1,721,029 research outputs found
Bi2Sr2Can-1CunO2((n+2))(+delta) thin films on c-axis oriented and vicinal substrates
No full textBi2Sr2Can-1CunO2(n+2)+delta (n = 1,2,3) thin films are prepared on c-axis oriented and vicinal substrates by pulsed-laser deposition. Optimization of substrate temperature, laser fluence and post-annealing conditions produces single-phase, well-oriented and smooth films. The surface morphology, texture, crystallinity and stoichiometry of Bi2Sr2CaCu2O8+delta (Bi-2212) thin films strongly depend on the fabrication parameters. On (001) SrTiO3 substrates, c-axis oriented Bi-2212 films with J(c) (60 K) = 2 x 10(6) A/cm(2) and T-c0 = 82 K are obtained. Vicinal Bi-2212 films grown on miscut substrates reveal anisotropic resistivities independent of film thickness (20-300 am), high J(c) anisotropy and strong in-plane and out-of-plane texture. (C) 2001 Elsevier Science B.V. All rights reserved
Magneto-optical investigation of bi-crystalline thin films of Y-123
No full textWe have performed a deliberate study on the transport properties of Y-123 thin films on bi-crystalline substrates containing low angle grain boundaries (LAGBs). By magneto-optical (MO) imaging we are able to map the magnetic field and current distribution of wide current bridges. A quantitative analysis of the distribution of currents over the entire grain boundary (GB) is presented. We discuss the effects of magnetic history on the transport properties of Y-123 thin films with LAGBs. The results are compared with artificially created two-dimensional (2D) current distributions. (C) 2004 Elsevier B.V. All rights reserved
Scanning tunneling spectroscopy of optimally doped and underdoped YBa2Cu3Ox thin films
No full textRecently, experimental and theoretical evidence was given that grain boundaries in cuprate high-temperature superconductors are charge carrier depleted zones. In order to examine, if a charge carrier reduction in YBa2Cu3Ox (YBCO) thin films is detectable by scanning tunneling spectroscopy (STS), we extensively studied optimally doped and underdoped (charge carrier reduced) YBCO films on SrTiO3 substrates. The current-voltage (I-U) characteristics of these pulsed laser deposited YBCO films were recorded by means of STS at growth island plateaus in an ultra-high vacuum (UHV). The outcome of our experiments shows that dI/dU spectra of optimally doped and underdoped films differ significantly. Since even in UHV the YBCO surface degrades, the influence of a degraded surface layer on the tunnel process is studied. It is discussed, to what extent the dI/dU spectra and the changes on doping, respectively, are related to the local density of states of the films. (C) 2002 Elsevier Science B.V. All rights reserved
Transport and ordering of polarons in CER manganites PrCaMnO
No full textThe temperature-dependent resistivity and the colossal resistance effect induced by strong electric fields of the small-band Pr(1-x)Ca(x)MnO(3) (PCMO) manganites are analysed with respect to the influence of the Ca doping, post-annealing, the prehistory of the electric stimulation, and the physical dimensions of the sample. Despite the phase separation between charge and orbital ordered and disordered phases, PCMO reveals the properties of a homogeneous medium with a conductivity governed by the hopping of small polarons if the electric field is not too strong. In contrast, high electric fields induce a structural transition which gives rise to a glassy behaviour in the transient regime. In the low resistance state the small activation energy of charge carrier hopping implies a transition to large polaron hopping
Spectral distribution of activation energies in YBa2Cu3O7-δ thin films
No full textLocal magnetic relaxation experiments are carried out on YBa2Cu3O7-delta thin films by magneto-optics. The current density is calculated quantitatively by an inversion scheme of Blot Savart's law which gives direct information about the time evolution of the current density with a high spatial resolution of about 5 mum. We determine the local activation energies U-0(x,y) by fitting different relaxation laws at different positions (x, y) in the film plane. This analysis yields a spectral distribution of the activation energies that is independent of the applied model. Possible mechanisms which could lead to such a distribution are discussed
Direct measurements of electrostatic potentials at grain boundaries: mechanism for current improvement in high-T-c superconductors
No full textGrain boundaries (GBs) in high-temperature superconductors represent the major factor limiting high current applications. Via magneto-optical imaging and direct transport measurements, we observed an increase of the intergranular J(c) in 4degrees and 8degrees [001] low-angle grain boundaries via substitution of Y by Ca in YBa2Cu3O7-x by up to 100%. Since Ca substitution leads to hole overdoping, these results suggest that charge depletion and band bending may be responsible for the suppression of superconductivity in GBs. By using electron holography, we directly observe ail electrostatic potential at the dislocation cores. The radius as well as the value of the potential are reduced in Ca-doped grain boundaries, directly reflecting the improved transport properties. (C) 2004 Elsevier B.V. All rights reserved
Direct evidence for negative grain boundary potential in Ca-doped and undoped YBa2Cu3O7-x
No full textUsing electron holography in a transmission electron microscope, we obtained direct evidence for the reduction of negative charge at grain boundary dislocations in Ca-doped YBa2Cu3O7 (YBCO) when compared to undoped YBCO. Because of the finite width of the valence band in the superconducting CuO2 planes, the negative grain boundary charge can lead to a depletion of electron holes available for superconductivity. A significant reduction in the size of the perturbed region in the Ca-doped samples appears to be the principal mechanism for the improved interfacial superconductivity
Modification of transport properties in low-angle grain boundaries via calcium doping of YBa2Cu3O delta thin films
No full textThe limiting factor for current transport in high-T-c superconductors including grain boundaries is the intergranular critical current density. There have been various attempts to enhance the transport properties of multicrystalline high-T-c superconductors and to understand the mechanisms leading to a reduction of the critical currents over grain boundaries (GB's). We have extensively studied the effects of Ca doping on textured thin films, establishing only a rather small overdoping of the bulk samples. Nevertheless, doping the YBa2Cu3Odelta films with 20% Ca, symmetrical [001] tilt GB's with misorientation angle theta =4 degrees and 8 degrees showed strong benefits to Ca additions. We found an increase in the critical current density [J(gb)(theta)] up to 40% and 100% for the 4 degrees and 8 degrees GB's, respectively. Considering a model of Gurevich and Pashitskii for current transport in low-angle GB's, the improvements can be attached to a reduction of the strain fields epsilon and the localized charges Q, leading to a reduced built-in potential \V-bi\ in the dislocation cores
Observation of dendritic flux instabilities in YNi2B2C thin films
No full textMagneto-optical imaging and magnetization measurements performed on thin films of the borocarbide superconductor YNi2B2C reveal the occurrence of magnetic flux instabilities upon reducing the applied magnetic field towards the remanent state. In contrast to other low-T-c materials such as Nb and MgB2, where similar instabilities occur in both increasing and decreasing magnetic fields, dendritic flux patterns are observed in YNi2B2C for decreasing fields only. Also in the magnetization measurements, a distinct asymmetry is evident between increasing and decreasing fields. The effect does not depend on the sweep rate of the field, but is strongly dependent on the maximum field applied before reduction. The observation of spontaneous flux instabilities in this additional family of low-temperature superconductors suggests that the responsible mechanism is universal to this class of materials. (C) 2004 American Institute of Physics
Comparative study of magnetic and electric field induced insulator-metal-transitions in Pr 1-x Ca xMnO 3 films
No full textThe insulator-metal (IM)-transition in Pr 1-x Ca xMnO 3 (PCMO) is of particular interest because it can be induced by a variety of external forces, such as magnetic and electric fields, photon exposure and hydrostatic pressure. In this paper, we present a comparative study of the IM-transition in magnetic and electric fields for epitaxial thin films prepared by pulsed laser deposition. The transport data as a function of applied field or temperature give strong evidence for the presence of electronic phase separation. However, the observed different IM-transitions in magnetic and electric fields indicate that two different areas of spatially inhomogeneous electronic ground states in the phase diagram of PCMO are involved. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 200673.50.-h Electronic transport phenomena in thin films, 75.47.-m Magnetotransport phenomena; materials for magnetotransport, 75.47.Lx Manganites, 71.30.+h Metal-insulator transitions and other electronic transitions,
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