Nano-domains and related phenomena in congruent lithium tantalate single crystals studied by scanning nonlinear dielectric microscopy

摘要

Nanodomains and their related phenomena in congruent lithium tantalate (CLT) single crystals are studied using scanning nonlinear dielectric microscopy (SNDM). We carried out two specific investigations: super higher order nonlinear dielectric spectroscopy studies on thick multi-domain congruent CLT single crystals and electrical conduction in nanodomains in thin CLT single crystals. First, without using a special sharp tip, we achieve improved lateral resolution in SNDM through the measurement of super higher order nonlinearity up to the fourth order. We also found a 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. Low frequencies (less than a few hundred hertz) do not enhance 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. Finally, electrical current flow behavior was investigated for nanodomains formed in a thin CLT single-crystal plate. When the nanodomains are relatively large, with diameters of about 100 nm, current flow is detected along the domain wall. However, when the nanodomains were about 40 nm or smaller in size, current flowed through the entire nanodomain. Schottky-like rectifying behavior was observed. A clear temperature dependence of the current is found, indicating that the conduction mechanism for nanodomains in CLT may involve thermally activated carrier hopping.