Roto-Flexo Effect Creates Polarization in Non-Polar Materials

Venkatraman Gopalan, Pennsylvania State Univ University Park DMR 0908718

Polar domains in nonpolar CaTiO3 crystals observed by optical Second Harmonic Generation Microscopy.

Appl. Phys. Lett. 100, 142902 (2012)

Phys. Rev. B. 85, 094107, (2012)

Phys. Rev. B. 85, 064117, (2012)

Phys. Rev. B. 86, 085315 (2012)

Polar materials are important for precision actuation, ultrasound detection, infrared imaging, and non-volatile memory. The most popular polar oxides for these applications today are ABO3 perovskites, where the displacement of a cation B or A with respect to the oxygen octahedral cage O6 generates a spontaneous polarization. Most perovskites, however, do not possess this phenomenon. Even when present, this polarization disappears above the Curie temperature. Alternate means of developing a polarization are desired, which can persist up to higher temperatures. A vast majority of perovskite oxides do possess oxygen octahedral rotations that can persist up to high temperatures (~1000 °C). Improper mechanisms result in a polarization through coupling to these rotations. One such mechanism has been discovered recently by the PIs, which is named the roto-flexo effect. This effect arises when there is a gradient of rotations near a surface or an interface. Through rotostriction effect, this gradient gives rise to a strain gradient, and through the flexoelectric effect, the strain gradient gives rise to a polarization. Initial estimates of this combined roto-flexo polarization in non-polar oxides such as strontium and calcium titanates (see figure), are significant, nearly a third to half the value of classic ferroelectrics such as barium titanate. The roto-flexo effect exists in all crystals with lattice rotations, and hence one can expect all twin walls and surfaces of such materials to be polar. The high mobility two-dimensional electron gas at non-polar oxide interfaces such as between LaAlO3 and SrTiO3 can have a strong roto-flexo polarization at the interface.

Carbon Nanotube at the Arts FestVenkatraman Gopalan, Pennsylvania State Univ University Park DMR 0908718

Jessica Leung and Arnab SenGupta from Gopalan group worked with children and their parents to build the carbon nanotube with balloons at the Arts Fest, State College, PA. (July 2012)