Electrocaloric effect in ferroelectric materials:From phase field to first-principles based effective Hamiltonian modeling

Electrocaloric effect(ECE)of ferroelectrics has attracted considerable interest due to its potential application in environmentally friendly solid-state refrigeration.The discovery of giant ECE in ferroelectric thin films has greatly renewed the research activities and significantly stimulated experimental and theoretical investigations.In this review,the recent progress on the theoretical modeling of ECE in ferroelectric and antiferroelectric materials are introduced,which mainly focuses on the phase field modeling and first-principles based effective Hamiltonian method.We firstly provide the theoretical foundation and technique details for each method.Then a comprehensive review on the progress in the application of two methods and the strategies to tune the ECE are presented.Finally,we outline the practical procedure on the development of multi-scale computational method without experiemtal parameters for the screening of optimized electrocaloric materials.

Generalised tanh-shaped hyperbolic potential: Klein–Gordon equation's bound state solution

The development of potential theory heightens the understanding of fundamental interactions in quantum systems.In this paper,the bound state solution of the modified radial Klein–Gordon equation is presented for generalised tanh-shaped hyperbolic potential from the Nikiforov–Uvarov method.The resulting energy eigenvalues and corresponding radial wave functions are expressed in terms of the Jacobi polynomials for arbitrary l states.It is also demonstrated that energy eigenvalues strongly correlate with potential parameters for quantum states.Considering particular cases,the generalised tanh-shaped hyperbolic potential and its derived energy eigenvalues exhibit good agreement with the reported findings.Furthermore,the rovibrational energies are calculated for three representative diatomic molecules,namely H2,HCl and O2.The lowest excitation energies are in perfect agreement with experimental results.Overall,the potential model is displayed to be a viable candidate for concurrently prescribing numerous quantum systems.

Missed ferroelectricity in methylammonium lead iodide

Methylammonium lead iodide,as related organometal halide perovskites,emerged recently as a particularly attractive material for photovoltaic applications.The origin of its appealing properties is sometimes assigned to its potential ferroelectric character,which remains however a topic of intense debate.Here,we rationalize from first-principles calculations how the spatial arrangement of methylammonium polar molecules is progressively constrained by the subtle interplay between their tendency to bond with the inorganic framework and the appearance of iodine octahedra rotations inherent to the perovskite structure.The disordered tetragonal phase observed at room temperature is paraelectric.We show that it should a priori become ferroelectric but that iodine octahedra rotations drive the system toward an antipolar orthorhombic ground state,making it a missed ferroelectric.

Nonreciprocal directional dichroism in multiferroics

Nonreciprocal directional dichroism in multiferroics,namely magnetoelectric coupling in the dynamic regime,is endowed with rich physics and promising applications,which are entangled with fundamental physical components,such as spin,orbital,lattice,charge,and topology.Such a linear nonreciprocal response behavior in the GHz-THz frequency range,represented by optical magnetoelectric effect and magnetochiral dichroism,occurs ubiquitously in material systems with the spontaneous breaking of space-time symmetry,and is subject to Onsager’s reciprocal theorem in the thermodynamic limit.Microscopically,these nonreciprocal responses are usually encoded by toroidization(chirality)and electromagnon(quasiparticle),thus establishing a comprehensive understanding of magnetoelectric coupling and irreversible dynamics.Herein,the basic mechanisms and emergent nonreciprocal directional dichroism in single-phase multiferroics are summarized.We expect that the present review will stimulate diverse possibilities toward nonreciprocal directional dichroism within and beyond multiferroics.