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Reversible non-180° domain switching is believed to be able to enhance the electromechanical performances of piezoelectrics. However, the slow improvement of strain performances of (K,Na)NbO3 (KNN) is caused by an inadequate understanding of reversible non-180° domain switching. Here, with an electric field-available sample holder, we directly observe the reversible domain wall motion by in situ transmission electron microscopy and non-180° domain switching in a compositionally graded KNN single crystal which has an ultrahigh large-signal piezoelectric coefficient. The characteristics of domain wall motion under varying electric fields are investigated. The analysis of high-angle annular dark-field images with an atomic resolution indicates that local compositional fluctuation, strain fluctuation and polarization rotation exist in the domains. We propose that the restoring force derives from the built-in flexoelectric effect caused by compositional fluctuation at the atomic scale. The new mechanism paves the way for enhancing the piezoelectric properties and designing high performance piezoelectrics. [ABSTRACT FROM AUTHOR]

Systems with both the quantum anomalous Hall (QAH) effect and the quantum anomalous valley Hall (QAVH) effect have wide appeal in fundamental research and practical quantum device applications. By using first-principles calculations, the topologically nontrivial phase and QAH effect were predicted in the ferromagnetic non-Dirac half-metal N2Pd4S6 monolayer. The N2Pd4S6 monolayer prefers out-of-plane magnetization and exhibits a Curie temperature of ∼80 K due to dual double-exchange interaction. Upon application of − 1% strain, the Curie temperature can reach up to ∼117 K. Furthermore, the N2Pd4S6 monolayer exhibits a non-Dirac band dispersion near the Fermi level. The introduction of magnetic exchange and spin–orbit coupling (SOC) together enables to realize topologically nontrivial phase and QAH state. Remarkably, the compressive strain induces spontaneous valley polarization at the non-high symmetry point, thereby enabling the unique QAVH effect. Reversible switching between the QAH and QAVH effect can be achieved in the N2Pd4S6 monolayer by applying of compression strain. [ABSTRACT FROM AUTHOR]

We present a theoretical and numerical study of drift current model in the ionosphere by incorporating the ohmic heating model and the magnetohydrodynamic (MHD) momentum equation. Based on these equations, the ionospheric electron temperature and drift current are investigated. The results indicate that the maximum change of electron temperature Δ T e is about 570 K, and the ratio is Δ T e / T e ~48 %. The maximum drift current density is 8 × 10−10 A ⋅ m −2, and its surface integral is 5.76 A. Diamagnetic drift current is the main form of current. The low collision frequency between charged particles and neutral particles has little effect on the current, and the collision frequency of electrons and ions is independent of the drift current. The current density profile is a flow ring. We present the effective conductivity as a function of the angle between the geomagnetic field and the radio wave; the model explains why the radiation efficiency in Kotik's experiment was strongest when the X wave is heating along the magnetic dip angle. [ABSTRACT FROM AUTHOR]

We present a theoretical and numerical study of the drift current and magnetic perturbation model in the ionosphere by incorporating the ohmic heating model and the magnetohydrodynamic (MHD) momentum equation. Based on these equations, the ionospheric electron temperature and drift current are investigated. The results indicate that the maximum change in electron temperature ΔTe is about 570 K, and the ratio is ΔTe/Te ∼ 48 %. The maximum drift current density is 8×10-10 A m -2 , and its surface integral is 5.76 A. Diamagnetic drift current is the main form of current. The low collision frequency between charged particles and neutral particles has little effect on the current, and the collision frequency of electrons and ions is independent of the drift current. The current density profile is a flow ring. We present the effective conductivity as a function of the angle between the geomagnetic field and the radio wave; the model explains why the radiation efficiency was strongest when the X wave is heating along the magnetic dip angle, as reported in recent observations by Kotik et al. (2013). We calculate the magnetic field variation in the heating region based on the MHD theory: the results show that the maximum magnetic field perturbation in the heating area is 48 pT. [ABSTRACT FROM AUTHOR]