Dataset of tensorial optical and transport properties of materials from the Wannier function method
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Thermal disorder and phonon softening in the ferroelectric phase transition of lead titanate
We report a molecular dynamics study of ab initio quality of the ferroelectric phase transition in crystalline PbTiO3. We model anharmonicity accurately in terms of potential energy and polarization surfaces trained on density functional theory data with modern machine learning techniques. Our simulations demonstrate that the transition has a strong order-disorder character, in agreement with diffraction experiments, and provide fresh insight into the approach to equilibrium across the phase transition. We find that the emergence and disappearance of the macroscopic polarization is driven by dipolar switching at the nanometer scale. We also computed the infrared optical absorption spectra in both the ferroelectric and the paraelectric phases, finding good agreement with the experimental Raman frequencies. Often, the almost ideal displacive character of the soft mode detected by Raman scattering in the paraelectric phase has been contrasted with the order-disorder character of the transition suggested by diffraction experiments. We settle this issue by showing that the soft mode coexists with a strong Debye relaxation associated with thermal disordering of the dipoles. The Debye relaxation feature is centered at zero frequency and appears near the transition temperature in both the ferroelectric and the paraelectric phases.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Real-Space Quantification of Exciton Localization in Acene Crystals Using Wannier Function Decomposition
Here, we introduce the Wannier function decomposition of excitons (WFDX) method to quantify exciton localization in solids within the ab initio Bethe-Salpeter equation framework. By decomposing each Bloch exciton wave function into products of single-particle electron and hole maximally localized Wannier functions, this real-space approach provides well-defined orbital- and spatial-resolved measures of both Frenkel and charge-transfer excitons at low computational cost. We apply WFDX to excitons in acene crystals, quantifying how the number of rings, the exciton spin state, and the center-of-mass momentum affect spatial localization. Additionally, we show how this real-space representation reflects structural nonsymmorphic symmetries that are hidden in standard reciprocal-space descriptions. We demonstrate how the WFDX framework can be used to efficiently interpolate exciton expansion coefficients in reciprocal-space and outline how it may facilitate evaluation of observables involving position operators, highlighting its potential as a general tool for both analyzing and computing excitonic properties in solids.
Tao, Zui [University of California, Berkeley, CA (↗
Impact of hole polaron formation on excitonic transitions in MgO from first principles
Here, we present a first-principles investigation of the excitonic properties of magnesia (MgO), an ionic insulator known to host hole polarons. We combine a density functional theory-based approach for structural relaxation in the presence of the hole and many-body perturbation theory to describe the excitonic properties. We determine that the hole polaron introduces new in-gap occupied states 0.6–0.8 eV above the valence band maximum that lead to two low-energy peaks in the optical spectrum. The predicted redshift of the lowest-energy transition due to polaron formation of 0.8 eV agrees well with the experimental Stokes shift of 0.8–0.9 eV. Analysis of the exciton wave function indicates that the electron-hole pair consists of a localized hole and delocalized electron, but that the wave function retains its Wannier-Mott character even in the presence of the hole polaron. Our study demonstrates that combining these previously established methods allows for a relatively computationally inexpensive approach to studying the exciton polaron in materials where only one charge carrier forms a polaron.
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