The prediction and experimental confirmation of magnetic skyrmions revolutionized the physics of nanoscale magnetism and opened new horizons for spintronics. In spite the inherently shorter and faster correlations of the electric polarization and challengingly smaller correlation lengths, the recent developments in electric skyrmionics follow these innovations. It is only 5 years since the first observation of a ferroelectric skyrmion in thin lead-strontium titanate superlattices [1].

The present molecular dynamics computational study [2)] reveals that the bulk crystal of the archetypal ferroelectric perovskite (barium titanate) can host peculiar 2-3 nm wide standalone polar columns stable till temperatures of several tens of Kelvins. They are spontaneously surrounded by a unique noncollinear polarization pattern (see figure) that has never been described before. We explained how and why this pattern is formed and stabilized. Since its invariant skyrmion topological charge is an integer with a sign opposite to that of the usual skyrmions, they named this soliton as ferroelectric antiskyrmion. They clarify that formation of antiskyrmions consists in breakdown of high-curvature 180-degree ferroelectric walls into triplets of lower-energy 71-degree walls. It is explained that this process is favored by a fortunate combination of the moderate anisotropy of the anharmonic electric susceptibility and the pronounced anisotropy of the polarization correlations in barium titanate crystals. These findings represent a clear milestone in the studies of topological defects in ferroelectrics.

Tetragonal tungsten bronzes are the second most important ferroelectric family after the perovskite one, and they give more versatility to play with multiferroic properties, as it has 5 different crystallographic sites to fill with different elements (see Figure 1). In this work we have used a broad band dielectric spectroscopy approach to study the dielectric response of one of these interesting compounds Ca_0.3Ba_0.7Nb₂O₆ (CBN3-30) within an impressive frequency range of 14 decades: from 1 Hz to 10¹⁴ Hz. We have proven that CBN-30 displays a ferroelectric phase transition of mixed displacive and order-disorder character, and that its paraelectric phase does not show traces of relaxor behaviour but precursor effects as polar fluctuations below about 550 K. This is partially attributed to the presence of Ca in the lattice, and its effect on maintaining the long-range polarization due to the Nb displacements along the main axis and suppressing the perpendicular displacements.

The analysis of the sub-MHz dielectric response together with infrared and Raman spectroscopy reveals that simultaneous polarization mechanisms are responsible for the phase transition. The main excitations have been phenomenologically assigned to phonons, to a soft anharmonic vibration of cationic origin, and to a relaxation in the GHz range related to polarization fluctuations of nanometric size. This GHz relaxation carries the main part of the permittivity at high temperatures in the paraelectric phase and on cooling it splits below T_C into several weaker excitations with different polarization correlation lengths. The comparison of the excitations found in CBN-30 with those of the famous (Sr,Ba)Nb₂O₆ reveals that these mechanisms are congruous, although in CBN-30 the main relaxation process behaves differently due to the different domain structure and the presence of more distorted oxygen octahedra network. The overall dielectric response was therefore explained by coexistence of several excitations with different thermal behaviors, corroborating the complexity of the tetragonal tungsten bronze structures.

Electron-hole plasma expansion with velocities exceeding c/50 and lasting over 10 ps at 300 K was evidenced by time-resolved terahertz spectroscopy [1]. This regime, in which the carriers are driven over >30 μm is governed by stimulated emission due to low-energy electron-hole pair recombination and reabsorption of the emitted photons outside the plasma volume. At low temperatures a speed of c/10 was observed in the regime where the excitation pulse spectrally overlaps with emitted photons, leading to strong coherent light-matter interaction and optical soliton propagation effects.

Pb(Zr_1-xTi_x)O₃ with very high content of Zr shows an antiferroelectric ground state and possesses an exceptional property: the coexistence of several built-in structural instabilities at high temperatures. This leads to the possibility of their successive condensation on cooling to trigger a sequence of phase transitions, instead of reaching directly the antiferroelectric state, as in pure PbZrO₃. This peculiar behaviour is more pronounced in compositions near the antiferroelectric morphotropic phase boundary and the tricritical point around room temperature, as Pb(Zr_0.95Ti_0.05)O₃ (PZT 95/5), where three phases are energetically available.

Raman scattering experiments performed out of the thermodynamical equilibrium revealed a complex and hysteretic thermal behaviour of the phase transition dynamics, due to the inhomogeneous microstructure and the coexistence of regions with different phases within the samples. A way to control the thermal development of these different phases is to selectively condense the different instabilities by an external parameter such as temperature, hence creating a sequence of phase transitions instead of a direct phase transition.

Our results, obtained under many different experimental conditions and specific pre-history, suggest that ceramics with composition near PZT 95/5 are potential materials for novel 3-state thermal switches. An innovative approach of phase-control is proposed, based on the thermal behaviour of the intermediate polar states observed and using small temperature gradients by appropriate heating-cooling cycles around room temperature.

Reference:
[1] E. Buixaderas, C. Milesi-Brault, P. Vaněk, J. Kroupa, F. Craciun, F. Cordero, C. Galassi, Peculiar Dynamics of Polar States at the Morphotropic Phase Boundary of Antiferroelectric Pb(Zr_1-xTi_x)O₃, Acta Materialia, 119208 (2023), online.

Our study delved into a theoretical exploration of charged domain walls in ferroelectric PbTiO₃, which were compensated by randomly distributed immobile charge defects located within a relatively broad slab. To achieve this, we employed a combination of atomistic shell-model simulations and continuous phase-field simulations based on the Ginzburg-Landau-Devonshire model. Our findings showed that domain walls form a zigzag pattern, and we examined their properties across a broad range of compensation-region widths, ranging from a few nanometers to over 100 nm, focusing in particular on understanding the zigzag modulation lengths in terms of material properties of PbTiO₃. The zigzag formation is accompanied by a local ferroelectric-polarization rotation, which we proposed as an efficient mechanism for local charge compensation. Our study provides a new understanding of the behavior of charged domain walls in ferroelectric materials and highlights the significance of the considered compensation charges in their formation. The insights gained from our study may contribute to the development of advanced ferroelectric materials with applications in the field of smart-materials for electronics.