values of the stray field and the period of the stripe magnetic domains that exceed a certain threshold
Side-gate leakage and field emission in all-graphene field effect transistors on SiO2/Si substrate
We fabricate planar all-graphene field-effect transistors with self-aligned side-gates at 100 nm from the main graphene conductive channel, using a single lithographic step. We demonstrate side-gating below 1V with conductance modulation of 35% and transconductance up to 0.5 mS/mm at 10 mV drain bias. We measure the planar leakage along the SiO2/vacuum gate dielectric over a wide voltage range, reporting rapidly growing current above 15 V. We unveil the microscopic mechanisms driving the leakage, as Frenkel-Poole transport through SiO2 up to the activation of Fowler-Nordheim tunneling in vacuum, which becomes dominant at high voltages. We report a field-emission current density as high as 1uA/um between graphene flakes. These findings are essential for the miniaturization of atomically thin devices
Side-gate leakage and field emission in all-graphene field effect transistors on SiO2/Si substrate
We fabricate planar all-graphene field-effect transistors with self-aligned
side-gates at 100 nm from the main graphene conductive channel, using a single
lithographic step. We demonstrate side-gating below 1V with conductance
modulation of 35% and transconductance up to 0.5 mS/mm at 10 mV drain bias. We
measure the planar leakage along the SiO2/vacuum gate dielectric over a wide
voltage range, reporting rapidly growing current above 15 V. We unveil the
microscopic mechanisms driving the leakage, as Frenkel-Poole transport through
SiO2 up to the activation of Fowler-Nordheim tunneling in vacuum, which becomes
dominant at high voltages. We report a field-emission current density as high
as 1uA/um between graphene flakes. These findings are essential for the
miniaturization of atomically thin devices.Comment: Research article, 8 pages, 5 figure
- …