Band Structure Of Materials Research Articles

Investigation of self-consistent site-dependent DFT + U effect on electronic band structure and optical properties of SiFe2O4 spinel

The first-principle investigation of SiFe2O4 (SFO) spinel was performed with the help of a plane-wave pseudopotential technique within the generalized gradient approximation (GGA) and local density approximation (LDA) as implemented in Quantum Espresso Simulation package. The Electronic band structure and optical properties of SFO spinel-type material have been investigated and discussed in this paper. The calculated band structure reveals that SFO spinel-type material is a direct bandgap semiconductor. Using GGA + U and LDA + U the band gap value so obtained is 3.52 eV and 2.96 eV respectively. The contribution to valence and conduction bands due to different bands was analyzed on the basis of the total and partial density of state. The Optical properties of SFO spinel-type material have been calculated and discussed in detail. The real, and the imaginary, part of the complex dielectric constants is found to be 6.52 and 5.42 at energies of 3.44 eV and 6.21 eV respectively. The refractive index and the reflectivity index at zero energy value were found to be 1.88 and 10% respectively. We found that SFO spinel-type material has good properties for optical devices.

Topology of the second-order constitutive model based on the Boltzmann–Curtiss kinetic equation for diatomic and polyatomic gases

The topological aspects of fluid flows have long been fascinating subjects in the study of the physics of fluids. In this study, the topology of the second-order Boltzmann–Curtiss constitutive model beyond the conventional Navier–Stokes–Fourier equations and Stokes’s hypothesis was investigated. In the case of velocity shear, the topology of the second-order constitutive model was shown to be governed by a simple algebraic form. The bulk viscosity ratio in diatomic and polyatomic gases was found to play an essential role in determining the type of topology: from an ellipse to a circle, to a parabola, and then finally to a hyperbola. The topology identified in the model has also been echoed in other branches of science, notably in the orbits of planets and comets and Dirac cones found in electronic band structures of two-dimensional materials. The ultimate origin of the existence of the topology was traced to the coupling of viscous stress and velocity gradient and its subtle interplay with the bulk viscosity ratio. In the case of compression and expansion, the topology of the second-order constitutive model was also found to be governed by a hyperbola. The trajectories of solutions of two representative flow problems—a force-driven Poiseuille gas flow and the inner structure of shock waves—were then plotted on the topology of the constitutive model, demonstrating the indispensable role of the topology of the constitutive model in fluid dynamics.

Role of Exchange-Correlation Functional in Bulk Bismuth

Presence of Bismuth (Bi) leads to topologically nontrivial band structure in many materials, especially in topological insulators. Traditionally Bi is known to be a semimetal but, quite surprisingly, in a recent experiment bulk Bi has been found to be a superconductor below 0.53 mK at ambient pressure. In order to have a closer look at the electronic properties of bulk Bi in the wake of this unexpected experimental evidence of superconducting phase, we have performed density-functional-theory (DFT) based first principle calculations using plane-wave basis set and with suitable ionic pseudopotentials. We have computed the band structure, density of states and Fermi surfaces for two different type of exchange-correlation (XC) functionals, namely Perdew-Zunger (PZ) and Perdew-Burke-Ernzerhof (PBE) type. Each of these XC functional has been considered without and with spin orbit (SO) interaction. After carefully examining the energy-convergence with respect to plane wave basis set and k-points in each case, the band structure has been calculated along the path Γ-L-T-Γ. Without SO coupling, electron pocket is found near ‘L’ and exactly at ‘Г’ and hole pocket is at ‘T’ for PZ type XC functional, while in the case of PBE-type electron pocket is found exactly at ‘L’ but the hole pocket to be near to ‘T’. With SO coupling, in PZ-type, electron pocket remains at same position, but hole pocket appears only at ‘Г’ point. Finally, when SO coupling is taken into account along with PBE-type XC functional electrons and holes are found at ‘L’ and at ‘T’ respectively. Furthermore, in this case we also observe an increase in the number of holes at ‘T’.

Dirac terahertz plasmonics in two and three dimensions

Material science at the beginning of the twenty-first century has largely been dominated by the study of graphene and other Dirac-type materials. Interesting physics manifest in the linear electronic bandstructure of Dirac-type materials, more so on the behavior of photons interacting with it. Moreover, with the recent discovery of three-dimensional Dirac semimetals (3DS), dimensionality becomes an important factor in the overall terahertz response. In this brief review, we look at some of the established physics behind the terahertz plasmonic response in Dirac materials, the contributions from their linear electronic bandstructure and dimensionality, as well as the current progress in device design and implementation. In particular, the strong terahertz response of Dirac material would have important implications in the development of terahertz-based communication systems and Internet-of-Things applications.

Periodic Electronic Structure Calculations with the Density Matrix Embedding Theory.

We developed a periodic version of density matrix embedding theory, DMET, with which it is possible to perform electronic structure calculations on periodic systems and compute the band structure of solid-state materials. Electron correlation can be captured by means of a local impurity model using various wave function methods, such as full configuration interaction, coupled cluster, and multiconfigurational methods. The method is able to describe not only the ground-state energy but also the quasiparticle band picture via the real space-momentum space implementation. We investigate the performance of periodic DMET in describing the ground-state energy as well as the electronic band structure for one-dimensional solids. Our results show that DMET is in good agreement with other many-body techniques at a cheaper computational cost. We anticipate that periodic DMET can be a promising first principle method for strongly correlated materials.

Pressure-induced topological phase transition in noncentrosymmetric elemental tellurium

Recent progress in understanding the electronic band topology and emergent topological properties encourage us to reconsider the band structure of well-known materials including elemental substances. Controlling such a band topology by external field is of particular interest from both fundamental and technological viewpoints. Here we report possible signatures of the pressure-induced topological phase transition from a semiconductor to a Weyl semimetal in elemental tellurium probed by transport measurements. Pressure variation of the periods of Shubnikov-de Haas oscillations, as well as oscillation phases, shows an anomaly around the pressure theoretically predicted for topological phase transition. This behavior is consistent with the pressure-induced band deformation and resultant band-crossing effect. Moreover, effective cyclotron mass is reduced toward the critical pressure, potentially reflecting the emergence of massless linear dispersion. The present result paves the way for studying the electronic band topology in well-known compounds and topological phase transition by the external field.

HICOLM: High-Performance Platform of Physical Simulations by Using Low Computational Cost Methods

For decades, computational simulation models have been used by scientists in search for new materials with technological applications in several areas of knowledge. For this, software based on several theoretical-computational models were developed in order to obtain an analysis of the physical properties at atomic levels. The objective of this work is proposing a widely functional software to analyze the physical properties of nanostructures based on carbon and condensed systems using theories of low computational cost. Therefore, a Fortran language computational program called HICOLM was developed, whose theoretical bases are based on two commonly known models (Tight-binding and Molecular Dynamics). The physical properties of condensed systems can be obtained in the thermodynamic equilibrium in several statistical ensembles, and possible to obtain an analysis of the properties of the material and its evolution in the time-dependent on its thermodynamic conditions like temperature and pressure. Moreover, from the tight-binding model, the HICOLM program is also capable of performing a physical analysis of carbon-based nanostructures from the calculation of the material band structure.

Electronic band structure of cuprous and silver halides: An all-electron GW study

Group IB metal halides (CuX and AgX, X=Cl, Br and I) are widely used in optoelectronic devices and photochemical catalysis due to their appropriate optical and electronic properties. First-principles calculations have confronted difficulties in accurately predicting their electronic band structures. Here we study CuX and AgX with many-body perturbation theory in the $GW$ approximation, implemented in the full-potential linearized augmented plane waves (FP-LAPW) framework. Comparing the quasi-particle band structures calculated with the default LAPW basis and the one extended by high-energy local orbitals (HLOs), denoted as LAPW+HLOs, we find that it is crucial to include HLOs to achieve sufficient numerical accuracy in $GW$ calculations of these materials. Using LAPW+HLOs in semi-local density functional approximation based $GW_0$ calculations leads to good agreement between theory and experiment for both band gaps and the splitting between metal (Cu or Ag) $d$ and X-$p$ states. It is indicated that quasi-particle band structures of CuX and AgX are crucially influenced by the numerical accuracy of $GW$ implementations, similar to what was found in ZnO [Jiang, H.; Blaha, P. Phys. Rev. B 2016, 93, 115203]. This work emphasizes the importance of numerical accuracy in the description of unoccupied states for quasi-particle band structure of materials with the $d^{10}$ electronic configuration.

First-principles investigation of structural, electronic, optical and thermal properties of Zinc doped SrTiO3

This work reports the first-principles study, based on the density functional theory (DFT), by using generalized gradient approximation (GGA) and ultra-soft pseudo-potential (USP), to explore effects of zinc (Zn) doping on structural, electronic, optical and thermal properties of cuboctahedral SrTiO3. We observe significant reduction in unit cell volume upon doping Zn into SrTiO3. Furthermore, Zn doping introduces new sates at Brillouin zone symmetry points turning the indirect band gap of host material into direct one. Replacement of Zn with Sr in host lattice repositions density of states at lower energies resulting in stronger interactions between Zn-atom and its neighbors as compared to interactions between Sr-atom and its surroundings. This refers to substantial modification in electronic band structure of host material by Zn doping. Physical properties of SrTiO3 also change significantly upon Zn doping in accordance with the electronic band structure. Significant changes in electronic structure and properties of SrTiO3 by Zn doping opens new prospects for potential applications of these materials in optoelectronics.

Supramolecular self-assembly production of porous carbon nitride nanosheets with excellent photocatalytic activity by a melamine derivative as doping molecule

Since doping has a good effect on improving the photocatalytic performance of carbon nitride (CN), the doping scheme has been extensively studied. In this work, a melamine derivative (2, 4-diamino-6-phenoxy-[1, 3, 5]-triazine, MA-phO) was synthesized and used as a new doping molecule to prepare the modified-CN materials by a facile supramolecular self-assembly method with melamine (MA) and cyanuric acid (CA). The “growth stoppers” effect caused by the doping molecule MA-phO in the supramolecular complex made the modified-CN materials more favorable for forming nanostructure and increasing the specific surface areas. The decomposition of phenyl group allowed C atoms to be incorporated into the CN skeleton which facilitated the separation of photogenerated h+/e− pairs. What's more, the addition of MA-phO could regulate the energy band structure of modified-CN materials. It has been found that the amount of doping molecule affected the photocatalytic performance of photocatalyst. Especially when the doping amount reached 0.5%, the prepared sample had the best photocatalytic activity for the organic pollutants. Significantly, design and synthesis of new melamine derivatives as doping molecules and pretreatment by using supramolecular self-assembly strategy may provide a facile approach for the preparation of tunable morphology, energy band structure and heteroatom-doped g-C3N4.

(Invited) Manipulation of the Energy Band Levels in Plasma-Produced Energy Materials

Progress in energy conversion and energy storage strongly rely on our ability to produce “new” and “old” materials with improved properties. Materials properties are always closely linked to the synthesis process and materials integration in application devices is also critical to the overall performance. Therefore, materials synthesis, properties and device integration are intimately interrelated [1]. In addition, the electronic energy band structure of materials largely determines their behaviour and impact the materials performance in application devices [2-4]. For instance, the bandgap of semiconductor is crucial for a large variety of applications in solar energy conversion. However, the full energy band diagram (EBD) requires accurate knowledge of a range of other critical parameters in addition to bandgap. These include the conduction and valence band edge location and Fermi level; together these are decisive for a very wide range of applications and the ability to manipulate the corresponding energy levels represent an attractive opportunity. Quantum confinement in nanoscale materials offers one possible avenue for the manipulation of the EBD whereby bandgap engineering, the ability to tailor the bandgap of nanoscale semiconductors, has inspired a considerable excitement at the visionary technological opportunities it presents. However, the impact of quantum confinement on the other energy band parameters has received little attention, in part due to measurement challenges as well as due to the higher sensitivity and precision required. Another potential avenue for EBD manipulation is also offered through doping, alloying as well as introducing “defects” in the crystal structure of materials. These, and in particular “defects”, are also as effective as quantum confinement strategies to modify energy levels to suit specific application. The synthesis of such materials, i.e. with strong quantum confinement and/or with engineered defects, has been explored and widely reported. However, many approaches have also revealed limitations closely bound to thermodynamic limit of solubility or crystal structure stability. Plasma-based synthesis however can break through the boundaries of these limitations and achieve composition that are beyond those expected by phase diagrams for instance. More specifically, atmospheric pressure microplasmas (APMs) are highly versatile and amenable to explore materials with untested composition or “exotic” structures and morphologies [5-7]. APMs can therefore contribute to progress materials science by pushing the boundaries beyond current synthetic capabilities. APM processes can be easily integrated in the application devices fabrication steps so that materials properties can be verified within the relevant application context [8-10]. Here we will report on various synthesis achievements with APMs demonstrating first the synthesis of both strongly quantum confined structures as well as materials with unexpected doping levels and chemical compositions [11-14]. We will then provide preliminary results on the ability of controlling defects in metal oxides. Corresponding changes in the EBD parameters will be evaluated, compared and discussed with a range of analytical techniques. These results will also offer the opportunity to discuss challenges in the measurement methods. Acknowledgments This work is supported by EPSRC (awards n. EP/R023638/1, EP/M024938/1, EP/M015211/1) References [1] Rocks C, Švrček V, Velusamy T, Macias-Montero M, Maguire P, Mariotti D: Nano Energy 50 (2018) 245 [2] Bera D, Qian L, Tseng TK, Holloway PH: Materials (Basel), 2010, 3, 2260. [3] Capasso F, Science 235 (1987), 172 [4] Smith AM, Nie S: Acc. Chem. Res. 43 (2010) 190 [5] Mariotti D, Belmonte T, Benedikt J, Velusamy T, Jain G, Švrček V: Plasma Processes and Polymers 13 (2016) 70 [6] Maguire P, Rutherford D, Macias-Montero M, Mahony C, Kelsey C, Tweedie M, Perez-Martin F, McQuaid H, Diver D, Mariotti D: Nano Letters 17 (2017) 1336 [7] Askari S, Mariotti D, Stehr JE, Benedikt J, Keraudy J, Helmersson U: Nano Letters 18 (2018) 5681 [8] Švrček V, Mariotti D, Shibata Y, Kondo M: Journal of Physics D: Applied Physics 43 (2010) 415402 [9] McKenna J, Patel J, Mitra S, Soin N, Švrček V, Mariotti D: The European Physical Journal-Applied Physics 56 (2011) 24020 [10]Švrček V, Kondo M, Kalia K, Mariotti D: Chemical Physics Letters 478 (2009) 224 [11]Ni C, Carolan D, Rocks C, Hui J, Fang Z, Padmanaban DB, Ni J, Xie D, Maguire P, Irvine JTS, Mariotti D: Green Chemistry 20 (2018) 2101 [12]Haq AU, Askari S, McLister A, Rawlinson S, Davis J, Chakrabarti S, Svrcek V, Maguire P, Papakonstantinou P, Mariotti D Nature Communications 10 (2019) 817 [13]Askari S, Haq AU, Macias-Montero M, Levchenko I, Yu F, Zhou W, Ostrikov K, Maguire P, Švrček V, Mariotti D: Nanoscale 8 (2016) 17141 [14]Askari S, Švrček V, Maguire P, Mariotti D: Advanced Materials 27 (2015) 8011

Symmetry indicators for topological superconductors

The systematic diagnosis of band topology enabled by the method of "symmetry indicators" underlies the recent advances in the search for new materials realizing topological crystalline insulators. Such an efficient method has been missing for superconductors because the band structure in the superconducting phase is not usually available. In this work, we establish symmetry indicators for weak-coupling superconductors that detect nontrivial topology based on the representations of the metallic band structure in the normal phase, assuming a symmetry property of the gap function. We demonstrate the applications of our formulas using examples of tight-binding models and density-functional-theory band structures of realistic materials.

Proximity exchange effects in MoSe2 and WSe2 heterostructures with CrI3 : Twist angle, layer, and gate dependence

Proximity effects in two-dimensional (2D) van der Waals heterostructures offer controllable ways to tailor the electronic band structure of adjacent materials. Exchange proximity in particular is important for making materials magnetic without hosting magnetic ions. Such synthetic magnets could be used for studying magnetotransport in high-mobility 2D materials, or magneto-optics in highly absorptive nominally nonmagnetic semiconductors. Using first-principles calculations, we show that the proximity exchange in monolayer MoSe$_2$ and WSe$_2$ due to ferromagnetic monolayer CrI$_3$ can be tuned (even qualitatively) by twisting and gating. Remarkably, the proximity exchange remains the same when using antiferromagnetic CrI$_3$ bilayer, paving the way for optical and electrical detection of layered antiferromagnets. Interestingly, the proximity exchange is opposite to the exchange of the adjacent antiferromagnetic layer. Finally, we show that the exchange proximity is confined to the layer adjacent to CrI$_3$, and that adding a separating hBN barrier drastically reduces the proximity effect. We complement our it ab initio results with tight-binding modeling and solve the Bethe-Salpeter equation to provide experimentally verifiable optical signatures (in the exciton spectra) of the proximity exchange effects.

Time-resolved\xa0XUV ARPES with tunable 24\u201333\u2009eV laser pulses at 30\u2009meV resolution

High harmonic generation of ultrafast laser pulses can be used to perform angle-resolved photoemission spectroscopy (ARPES) to map the electronic band structure of materials with femtosecond time resolution. However, currently it is difficult to reach high momenta with narrow energy resolution. Here, we combine a gas phase extreme ultraviolet (XUV) femtosecond light source, an XUV monochromator, and a time-of-flight electron analyzer to develop XUV-based time-resolved ARPES. Our technique can produce tunable photon energy between 24–33 eV with an unprecedented energy resolution of 30 meV and time resolution of 200 fs. This technique enables time-, energy- and momentum-resolved investigation of the nonequilibrium dynamics of electrons in materials with a full access to their first Brillouin zone. We evaluate the performance of this setup through exemplary measurements on various quantum materials, including WTe2, WSe2, TiSe2, and Bi2Sr2CaCu2O8+δ.

Complex frequency band structure of periodic thermo-diffusive materials by Floquet–Bloch theory

This work deals with the micromechanical study of periodic thermo-diffusive elastic multi-layered materials, which are of interest for the fabrication of solid oxide fuel cells. The focus is on the dynamic regime that is investigating the dispersive wave propagation within the periodic material. In this framework, a generalization of the Floquet–Bloch theory is adopted, able to determine the complex band structure of such materials. The infinite algebraic linear system, obtained by exploiting both bilateral Laplace transform and Fourier transform, is replaced by its finite counterpart, resulting from a proper truncation at a finite number of considered unknowns and equations. A regularization technique is herein useful to get rid of the Gibbs phenomenon. The solution of the problem is, finally, found in terms of complex angular frequencies, corresponding to a finite sequence of eigenvalue problems for given values of the wave vector. The paper is complemented by numerical examples taking into account thermo-mechanical coupling. The frequency band structure of the periodic thermo-diffusive elastic material is found to be strongly influenced by the interaction between thermal and mechanical phenomena.

Thermionic emission laws for general electron dispersion relations and band structure data

In this article, a thermionic electron emission theory for general electron dispersion relations E(k) is presented, relating electron energy E to wave-vector magnitude k = |k|. This theory does not require the construction of a model Hamiltonian for the electrode's materials, like Dirac or Weyl Hamiltonians. Instead, use is made of the material's band structure data, e.g., the parabolic E(k) approximation for the Richardson–Dushman equation and linear E(k), as used for graphene and 3D Dirac semimetals. This new theory confirms previous findings on parabolic E(k), e.g., that the emission current is independent of effective electron mass in the material as long as it is larger than real electron mass m0. For effective mass lower than m0, the emission is reduced and tends to zero for vanishing effective mass. For materials with negative electron affinity, additional terms arise in the emission current equation. It turns out that the linear E(k) dispersion, e.g., for Dirac semimetals, does not have the potential to surpass the Richardson emission in materials with the same work function. In addition, a more rigorous electron emission theory is established by utilizing real anisotropic band structure data En(k) for electrode materials. For collimated electron emission normal to the surface, the transverse electron velocities tend to zero, i.e., the transverse derivatives of En(k) have to be comparatively small. If stable electrode materials of this kind can be realized, a considerable increase of electron emission by a factor 100 or more can be achieved, compared to the Richardson–Dushman theory, especially for small lattice constants perpendicular to the emission direction.

Engineered ‘Nanomaterials by design’ theoretical studies experimental validations current and future prospects

Modulating the electron energy band structure of a nano crystalline material by varying its size, shape, and constituent species amounts to practically designing the nano size material building blocks for arriving at a known set of related physico-chemical properties in terms of the internal electronic structures for a given organization of the constituent species via covalent and non-covalent interactions operating at different length scales. In order to explore further possibilities of using synergistic combinations of nano structured materials derived from inorganic, organic and polymeric species particularly knowing through their chemical bonds involved in different forms, it is equally necessary to know about the interaction pathways among the constituent species, as mentioned above, in addition to the biomolecular species, where they form a variety of 3-d supramolecular organizations arising out of self-assembly and self-organization. After having a clear picture of these basic processes involved in the internal and external organization of the hierarchical supramolecular structures, the next step is to explore the prospects of incorporating some sort of intelligent features starting from using the biomolecular species like polypeptides, proteins and enzymes. What is emerging from the current developments taking place in the related areas can be foreseen from this review particularly viewed from material science point of view.

Superstructure-Induced Splitting of Dirac Cones in Silicene.

Atomic scale engineering of two-dimensional materials could create devices with rich physical and chemical properties. External periodic potentials can enable the manipulation of the electronic band structures of materials. A prototypical system is (3×3)-silicene/Ag(111), which has substrate-induced periodic modulations. Recent angle-resolved photoemission spectroscopy measurements revealed six Dirac cone pairs at the Brillouin zone boundary of Ag(111), but their origin remains unclear [Proc. Natl. Acad. Sci. USA 113, 14656 (2016)]. We used linear dichroism angle-resolved photoemission spectroscopy, the tight-binding model, and first-principles calculations to reveal that these Dirac cones mainly derive from the original cones at the K (K^{'}) points of free-standing silicene. The Dirac cones of free-standing silicene are split by external periodic potentials that originate from the substrate-overlayer interaction. Our results not only confirm the origin of the Dirac cones in the (3×3)-silicene/Ag(111) system, but also provide a powerful route to manipulate the electronic structures of two-dimensional materials.

Electronic band structures of group-V two-dimensional materials

We systematically study the electronic properties of two-dimensional group-V materials, i.e. phosphorene, arsenene, antimonene, and bismuthene. The density functional calculations are performed using generalized gradient approximation. We first clarify that the α structure is the most stable in the cases of phosphorene and bismuthene and the β structure is the most stable in the cases of arsenene and antimonene. We next analyze the band structures based on group theory. As a result, we find that all the bands are doubly degenerated at the boundaries of the first Brillouin zone in the α structures and the degeneracies are due to the symmetry of the materials. The band gaps of the β structures are found to be larger than those of the α structures. This tendency in the band gaps is explained based on the fact that the band gap increases as the sp hybridization becomes large. We find that buckling occurs in the α structures of antimonene and bismuthene due to the electron transfer from the higher atom to the lower atom.

First-principles calculation of influence of nitrogen substituting for oxygen on the crystal structures and electronic band structures of Sr3MgSi2O8-σNσ

In this work, influence of the nitrogen substituting for oxygen on the crystal structures and electronic band structures of Sr3MgSi2O8-σNσ (σ = 0, 0.125 and 0.250) materials had been investigated by the first-principles calculation. For the initial Sr3MgSi2O8 without nitrogen substitution, the band gap is about 5.889 eV between the conduction band and valence band. Analysis of its density of states (DOS) indicates that the conduction band is mainly constituted by Sr-d orbit. The valence band is mainly constituted by O-p orbit. After the nitrogen substituting for oxygen occurs, the band gap gradually decreases to 4.421 eV in the case σ = 0.250. Top of the valence band is still locating near 0 eV while bottom of the conduction band drops down to 4.421 eV. On the other hand, it can be observed that the N-p orbit hybridized into the top of the valence band which induced a new band above the O-p orbit. For Sr3MgSi2O8-σNσ materials with rare earth ions or transition metal ions serving as luminescence centres, this condition would influence the charge transfer process between the host materials and the luminescence centres. In addition, the energy needed for the substitution of one nitrogen anion for one oxygen anion is calculated to 89.50 eV. This illuminates clearly that the preparation condition of nitrogen oxide compounds is harsh. This work is helpful for better understanding the influence of nitrogen substituting for oxygen on the crystal structures and electronic band structures of Sr3MgSi2O8-σNσ materials.