Domains within Domains and Walls within Walls: Evidence for Polar Domains in Cryogenic SrTiO_3

摘要

Resonant piezoelectric spectroscopy shows polar resonances in paraelectric SrTiO_3 at temperatures below 80 K. These resonances become strong at T ＜ 40 K. The resonances are induced by weak electric fields and lead to standing mechanical waves in the sample. This piezoelectric response does not exist in paraelastic SrTiO_3 nor at temperatures just below the ferroelastic phase transition. The interpretation of the resonances is related to ferroelastic twin walls which become polar at low temperatures in close analogy with the known behavior of CaTiO_3. SrTiO_3 is different from CaTiO_3, however, because the wall polarity is thermally induced; i.e., there exists a small temperature range well below the ferroelastic transition point at 105 K where polarity appears on cooling. As the walls are atomistically thin, this transition has the hallmarks of a two-dimensional phase transition restrained to the twin boundaries rather than a classic bulk phase transition.