Scanning nonlinear dielectric microscopy is a powerful technique for measuring the domain structure of ferroelectrics. We observed congruent LiTaO 3 and found the marked enhancement of nonlinear dielectric ''constants'' when the applied tip-sample voltage exceeded a particular threshold value. This is due to domain nucleation activated by a huge electric field under the tip. Moreover, low frequencies (less than a few hundred Hz) did not enhance the nonlinearity. An effectively lower electric field caused by ion conduction in the sample under the tip is a possible reason for the frequency-dependent characteristics of the enhanced nonlinearity for the applied voltage.
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