The quantum paraelectricity of perovskite oxides from first-principle calculation

Abstract

Perovskites have been widely utilized in various applications due to their ability to transition into different phases such as ferroelectricity, ferromagnetism, and superconductivity, depending on external conditions. A new class of phase known as quantum paraelectricity (QPE) has been discovered, which do not undergo phase transitions even at low temperatures away from conventional ferroelectric materials. The underlying cause of this behavior is attributed to quantum fluctuations occurring at low temperature, and extensive research is being conducted to microscopically elucidate the properties of QPE in terms of quantum fluctuation. A representative study is proceeded in the SrTiO3 by highlighting the interaction between lattice elongation and ferroelectric soft modes based on the first principle calculations. In our research, we investigated QPE behavior in the specific perovskite oxides, such as KTaO3 and EuTiO3, and aimed to unravel the nature of QPE behavior. For this, we first studied the methodology of the aforementioned study on SrTiO3 to verify our numerical configurations, and then applied the methodology to our chosen QPE candidates (KTaO3 and EuTiO3). The specific methodology involved the following steps: We used four functionals (LDA, PBE, PBEsol, HSE06) to find the one that best describes the ferroelectric soft mode. For this, we performed DFPT (Density Functional Perturbation Theory) and then confirmed whether these methods achieved an unstable equilibrium at the equilibrium point. Subsequently, to incorporate quantum fluctuations of the material, we introduced the lattice–Schr¨odinger equation, calculated the mode frequencies from this, and used these frequencies as a measure of QPE characteristics. We can confirm QPE for the above two materials from PBE functionals. In the case of KTaO3, unlike SrTiO3, the influence of lattice elongation was negligible, and due to the cubic symmetry of the structure, there were no modes preferred in specific axial directions. Therefore, the mode could be described as a linear combination of modes in each axial direction as 3D quantum harmonic oscillator model. In the case of EuTiO3, the interaction between the spins on Eu and the lattice affects the QPE iproperties. Furthermore, by analyzing the cause of the variation in mode frequencies depending on the degree of spin interaction, we identified that this variation was related to the electron occupancy of the 3d-orbitals of the transition metal. In conclusion, we verified that both materials show QPE based on the first-principle calculation.