Free particle charge-dipole interaction

The EDM r/e,p,nu of an electron, proton, or neutron is neccessarily aligned along the spin direction a of the particle. In essence, an EDM measurement in an atom or molecule involves polarizing the system with an applied external electric field and searching for the interaction E between the electronic or nuclear EDM and the polarized atom/molecule. Schiff s theorem [18] states that q = 0 if the atom/molecule is made of point particles bound by electrostatic forces. In other words, the electronic or nuclear EDM does not see the applied field because it is shielded out by the other charged particles. This theorem is important for its loopholes nuclei are not point particles and the electric dipole interaction is not screened when the electrons are relativistic. Consequently, q is not zero if the atom/molecule is well chosen [19,20]. Eor example the best measurement of the proton EDM comes from a measurement on TIE molecules [21], where the large size of the T1 nucleus ends up giving q 1 for the nuclear spin EDM interaction. The upper limit on the neutron EDM is known both directly, from measurements on free neutrons [22], and indirectly from nuclear spin measurements on Hg atoms [23]. 

The electrostatic polarization theory is commonly employed to describe ER response. The model assumes that ER fluids are dispersions of nonionic polarizable particles in a low dielectric medium and that free charges and charge-transfer electrochemical processes can be neglected. This model is based on the fact that, due to the permittivity mismatch between the particles 6p and the continuous phase e, the dipolar particles are polarized and aligned with the neighboring particles. When an electric field is superimposed on the point dipole interaction, the orientation of the dipoles in relation to the exter-... 

Fig. 4.35 (a) Ensemble of non-interacting ferroelectric nanoparticles covered outside by the ambient free charges a. AH particle radii R are less than the correlation radius so that the dipole moments inside the particle are aligned due to the correlation effects, (b) A given nanoparticle, where the arrows inside the particle indicate the absolute value of dipole moments in different points [117]... 

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