Electrostatics liberating restrictions on ferroelectric by unification of polar discontinuity e^–h⁺ layers and criteria of intrinsicality

Abstract

Electron e^– and hole h⁺ layers at polar discontinuities as a thermodynamically stable state previously predicted are recently shown by many experiments. We discuss that these layers including those at SrTiO₃/LaAlO₃ support our previous prediction of the substantially reduced electrostatic restrictions on ferroelectricity such as size effect and domains when the origins of these conductions are intrinsic. For this, the criteria of the intrinsicality of these layers, i.e., not due to conventional mechanisms such as defects and adsorbates, are presented. In addition, we discuss that this reduction of the electrostatic restriction is the same as recently proposed hyperferroelectricity and show that hyperferroelectricity is a universal properties of general ferroelectric.

KEYWORDS:

• Polar discontinuity
• conduction domain
• size effect
• hyperferroelectric

Notes

1 F is given by F = ∫dv F_ferro(P_S) + ∫dvρϕ/2 + E_K + E^{surf/domain_b}, where F_ferro, ρ, ϕ, E_K and E^{surf/domain_b} are free energy density of bulk ferroelectric phase, charge density, electrostatic potential, kinetic energy of electrons and holes measured from fermi level, and the energy of domain boundary formation. In Eq. (2)(2) $$\begin{array}{c}\Delta F_{\mathrm{I}}=l_{\mathrm{f}}{P}_{\mathrm{S}}{}^2\left(T=T_{\mathrm{C}}\right)/2{\epsilon }_{0,}\\ \Delta F_{\text{eh}}=l_{\mathrm{e}}{P}_{\mathrm{S}}{}^2(T\hspace{0.17em}\approx \hspace{0.17em}{T}_{\mathrm{C}})/{\epsilon }_0+ \left(E\mathrm{g}+2E_{\mathrm{k}}\right)P_{\mathrm{S}}(T\hspace{0.17em}\approx \hspace{0.17em}{T}_{\mathrm{C}})/q,\end{array}$$(2) , the 1^st and 4^th term are not included.

Additional information

Funding

The support of JPSJ KAKENHI Grant No. JP19K21853 is acknowledged.