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Register a new userFirst-Principles Supercell Calculations of Small Polarons with Proper Account for Long-Range Polarization Effects
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We present a density functional theory (DFT) based supercell approach for modeling small polarons with proper account for the long-range elastic response of the material. Our analysis of the supercell dependence of the polaron properties (e.g., atomic structure, binding energy, and the polaron level) reveals long-range electrostatic effects and the electron-phonon interaction as the two main contributors. We develop a correction scheme for DFT polaron calculations that significantly reduces the dependence of polaron properties on the DFT exchange-correlation functional and the size of the supercell in the limit of strong electron-phonon coupling. Using our correction approach, we present accurate all-electron full-potential DFT results for small polarons in rocksalt MgO and rutile TiO$_2$.
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We develop a formalism and a computational method to study polarons in insulators and semi-conductors from first principles. Unlike in standard calculations requiring large supercells, we solve a secular equation involving phonons and electron-phonon matrix elements from density-functional perturbation theory, in a spirit similar to the Bethe-Salpeter equation for excitons. We show that our approach describes seamlessly large and small polarons, and we illustrate its capability by calculating wavefunctions, formation energies, and spectral decomposition of polarons in LiF and Li2O2.
Localized Wannier functions provide an efficient and intuitive framework to compute electric polarization from first-principles. They can also be used to represent the electronic systems at fixed electric field and to determine dielectric properties of insulating materials. Here we develop a Wannier-function-based formalism to perform first-principles calculations at fixed polarization. Such an approach allows to extract the polarization-energy landscape of a crystal and thus supports the theoretical investigation of polar materials. To facilitate the calculations, we implement a quasi-Newton method that simultaneously relaxes the internal coordinates and adjusts the electric field in crystals at fixed polarization. The method is applied to study the ferroelectric behavior of $mathrm{BaTiO_3}$ and $mathrm{PbTiO_3}$ in tetragonal phases. The physical processes driving the ferroelectricity of both compounds are examined thanks to the localized orbital picture offered by Wannier functions. Hence, changes in chemical bonding under ferroelectric distortion can be accurately visualized. The difference in the ferroelectric properties of $mathrm{BaTiO_3}$ and $mathrm{PbTiO_3}$ is highlighted. It can be traced back to the peculiarities of their electronic structures.
Phonon Hall effect (PHE) has attracted a lot of attention in recent years with many theoretical and experimental explorations published. While experiments work on complicated materials, theoretical studies are still hovering around the phenomenon-based models. Moreover, previous microscopic theory was found unable to explain large thermal Hall conductivity obtained by experiments in strontium titanate (STO). Therefore, as a first attempt to bridge this gap, we implement first-principles calculations to explore the PHE in real materials. Our work provides a new benchmark of the PHE in sodium chloride (NaCl) under a large external magnetic field. Moreover, we demonstrate our results in barium titanate (BTO), and discuss the results in STO in detail about their deviation from experiments. As a possible future direction, we further propose that the inner electronic Berry curvature plays an important role in the PHE in STO.
We report a systematic first-principles investigation of the influence of different magnetic insulators on the magnetic proximity effect induced in graphene. Four different magnetic insulators are considered: two ferromagnetic europium chalcogenides namely EuO and EuS and two ferrimagnetic insulators yttrium iron garnet (YIG) and cobalt ferrite (CFO). The obtained exchange-splitting varies from tens to hundreds of meV. We also find an electron doping induced by YIG and europium chalcogenides substrates, that shift the Fermi level up to 0.78 eV and 1.3 eV respectively, whereas hole doping up to 0.5 eV is generated by CFO. Furthermore, we study the variation of the extracted exchange and tight binding parameters as a function of the EuO and EuS thicknesses. We show that those parameters are robust to thickness variation such that a single monolayer of magnetic insulator can induce a large magnetic proximity effect on graphene. Those findings pave the way towards possible engineering of graphene spin-gating by proximity effect especially in view of recent experiments advancement.
We propose a guideline for exploring substrates that stabilize the monolayer honeycomb structure of silicene and germanene while simultaneously preserve the Dirac states: in addition to have a strong binding energy to the monolayer, a suitable substrate should be a large-gap semiconductor with a proper workfunction such that the Dirac point lies in the gap and far from the substrate states when their bands align. We illustrate our idea by performing first-principles calculations for silicene and germanene on the Al-terminated (0001) surface of Al2O3 . The overlaid monolayers on Al-terminated Al2O3(0001) retain the main structural profile of the low-buckled honeycomb structure via a binding energy comparable to the one between silicene and Ag(111). Unfolded band structure derived from the k-projection method reveals that gapped Dirac cone is formed at the K point due to the structural distortion and the interaction with the substrate. The gaps of 0.4 eV and 0.3 eV respectively for the supported silicene and germanene suggest that they may have potential applications in nanoelectronics.
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