Polarons, free charge localisation and effective dielectric permittivity in oxides

This review will deal with several types of free charge localisation in oxides and their consequences on the effective dielectric spectra of such materials. The first one is the polaronic localisation at the unit cell scale on residual impurities in ferroelectric networks. The second one is the collective localisation of free charge at macroscopic interfaces like surfaces, electrodes and grain boundaries in ceramics. Polarons have been observed in many oxide perovskites mostly when cations having several stable electronic configurations are present. In manganites, the density of such polarons is so high as to drive a net lattice of interacting polarons. On the other hand, in ferroelectric materials like BaTiO3 and LiNbO3, the density of polarons is usually very small but they can influence strongly the macroscopic conductivity. The contribution of such polarons to the dielectric spectra of ferroelectric materials is described. Even residual impurities as for example Iron can induce well defined anomalies at very low temperatures. This is mostly resulting from the interaction between localised polarons and the highly polarisable ferroelectric network in which they are embedded. The case of such residual polarons in SrTiO3 will be described in more details, emphasizing the quantum polaron state at liquid helium temperatures. Recently, several non-ferroelectric oxides have been shown to display giant effective dielectric permittivity. It is first shown that the frequency/temperature behaviour of such parameters is very similar in very different compounds (donor doped BaTiO3, CaCu3Ti4O12, LuFe2O4,Li doped NiO,...). This similarity calls for a common origin of the giant dielectric permittivity in these compounds. A space charge localisation at macroscopic interfaces can be the key for such extremely high dielectric permittivity.Comment: 17 pages, 11 figure

Dielectric and polarization experiments in high loss dielectrics: a word of caution

The recent quest for improved functional materials like high permittivity dielectrics and/or multiferroics has triggered an intense wave of research. Many materials have been checked for their dielectric permittivity or their polarization state. In this report, we call for caution when samples are simultaneously displaying insulating behavior and defect-related conductivity. Many oxides containing mixed valent cations or oxygen vacancies fall in this category. In such cases, most of standard experiments may result in effective high dielectric permittivity which cannot be related to ferroelectric polarization. Here we list few examples of possible discrepancies between measured parameters and their expected microscopic origin

Controlling internal barrier in low loss BaTiO3 supercapacitors

Supercapacitor behavior has been reported in a number of oxides including reduced BaTiO3 ferroelectric ceramics. These so-called giant properties are however not easily controlled. We show here that the continuous coating of individual BaTiO3 grains by a silica shell in combination with spark plasma sintering is a way to process bulk composites having supercapacitor features with low dielectric losses and temperature stability. The silica shell acts both as an oxidation barrier during the processing and as a dielectric barrier in the final composite

From bound states to resonances: analytic continuation of the wave function

Single-particle resonance parameters and wave functions in spherical and deformed nuclei are determined through analytic continuation in the potential strength. In this method, the analyticity of the eigenvalues and eigenfunctions of the Schroedinger equation with respect to the coupling strength is exploited to analytically continue the bound-state solutions into the positive-energy region by means of Pade' approximants of the second kind. The method is here applied to single-particle wave functions of the $^{154}Sm$ and $^{131}Eu$ nuclei. A comparison of the results with the direct solution of the Schroedinger equation shows that the method can be confidently applied also in coupled-channel situations requiring high numerical accuracy.Comment: 13 pages, 3 figure

Magnetic field tuning of polaron losses in Fe doped BaTiO3 single crystals

Artificial tuning of dielectric parameters can result from interface conductivity in polycrystalline materials. In ferroelectric single crystals, it was already shown that ferroelectric domain walls can be the source of such artificial coupling. We show here that low temperature dielectric losses can be tuned by a dc magnetic field. Since such losses were previously ascribed to polaron relaxation we suggest this results from the interaction of hopping polarons with the magnetic field. The fact that this losses alteration has no counterpart on the real part of the dielectric permittivity confirms that no interface is to be involved in this purely dynamical effect. The contribution of mobile charges hopping among Fe related centers was confirmed by ESR spectroscopy showing maximum intensity at ca T\sim40 K.Comment: Submitte

A material that “lends itself to fine interpretations”. The status of plaster in Marino Marini’s Oeuvre

The purpose of this paper aims to clarify the use of plaster in Marini’s artistic practice, both when the plaster is a unique piece, and when it is in relation with other materials. Even if Marini often used plaster in a ‘traditional’ way to make casts suitable for the translations to bronze or other materials, these sculptures are very often considered by the artist not only as translation tools and ‘mediation’ between the idea and the final plastic artwork, or academic exercise. For the artist, sculptures in plaster are also works with a proper autonomy that can render them final and definitive, as is evident from sculptures such as Piccola Pomona or Danzatrice, both made in the 1940s