Maxwell-Wagner Relaxations

Non-intrinsic effects can give rise to colossal dielectric constants, e.g., the long-known Maxwell-Wagner polarization, which arises from charge accumulation at interfaces. Interfaces of any kind can generate very high values of the dielectric constant because they act as parallel-plate capacitors with very small plate distances, thus having high capacitances. They can occur at the surface of the sample, e.g., due to the formation of a Schottky diode at the electrode/sample interface. Also internal interfaces can arise, e.g., from grain boundaries in ceramic samples, planar crystal defects (e.g., twin boundaries), but also spontaneously via electronic phase separation (e.g., charge stripe-ordering) or other mechanisms. Especially the latter "quasi-intrinsic" effects are of high interest for application.

Maxwell-Wagner relaxations lead to a strong frequency-dependence of the dielectric properties, which can be modeled by equivalent circuits (see figure below). At high frequencies the capacitor caused by the interface becomes shorted and the intrinsic bulk behavior is observed.

colossal epsilon

Frequency-dependent dielectric response as arising from the equivalent-circuit indicated in the figure. It describes a typical material with interfacial polarization giving rise to colossal values of the dielectric constant. The dashed lines show the intrinsic bulk contribution, including an element ("UDR") for hopping conductivity of localized charge carriers as often found, e.g., in transition-metal oxides. [from P. Lunkenheimer et al., Phys. Rev. B 66, 052105 (2002).]

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