Design of new Mott multiferroics via complete charge transfer: promising candidates for bulk photovoltaics

摘要

Optimal materials to induce bulk photovoltaic effects should lack inversionsymmetry and have an optical gap matching the energies of visible radiation.Ferroelectric perovskite oxides such as BaTiO$_3$ and PbTiO$_3$ exhibitsubstantial polarization and stability, but have the disadvantage ofexcessively large band gaps. We use both density functional theory anddynamical mean field theory calculations to design a new class of Mottmultiferroics--double perovskite oxides $A_2$VFeO$_6$ ($A$=Ba, Pb, etc). Whileneither perovskite $A$VO$_3$ nor $A$FeO$_3$ is ferroelectric, in the doubleperovskite $A_2$VFeO$_6$ a `complete' charge transfer from V to Fe leads to anon-bulk-like charge configuration--an empty V-$d$ shell and a half-filledFe-$d$ shell, giving rise to a polarization comparable to that of ferroelectric$A$TiO$_3$. Different from nonmagnetic $A$TiO$_3$, the new double perovskiteoxides have an antiferromagnetic ground state and around room temperatures, areparamagnetic Mott insulators. Most importantly, the V $d^0$ state significantlyreduces the band gap of $A_2$VFeO$_6$, making it smaller than that of$A$TiO$_3$ and BiFeO$_3$ and rendering the new multiferroics a promisingcandidate to induce bulk photovoltaic effects.