Ferroelectric glass of spheroidal dipoles with impurities: polar nanoregions, response to applied electric field, and ergodicity breakdown

Abstract

Using molecular dynamics simulation, we study dipolar glass in crystals composed of slightly spheroidal, polar particles and spherical, apolar impurities between metal walls. We present physical pictures of ferroelectric glass, which have been observed in relaxors, mixed crystals (such as KCN KBr ), and polymers. Our systems undergo a diffuse transition in a wide temperature range, where we visualize polar nanoregions (PNRs) surrounded by impurities. In our simulation, the impurities form clusters and their space distribution is heterogeneous. The polarization fluctuations are enhanced at relatively high T depending on the size of the dipole moment. They then form frozen PNRs as T is further lowered into the nonergodic regime. As a result, the dielectric permittivity exhibits the characteristic features of relaxor ferroelectrics. We also examine nonlinear response to cyclic applied electric field and nonergodic response to cyclic temperature changes (ZFC/FC), where the polarization and the strain change collectively and heterogeneously. We also study antiferroelectric glass arising from molecular shape asymmetry. We use an Ewald scheme of calculating the dipolar interaction in applied electric field.

Spin Symmetry Breaking: Superparamagnetic and Spin Glass-Like Behavior Observed in Rod-Like Liquid Crystalline Organic Compounds Contacting Nitroxide Radical Spins

“…For the mechanism of such super-para-magnetic relaxors, the presence of DM interactions are suggested, which will give weak ferromagnetism perpendicular to the polarization of PNR [61]. In addition, the MD simulation showed that the addition of spherical apolar impurity to spheroidal polar particles produces PNR to show the typical behavior of relaxors [63]. This seems analogous to the formation of ferroelectric domains in LC nitroxide radical samples by an impurity effect (See Section 4.2).…”

Self-organization into ferroelectric and antiferroelectric crystals via the interplay between particle shape and dipolar interaction

“…We also introduce a dipole moment to spheroid i in parallel to n i as µ i = µ 0 n i . Then the dipolar interaction is described as (43)…”

“…Motion and Numerical Units. The numerical integration of particle positions and orientations is obtained by solving the equation of motion for uniaxial molecules (43,62,63), mr i = −∂U/∂r i and I(…”

“…· ∂U/∂n i under an appropriate ensemble (below). Here, ↔ 1 is the unit tensor and the inertia moment I is assumed to be that of corresponding spheroids: I = σ 2 (1 + p)m/20 with p = (1 + 2η) 1/6 being the aspect ratio (43,62). All of the quantities in this article are presented in dimensionless form, where the units of energy, length, and mass are , σ, and m, respectively.…”

“…To realize this by a simple particlebased model, we construct a system consisting of spheroidlike Lennard-Jones particles with a point dipole, whose shape anisotropy (i.e., the aspect ratio characterized by an anisotropy parameter η) is introduced as a key physical parameter to control the short-range repulsive interaction (Materials and Methods). So far there have been many particle-based modelings of ferroelectric ordering (39)(40)(41)(42)(43)(44), but no reports on antiferroelectric ordering and phase controllability. Our model may not be universal [e.g., not directly applied to perovskite-type solids where a point dipole moment cannot be well defined even approximately (45)], but captures essential physics behind self-organization of dipoles into two types of long-range dipolar order into different crystalline lattices: competing orderings.…”