Band Structure Of Materials Research Articles

Auger losses in dilute InAsBi

Density functional theory is used to determine the electronic band structure and eigenstates of dilute InAsBi bulk materials. The results serve as input for fully microscopic many-body models calculating the composition and carrier density dependent losses due to Auger recombination. At low to intermediate carrier concentrations, the Auger loss coefficients are found to be in the range of 10−27cm6/s for a low Bi content and around 10−25cm6/s for compositions suitable for long wavelength emission. It is shown that due to the fact that in InAsBi, the spin-orbit splitting is larger than the bandgap for all Bi contents, the Bi-dependent increase in the spin-orbit splitting does not lead to a significant suppression of the losses. Instead, unlike in GaAsBi, a mostly exponential increase in the losses with the decreasing bandgap is found for all compositions.

Influence of the Diagonal and Off-Diagonal Electron–Phonon Interactions on the Formation of Local Polarons and Their Band Structure in Materials with Strong Electron Correlations

For systems with strong electron correlations and strong electron–phonon interaction, we analyze the electron–phonon interaction in local variables. The effects of the mutual influence of electron–electron and electron–phonon interactions that determine the structure of local Hubbard polarons are described. Using a system containing copper–oxygen layers as an example, we consider the competition between the diagonal and off-diagonal interactions of electrons with the breathing mode as the polaron band structure is formed within a corrected formulation of the polaron version of the generalized tight-binding method. The band structure of Hubbard polarons is shown to depend strongly on the temperature due to the excitation of Franck–Condon resonances. For an undoped La2CuO4 compound we have described the evolution of the band structure and the spectral function from the hole dispersion in an antiferromagnetic insulator at low temperatures with the valence band maximum at point (π/2, π/2) to the spectrum with the maximum at point (π, π) typical for the paramagnetic phase. The polaron line width at the valence band top and its temperature dependence agree qualitatively with angle-resolved photoemission spectroscopy for undoped cuprates.

Size-induced axial band structure and directional flow of a ternary-size granular material in a 3-D horizontal rotating drum

Differences in the material property of the granular material induce segregation which inevitably influences both natural and industrial processes. To understand the dynamical segregation behavior, the band structure, and also the spatial redistribution of particles induced by the size differences of the particles, a ternary-size granular mixture in a three-dimensional rotating drum operating in the rolling flow regime is numerically simulated using the discrete element method. The results demonstrate that (i) the axial bands of the medium particles are spatially sandwiched in between those of the large and small ones; (ii) the total mass in the active and passive regions is a global parameter independent of segregation; (iii) nearly one-third of all the particles are in the active region, with the small particles having the highest mass fraction; (iv) the axial bands initially appear near the end wall, then become wider and purer in the particular species with time as more axial bands form toward the axial center; and (v) the medium particle type exhibits segregation later and has the narrowest axial bandwidth and least purity in the bands. Compared to the binary-size system, the presence of the medium particle type slightly increases the total mass in the active region, leads to larger mass fractions of the small and large particle types in the active region, and enhances the axial segregation in the system. The results obtained in the current work provide valuable insights regarding size segregation, and band structure and formation in the rotating drum with polydisperse particles.

A Perspective on the Application of Spatially Resolved ARPES for 2D Materials.

In this paper, a perspective on the application of Spatially- and Angle-Resolved PhotoEmission Spectroscopy (ARPES) for the study of two-dimensional (2D) materials is presented. ARPES allows the direct measurement of the electronic band structure of materials generating extremely useful insights into their electronic properties. The possibility to apply this technique to 2D materials is of paramount importance because these ultrathin layers are considered fundamental for future electronic, photonic and spintronic devices. In this review an overview of the technical aspects of spatially localized ARPES is given along with a description of the most advanced setups for laboratory and synchrotron-based equipment. This technique is sensitive to the lateral dimensions of the sample. Therefore, a discussion on the preparation methods of 2D material is presented. Some of the most interesting results obtained by ARPES are reported in three sections including: graphene, transition metal dichalcogenides (TMDCs) and 2D heterostructures. Graphene has played a key role in ARPES studies because it inspired the use of this technique with other 2D materials. TMDCs are presented for their peculiar transport, optical and spin properties. Finally, the section featuring heterostructures highlights a future direction for research into 2D material structures.

Doubly Screened Hybrid Functional: An Accurate First-Principles Approach for Both Narrow- and Wide-Gap Semiconductors.

First-principles prediction of electronic band structures of materials is crucial for rational material design, especially in solar-energy-related materials science. Hybrid functionals that mix the Hartree-Fock exact exchange with local or semilocal density functional approximations have proven to be accurate and efficient alternatives to more sophisticated Green's function-based many-body perturbation theory. The optimal fraction of the exact exchange, previously often treated as an empirical parameter, is closely related to the screening strength of the system under study. From a physical point of view, the screening has two extreme forms: the dielectric screening [1/ϵM] that is dominant in wide-gap materials and the Thomas-Fermi metallic screening [exp(-ζ r) ] that is important in narrow-gap semiconductors. In this work, we have systematically investigated the performances of a nonempirical doubly screened hybrid (DSH) functional that considers both screening mechanisms and found that it excels all other existing hybrid functionals and describes the band gaps of narrow-, medium-, and wide-gap insulating systems with comparably good performances.

(Invited) Constructing Efficient Photocatalysts Based on the Bonding Difference

Solar-driven photocatalysis represents a promising pathway of utilizing intermittent solar energy to produce storable chemical fuels from water splitting/CO2 reduction or to decompose pollutants in air/water environment. Photocatalysis can be divided into three basic steps, namely light absorption, charge separation and surface catalysis. Most stable photocatalysts suffer from the narrow visible light absorption range and/or high recombination probability of photogenerated charge carriers. Various strategies have been developed to address these challenges in the past decades. Controlling the compositions/defects of photocatalysts is an essential way of changing band structure of materials for a wide visible light absorption. Our ongoing studies on this topic demonstrate the substantial role of homogeneous distribution of electronic structure modifiers (dopants, defects, or disordering) in realizing band-to-band redshift of the absorption edge towards visible light region. Besides taking advantage of the natural diffusion pathways of some materials for electronic structure modifiers, another strategy we developed is weakening strong cation-anion bonds for an easy substitutional doping or introduction of anion vacancies. Some progress in producing a series of red anatase TiO2 and graphitic carbon nitride with strong visible light absorption has achieved. Constructing heterostructures of photocatalysts represents an important way of promoting the separation of photogenerated charge carriers. On the basis of the different bonding nature of different bonds in complex oxides, a series of complex oxide-binary oxide heterostructures were prepared by an in-situ process. As a consequence, the promoted charge separation is responsible for the enhanced photocatalytic activity.

An electrochemically modulated single-walled carbon nanotube network for the development of a transparent flexible sensor for dopamine

Transparent and flexible single-walled carbon nanotube (SWCNT) network electrodes were used for the determination of neurotransmitters. The surface of SWCNT network film was oxidised by multiple electrochemical anodic scans, thereby the electronic structure of SWCNTs network film was modulated with p-typed doping. Herein, we analysed the electronic structure in-depth by resonant Raman spectroscopy with two different wavelength lasers (514 and 633 nm) and UV–vis-NIR absorption spectroscopy. From this analysis, it is found that the electrochemical oxidation process selectively occurred on metallic SWCNTs surface in the mixture of metallic and semiconducting SWCNTs. Interestingly, after electrochemical oxidation, the charge transfer resistance was observed to have decreased by 52%, compared to that of a SWCNT network electrode before treatment (4.7 kΩ). This is probably due to the suppression of Schottky barrier of junctions between metallic and semiconducting SWCNT bundles. The enhanced electrocatalytic activity of a SWCNT network film by electrochemical doping (Eox-SWCNT network film) has enabled discrimination of dopamine (DA) from a mixture with ascorbic acid (AA) and uric acid (UA). The limit of detection was estimated to be 0.51 μM DA in the presence of 100 μM AA and 10 μM UA, and the DA sensing activity was not significantly affected even using a bent configuration of Eox-SWCNT network film. The simple and eco-friendly electrochemical doping with multiple cyclic voltammetric scans can be a powerful tool to modulate the electronic energy in the band structure of carbon-related materials.

Realization of a topological phase transition in a gyroscopic lattice

Topological metamaterials exhibit unusual behaviors at their boundaries, such as unidirectional chiral waves, that are protected by a topological feature of their band structure. The ability to tune such a material through a topological phase transition in real time could enable the use of protected waves for information storage and readout. Here we dynamically tune through a topological phase transition by breaking inversion symmetry in a metamaterial composed of interacting gyroscopes. Through the transition, we track the divergence of the edge modes' localization length and the change in Chern number characterizing the topology of the material's band structure. This work provides a new axis with which to tune the response of mechanical topological metamaterials.

Topological classification of Chern-type insulators by means of the photonic Green function

The Chern topological numbers of a material system are traditionally written in terms of the Berry curvature which depends explicitly on the material band structure and on the Bloch eigenwaves. Here, we demonstrate that it is possible to calculate the gap Chern numbers of a photonic platform without having any detailed knowledge of its band structure, relying simply on the system photonic Green function. It is shown that the gap Chern number is given by an integral of the photonic Green function along a line of the complex frequency plane parallel to the imaginary axis. Our theory applies to arbitrary frequency dispersive fully three-dimensional photonic crystals, as well as to the case of electromagnetic continua with no intrinsic periodicity.

Tunable Lifshitz Transitions and Multiband Transport in Tetralayer Graphene.

As the Fermi level and band structure of two-dimensional materials are readily tunable, they constitute an ideal platform for exploring the Lifshitz transition, a change in the topology of a material's Fermi surface. Using tetralayer graphene that host two intersecting massive Dirac bands, we demonstrate multiple Lifshitz transitions and multiband transport, which manifest as a nonmonotonic dependence of conductivity on the charge density n and out-of-plane electric field D, anomalous quantum Hall sequences and Landau level crossings that evolve with n, D, and B.

Gd impurities effect on $$\hbox {Co}_{2}\hbox {CrSi}$$ Co 2 CrSi alloy: first-principle calculations

First-principle calculations have been performed to study Gd impurities doping effect on the physical properties of the Heusler half-metallic ferromagnet \(\hbox {Co}_{2}\hbox {CrSi}\) using the density functional theory in the local spin density approximation with an additional Hubbard correlation term for the rare-earth 4f states. The results show that the gadolinium moment is aligned antiparallel to that of transition metal atoms for both \(\hbox {Co}_{16} \hbox {Cr}_{7} \hbox {GdSi}_{8}\) and \(\hbox {Co}_{15}\hbox {GdCr}_{8}\hbox {Si}_{8}\). The analysis of the doped material band structures shows that the half-metallic properties are completely conserved if Gd substitutes Cr atoms, while the minority-spin gap is filled and half-metallicity is lost when Gd substitutes Co atoms.

Quantum multicriticality in disordered Weyl semimetals

In electronic band structure of solid state material, two band touching points with linear dispersion appear in pair in the momentum space. When they annihilate with each other, the system undergoes a quantum phase transition from three-dimensional Weyl semimetal (WSM) phase to a band insulator phase such as Chern band insulator (CI) phase. The phase transition is described by a new critical theory with a `magnetic dipole' like object in the momentum space. In this paper, we reveal that the critical theory hosts a novel disorder-driven quantum multicritical point, which is encompassed by three quantum phases, renormalized WSM phase, CI phase, and diffusive metal (DM) phase. Based on the renormalization group argument, we first clarify scaling properties around the band touching points at the quantum multicritical point as well as all phase boundaries among these three phases. Based on numerical calculations of localization length, density of states and critical conductance distribution, we next prove that a localization-delocalization transition between the CI phase with a finite zero-energy density of states (zDOS) and DM phase belongs to an ordinary 3D unitary class. Meanwhile, a localization-delocalization transition between the Chern insulator phase with zero zDOS and a renormalized Weyl semimetal (WSM) phase turns out to be a direct phase transition whose critical exponent $\nu=0.80\pm 0.01$. We interpret these numerical results by a renormalization group analysis on the critical theory.

Defect state of indium-doped bismuth molybdate nanosheets for enhanced photoreduction of chromium(vi) under visible light illumination.

The construction of defect states is an effective method for regulating the energy band structure of photocatalytic semiconductor materials. Establishing defect states effectively is a particularly significant strategy to improve the photoelectric conversion efficiency. Herein, we report a peculiar method to manufacture defect states in indium-doped Bi2MoO6 by changing the valence state of the doped indium element. By increasing the amount of doped indium, different doping forms and valence states are produced, which causes distortion of crystal structure, generation of oxygen vacancies and variation in the elemental valence state. Specifically, when Bi is substituted with In(3-x)+, the acceptor energy level becomes higher than the valence band due to the p-type-like doping, which widens the width of the valence band and drives the up-shift of the valence band edges, resulting in narrowed bandgap and improved photoresponse ability. In addition, the photoelectrochemical performance tests using the In-doped Bi2MoO6 show that low-valent indium doping effectively improves the carrier separation efficiency, reduces the transmission impedance, and greatly improves the photocatalytic reduction performance of Cr(vi). This study provides a new insight to the design of efficient photocatalysts for the green and sustainable treatment of Cr(vi) pollution.

Impact of Coinage Metal Insertion on the Thermoelectric Properties of GeTe Solid-State Solutions

A comprehensive study on the thermoelectric effect of Ag substitution in GeTe solid solutions, a congenital base for highly efficient TAGS-m [(GeTe)m(AgSbTe2)100–m] thermoelectric materials, was performed. First-principles calculations were carried out to probe the changes arising from doping on the electronic band structure of GeTe, which exhibits a rhombohedral (r) structure at temperatures lower than 700 K. Aliovalent Ag substitution in GeTe increases the hole concentration and decreases the thermoelectric figure of merit (zT) due to the reduction of the Seebeck coefficient, which is ascribed mainly to the lowering of the Fermi level together with the loss of band degeneracy. The band structure and effective mass calculations of these doped materials also point to a soaring contribution from several hole pockets in the valence band. First-principles calculations carried out with two other group 11 transition metals (Cu, Au) reveal that silver substitution has the lowest impact on the thermopower of r-GeTe. A mean zT of ∼0.85 at 773 K is achieved for Ge1–xAgxTe (x ≤ 0.04) solid solutions. The study highlights the limits in doping just a coinage metal to GeTe and recapitulates the need for pair substitution to enhance the thermoelectric properties of GeTe-based solid-state solutions.

Effects of High-Pressure High-Temperature Sintering on the Band Gap and Thermoelectric Properties of PbSe

In this study, PbSe bulk samples were prepared by a high-pressure high-temperature (HPHT) sintering technique, and the phase compositions, band gaps and thermoelectric properties of the samples were systematically investigated. The sintering pressure exerts a significant influence on the preferential orientation, band gap and thermoelectric properties of PbSe. With increasing pressure, the preferential orientation decreases, mainly due to the decreased crystallinity, while the band gap first decreases and then increases. The electrical conductivity and power factor decrease gradually with increasing pressure, mainly attributed to the decreased carrier concentration and mobility. Consequently, the sample prepared by 2 GPa shows the highest thermoelectric figure-of-merit, ZT, of 0.55 at ∼ 475 K. The ZT of the HPHT-sintered PbSe could be further improved by properly doping or optimizing the HPHT parameters. This study further demonstrates that the sintering pressure could be another degree of freedom to manipulate the band structure and thermoelectric properties of materials.

An isogeometric approach for analysis of phononic crystals and elastic metamaterials with complex geometries

An isogeometric analysis (IGA) framework is presented to construct and solve dispersion relations for generating the band structure of periodic materials with complicated geometries representing phononic crystals and elastic metamaterials. As the dispersive properties depend on the microstructural geometry, an accurate representation of microstructural geometrical features is paramount. To this end, the ability of isogeometric analysis to exactly model complex curved geometries is exploited, and wave propagation in infinitely periodic solids is combined with isogeometric analysis. The benefits of IGA are demonstrated by comparing the results to those obtained using standard finite element analysis (FEA). It is shown that the IGA solutions can reach the same level of accuracy as FEA while using significantly fewer degrees of freedom. IGA is applied to phononic crystals and elastic metamaterials and the band structure for a variety of unit cells with complex microstructural geometries is investigated to illustrate the desirable dispersive effects in these metamaterials.

High Performance Thermoelectric Materials: Progress and Their Applications

AbstractThermoelectric (TE) materials have the capability of converting heat into electricity, which can improve fuel efficiency, as well as providing robust alternative energy supply in multiple applications by collecting wasted heat, and therefore, assisting in finding new energy solutions. In order to construct high performance TE devices, superior TE materials have to be targeted via various strategies. The development of high performance TE devices can broaden the market of TE application and eventually boost the enthusiasm of TE material research. This review focuses on major novel strategies to achieve high‐performance TE materials and their applications. Manipulating the carrier concentration and band structures of materials are effective in optimizing the electrical transport properties, while nanostructure engineering and defect engineering can greatly reduce the thermal conductivity approaching the amorphous limit. Currently, TE devices are utilized to generate power in remote missions, solar–thermal systems, implantable or/wearable devices, the automotive industry, and many other fields; they are also serving as temperature sensors and controllers or even gas sensors. The future tendency is to synergistically optimize and integrate all the effective factors to further improve the TE performance, so that highly efficient TE materials and devices can be more beneficial to daily lives.

Finite size and volume effects in line node semimetals: A first-principles investigation

We systematically study the bulk and finite size effects on the band structure of prototypical line node materials using density functional theory based computations. For the bulk system, we analyze quantum oscillations with changes in the Fermi surface topology. We show that ultra thin slabs are gapped, with the first signatures of the line node appearing beyond a critical thickness. Further, motivated by possibility of tuning the bulk line node, we find that the line node radius is rather sensitive to bulk volume change. Based on this observation, we propose that application of pressure or suitable substrate engineering can be used as an effective means to control and tune the line node and the accompanying surface drumhead states.

Band structure of a two-dimensional Dirac semimetal from cyclotron resonance

Knowing the band structure of materials is one of the prerequisites to understand their properties. Therefore, especially in the last decades, angle-resolved photoemission spectroscopy (ARPES) has become a highly demanded experimental tool to investigate the band structure. However, especially in thin film materials with a layered structure and several capping layers, access to the electronic structure by ARPES is limited. Therefore, several alternative methods to obtain the required information have been suggested. Here, we directly invert the results by cyclotron resonance experiments to obtain the band structure of a two-dimensional (2D) material. This procedure is applied to the mercury telluride quantum well with critical thickness which is characterized by a 2D electron gas with linear dispersion relations. The Dirac-like band structure in this material could be mapped both on the electron and on the hole side of the band diagram. In this material, purely linear dispersion of the hole-like carriers is in contrast to detectable quadratic corrections for the electrons.

Band structure and optical properties of polyaniline polymer material

The present contribution reports on the electronic structure and optical properties of the polyaniline polymer material. The calculations are performed using ab initio total energy calculations within the full-potential linearized augmented plane wave method in the framework of the density functional theory. The generalized gradient approximation is used to treat the effects of exchange and correlation. The fundamental band-gap energy and static and high-frequency dielectric constants of the polymer of interest are predicted. The optical spectra against the photon incident energy along both the x- and z-axis are presented and examined.