Turbulence-induced changes in degree of polarization,degree of coherence and spectrum of partially coherent electromagnetic beams

Based on a recently formulated unified theory of coherence and polarization, a method is described to study turbulence-induced changes in the polarization, the coherence and the spectrum of partially coherent electromagnetic beams on propagation. The electromagnetic Gaussian Schell-model beam is taken as a typical example of partially coherent electromagnetic beams, and the closed-form expressions for the degree of polarization, the degree of coherence and the spectrum of electromagnetic Gaussian Schell-model beams propagating through atmospheric turbulence are derived in the quadratic approximation of Rytov＇s phase structure function. Some interesting results are obtained, which are illustrated by numerical examples and are explained in physics.

Valley piezoelectricity promoted by spin-orbit coupling in quantum materials

Quantum materials have exhibited attractive electro-mechanical responses,but their piezoelectric coefficients are far from satisfactory due to the lack of feasible strategies to benefit from the quantum effects.We discovered the valley piezoelectric mechanism that is absent in the traditional piezoelectric theories yet promising to overcome this challenge.A theoretical model was developed to elucidate the valley piezoelectricity in 2D materials as originating from the strong spin-orbit coupling.Consistent analytical and density-functional-theory calculations validate the model and unveil the crucial dependence of valley piezoelectricity on valley/spin splitting and hybridization energy.Up to 50%of electro-mechanical responses in our tested twodimensional systems are attributed to the valley piezoelectric mechanisms.Rational strategies including doping,passivation,and external strain are proposed to optimize piezoelectricity,with a more than 127%increase in piezoelectricity demonstrated by density-functional-theory simulations.The general valley piezoelectric model not only opens an opportunity to achieve outstanding piezoelectricity via optimizing intrinsic variables but also makes the large family of valley materials promising for piezoelectric sensing and energy harvesting.

Spontaneous ferroelectricity in strained low-temperature monoclinic Fe3O4： A first-principles study

As a single-phase multiferroic material, Fe3O4 exhibits spontaneous ferroelectric polarization below 38 K. However, the nature of the ferroelectricity in Fe3O4and effect of external disturbances such as strain on it remains ambiguous. Here, the spontaneous ferroelectric polarization of low-temperature mon- oclinic Fe3O4 was investigated by first-principles calculations. The pseudo-centrosymmetric FeB42- FEB43 pair has a different valence state. The noncentrosymmetric charge distribution results in fer- roelectric polarization. The initial ferroelectric polarization direction is in the -x and -z directions. The ferroelectricity along the y axis is limited owing to the symmetry of the Cc space group. Both the ionic displacement and charge separation at the FeB42-FeB43 pair are affected by strain, which further influences the spontaneous ferroelectric polarization of monoclinic Fe3O4. The ferroelectric polarization along the z axis exhibits an increase of 45.3% as the strain changes from 6% to -6%.