Observing ferroelastic switching in Hf_(0.5)Zr_(0.5)O_(2) thin film

Hafnium zirconium oxides(HZO),which exhibit ferroelectric properties,are promising materials for nanoscale device fabrication due to their high complementary metal-oxide-semiconductor(CMOS) compatibility.In addition to piezoelectricity,ferroelectricity,and flexoelectricity,this study reports the observation of ferroelasticity using piezoelectric force microscopy(PFM) and scanning transmission electron microscopy(STEM).The dynamics of 90° ferroelastic domains in HZO thin films are investigated under the influence of an electric field.Switching of the retentive domains is observed through repeated wake-up measurements.This study presents a possibility of enhancing polarization in HZO thin films during wake-up processes.

Exotic Dielectric Behaviors Induced by Pseudo-Spin Texture in Magnetic Twisted Bilayer

Twisted van der Waals bilayers provide an ideal platform to study the electron correlation in solids. Of particular interest is the 30° twisted bilayer honeycomb lattice system, which possesses an incommensurate moiré pattern,and uncommon electronic behaviors may appear due to the absence of phase coherence. Such a system is extremely sensitive to further twist and many intriguing phenomena will occur. Based on first-principles calculations we show that, for further twist near 30°, there could induce dramatically different dielectric behaviors of electron between left and right-twisted cases. Specifically, it is found that the left and right twists show suppressed and amplified dielectric response under vertical electric field, respectively. Further analysis demonstrate that such an exotic dielectric property can be attributed to the stacking dependent charge redistribution due to twist,which forms twist-dependent pseudospin textures. We will show that such pseudospin textures are robust under small electric field. As a result, for the right-twisted case, there is almost no electric dipole formation exceeding the monolayer thickness when the electric field is applied. Whereas for the left case, the system could even demonstrate negative susceptibility, i.e., the induced polarization is opposite to the applied field, which is very rare in the nature. Such findings not only enrich our understanding on moiré systems but also open an appealing route toward functional 2D materials design for electronic, optical and even energy storage devices.

Concepts of the half-valley-metal and quantum anomalous valley Hall effect

Valley,the energy extrema in the electronic band structure at momentum space,is regarded as a new degree of freedom of electrons,in addition to charge and spin.The studies focused on valley degree of freedom now form an emerging field of condensed-matter physics,i.e.,valleytronics,whose development is exactly following that of spintronics,which focuses on the spin degree of freedom.

Missed ferroelectricity in methylammonium lead iodide

Methylammonium lead iodide,as related organometal halide perovskites,emerged recently as a particularly attractive material for photovoltaic applications.The origin of its appealing properties is sometimes assigned to its potential ferroelectric character,which remains however a topic of intense debate.Here,we rationalize from first-principles calculations how the spatial arrangement of methylammonium polar molecules is progressively constrained by the subtle interplay between their tendency to bond with the inorganic framework and the appearance of iodine octahedra rotations inherent to the perovskite structure.The disordered tetragonal phase observed at room temperature is paraelectric.We show that it should a priori become ferroelectric but that iodine octahedra rotations drive the system toward an antipolar orthorhombic ground state,making it a missed ferroelectric.