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A region with a consistent orientation of spontaneous electric polarization. Antiparallel domains can give opposite signs of piezoresponse force microscopy response.

Analytical and experimental investigation distinguishing electrostatic and electromechanical image-contrast regimes in piezoresponse force microscopy.

A surface slope converts some vertical displacement into lateral motion of the tip and changes the normal contact geometry. Roughness also changes friction, contact area and local contact stiffness, and can mix cantilever bending, buckling and torsion. A lateral image that follows a height gradient can therefore appear even when the relevant in-plane polarization component is absent.

Record topography and both piezoresponse force microscopy channels simultaneously, and compare the lateral signal with the local slope rather than height alone. Reverse the scan direction to identify frictional contributions, rotate the sample relative to the cantilever, and compare the vector transformation expected for an in-plane response with the geometrical cross-talk. Repeat on a smoother region or a polished specimen, at controlled force and drive amplitude. Track contact resonance or compare sufficiently far from it so that local stiffness does not masquerade as a change in response.

A useful control is a nonpiezoelectric surface of similar roughness. Electrically switching a domain while leaving its topography unchanged provides a further comparison when feasible. No single trace–retrace test proves the absence of cross-talk: genuine in-plane response and slope-induced signals can both transform under rotation, so several controls must agree.

In piezoresponse force microscopy, a conducting atomic force microscopy tip contacts a sample with a counter-electrode. Apply $V(t)=V_{\mathrm{dc}}+V_{\mathrm{ac}}\cos \omega t$. The converse piezoelectric effect produces an oscillatory displacement, approximately $u_{\omega }=d_{\mathrm{eff}}{V}_{\mathrm{ac}}$. Contact with the moving surface bends the cantilever, and a lock-in amplifier records the displacement amplitude and phase relative to the drive.

For opposite out-of-plane ferroelectric domains, the effective piezoelectric coefficient changes sign. Their vertical response therefore differs by approximately $180^{\circ }$ in phase. Equivalent antiparallel domains can have similar amplitudes, while cancellation or reduced response near a wall can produce an amplitude minimum. Phase identifies the relative response sign; an uncalibrated detector phase is not an absolute polarization direction.

Use a small drive below the switching threshold while imaging, a stable contact force and a well-characterized contact resonance. Electrostatic forces, contact stiffness and surface charging can also create signals at the drive frequency; observing contrast alone is insufficient evidence for polarization switching.^[ref]