1,720,983 research outputs found
Josephson effect in TiO2 nanostructured films
No full textHigh resistivity TiO2 nanostructured films exhibit photovoltaic dc voltages within the range1–10mVwith strong sensitivity to low energy radiation. Close similarities of these dc photovoltages with those related to Josephson coupling in granular films and high-Tc cuprates suggest large infrared photon absorption with pair transitions between superconducting grains. We give theoretical support to the existence of a high-Tc state within the TiO2 grains which can be the basis of resonant radiation absorption through Josephson intergrain coupling
Internal field-assisted thermally activated hopping and tunnelling in insulators and composite materials
No full textThe vortex structure of Dirac's equation for superconducting pairs in a magnetic field
No full textIt has been suggested recently that one can attach an electrodynamic meaning to quantum states of particles in a magnetic field described by Dirac's equation, the electric and magnetic fields and currents being linearly related to the components of the four-vector wavefunctions. As such one can calculate the internal fields from knowledge of quantum states and conversely knowledge of the fields allows to reconstruct the wavefunctions. In this paper we present the internal fields and currents in superconductors in a magnetic field by using this method, corresponding to Landau's states of lowest energy. We find that such states exhibit vortex structure with a London's behaviour at distances sufficiently far from their center; the structure of an isolated vortex is studied through the form of the field and current corresponding isomorphically to the state. The lower and upper critical fields arise from the condition of overlap of orbits within a length of the order of the London's and coherence length respectively. The lattice of vortices around the upper field is found. Some properties of excited states are also discussed. This method could reveal useful in visualization techniques of quantum states in type II superconductors
Diffraction effects in the propagation of polarizable layers
No full textThe very low transmission of light through holes smaller than the wavelength has been found to be enhanced for subwavelength apertures in metallic surfaces with periodic corrugations. This effect has been attributed to the interaction of light with surface plasmons. Similar effects obtained subsequently for non-metallic surfaces have been attributed to evanescent waves on the surface produced by the diffracted Bloch waves from different points in the array. We present an exact solution of Maxwell’s equations in the discrete dipole approximation (DDA) for a periodic array of polarizable point dipoles in a layer. Metallic as well as non metallic layers are described. When the wavelength is smaller than the lattice period there is a Bragg’s scattered wave, while for subwalength conditions an evanescent wave on the surface appears. The transmission/reflection coefficients are found to oscillate as a function of frequency, with resonances occurring in a broad range of frequencies depending on the polarizability, at which the evanescent field is enhanced. A detailed study is presented for nanostructured arrays. We find that this model agrees with features observed in experiments through hole arrays supporting the role played by diffraction during light transmission through such arrays without invoking surface plasmons and providing a base to analyze more complex geometries
Internal spinor fields and London's equation
No full textAn equivalence between Dirac's equation of an electron subject to an applied magnetic and Maxwell's equations by introducing electric and magnetic fields linearly related to the spinor wavefunctions. The internal magnetic field adds to the applied field and screens it due to its diamagnetic character. For a superconductor this screening is the origin of the expulsion of magnetic vortices and turns into a non-linear Ampere's law, of which London's equation is an approximate linearized form obtained on fixing the value of the internal field to its average value. This equation can be used to predict the critical field, intermediate normal/superconductor phases as in type I materials and non-linear effects beyond London's state
Quantum percolation in high Tc superconductors
No full textThis book gives an overview of the methods and results obtained so far by studying the characteristics and properties of nanoscale self-organized networks. It demonstrates the universality of the network approach over a range of disciplines, from protein folding to the newest electronic materials
Infrared radiation effects in TiO2 nanostructured films
No full textWe present results of the influence of infrared radiation on metal–TiO2–metal junctions where the metal electrodes are composed of conducting SnO–SnO2 on glass substrates. An amount of 10–30 micron oxide films are obtained by deposition of TiO2 from a colloidal suspension followed by heating sinterization of grains. The morphology of the films have been studied by AFM analysis and result in an array of weakly separated grains of average diameter 30nm. In the dark, the devices show open-circuit voltages of few millivolt, with slight increase under solar light illumination. The voltages exhibit strong sensitivity, oscillatory behaviour and polarity reversibility due to external changes of the radiation. Such a photovoltaic effect can be explained either as a mesoscopic voltage arising from the nanograins under thermal radiation or as resonances due to pair tunnelling between the nanograins where high Tc locally develops
Quantum percolation metal-insulator transition in high Tc oxides
No full textThe electric behaviour of high Tc oxides is analyzed on assuming a model of granular metals, in which conducting islands of radius d are separated by insulating barriers of width s. The conductivity as a function of temperature, averaged over conducting channels with a probability distribution is determined, on considering tunnelling across the barriers and the charging energy
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