Features Of Band Structure Research Articles

Study the Influence of High Pressure on the Optical and Electronic Properties of Composite ZnO Using (DFT-GGA)

In this article, the lattice constants, band structure and optical characteristics of ZnO wurtzite structure under various pressures were studied using the generalized gradient approximation (GGA) method. This method is based on the functional density (DFT) Theory, according to the first principle. The results show that as the pressure increases and the band gap increases, the lattice constants (a and c) decrease. As the pressure increases, the minimum conduction band will move to a higher energy level, and the maximum valence band will move to a lower energy level, thereby increasing the energy band difference. As the pressure increases, the shape of the optical parameter curve remains almost unchanged, and all peaks move to higher energies. The state density, dielectric function, reflectance and absorption coefficient are also calculated. The overview of the spectrum and optical properties is discussed, including the imaginary part of the dielectric function, reflectance and absorption coefficient of wurtzite-type ZnO under environmental conditions. The optical constants indicate that the phase of ZnO wurtzite structure is transparent. We noticed that our measurements are comparable to those observed in the literature.

Simulation and Numerical Modelling of CIGSSe-Based Solar Cells by AFORS-HET

A general equation to determine properties of penternary solar cell based on Cu (In, Ga) (Se, S) 2 (CIGSSe) with a double buffer layer ZnS/Zn0.8Mg0.2O(ZMO) were derived. Numerical analysis of a (CIGSSe) solar cell with a double buffer layer ZnS/ZMO, CdS free absorber layer, were investigated using the AFORS-HET software simulation. Taking into consideration the effect of thickness and doping concentration for the CIGSSe absorption layer, ZnS buffer layer and ZnO:B(BZO) window layer on the electron transport, short circuit current density (Jsc) and open circuit voltage (Voc); numerical simulation demonstrated that the changes in band structure characteristics occurred. The solar energy conversion efficiency is 28.34%, the filling factor is 85.59%, the open circuit voltage is 782.3 mV, the short circuit current is 42.32 mA. then we take the range of the gradient between the ratio of x and y for the absorption layer, and the best result of Voc, Jsc, FF, Eff equal (838.7 mV, 40.94 mA/cm2, 86.23%, 29.61%) respectively at x= 0, y= 0.26.

Semimetallic square-octagon two-dimensional polymer with high mobility

The electronic properties of $\pi$-conjugated two-dimensional (2D) polymers near the Fermi level are determined by structural topology and chemical composition. Thus, tight-binding (TB) calculations of the corresponding fundamental network can be used to explore the parameter space to find configurations with intriguing properties before designing the the atomistic 2D polymer network. The vertex-transitive \textbf{fes} lattice, which is also called square-octagon lattice, is rich in interesting topological features including Dirac points and flat bands. Herein, we study its electronic and topological properties within the TB framework using representative parameters for chemical systems. Secondly, we demonstrate that the rational implementation of band structure features obtained from TB calculations into 2D polymers is feasible with a family of 2D polymers possessing \textbf{fes} structure. A one-to-one band structure correspondence between fundamental network and 2D polymers is found. Moreover, changing the relative length of linkers connecting the triangulene units in the 2D polymers reflect tuning of hopping parameters in the TB model. These perturbations allow to open sizeable local band gaps at various positions in the Brillouin zone. From analysis of Berry curvature flux, none of the polymers exhibits a large topologically non-trivial band gap. However, we find a particular configuration of semimetallic characteristics with separate electron and hole pockets, which possess very low effective masses both for electrons (as small as $m^*_\mathrm{e} = 0.05$) and holes (as small as $m^*_\mathrm{h} = 0.01$).

First-principles calculation to investigate the influence of shear deformation on the electronic structure and optical properties of hydrogenated silicene

The CASTEP program based on density functional theory was used to study the effect of shear deformation on the structure, electronic and optical properties of hydrogenated silicene. The calculation results show that the hydrogenated silicene can still remain stable under the action of shear deformation. After the hydrogenation of silicene, the forbidden band width increases to 2.204 eV, and the electrical conductivity is significantly improved. Shear deformation significantly affects the energy band structure characteristics of hydrogenated silicene. The resulting band gap gradually decreases as the deformation increases. The shear deformation causes the red shift of the imaginary part of the dielectric function of the hydrogenated silicene to the direction of lower energy. Shear deformation improves the luminous efficiency of the system. The peak range of the reflection spectrum gradually widens with the increase of the deformation. These properties may be applied to optoelectronic devices in the future.

High-harmonic generation in Weyl semimetal \u03b2-WP2 crystals

As a quantum material, Weyl semimetal has a series of electronic-band-structure features, including Weyl points with left and right chirality and corresponding Berry curvature, which have been observed in experiments. These band-structure features also lead to some unique nonlinear properties, especially high-order harmonic generation (HHG) due to the dynamic process of electrons under strong laser excitation, which has remained unexplored previously. Herein, we obtain effective HHG in type-II Weyl semimetal β-WP2 crystals, where both odd and even orders are observed, with spectra extending into the vacuum ultraviolet region (190 nm, 10th order), even under fairly low femtosecond laser intensity. In-depth studies have interpreted that odd-order harmonics come from the Bloch electron oscillation, while even orders are attributed to Bloch oscillations under the “spike-like” Berry curvature at Weyl points. With crystallographic orientation-dependent HHG spectra, we further quantitatively retrieved the electronic band structure and Berry curvature of β-WP2. These findings may open the door for exploiting metallic/semimetallic states as solid platforms for deep ultraviolet radiation and offer an all-optical and pragmatic solution to characterize the complicated multiband electronic structure and Berry curvature of quantum topological materials.

Nonlinear plasmonic response in atomically thin metal films.

Nanoscale nonlinear optics is limited by the inherently weak nonlinear response of conventional materials and the small light-matter interaction volumes available in nanostructures. Plasmonic excitations can alleviate these limitations through subwavelength light focusing, boosting optical near fields that drive the nonlinear response, but also suffering from large inelastic losses that are further aggravated by fabrication imperfections. Here, we theoretically explore the enhanced nonlinear response arising from extremely confined plasmon polaritons in few-atom-thick crystalline noble metal films. Our results are based on quantum-mechanical simulations of the nonlinear optical response in atomically thin metal films that incorporate crucial electronic band structure features associated with vertical quantum confinement, electron spill-out, and surface states. We predict an overall enhancement in plasmon-mediated nonlinear optical phenomena with decreasing film thickness, underscoring the importance of surface and electronic structure in the response of ultrathin metal films.

Dirac Hierarchy in Acoustic Topological Insulators

Dirac cones are essential features of the electronic band structure of materials like graphene and topological insulators (TIs). Lately, this avenue has found a growing interest in classical wave physics by using engineered artificial lattices. Here, we demonstrate an acoustic 3D honeycomb lattice that features a Dirac hierarchy comprising an eightfold bulk Dirac cone, a 2D fourfold surface state Dirac cone, and a 1D twofold hinge state Dirac cone. The lifting of the Dirac degeneracy in each hierarchy authorizes the 3D lattice to appear as a first-order TI with 2D topological surface states, a second-order TI exhibiting 1D hinge states, and a third-order TI of 0D midgap corner states. Analytically we discuss the topological origin of the surface, hinge, and corner states, which are all characterized by out-of-plane and in-plane winding numbers. Our study offers new routes to control sound and vibration for acoustic steering and guiding, on-chip ultrasonic energy concentration, and filtering to name a few.

Explaining improved photocatalytic activity of Double-walled TiO2 Nanotube: A hybrid density functional calculation

The single-walled and double-walled TiO2 NTs rolled up with rectangular anatase (101) slab, have been investigated using modified hybrid density functional (m-B3LYP) with all-electron Gaussian basis set. We have considered single-walled and double-walled TiO2 NTs only with chirality of (0, n) and (0, m)@(0, n), respectively. Their equilibrium geometric structure, stability, electronic structure and band gap edge position relative to water splitting redox potential have been calculated. For single-walled and double-walled TiO2 NTs, the diameters and Ti-O bond lengths remain unchanged compared to TiO2 slab. To analyze the stability of TiO2 NTs, we have calculated the strain energies and formation energies which decrease with the increased diameters, and simulated the binding energies for double-walled ones to describe two walls interaction. when increasing the NTs diameters, the band gap widths of single-walled TiO2 NTs decrease with the valence band and conduction band moving downward. The double-walled TiO2 NTs exhibit II-type band structure characteristics, and their band gap widths are considerable smaller than that of single-walled ones, with the band gap edge straddling the water splitting redox potential. Based on these important findings, we can explain the mechanism of improved photocatalytic activity of double-walled TiO2 NTs as compared to the single-walled ones.

Phosphorene and phosphorene oxides as a toxic gas sensor materials: a theoretical study

A systematic study of the adsorption of several harmful gases (CO2, NO, SO2, NH3 y H2S) onto black phosphorene and three different black phosphorene oxides (BPO) is carried out through density functional theory calculations. In general, it is shown that BPOs are more suitable adsorbents than pure black phosphorene. Smaller values of adsorption energy correspond to CO2 molecules, whilst those exhibiting larger ones are NH3, H2S, NO y SO2. It is found that SO2 shows the greater difference in electronic charge transfer as well as the longer time of recovery among all species, being an electron acceptor molecule. Besides, it is revealed that physisorption induces changes of different order in the electronic, magnetic and optical responses of phosphorene systems involved. Greater changes in the electronic structure are produced in the case of NO adsorption. In that case, semiconductor nature and magnetization features of black phosphorene band structure become significantly modified. Moreover, a notorious effect of an externally applied electric field on the molecule adsorption onto BPOs has been detected. In accordance, adsorption energy changes with the applied electric field direction, in such a way that the higher value is favored through an upwards-directed orientation of NO y SO2 adsorbates. Results presented could help to enhancing the understanding of BPOs as possible candidates for applications in gas sensing.

Bilayer graphene encapsulated within monolayers of WS2 or Cr2Ge2Te6 : Tunable proximity spin-orbit or exchange coupling

Van der Waals (vdW) heterostructures consisting of bilayer graphene (BLG) encapsulated within monolayers of strong spin-orbit semiconductor WS$_2$ or ferromagnetic semiconductor Cr$_2$Ge$_2$Te$_6$ (CGT), are investigated. By performing realistic first-principles calculations we capture the essential BLG band structure features, including layer- and sublattice-resolved proximity spin-orbit or exchange couplings. For different relative twist angles (0 or 60$^{\circ}$) of the WS$_2$ layers, and the magnetizations (parallel or antiparallel) of the CGT layers, with respect to BLG, the low energy bands are found and characterized by a series of fit parameters of model Hamiltonians. These effective models are then employed to investigate the tunability of the relevant energy dispersions by a gate field. For WS$_2$/BLG/WS$_2$ encapsulation we find that twisting allows to turn off the spin splittings away from the $K$ points, due to opposite proximity-induced valley-Zeeman couplings in the two sheets of BLG. Close to the $K$ points the electron spins are polarized out of the plane. This polarization can be flipped by applying a gate field. As for magnetic CGT/BLG/CGT structures, we realize the recently proposed spin-valve effect, whereby a gap opens for antiparallel magnetizations of the CGT layers. Furthermore, we find that for the antiferromagnetic orientation the electron states away from $K$ have vanishingly weak proximity exchange, while the states close to $K$ remain spin polarized in the presence of an electric field. The induced magnetization can be flipped by changing the gate field. These findings should be useful for spin transport, spin filtering, and spin relaxation anisotropy studies of BLG-based vdW heterostructures.

In-plane wave propagation analysis for waveguide design of hexagonal lattice with Koch snowflake

In this study, the band structure and directional characteristics of the hexagonal lattice with Koch snowflake are investigated. The finite element method and Bloch theorem are used to analyze the wave propagation in hexagonal lattice with Koch snowflake for different fractal orders. The effects of the geometric parameters on band structures are discussed in detail. The group velocities at given frequencies are calculated for analyzing the directional characteristics of the wave propagation. The dynamic responses of the periodic lattice are investigated via numerical simulation to confirm the directional features. The vibration experiments are conducted to verify the existence of band gap and vibration suppression performances of the structures, which are used to design the waveguide in the lattices. The results demonstrate that the hexagonal lattice with Koch snowflake for different fractal orders can tune the elastic wave behaviors in a specific way. As the fractal order increases, multiple band gaps appear and the band structure has self-similarity in the low-frequency range. The directional characteristics of wave propagation in the hexagonal lattice with Koch snowflake can be used to further expand the vibration attenuation range. The results also show the potential importance and feasibility of using the hexagonal lattice with Koch snowflake for vibration isolation and control using the hexagonal lattice with Koch snowflake.

Electronic transitions and energy band structure of CuGaxAl1-xSe2 crystals

Optical reflectance spectra have been investigated at photon energies higher than the band gap of CuGa x Al 1-x Se solid solutions with x values varying from 0 to 1. The spectral dependence of the real ε 1 and imaginary ε 2 dielectric functions was calculated from experimental reflectivity spectra by means of Kramers–Kronig relations. The behavior of features observed in reflectivity spectra and in the spectral dependence of the dielectric functions was analyzed as a function of the solid solution composition. The experimentally observed peaks have been tabulated and related to the electronic band structure of materials computed in previous works. • Optical reflectance spectra of CuGa x Al 1-x Se 2 crystal measured in a wide spectral range. • Dielectric functions calculated on the basis of Kramers–Kronig relations. • Electronic transitions tabulated according to the energy band structure. • Trends in the behavior of band structure features with changing the composition established.

Flat band, spin-1 Dirac cone, and Hofstadter diagram in the fermionic square kagome model

We study characteristic band structures of the fermions on a square kagome lattice, one of the two-dimensional lattices hosting a corner-sharing network of triangles. We show that the band structures of the nearest-neighbor tight-binding model exhibit many characteristic features, including a flat band which is ubiquitous among frustrated lattices. On the flat band, we elucidate its origin by using the molecular-orbital representation and also find localized exact eigenstates called compact localized states. In addition to the flat band, we also find two spin-1 Dirac cones with different energies. These spin-1 Dirac cones are not described by the simplest effective Dirac Hamiltonian because the middle band is bended and the energy spectrum is particle-hole asymmetric. We also investigated the Hofstadter problem on a square kagome lattice in the presence of an external field and find that the profile of the Chern numbers around the modified spin-1 Dirac cones coincides with the conventional one.

Mirror symmetry breaking and lateral stacking shifts in twisted trilayer graphene

We construct a continuum model of twisted trilayer graphene using {\it ab initio} density-functional-theory calculations, and apply it to address twisted trilayer electronic structure. Our model accounts for moir\'e variation in site energies, hopping between outside layers and within layers. We focus on the role of a mirror symmetry present in ABA graphene trilayers with a middle layer twist. The mirror symmetry is lost intentionally when a displacement field is applied between layers, and unintentionally when the top layer is shifted laterally relative to the bottom layer. We use two band structure characteristics that are directly relevant to transport measurements, the Drude weight and the weak-field Hall conductivity, and relate them via the Hall density to assess the influence of the accidental lateral stacking shifts currently present in all experimental devices on electronic properties, and comment on the role of the possible importance of accidental lateral stacking shifts for superconductivity in twisted trilayers.

Band shifting and magnetic anisotropy switching induced by electric field in CrI3/1T′-MX2 heterojunction

Manipulating magnetism by the electric field (EF) is a well-established technology that has been applied in two-dimensional materials systems. Here, based on first-principles calculations, we study the influence of the EF on the band structure and magnetic characteristics of the CrI3/1T′-MX2 heterojunction. The results show amounts of charge exchange at the interface accompanying an obvious band shifting at the Γ point around the Fermi level. The magnetic moment of CrI3/1T′-MX2 is visibly increased under an external EF relative to isolated CrI3. Particularly, the magnetic anisotropy of CrI3/1T′-MX2 switching from out-of-plane to in-plane can be modulated by the EF, while this change does not occur in isolated CrI3. In addition, the topological properties of the heterojunction are still preserved by a nontrivial topological invariant 2 = 1. Our findings provide theoretical guidance for the spintronic application of CrI3/1T′-MX2 materials.

Optoelectronic properties exploration of native point defects on GaN nanowires

The formation energy, band structure and absorption features of perfect and p-type doped GaN nanowires with various point defect types (vacancy, interstitial and substitutional defects) at different positions (core, subsurface and surface) were calculated based on the first-principles calculations. The results show that NGa substitutional defects are the surface defects with the lowest formation energy in GaN nanowires. The nanowires with defects generate new impurity levels in the band structure. The emergence of NGa has turned p-type doped GaN nanowires into indirect bandgap semiconductors. At the peak of the ultraviolet absorption spectrum, all types of defects could damage the absorption characteristics of the nanowires.

Into the valley

Quantum Optics The topological features of band structures provide robust propagation paths for classical and quantum states of light. Another ingredient of band structure is the valley degree of freedom, in which the propagation path of light is dependent on its polarization state. Chen et al. used both topology and valley-dependent transport to design and fabricate a photonic crystal structure capable of forming the basis of optical quantum circuits. The authors demonstrate the input of left- and right-circularly polarized states of quantum light, their separation into different paths, and controlled interference as the single photons are brought back together again. The results provide a route for developing complex optical quantum circuits for on-chip quantum information processing. Phys. Rev. Lett. 126 , 230503 (2021).

InSe/Te van der Waals Heterostructure as a High-Efficiency Solar Cell from Computational Screening.

Designing the electronic structures of the van der Waals (vdW) heterostructures to obtain high-efficiency solar cells showed a fascinating prospect. In this work, we screened the potential of vdW heterostructures for solar cell application by combining the group III–VI MXA (M = Al, Ga, In and XA = S, Se, Te) and elementary group VI XB (XB = Se, Te) monolayers based on first-principle calculations. The results highlight that InSe/Te vdW heterostructure presents type-II electronic band structure feature with a band gap of 0.88 eV, where tellurene and InSe monolayer are as absorber and window layer, respectively. Interestingly, tellurene has a 1.14 eV direct band gap to produce the photoexcited electron easily. Furthermore, InSe/Te vdW heterostructure shows remarkably light absorption capacities and distinguished maximum power conversion efficiency (PCE) up to 13.39%. Our present study will inspire researchers to design vdW heterostructures for solar cell application in a purposeful way.

Chiral topological superconducting state with Chern number C = −2 in Pb3Bi/Ge(111)

Materials realization of chiral topological superconductivity is a crucial condition for observing and manipulating Majorana fermions in condensed matter physics. Here we develop a tight-binding description of Pb$_3$Bi/Ge(111), identified recently as an appealing candidate system for realizing chiral $p$-wave topological superconductivity [Nat. Phys. 15, 796 (2019)]. We first show that our phenomenological model can capture the two main features of the electronic band structures obtained from first-principles calculations, namely, the giant Rashba splitting and type-II van Hove singularity. Next, when the $s$-wave superconducting property of the parent Pb system is explicitly considered, we find the alloyed system can be tuned into a chiral topological superconductor with Chern number $\mathcal{C} = -2$, resulting from the synergistic effect of a sufficiently strong Zeeman field and the inherently large Rashba spin-orbit coupling. The nontrivial topology with $\mathcal{C} = -2$ is further shown to be detectable as two chiral Majorana edge modes propagating along the same direction of the system with proper boundaries. We finally discuss the physically realistic conditions to establish the predicted topological superconductivity and observe the corresponding Majorana edge modes, including the influence of the superconducting gap, Land\'{e} $g$-factor, and critical magnetic field. The present study provides useful guides in searching for effective $p$-wave superconductivity and Majorana fermions in two-dimensional or related interfacial systems.

Band Engineering of Dirac Semimetals Using Charge Density Waves.

New developments in the field of topological matter are often driven by materials discovery, including novel topological insulators, Dirac semimetals, and Weyl semimetals. In the last few years, large efforts have been made to classify all known inorganic materials with respect to their topology. Unfortunately, a large number of topological materials suffer from non-ideal band structures. For example, topological bands are frequently convoluted with trivial ones, and band structure features of interest can appear far below the Fermi level. This leaves just a handful of materials that are intensively studied. Finding strategies to design new topological materials is a solution. Here, a new mechanism is introduced, which is based on charge density waves and non-symmorphic symmetry, to design an idealized Dirac semimetal. It isthen shown experimentally that the antiferromagnetic compound GdSb0.46 Te1.48 is a nearly ideal Dirac semimetal based on the proposed mechanism, meaning that most interfering bands at the Fermi level are suppressed. Its highly unusual transport behavior points to a thus far unknown regime, in which Dirac carriers with Fermi energy very close to the node seem to gradually localize in the presence of lattice and magneticdisorder.