Duality of Topological Defects in Hexagonal Manganites

Abstract

We show that the spontaneous symmetry breaking in multiferroic hexagonal manganites can be chemically manipulated to yield two complementary ground states: the well-known ferroelectric P6(3)cm and an antipolar P3c phase. Both symmetry breakings yield topologically protected vortex defects, with the antipolar vortices dual to those of the ferroelectric. This duality stems from the existence of 12 possible angles of MnO5 tilting, and broad strain-free walls with low energy spontaneously emerge through an intermediate P3c1 state, providing a complete unified symmetry description.

Defect Chemistry as a Crystal Structure Design Parameter: Intrinsic Point Defects and Ga Substitution in InMnO3

“…Note that the P 3c1 structure has not been observed as a bulk phase in stoichiometric InMnO 3 , but was recently reported at the walls separating the partially undistorted antipolar domains in the Ga-doped material. 15 The presence of an oxygen vacancy causes significant changes in the density of states, particularly near the Fermi level, as shown in Fig. 3(c).…”

“…Previous ab initio studies 14,15 found that the ground-state energetics are sensitive to the choice of exchange-correlation functional. For spin-polarized Perdew-Burke-Ernzerhof functionals (PBE), 36 for example, a non-polar ground state is slightly favored for stoichiometric InMnO 3 , whereas the spin-polarized PBE plus Hubbard U (GGA+U) method yields a ferroelectric ground state, in agreement with the likely experimental situation.…”

“…It was established in earlier work 10,15 that the nature of domain formation in hexagonal manganites, in particular the width of domain walls and the concentration of domain wall intersections, is dependent on the details of the energy barrier connecting polar and non-polar states. In particular, the different polar domains in the P 6 3 cm ground state are separated by non-polar (P3c) domain walls.…”

“…1(c)), which is a subgroup of both P 6 3 cm and P3c. 15 While the ferroelectric P 6 3 cm structure is well established to be the ground state for most members of the series, the behavior of InMnO 3 has been controversial. The first fluxgrown samples were reported from x-ray diffraction measurements to have the high-symmetry P 6 3 /mmc structure, 16 whereas polycrystalline samples annealed at 1000°C in an evacuated sealed quartz tube indicated a unit-cell tripling that was interpreted in terms of the polar P 6 3 cm structure.…”

“…(We note that, while distinguishing between the P 6 3 cm, P3c and P 3c1 space groups is difficult using XRD, particularly when they coexist in a single sample, HAADF-TEM experiments image the In displacements and can thus detect the local structural details corresponding to the different symmetries. 15 In addition, it was shown that substitution of Ga on the Mn site could be used to tune between the various phases. It now seems likely that ideal, stoichiometric InMnO 3 has the P 6 3 cm ferroelectric ground state, with the non-polar P3c phase, as well as the low-symmetry ferroelectric P 3c1 and possibly even the high-symmetry paraelectric reference P 6 3 /mmc structures accessible by modifying the defect concentration or overall stoichiometry.…”