Observation of Quasi-Two-Dimensional Polar Domains and Ferroelastic Switching in a Metal, Ca₃Ru₂O₇

Abstract

Polar domains arise in insulating ferroelectrics when free carriers are unable to fully screen surface-bound charges. Recently discovered binary and ternary polar metals exhibit broken inversion symmetry coexisting with free electrons that might be expected to suppress the electrostatic driving force for domain formation. Contrary to this expectation, we report the first direct observation of polar domains in single crystals of the polar metal CaRuO. By a combination of mesoscale optical second-harmonic imaging and atomic-resolution scanning transmission electron microscopy, the polar domains are found to possess a quasi-two-dimensional slab geometry with a lateral size of ∼100 μm and thickness of ∼10 nm. Electronic structure calculations show that the coexistence of electronic and parity-lifting orders arise from anharmonic lattice interactions, which support 90° and 180° polar domains in a metal. Using in situ transmission electron microscopy, we also demonstrate a strain-tuning route to achieve ferroelastic switching of polar metal domains.

Probing ferroelectricity in highly conducting materials through their elastic response: Persistence of ferroelectricity in metallic BaTiO3−δ

“…This is a manifestation of the fact that probing the permanence of polar distortions in the bulk of a ferroelectric material with high electrical conductivity cannot be done with the usual methods directly sensitive to the electric polarization, like dielec- tric permittivity, hysteresis loop or thermally stimulated depolarization current, because of the screening effect of the free charges. It is then clear that the anelastic spectra provide a practical and valuable tool for studying highly doped ferroelectrics, in addition to the other techniques used so far, notably optical SHG 15,17,18,46 and Rietveld analysis of diffraction data. 14 The nonlinear optical technique also allows polar domains to be mapped, 15,46 though it is not obvious to quantitatively relate the magnitude of the SHG signal with the spontaneous polarization or piezoelectric coefficients.…”

Ferroelectric enhancement of superconductivity in compressively strained SrTiO3 films

“…Superconductivity requires free carriers, with the superconducting energy gap depending strongly on the density of states at the Fermi level, while free charges tend to rapidly screen electric dipole fields and prevent longrange spontaneous electric polarization. Indeed, only recently have long-sought examples of so-called "polar metals" been discovered [6][7][8][9]. Because of this discordance, the fundamental relationship between ferroelectricity and superconductivity in SrTiO 3 is still a matter of debate.…”

Atomic and electronic structure of domains walls in a polar metal

“…Recent studies of the polar metal Ca3Ru2O7 have verified that despite the absence of depolarization fields, polar metals support a rich quasi-two-dimensional polar domain structure that coexists with metallic conductivity [10]. These structures are similar to those found in the isostructural and insulating ferroelectrics (Sr,Ca)3Ti2O7 [1,11,12] and Ca3(Mn,Ti)2O7 [13] where polar displacements arise from anharmonic coupling of oxygen octahedral tilt modes [14][15][16].…”