Terahertz Dielectric Response and Coupled Dynamics of Ferroelectrics and Multiferroics From Effective Hamiltonian Simulations

Abstract

Ferroelectric and multiferroic materials form an important class of functional materials. Over the last twenty years, first-principles-based effective Hamiltonian approaches have been successfully developed to simulate these materials. In recent years, effective Hamiltonian approaches were further combined with molecular dynamics methods to investigate terahertz dynamical properties of various perovskites. With this combination, a variety of ferroelectric and multiferroic materials, including BaTiO3, Ba(Sr,Ti)O3, Pb(Zr,Ti)O3, BiFeO3, and SrTiO3 bulks and films have been simulated, which led to the understanding of complex phenomena and discovery of novel effects. In this review, we first provide technical details about effective Hamiltonians and molecular dynamics simulation. Then, we present applications of the combination of these two techniques to different perovskites. Finally, we also briefly discuss possible future directions of this approach.

Subterahertz dielectric relaxation in lead-free Ba(Zr,Ti)O3 relaxor ferroelectrics

“…At any given time t , d ( t ) is obtained by summing over every dipole of the system, and therefore contains contributions from different compositional regions that have different dipole dynamics, including relaxational dynamics. Note the important technical differences with respect to previous studies 20 32 33 34 35 38 39 : the total molecular dynamics simulation time is extended to 16 ns, which is about eight times longer than the simulation time used in refs 20 , 32 , 33 , 34 , 35 , 38 , 39 , to accurately mimic the low-frequency part of the dielectric response; and the real part of the dielectric response is presently investigated in detail, in addition to the imaginary part.…”

“… 4 and successfully used to model and study different static and dynamical properties of the BZT systems 18 19 20 . This Hamiltonian is presently implemented within the molecular dynamics technique described in refs 20 , 31 , 32 , 33 , 34 , 35 , and is applied to a 12 × 12 × 12 supercell made from disordered Ba(Zr 0.5 Ti 0.5 )O 3 solid solution. The complex dielectric susceptibility, χ lm ( ν ), can be obtained from the molecular dynamics simulations via refs 20 , 31 , 33 , 36 , 37 :…”

Fano resonance and dipolar relaxation in lead-free relaxors

“…During the MD simulations, the temperatures of all these structural degrees of freedom are controlled by EvansHoover thermostats 56,59 . For each MD simulation, we first run 10 5 MD steps of NPT (isothermal-isobaric ensemble) simulations to equilibrate the system at a chosen temperature and pressure.…”

“…[54][55][56][57] to obtain complex dielectric responses of disordered BZT for temperatures ranging from 10 to 1,000 K (see the Methods section). At the end of the MD simulations, the complex electric susceptibility, w ab (n), can be obtained using the following equation [56][57][58][59] :…”

Electric field control of magnetism: multiferroics and magnetoelectrics

“…Modeling of the dynamics of ferroelectric switching [85] and its effect on magnetic order [86], both of which are on time-and length-scales that are far outside the ranges accessible using density functional methods, have now become feasible. Such models in combination with molecular dynamics start to allow calculation of dynamical magnetoelectric responses in the THz region [87], which is particularly timely as it coincides with advances in experimental methods for generating THz radiation mentioned above. Finally, the ongoing development of new theoretical concepts, such as the magnetoelectric multipole as an order parameter for phase transitions that break both space-inversion and time-reversal [88,89], as well as the production of practical computational tools for their calculation look very promising in terms of pushing the limits of computationally driven materials discovery [22,[90][91][92].…”