Understanding and controlling surface charge-induced polarization switching have attracted interest of researchers extensively, and the rich physical interactions between ionic and atomic displacements play a significant role in polarization reversal. In this work, we investigated the mechanism of surface charge-induced polarization switching in PbZr0.2Ti0.8O3 and BiFeO3 thin films by phase-field simulations. We observed ferroelectric polarization reversal by applying positive/negative charge on the top of a thin film, because the double well of free energy becomes asymmetric by applying surface charge. It is found that the nucleation of switching always starts from the surface and gradually expands into the whole film. In BiFeO3 thin films, the formation of an anti-vortex domain provides topological protection for hindering surface charge-induced polarization switching. The present study, therefore, contributes to a better understanding of charge-induced polarization switching and provides guidance for the experimental design of reversible electronic devices by selecting the appropriate polarity region.
A transverse quantum-dot (QD) shuttle (T-QDS) is proposed where the QD oscillating direction is perpendicular to the electron transmission direction, different from the usual QD shuttle where the oscillating direction is parallel to the electron transmission direction. Both the electrical and mechanical degrees of freedom are dealt with by using the full quantum mechanical approaches. We derive the modified rate equations and numerically investigate the quantum properties of the T-QDS. First, as a comparison, we study the time-dependent evolutions of the electron-occupation probabilities and the currents flowing through the classical T-QDS. It is shown that the current shows the time-dependent oscillation in phase with the oscillation of the T-QDS. Then, we turn to study the quantum properties of the quantum T-QDS. Sharply different from the classical T-QDS, no oscillations of the probabilities and the current are observed due to the quantum uncertainty of the space position of the quantum T-QDS. It is demonstrated that the effects induced by the quantized oscillation of the quantum T-QDS are reflected as the renormalization of the tunneling rates between the QD and the leads. Notably, the two tunneling rates of the QD with the left and right leads depend on the relative space positions of the leads and the QD in different ways. It is the interplay of the two tunneling rates that eventually determines the stationary current of the T-QDS. Moreover, we study the influences induced by the temperature, which can also be considered via the renormalized tunneling rates.
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