NNSE508_EM-L7-Dielectrics

NNSE508_EM-L7-Dielectrics - 1 Lecture contents Dielectrics....

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NNSE 508 EM Lecture #7 1 Lecture contents Dielectrics. Polarization Linear polarizability Clausius-Masotti equation Frequency dependence of polarizability EM/optical properties of dielectrics Ferroelectrics and piezoelectrics
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NNSE 508 EM Lecture #7 2 Charge and Polarization Polarization may be thought of as a bulk movement of the positive charges relative to the negative charges resulting in the bound charge density ρ b . Consider three cases: • No polarization. Charge density (ρ b ) in the medium is zero since the positive (ρ + ) and negative (ρ - ) distributions overlap. • Uniform polarization. The relative shift of the charge densities leads to the appearance of surface charge densities (σ) The positive and negative charge densities in the bulk still cancel. • Nonuniform polarization. The positive charge density is stretched out as well as displaced to the right. The charge density on the positive surface is greater than that on the negative surface. The polarisation increases to the right. b P  
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NNSE 508 EM Lecture #7 3 Dielectrics. Polarizability • Three basic mechanisms of polarization: – Dipolar (molecular) polarizability due to reorientation (most significant in liquids and gases) – Ionic polarizability due to displacements of the positive and negative ions – Results in lattice distortions – May give rise to ferroelectricity – Atomic (electronic) polarizability due to redistribution of charge in any atom
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NNSE 508 EM Lecture #7 4 Polarizability • Problem: find electric response of dielectric material on electric field, in other words find polarization ( P ) vs. external electric field • Linear response: • Dipole moment of atom • Simplest case: linear scalar polarizability of atom, a (can be tensor) • Next, we need to find E loc   (1) (2) 2 0 ... P E E c 2 .. { } P E E CGS cc 0 PE c   P E + - + - - Electric Field   p lq e rn r dv   loc pE * 
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NNSE 508 EM Lecture #7 5 Polarizability of atom loc pE a
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This note was uploaded on 11/08/2011 for the course NNSE 508 taught by Professor Sergeoktyabrsky during the Spring '11 term at SUNY Albany.

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NNSE508_EM-L7-Dielectrics - 1 Lecture contents Dielectrics....

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