Abstract
During switching, the microstructure of a ferroelectric normally adapts to align internal dipoles with external electric fields. Favorably oriented dipolar regions (domains) grow at the expense of those in unfavorable orientations and this is manifested in a predictable field-induced motion of the walls that separate one domain from the next. Here, the discovery that specific charged 90°domain walls in copper-chlorine boracite move in the opposite direction to that expected, increasing the size of the domain in which polarization is anti-aligned with the applied field, is reported. Polarization-field (P-E) hysteresis loops, inferred from optical imaging, show negative gradients and non-transient negative capacitance, throughout the P-E cycle. Switching currents (generated by the relative motion between domain walls and sensing electrodes) confirm this, insofar as their signs are opposite to those expected conventionally. For any given bias, the integrated switching charge due to this anomalous wall motion is directly proportional to time, indicating that the magnitude of the negative capacitance component should be inversely related to frequency. This passes Jonscher's test for the misinterpretation of positive inductance and gives confidence that field-induced motion of these specific charged domain walls generates a measurable negative capacitance contribution to the overall dielectric response.
Highlights
Most recently, heating has been seen to cause high symanomalous wall motion is directly proportional to time, indicating that the metry labyrinthine ferroelectric domain magnitude of the negative capacitance component should be inversely related to frequency
We report observations that challenge conventional expectations of dipolar switching. We find both normal and anomalous field-induced domain wall motion in improper ferroelectric copper–chlorine (Cu–Cl) boracite (Cu3B7O13Cl) single crystals
Summary and Outlook While the energetics responsible for anomalous domain wall motion in the Cu–Cl boracite system are somewhat uncertain, the ability to induce polarization that is anti-aligned with the electric field creating it is an unprecedented observation; the fact that this generates a measurable negative capacitance contribution to the overall dielectric response should be of great interest fundamentally and for device applications, in which negative capacitance can be exploited
Summary
Establishing the nature of the domain states (and associated dipole orientations), in the specific Cu–Cl boracite crystals investigated is critical for making valid statements about electric field-induced changes in polarization. Shear variants were found to abut along pc and pc line vectors on the crystal surface (Figure 1a), consistent with the expected {100}pc 180° and {110}pc 90° domain walls, respectively These domain wall planes are oriented perpendicularly to the crystal surface, and this is explicitly confirmed by through-focus optical microscopy (see Movie S1 and Figure S2, Supporting Information). It should be reiterated that the crystallographic planes, along which these domain walls lie, are perpendicular to the surface (Figure S2, Supporting Information) and that imaging was done in transmission mode; the change in position of the line trace of the domain wall can be taken as a proxy for the change in volume of the domains within the interelectrode gap, induced by the applied field. While wall motion changed, the sense in which field-aligned polarization developed during switching was conventional at all points
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