Accurate semiempirical analytical formulas for spontaneous polarization by crystallographic parameters of SrTiO3-BaTiO3 system by ab initio calculations

In this paper, ab initio calculations are used to determine polarization difference in zinc blende (ZB), hexagonal (H) and wurtzite (WZ) AlN-GaN and GaN-InN superlattices. It is shown that a polarization difference exists between WZ nitride compounds, while for H and ZB lattices the results are consistent with zero polarization difference. It is therefore proven that the difference in Berry phase spontaneous polarization for bulk nitrides (AlN, GaN and InN) obtained by Bernardini et al. and Dreyer et al. was not caused by the different reference phase. These models provided absolute values of the polarization that differed by more than one order of magnitude for the same material, but they provided similar polarization differences between binary compounds, which agree also with our ab initio calculations. In multi-quantum wells (MQWs), the electric fields are generated by the well-barrier polarization difference; hence, the calculated electric fields are similar for the three models, both for GaN/AlN and InN/GaN structures. Including piezoelectric effect, which can account for 50% of the total polarization difference, these theoretical data are in satisfactory agreement with photoluminescence measurements in GaN/AlN MQWs. Therefore, the three models considered above are equivalent in the treatment of III-nitride MQWs and can be equally used for the description of the electric properties of active layers in nitride-based optoelectronic devices.

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Examination of permittivity for depolarization field of ferroelectric by ab initio calculation, suggesting hidden mechanisms

Scientific Reports ◽

10.1038/s41598-021-81237-0 ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

Yukio Watanabe

Keyword(s):

Ab Initio Calculations ◽

Ab Initio ◽

Spontaneous Polarization ◽

Ab Initio Calculation ◽

Surface Effects ◽

Atomic Scale ◽

Free Standing ◽

Unit Cells ◽

Scale Surface

AbstractElectrostatics of depolarization field Ed in relation to the polarization is studied. In particular, the value of permittivity for Ed (εd) in prototypical situations of ferroelectrics, including Mehta formula, is examined by ab initio calculations. By using spontaneous polarization PS corresponding to accurate experiment ones, we show εd = 1, which suggests that the results of εd ≫ 1 indicate hidden mechanisms; εd = 1 suggests that the effect of Ed is significant to induce intriguing important phenomena overlooked by εd ≫ 1. A bridge between εd = 1 and εd ≫ 1, i.e. the consistency of εd = 1 with conventional results is presented. The exact electrostatic equality of head-to-head–tail-to-tail domains to free-standing ferroelectrics is deduced. Hence, most stoichiometric clean freestanding monodomain ferroelectrics and head-to-head–tail-to-tail domains are shown unstable regardless of size, unless partially metallic. This verifies the previous results in a transparent manner. This conclusion is shown consistent with a recent hyperferroelectric LiBeSb and “freestanding” monolayer ferroelectrics, of which origin is suggested to be adsorbates. In addition, this restriction is suggested to break in externally strained ultrathin ferroelectrics. The macroscopic formulas of Ed are found valid down to a several unit-cells, when electronic and atomic-scale surface effects are unimportant and accurate PS is used.

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