Carrier-mediated control over the soft mode and ferroelectricity in BaTiO3

Abstract

We calculate the effects of conduction band electrons (CBEs), introduced by doping or photoexcitation, on the ferroelectricity and phonon dynamics of BaTiO3 (BTO). We show that CBEs destabilize ferroelectricity, which would lower the Curie temperature and coercive field, and might help to improve the speed or efficiency with which polarization domains can be switched in ferroelectric devices. We show that CBEs lower the frequencies of the A1 soft/ferroelectric modes in BTO's ferroelectric phases, and raise the soft mode frequency in its paraelectric phase. We also show that femtosecond laser pulses could be used to selectively excite a coherent A1 ferroelectric-mode phonon. This would allow this much-studied excitation to be monitored by pump-probe spectroscopy as it decays into other modes. We show that many of the properties of doped and undoped BTO have simple and intuitive explanations, within an ionic picture of BTO's bonding, if it is assumed that ferroelectricity is not driven by long range interactions, but by the attraction between Ti and O neighbours. Most of the effects of CBEs are consequences of them reducing ions' charges and increasing their polarizabilities, thereby weakening the Ti-O attraction. As the CBE density increases, so does the density of delocalized interstitial electrons, which would increase conductivity. We argue that a polar metallic phase exists if the threshold for metallic conductivity can be reached before the CBEs make polar distortions energetically unfavourable by critically weakening the Ti-O attraction.

Interplay between ferroelectricity and metallicity in BaTiO3

“…18 The specific effects of doping on chemical bonding and the effect of electron-doping on ferroelectricity and phonon dynamics of BaTiO 3 were discussed by Hickox-Young et al and Gu et al, respectively. 19,20 In 2016, Benedek et al compared the effect of doping in LiOsO 3 and ATiO 3 perovskites (A = Ba, Sr, Ca). 6 They showed that the ferroelectricity is suppressed by electron doping in BaTiO 3 , whereas the non-centrosymmetricity in metallic LiOsO 3 and pseudocubic CaTiO 3 (with octahedral rotations not allowed) is robust to addition of charge carriers because of the local-bonding nature of the mechanism underlying the off-centering of the ions.…”

Quantum fluctuation of ferroelectric order in polar metals

“…On the other hand, it would lower the energy cost of quantum fluctuation and therefore further enhance it (as if t/J is effectively increased) and consequently the polaron formation. Nonetheless, given the high energy barrier between the local dipolar modes (~105 meV for BaTiO 3 46,64 ), it is convenient to also drop the much weaker local fluctuation H R , considering that the corresponding fluctuation rate R must be orders of magnitude smaller than the eV energy scale of t and J. In other words, the time scale for such intrinsic fluctuation between dipolar modes is so long that it cannot possibly affect the much faster quantum polaron formation in any meaningful way.…”

Quantum fluctuation of ferroelectric order in polar metals

“…While the introduction of H MD 36 and long-range H DD can slightly modify the internal shape of the polarons, it would not affect the qualitative trend since their energy is much smaller than that of the cooperative ligand displacement 52 . Similarly, given the already strong quantum fluctuation due to the eV-scale kinetic energy, it is convenient to also drop the much weaker local fluctuation H R , considering the small ∼20meV potential barriers between the local dipole modes in real materials 53 .…”