Origin of Perovskite Multiferroicity and Magnetoelectric-Multiferroic Effects—The Role of Electronic Spin in Spontaneous Polarization of Crystals

In this semi-review paper, we show that the multiferroic properties of perovskite ABO3 crystals with B(dn), n > 0, centers are fully controlled by the influence of the electronic spin on the local dipolar instability that triggers the spontaneous polarization of the crystal. Contrary to the widespread statements, the multiferroicity of these crystals does not emerge due to the addition of unpaired electrons (carrying magnetic moments) to the spontaneously polarizing crystal; the spin states themselves are an important part of the local electronic structure that determines the very possibility of the spontaneous polarization. This conclusion emerges from vibronic theory, in which the ferroelectricity is due to the cooperative interaction of the local dipolar distortions induced by the pseudo-Jahn-Teller effect (PJTE). The latter requires sufficiently strong vibronic coupling between ground and excited electronic states with opposite parity but the same spin multiplicity. The detailed electronic structure of the octahedral [B(dn)O6] center in the molecular orbital presentation shows how this requirement plays into the dependence of the possible perovskite magnetic, ferroelectric, and multiferroic properties on the number of d electrons, provided the criterion of the PJTE is obeyed. Revealed in detail, the role of the electronic spin in all these properties and their combination opens novel possibilities for their manipulation by means of external perturbations and exploration. In particular, it is shown that by employing the well-known spin-crossover phenomenon, a series of novel effects become possible, including magnetic-ferroelectric (multiferroic) crossover with electric-multiferroic, magnetic-ferroelectric, and magneto-electric effects, some of which have already been observed experimentally.

Coexistence of Flexo- and Ferro-Electric Effects in an Ordered Assembly of BaTiO3 Nanocubes

It has been reported that the flexoelectric effect could be dominant in the nanoscale. The discrepancy between theory and experiments on the frequency dependence of the dielectric constant of an ordered assembly of BaTiO3 nanocubes is nearly resolved by assuming the coexistence of flexo- and ferro-electric effects. Although flexoelectric polarizations perpendicular to the applied alternating electric field contribute to the dielectric constant, those parallel to the electric field do not contribute because the magnitude of the flexoelectric polarization does not change due to the mismatch of strain at the interface of the nanocubes. On the other hand, some dielectric response is possible for the ferroelectric component of the polarization parallel to the electric field.

Critical Assessment of the Electric Effect in Electric Arc Welding

This work provides a critical assessment of electric effects on the microstructure evolution at the heat-affected zone in electric arc welding. Electric effects are the interactions between electromagnetic fields and materials’ microstructures. They differ from the arc effect and the Joule heating effect by providing an alternative contribution to nucleation, grain growth, recrystallisation and tempering. The influence of the electric effect on grain size, defects, anisotropic properties, precipitates and residual stress has been examined kinetically and thermodynamically. The use of adaptable electric current densities, pulse durations, pulse frequencies and electrode movements is suggested to achieve desirable microstructures and mechanical properties for the weldments.

Strain-Mediated Magneto-Electric Effects in Coaxial Nanofibers of Y/W-Type Hexagonal Ferrites and Ferroelectrics

Nanofibers of Y- or W-type hexagonal ferrites and core–shell fibers of hexagonal ferrites and ferroelectric lead zirconate titanate (PZT) or barium titanate (BTO) were synthesized by electrospinning. The fibers were found to be free of impurity phases, and the core–shell structure was confirmed by electron and scanning probe microscopy. The values of magnetization of pure hexagonal ferrite fibers compared well with bulk ferrite values. The coaxial fibers showed good ferroelectric polarization, with a maximum value of 0.85 μC/cm2 and 2.44 μC/cm2 for fibers with BTO core–Co2W shell and PZT core–Ni2Y shell structures, respectively. The magnetization, however, was much smaller than that for bulk hexaferrites. Magneto-electric (ME) coupling strength was characterized by measuring the ME voltage coefficient (MEVC) for magnetic field-assembled films of coaxial fibers. Among the fibers with Y-type, films with Zn2Y showed a higher MEVC than films with Ni2Y, and fibers with Co2W had a higher MEVC than that of those with Zn2W. The highest MEVC of 20.3 mV/cm Oe was measured for Co2W–PZT fibers. A very large ME response was measured in all of the films, even in the absence of an external magnetic bias field. The fibers studied here have the potential for use in magnetic sensors and high-frequency device applications.

Transepithelial potential difference governs epithelial homeostasis by electromechanics

Studies of electric effects in biological systems, from the historical experiments of Galvani 1 and the ground-breaking work on action potential2 to studies on limb regeneration3 or wound healing4, share the common feature of being concerned with transitory behavior and not addressing the question of homeostasis. Here using a novel microfluidic device, we study how the homeostasis of confluent epithelial tissues is modified when a trans-epithelial electric potential (TEPD) different from the natural one is imposed on an epithelial layer. We show that epithelial fate is dependent on TEPD of few Volts/cm similar to the endogenous one. When the field direction matches the natural one, we can restore a perfect confluence in an epithelial layer turned defective either by E-cadherin knock-out or by weakening cell-substrate adhesion; additionally, the tissue pushes on the substrate with kilo-Pascals stress, inducing active cell response such as death and differentiation. When the field is opposite, homeostasis is destroyed by the perturbation of junctional actin and cell shapes, and the formation of dynamical mounds5, while the tissue pulls with similar strengths. Most of these observations can be quantitatively explained by an electro-hydrodynamic theory involving local electro-osmotic flows. We expect this work to motivate further studies on long time effects of electromechanical pathways with important tissue engineering applications.

Study of the Seismoelectric Effect of the Second Kind Using Molecular Sensors

The article is devoted to the study of the potential possibilities of using molecular-electronic sensors of seismic waves for field work using the seismoelectric method to explore the hydrocarbon deposits. The introduction provides an analytical review of the current state of research based on data from science magazines and patents. It is shown that at present, seismoelectric effects are at the stage of experimental implementation into the practice of field work for oil and gas geophysical prospecting. Further in the article, theoretical estimates and results of mathematical modeling of the manifestation of seismoelectric (SE) phenomena in the regions of hydrocarbon anomalies are presented, numerical estimates of the values of the seismic and secondary electromagnetic fields are given. The analysis of the results (on a tank and real gas condensate field) showed that the use of molecular-electronic geophones, which have a higher sensitivity and operate in a wider frequency range (up to 0.1 Hz), allows one to obtain higher signal-to-noise ratio. Thus, it has been experimentally established that the use of molecular sensors for recording seismic electric effects when searching for deposits is more preferable when carrying out field work.

Seebeck–Peltier Transition Approach to Oncogenesis

In this paper, a non-equilibrium thermodynamic approach to cancer is developed. The thermo-electric effects in the cell membrane are analysed, in relation to the Seebeck-like and the Peltier-like effects. The role of the cell membrane electric potential is studied from a thermodynamic viewpoint, pointing out the relation between the proliferation rate and the membrane potential, the existence of a thermodynamic threshold for the mitotic activity, the relation between metastases and membrane potential and the comprehension of the role of ions fluxes in the cell behaviour.