In many publicationd, explanation of AC conduction mechanism given in the terms of a polaron hopping conduction mechanism. Why then do polarons increase the conductivity of ceramics?
Depending on the field You are working in, one of several definitions will be used, however, in general polarons are spin-bearing charge carriers (when considering organic semiconductors, these typically are linked with radical cations / radical anions).
Macroscopic conductivity of a material is the product of the number of polarons (or other types of charge carriers, such as bipolarons or solitons) in the material (which can be influenced by doping) and their mobility (which is tied to the nature of the material), across the bulk of the material.
Polarons are charge carriers together with the local deformation. They are heavy. They decrease conductivity compared to the materials without deformation, but for some materials the deformation cannot be neglected. It is the only way to go.
Dear Sandeep
Polaron is a quasi particle used in condensed matter to describe the interaction of electron with atom in solids. In fact it is self trapped electron which moves from one place to another by lattice distortion associated with it. The mobility of polaron is decreased and effective mass is increased as compared to electron. It requires thermal energy in addition to electric field to move as hopping is required for conduction to take place. Conductivity in this process is low and increases with temperature. This concept has been used in amorphous semiconductors also. The magnitude of conductivity is material sensitive. In ceramic samples also conductivity will depend on the material.
Dear Sandeep Kumar
Have a nice time and good day
The polaron concept has been studied in both ordered and disordered solids. A polaron is a quasi particle used to understand the interactions between electrons and atoms in the solid materials. The general concept of a polaron has been extended to describe other interactions between the electrons and ions. The polaron radius,Rp must be greater than the radius of the atom, on which the electron is localized, but less than the distance,r, separating these sites. Note that, the size of polarons should decrease when the number of atoms was increased.
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Estimation of some physical characteristics of chalcogenide bulk Cd 50 S(50-x) Se x glassy systems. Available from: https://www.researchgate.net/publication/281495010_Estimation_of_some_physical_characteristics_of_chalcogenide_bulk_Cd_50_S50-x_Se_x_glassy_systems [accessed May 1, 2016].
Article Estimation of some physical characteristics of chalcogenide ...
Dear Sandeep,
A polaron is simply a combination of a defect state and a trapped electron. This whole combination moves with increased effective mass and decreased mobility. As for as ac conductivity is concerned there is two types of hoping : single polaron hoping and bipolaron hoping. In single polaron hoping only one electron moves back and forth between two defect states whereas in bipolaron hoping two electrons hop simultaneously between two randomly created defect states.
Following are the papers which describe the mechanism of ac conduction on the basis of polaron hoping.
Article A. C. Conduction in glassy alloys of Se90Sb10-xAgx
Article Density of Charged Defect States Using A.C. Conductivity Mea...
Dear Sandeep,
The polaron is one quasiparticle formed with the coupling between one electron and phonon of a crystal lattice (no with one atom in particular). It is possible to distinguish two kind of polarons, weak and strong, taking into account the value of their coupling constant. The effective mass of these particles is higher or much higher depending of the coupling (quadratic in the weak and to the fourth power) with respect to electronic mass band. Thus it is immediate to see that the hopping for these quasiparticles is lower ( or much lower) than for the band electrons.
This is quite well known, although there are still problems for solving models with them.
References:
A. A. Abrikosov, L. P. Gorkov and I. E. Dzyaloshinski, Methods of Quantum Field Theory in Statistical Physics, Prentice-Hall 1963.
A. L. Fetter and J. D. Walecka, Quantum Theory of Many-Particle Systems, McGraw-Hill 1971.
G. D. Mahan, Many-Particle Physics, Plenum Press 1981.
J. W. Negele and H. Orland, Quantum Many Particle Systems, Perseus Books 1998.
Ph. A. Martin and F. Rothen, Many-Body Problems and Quantum Field Theory, Springer-Verlag 2002.
H. Bruus and K. Flensberg, Many-Body Quantum Theory in Condensed Matter Physics, Oxford University Press 2004.
Dear Madhu,
There are some methods for direct measurement of polaron mobility (such as high-frequency AC techniques). Indirect methods are, however, more common and typically involve measuring the conductivity of a material (e.g. two-point or four-point probe measurements) and the concentration of polarons (e.g. EPR spectroscopy) in that material to calculate the average polaron mobility. There are a number of variations here and investigations of the dependence of charge carrier (e.g. polaron) mobility on doping level or temperature are rather common topics.
the interchange of single and double bonds leads to changes in energy, the solitons cannot be stable because the structure is not symmetric. Instead, double defects are required for such non-degenerate polymers. Such double defects are called polarons; they can be considered as a bound state of a charged soliton and a neutral soliton
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