Structural instabilities under electron doping in SrTiO3

by Ulrich Aschauer (ETH Zürich)

Monday 23 Nov 2015, 11:00 → 12:00 Europe/Stockholm

112:028 (Nordita South)

Perovskite oxides often exhibit structural distortions that lower the energy by optimization of the bonding compared to the perfect cubic structure. For titanates these distortions are typically the polar ferroelectric distortion as observed in BaTiO3 or octahedral rotations such as observed in CaTiO3. These two distortions are normally considered to be competing and excluding one another. SrTiO3, which chemically lies in between CaTiO3 and BaTiO3, however exhibits both instabilities at the same time. The ferroelectric instability is however so weak that it is suppressed by quantum fluctuations, making the material not a ferroelectric but instead a so-called quantum paraelectric. Here we use density functional theory (DFT) calculations to study the interaction between the two structural instabilities in SrTiO3 and find that they also compete but that no complete suppression is observed. We then study their behavior and interaction under electron doping and find that the ferroelectric mode is strongly suppressed even by very small amounts of charge carriers, whereas that octahedral distortions are not significantly affected. The potential role of the polar instability in the superconducting mechanism will also be discussed.