Computational Design of Flat-Band Material

Investigations on the thermoelectric and thermodynamic properties of quaternary coinage metal HgSBr

We present the findings of a thorough first-principles analysis of physical parameters related to the ground state, elastic, electronic, optical, thermodynamic, and transport properties of the quaternary coinage metal-based compound CuHgSBr using the WIEN2k package. The computed equilibrium lattice parameters align well with their experimental equivalents, providing strong support for the validity of the findings. We performed numerical and computational calculations to estimate the elastic constants for the orthorhombic structure with space group Pbam. The band structure analysis of CuHgSBr reveals an indirect band gap semiconductor of 0.76 eV, classifying it as a p-type semiconductor. We also calculated the optical properties within the energy range of 0-13.56 eV. Moreover, we investigated the effective mass, exciton binding energy, and exciton Bohr radius, which indicated that CuHgSBr exhibits a weak exciton binding energy and belongs to the Mott-Wannier type exciton category. Using the Boltzmann transport theory, along with the constant relaxation time and Slack equations, we determined the thermoelectric properties and lattice thermal conductivity of CuHgSBr. Notably, the figure of merit at 800 K is calculated to be 0.54, which is encouraging for potential thermoelectric applications. The comprehensive research study we conducted provides valuable insights for experimental research across multiple physical properties, as the material is being theoretically examined for the first time in this full prospectus.

Thin-Film Ferroelectrics

Over the last 30 years, the study of ferroelectric oxides has been revolutionized by the implementation of epitaxial-thin-film-based studies, which have driven many advances in the understanding of ferroelectric physics and the realization of novel polar structures and functionalities. New questions have motivated the development of advanced synthesis, characterization, and simulations of epitaxial thin films and, in turn, have provided new insights and applications across the micro-, meso-, and macroscopic length scales. This review traces the evolution of ferroelectric thin-film research through the early days developing understanding of the roles of size and strain on ferroelectrics to the present day, where such understanding is used to create complex hierarchical domain structures, novel polar topologies, and controlled chemical and defect profiles. The extension of epitaxial techniques, coupled with advances in high-throughput simulations, now stands to accelerate the discovery and study of new ferroelectric materials. Coming hand-in-hand with these new materials is new understanding and control of ferroelectric functionalities. Today, researchers are actively working to apply these lessons in a number of applications, including novel memory and logic architectures, as well as a host of energy conversion devices.

Possible Three-Dimensional Topological Insulator in Pyrochlore Oxides

A Kene–Mele-type nearest-neighbor tight-binding model on a pyrochlore lattice is known to be a topological insulator in some parameter region. It is an important task to realize a topological insulator in a real compound, especially in an oxide that is stable in air. In this paper we systematically performed band structure calculations for six pyrochlore oxides A2B2O7 (A = Sn, Pb, Tl; B = Nb, Ta), which are properly described by this model, and found that heavily hole-doped Sn2Nb2O7 is a good candidate. Surprisingly, an effective spin–orbit coupling constant λ changes its sign depending on the composition of the material. Furthermore, we calculated the band structure of three virtual pyrochlore oxides, namely In2Nb2O7, In2Ta2O7 and Sn2Zr2O7. We found that Sn2Zr2O7 has a band gap at the k = 0 (Γ) point, similar to Sn2Nb2O7, though the band structure of Sn2Zr2O7 itself differs from the ideal nearest-neighbor tight-binding model. We propose that the co-doped system (In,Sn)2(Nb,Zr)2O7 may become a candidate of the three-dimensional strong topological insulator.

Flatband in a three-dimensional tungsten nitride compound

Herein, the flatband of a W₁N₂ crystal is theoretically investigated. It is revealed that the flatband can be well-described by a tight-binding model of the N₁₂ skeleton, where the dispersion of the flatband is governed by both the ppσ bonding strength (V_ppσ) and the ppπ bonding strength (V_ppπ) between the nearest-neighbor N atoms. It is also found that the proper strength of the ppπ bonding between neighboring N atoms plays a prime role in the formation of the flatband. In addition, when the compound is doped with holes, the electrons at the flatband are fully polarized, showing a ferromagnetic character. This behavior has a weak correlation with the on-site Coulomb interaction U. Moreover, several three-dimensional compounds possessing flatbands in the whole k space are predicted.

Flat-Band in Pyrochlore Oxides: A First-Principles Study

Using a first-principles electronic band calculation, we obtained a quasi flat-band near the Fermi level for the six pyrochlore oxides, A₂B₂O₇. These quasi flat-bands are mostly characterized by the s-orbitals of the A-site. The band structures of these oxides are well described by the non-interacting Mielke model. Spin-polarized calculations showed that the ground state of these compounds was ferromagnetic after appropriate carrier doping, despite the absence of the magnetic element.