Vortex Polarization Instabilities in PbTiO3 Nanowires

G. Pilania and R. Ramprasad

Chemical, Materials and Biomolecular Engineering Institute of Materials Science

University of Connecticut, Storrs, CT

Ferroelectricity in Nanostructures Critical Size & Polarization States

Lateral Polarization in BaTiO3 nanowires Spanier et al, Nano Lett. 6, 735 (2006)

0.8 nm

Ferroelectric Nanostructures Vortex (Non-rectilinear) Polarization

PFM results indicate possible presence of non-rectilinear polarization in PZT nanodots

Rodriguez et al (Nanoletters, 2009)

Prosendeev & Bellaiche (PRB 2007)

Aguado-Puente et al (PRL, 2008)

Computations indicate the presence of non-rectilinear polarization in ferroelectric nanostructures

BaTiO3 Nanowires – Our DFT Study

Axial polarization instability above 1.2 nm

ferroelectric paraelectric

Vortex polarization instability above 1.6 nm

Also see: Geneste et. al, APL 88, 112906 (2006); Shimada et al, PRB 79, 024102 (2009)

PbTiO3 Nanowires – Our DFT Study Vortex polarization at

equilibrium in TiO2-terminated nanowires above 1.6 nm

Ground State Polarization & Atomic Configurations 4x4 TiO2-terminated PbTiO3 Nanowire

PbTiO3 Nanowires vs. Terminations Strain-induced phase transition: vortex axial polarization

Four possible switchable polarization states Vortex (clockwise/counter-clockwise), Axial (positive/negative)

Vortex Instability vs. “Soft-mode”

Atomic displacement vector with respect to reference

paraelectric state

Zone-center phonon eigenvectors of reference

paraelectric state

“Vortex” modes: imaginary

Summary PbTiO3 nanowires display switchable rectilinear (axial) and

non-rectilinear (vortex) polarization configurations