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International audience; The archetypical antiferroelectric, PbZrO3, is currently attracting a lot of interest, but no consensus can be clearly established on the nature of its ground state as well as on the influence of external stimuli over its physical properties. Here, the antiferroelectric state of 45-nm-thick epitaxial thin films of PbZrO3 is established by observing the characteristic structural periodicity of antiparallel dipoles at the atomic scale, combined with clear double hysteresis of the polarization-electric field response related to antiferroelectric–to–ferroelectric phase transitions. Surprisingly, while the antiferroelectric state is identified as the ground state, temperature-dependent measurements show that a transition to a ferroelectric-like state appears in a large temperature window (100 K). Atomistic simulations further confirm the existence, and provides the origin, of such ferroelectric state in the films. Electric-field-induced ferroelectric transitions are also detected by the divergence of the piezoresponse force microscopy response. Using this technique, we further reveal the signature of a ferroelectric ground state for 4-nm-thick PbZrO3 films. Compared with bulk crystals, these results suggest a more complex competition between ferroelectric and antiferroelectric phases in epitaxial thin films of PbZrO3.

First-principles calculations are performed to investigate and analyze properties of (001) thin films made of the most complex perovskite system, namely, $\mathrm{Na}\mathrm{Nb}{\mathrm{O}}_{3}$ (NNO), and are subject to epitaxial strain. In particular, an energy-versus-misfit strain phase diagram is constructed and reveals the existence of three different ground states for different strain regimes. For large compressive strain and up to moderate tensile strain, a monoclinic $Cc$ phase occurred, with its polarization and axis of antiphase tilting both rotating within a ($\overline{1}10$) plane with the magnitude of the strain. For large tensile strain, a ferroelectric orthorhombic state of $Pmc{2}_{1}$ symmetry emerges with polarization lying along the [110] in-plane direction together with an octahedral tilting adopting the ${a}^{\ensuremath{-}}{a}^{\ensuremath{-}}{c}^{+}$ pattern and an antiferroelectric vector associated with the reciprocal zone-border $X$ point. Finally, in between and for a narrow region of strain, a complex ground state is found. It has orthorhombic $Pca{2}_{1}$ symmetry, for which a complex tilting pattern coexists with polarization pointing along the [001] out-of-plane direction and antiferroelectric displacement associated with the $\mathrm{\ensuremath{\Delta}} k$ point located halfway between the zone-center $\mathrm{\ensuremath{\Gamma}}$ and zone-border $X$ point. Ferroelectric, antiferroelectric, and antiferrodistortive properties are also reported and discussed as a function of misfit strain.

International audience; Ferroics include diverse degrees of freedom (such as structural distortions and magnetic moments) among which cross couplings occur, rendering a large variety of interesting phenomena. Determining such couplings, based on symmetry analysis, is not only important to interpret observed phenomena but can also result in novel predictions to be then experimentally checked. Often, such energetic couplings are difficult to construct without a deep knowledge of group theoretical symmetry principles. In the present review, a crash course towards the derivation of energetic couplings, without using much the group theoretical language, is provided. Rather, the present approach relies on a graphical technique and suitable symbolic language, which naturally yields some known couplings (resulting in, e.g., spin/dipole canting, magnetically driven polarization and antipolar/antiferroelectric states). This review also reports and discusses other symmetry-allowed energetic terms, including some leading to the occurrence of an electric polarization in a variety of materials, and "exotic" ones that generate complex phases and phenomena in, e.g., nanostructures and heterostructures.

An atomistic effective Hamiltonian, along with a presently developed analytical model, are employed to investigate and analyze low-frequency polar, antipolar and antiferrodistortive phonons at finite temperature in a prototypical hybrid improper ferroelectric, that is, the $({\mathrm{BiFeO}}_{3})/({\mathrm{NdFeO}}_{3})$ [abbreviated as ${(\mathrm{BFO})}_{1}/{(\mathrm{NFO})}_{1}$] 1:1 superlattice. In the high-temperature paraelectric phase, phonons having both polar and antipolar characters are found to exist, as a result of a bilinear coupling between different cation motions, with these phonons having frequencies that are basically independent of temperature. In contrast, phonons having fluctuations of either in-phase or antiphase octahedral tiltings are soft in this high-temperature phase (with these two fluctuations being uncoupled), which results in their condensation below some critical temperature and the emergence of a low-temperature phase. In this latter low-temperature phase, trilinear energetic couplings between these two types of octahedral tiltings and Bi and Nd cations' motions lead to the appearance of a spontaneous polarization, consistent with the nature of hybrid improper ferroelectricity. These trilinear energetic couplings also yield an increase in the number of phonons possessing both polar and antipolar characters in the low-temperature phase, with most of these phonons softening when approaching the ferroelectric-to-paraelectric transition from below, as a result of the fact that they also exhibit antiferrodistortive features. The different temperature behaviors of polar modes at high versus low temperatures emphasize the uniqueness of hybrid improper ferroelectrics.

Materials possessing antipolar cation motions are currently receiving a lot of attention because they are fundamentally intriguing while being technologically promising. Most studies devoted to these complex materials have focused on their static properties or on their zone-center phonons. As a result, some important dynamics of antipolar cation distortions, such as the temperature behavior of their phonon frequencies, have been much less investigated, despite the possibility to exhibit unusual features. Here, we report the results and analysis of atomistic simulations revealing and explaining such dynamics for BiFeO3 bulks being subject to hydrostatic pressure. It is first predicted that cooling such material yields the following phase transition sequence: the cubic paraelectric Pm $$\bar 3$$ m state at high temperature, followed by an intermediate phase possessing long-range-ordered in-phase oxygen octahedral tiltings, and then the Pnma state that is known to possess antipolar cation motions in addition to in-phase and antiphase oxygen octahedral tiltings. Antipolar cation modes are found to all have high phonon frequencies that are independent of temperature in the paraelectric phase. On the other hand and in addition to antipolar cation modes increasing in number, some phonons possessing antipolar cation character are rather soft in the intermediate and Pnma states. Analysis of our data combined with the development of a simple model reveals that such features originate from a dynamical mixing between pure antipolar cation phonons and fluctuations of oxygen octahedral tiltings, as a result of a specific trilinear energetic coupling. The developed model can also be easily applied to predict dynamics of antipolar cation motions for other possible structural paths bringing Pm $$\bar 3$$ m to Pnma states. Temperature dependent structural dynamics in pressurized BiFeO3 are simulated and analyzed for ferroelectric-related phases. Kinnary Patel and coworkers from the University of Arkansas in US and Southern Federal University in Russia performed atomistic simulations on the dynamics of antipolar distortions, which are cation distortions against ferroelectric polarization, with respect to temperature in BiFeO3 under hydrostatic pressure. The material undergoes a paraelectric phase, an intermediate phase with long-range-ordered FeO6 octahedral tilting and a motion-mixed phase with both antipolar distortion and FeO6 octahedral tilting upon cooling. In paraelectric phase, the antipolar distortions always have high frequencies independent of temperature while they become soft with low frequencies in the intermediate and the motion-mixed phases. These results can be applied to predict dynamics of antipolar motions on the phase evolutions for other materials with similar crystal structures.

An atomic interaction is identified in all perovskite compounds, such as $AB{\mathrm{O}}_{3}$ oxides, that can potentially result in unconventional structures. The term is harmonic in nature and couples the motions of the $A$ cations with the rotations of the oxygen octahedra in the perovskite lattice. When strong enough, this coupling leads to hybrid normal modes that present both (anti)polar and rotational characters, which are keys to understand a variety of exotic phases. For example, we show that not only does this new coupling explain the long-period soft phonons characterizing prototype antiferroelectric ${\mathrm{PbZrO}}_{3}$, but it also provides us with an unified description of the complex antipolar structures of a variety of perovskites, including the possible occurrence of incommensurate phases. This coupling is further demonstrated to result, in the continuum limit, in an energy invariant adopting an analytical form that has been previously overlooked, to the best of our knowledge.

Date Presented 3/31/2017 The objective of this research study was to explore if driving decreased or increased older adults’ quality of life. This research was conducted to compare the differences in quality of life between older adults who are currently driving with those who have ceased driving. Primary Author and Speaker: Sharon Sason Additional Authors and Speakers: Nastassia Dietrich, Kinnary Patel Contributing Authors: Debora Oliveira

Using embedded Gaussian orthogonal random matrix ensemble of two-body interactions (EGOE(2)), it is demonstrated that finite interacting boson systems in the dense limit exhibit average-fluctuations separation with the smoothed state density being a Gaussian (with corrections) and the fluctuations are of GOE type.

The Fe-based multicomponent amorphous alloys (also referred to as metallic glasses) are known to exhibit soft magnetic properties and, it makes them important for many technological applications. However, metallic glasses are in a thermodynamically metastable state and in case of high temperature operating conditions, the thermally activated crystallization would be detrimental to their magnetic properties. The study of crystallization kinetics of metallic glasses gives useful insight about its thermal stability. In the present work, crystallization study of Fe67Co18B14Si1 (2605CO) metallic glass has been carried out using differential scanning calorimetry (DSC) technique. Mossbauer study has also been undertaken to know the phases formed during the crystallization process. The alloy shows two-stage crystallization. The activation energy has been derived using the Kissinger method. It is found to be equal to 220 kJ/mol and 349 kJ/mol for the first and second crystallization peaks, respectively. The Mossbauer study indicates the formation of α-(Fe, Co) and (Fe, Co)3B phases in the alloy.

Spectral averaging methods have been used in nuclear physics for many purposes but always using spherical symmetry. We demonstrate its use in calculating the state density and level density of deformed nuclei using Nilsson model single-particle scheme. The calculated results are compared with statistical Fermi-gas model to evaluate standard Fermi-gas parameters; these are then compared with experimental data. The spin-cutoff factors and parity ratios have also been calculated.