Inverse Band Structure Design via Materials Database Screening: Application to Square Planar Thermoelectrics

Structure and electronic properties of LnScO3 compounds: A GGA + U calculation

In this work, an inverse design method for multiinput multi-output (MIMO) metastructured devices is developed. Large-scale inverse design problems are difficult to solve directly and often require heuristic methods or design optimization to find a solution. Inherent errors introduced by heuristic methods makes design optimization a more promising route to the realization of high performance devices. Here, a fast frequency domain solver for grids of Y-parameter matrices is developed. The solver is used together with an adjoint-based optimization routine to solve inverse metastructured design problems. The design procedure is demonstrated through the realization of a planar beamforming network for a multi-beam antenna.

Topological materials are characterized by an electronic band structure with nontrivial topological properties. In this paper, we introduce a basis of operators for the linear space of operators spanned by charge-neutral fermion bilinears. These band projected density operators are constructed using directly the eigenfunctions of the electronic energy band structure and there is no need to assume a flat Berry curvature. As a result, our set of operators has a wider range of validity and is sensitive to physical phenomena which are not detectable in the flat curvature limit. In particular, we show that the Berry monopole configuration of Weyl semimetal give rises to a nonvanishing Jacobiator for these band projected density operators, implying the emergence of nonassociativity at the location of the Weyl nodes. The resulting nonassociativity observes the fundamental identity, the defining property of the Nambu bracket and so one may call this a nonassociative Nambu geometry. We also derive the corresponding uncertainty principle.

Band structure of an electron in a kind of periodic potentials with singularities

Ground state, structural, and magnetic phase transitions of strained BiVO3 were investigated using the first-principles calculation based on the self-interaction corrected local density approximation + U functional. The calculated total energy indicated that the ground state structure for BiVO3 was a monoclinic P21/a phase and ferromagnetic ordering is predicted to be the magnetic ground state. Furthermore, it was found that BiVO3 are metastable compounds under strain effects, where several structural phase transitions were predicted from the monoclinic P 21 / a phase to the orthorhombic P b n m phase (at compressive volumetric strain) and to monoclinic C m and C c phases (at expansive volumetric strain). Furthermore, calculations on electronic band structure and density of states for the monoclinic P21/a phase and the monoclinic C c phase of BiVO3 with ferromagnetic ordering were conducted. The calculated band structure predicted a small energy gap with a direct energy bandgap of 0.456 eV and an indirect energy gap of 0.466 eV. Interestingly, the obtained results revealed that the BiVO3 calculated density of states exhibited Mott-Hubbard insulator type. Therefore, the ground state monoclinic C c phase of strained BiVO3 with ferromagnetic ordering is suggested as a new candidate for multiferroic materials at room temperature and a visible light driven photocatalytic/photovoltaic effect. The obtained results in this study could provide valuable theoretical information on BiVO3 under strain effects, which could open new opportunities for future research and technological applications.

Electronic and phononic modulation of MoS2 under biaxial strain

Designing photonic topological insulators (PTIs) is highly non-trivial because it requires inversion of band symmetries around the band gap, which was so far done using intuition combined with meticulous trial and error. Here we take a completely different approach: we consider the design of PTIs as an inverse design problem and use topology optimization to maximize the transmission through an edge mode past a sharp bend. Two design domains composed of two different but initially identicalC6ν-symmetric unit cells define the geometrical design problem. Remarkably, the optimization results in a PTI reminiscent of the shrink-and-grow approach to quantum-spin-Hall PTIs but with notable differences in the crystal structure as well as qualitatively different band structures and with significantly improved performance as gauged by the band-gap sizes, which are at least 50% larger than in previous designs. Furthermore, we find a directional β-factor exceeding 99% and very low losses for sharp bends. Our approach allows the introduction of fabrication limitations by design and opens an avenue towards designing PTIs with hitherto-unexplored symmetry constraints.

Anatase TiO2 (A-TiO2) has been a prominent material in the field of solar-to-chemical energy conversion due to its outstanding physical and chemical properties, which are closely related to its electronic structure. Nevertheless, the valence band (VB) structure of A-TiO2(101), the most stably faceted A-TiO2, has seldom been experimentally investigated. In this work, polarization-dependent angle-resolved photoelectron spectroscopy (ARPES) in combination with density functional theory calculations has been used to investigate the VB of A-TiO2(101). σ, π, and Pπ bands have been detected in the 4–9 eV binding energy range. The three bands along Γ-Q are rather flat, such that the band center energies vary within a narrow range of 0.5 eV. However, the bands along Γ-M are much more dispersive, displaying significant anisotropy arising from an anisotropic atomic structure. The calculated band structure is in reasonable agreement with the measurements. The π bands along Γ-M exhibit apparent polarization dependence, which is attributed to the different orbital symmetries of the initial states based on the photoemission matrix element effect. The anisotropic valence band structure results in the anisotropic hole effective mass (mh). The direct measurements of the anisotropic valence band structure of A-TiO2(101) will be beneficial to future research and applications of this material.

Physical properties of the Si\(_{1-x}\)Ge\(_{x}\) alloys ($x$ being the composition of Ge) can be understood and predicted from their electronic band structures. In this paper, electronic band structures of the Si\(_{1-x}\)Ge\(_{x}\) alloys are calculated using the first-principles density functional theory. The supper cell approach employed in our calculations leads to folding of electronic bands into the smaller Brillouin zone of the supercell, especially at the \(\Gamma\) point. This often leads to the misinterpretation that the materials have direct band gap. The problem can be resolved by an unfolding band technique which allows us to recover the primitive cell picture of band structure of Si\(_{1-x}\)Ge\(_{x}\). As a result, unfolded electronic bands correctly show an indirect band gap with the valence band maximum (VBM) at the $\Gamma $ point and the conduction band minimum (CBM) shifted away from \(\Gamma\). CBM is gradually shifted from a point along \(\Gamma X\) symmetry line (associated with Si) to the L point (associated with Ge) with the increased Ge composition \(x\) and the switching occurs at \(x\) in the range of 0.6\(\sim\)0.8 which is in accordance with the calculation using \textbf{\textit{kp}} method. Moreover, the additional electron pockets appear and develop at \(\Gamma\) and $L$. This provides more comprehensive understanding for our recent experimental observations on the shift of the absorption energy assigned to $E1$ direct transitions within \(L\) and \(\Gamma\) points in the Brillouin zone of Si\(_{1-x}\)Ge\(_{x}\) alloy nanocrystals.

A first principle study of pressure-induced effects on phase transitions, band structures and elasticity of zinc oxide

Deep learning-based inverse design of lattice metamaterials for tuning bandgap

The electronic energy band structures of the La-doped superconductors TlBa1.2 La0.8 CuO5 + δ were calculated. The effect of the oxygen content on their electronic structures was studied. The results show that compared with those for TlBa2CuO5, the La doping at the Ba site results in the moderate change in the band structures and the decrease in the densities of states, but the increase in the oxygen content caused by the La doping results in the great change in the band structures and the densities of states near the Fermi surface. The low oxygen content causes the degree of complexity of the band structures and the densities of states near the Fermi level Ef to increase and the high oxygen content causes them to decrease. The oxygen content plays a dominant role in TlBa1.2La0.8CuO5 + δ. In addition, the Cu-O planes are most sensitive to the increase in the oxygen content. © 1996 John Wiley & Sons, Inc.

Present paper deals with a critical and comprehensive analysis of the dependence of photo emission (PE) electronic band gap and optical absorption (OA) excitonic band gap on the size of CdSe QD, via connecting it with excitonic absorbance wavelength. Excitonic absorbance wavelength is determined through an empirical fit of established experimental evidences. Effective excitonic charge and Bohr radius is determined as a function of size. Increase in size of the CdSe QD results in greater Bohr radius and smaller effective excitonic charge. Excitonic binding energy as a degree of size of QD is also calculated which further relates with the difference in PE electronic and OA optical band gaps. It is also shown that with increase in size of CdSe QD, the excitonic binding energy decreases which consequently increases differences in two band gaps. Our results are very well comparable with the established results. Explanation for the origin of the unusual optical properties of CdSe QD has been also discussed.

Generalized Bloch wave functions of bulk structures, which are composed of not only propagating waves but also decaying and growing evanescent waves, are known to be essential for defining the open boundary conditions in the calculations of the electronic surface states and scattering wave functions of surface and junction structures. Electronic complex band structures being derived from the generalized Bloch wave functions are also essential for studying bound states of the surface and junction structures, which do not appear in conventional band structures. We present a novel calculation method to obtain the generalized Bloch wave functions of periodic bulk structures by solving a generalized eigenvalue problem, whose dimension is drastically reduced in comparison with the conventional generalized eigenvalue problem derived by Fujimoto and Hirose [Phys. Rev. B 67, 195315 (2003)]. The generalized eigenvalue problem derived in this work is even mathematically equivalent to the conventional one, and, thus, we reduce computational cost for solving the eigenvalue problem considerably without any approximation and losing the strictness of the formulations. To exhibit the performance of the present method, we demonstrate practical calculations of electronic complex band structures and electron transport properties of Al and Cu nanoscale systems. Moreover, employing atom-structured electrodes and jellium-approximated ones for both of the Al and Si monatomic chains, we investigate how much the electron transport properties are unphysically affected by the jellium parts.

Effect of ion irradiation on GaAs core-level electron binding energies and band structure