Band Dispersion Relation Research Articles

Optimization of high optical gain in type-II In0.70Ga0.30As/GaAs0.40Sb0.60 lasing nano-heterostructure for SWIR applications

Most of the nano-heterostructures exhibiting lasing action in NIR (near infra-red) region that have been modeled and simulated are based on type-I category. The nano-scaled lasing heterostructures, however, of type-II category operating in SWIR (short wave infra-red) region have not been well studied. In this paper, for SWIR generation, an M-shaped type-II In0.70Ga0.30As/GaAs0.40Sb0.60 symmetric lasing nano-heterostructure has been designed. In order to simulate the optical gain, firstly the wave functions associated with conduction and valence sub-bands, carrier densities within the bands, energy band dispersion relations for the quantum well structure, optical matrix elements and finally optical gain have been studied by utilizing the six band k.p method. For the injected carrier concentration of 5 × 1012/cm2, the optimized optical gain within TE mode is as high as ∼9000/cm at the wavelength of ∼1.95 μm, thus providing a very important alternative material system for the generation of SWIR wavelength region.

A dye-sensitized visible light photocatalyst-Bi24O31Cl10

The p-block semiconductors are regarded as a new family of visible-light photocatalysts because of their dispersive and anisotropic band structures as well as high chemical stability. The bismuth oxide halides belong to this family and have band structures and dispersion relations that can be engineered by modulating the stoichiometry of the halogen elements. Herein, we have developed a new visible-light photocatalyst Bi24O31Cl10 by band engineering, which shows high dye-sensitized photocatalytic activity. Density functional theory calculations reveal that the p-block elements determine the nature of the dispersive electronic structures and narrow band gap in Bi24O31Cl10. Bi24O31Cl10 exhibits excellent visible-light photocatalytic activity towards the degradation of Rhodamine B, which is promoted by dye sensitization due to compatible energy levels and high electronic mobility. In addition, Bi24O31Cl10 is also a suitable photoanode material for dye-sensitized solar cells and shows power conversion efficiency of 1.5%.

Novel band structures in silicene on monolayer zinc sulfide substrate

Opening a sizable band gap in the zero-gap silicene without lowering the carrier mobility is a key issue for its application in nanoelectronics. Based on first-principles calculations, we find that the interaction energies are in the range of −0.09‒0.3 eV per Si atom, indicating a weak interaction between silicene and ZnS monolayer and the ABZn stacking is the most stable pattern. The band gap of silicene can be effectively tuned ranging from 0.025 to 1.05 eV in silicene and ZnS heterobilayer (Si/ZnS HBL). An unexpected indirect–direct band gap crossover is also observed in HBLs, dependent on the stacking pattern, interlayer spacing and external strain effects on silicene. Interestingly, the characteristics of Dirac cone with a nearly linear band dispersion relation of silicene can be preserved in the ABS pattern which is a metastable state, accompanied by a small electron effective mass and thus the carrier mobility is expected not to degrade much. These provide a possible way to design effective FETs out of silicene on a ZnS monolayer.

Topological states inBi2Se3surfaces created by cleavage within a quintuple layer: Analysis in terms of the Shockley criterion

In topologically trivial materials, the existence of surface states within the bulk band gap depends on the surface condition. A surface state exists if the system is Shockley inverted, and vice versa; furthermore, this surface condition in a superlattice can be tuned from one case to the other by varying the terminating atomic plane within a superlattice period. By contrast, we demonstrate here based on first-principles calculations for ${\text{Bi}}_{2}$${\text{Se}}_{3}$ cleaved at different atomic planes within a quintuple layer that topological surface states always span the bulk band gap in accordance with the topologically nontrivial nature of ${\text{Bi}}_{2}$${\text{Se}}_{3}$. However, the number of surface bands, the band dispersion relations, and the degree of spin polarization are strongly dependent on the cleavage plane. Multiple Dirac cones can exist at the zone center and/or zone boundary, and a massive but topologically nontrivial Dirac cone is observed for a Se-terminated surface. The results confirm the robustness of the topological nature of the system, but the details of the topological surface states themselves can vary substantially within the broad topological constraint. The persistent existence of topological states within the gap is discussed in terms of the Shockley criterion.

Comparative investigations of surface plasmon polaritons on dielectric and metallic gratings

We investigate the properties of Surface Plasmon Polaritons (SPPs) on two kinds of structures: metallic and dielectric gratings loaded on a metallic layer (MM grating and DM grating). The rigorous coupled wave analysis (RCWA) is used for all the simulation in this article. The characteristics of band gap, dispersion relation, reflectivity curves and resonant electric field distributions are comparative simulated and analyzed in MM and DM gratings. Compared with the sticky short-wavelength band edge and complicated surface electric field distributions of MM grating, it is found that the band gap edges of DM grating could be controlled flexibly and the surface fields are more smoothly, which can be used to facilitate the spectral tunability and tailor the optical response of DM structure. This work would provide help for understanding the intrinsic reason of the difference of SPPs in two kinds of structures and basis for choosing suitable structure (MM or DM) of SPPs devices aiming at different functions, especially for designing the dielectric loaded surface plasmon polaritions (DLSPPs) devices.

Photonic band gaps in one-dimensional magnetized plasma photonic crystals with arbitrary magnetic declination

In this paper, the properties of photonic band gaps and dispersion relations of one-dimensional magnetized plasma photonic crystals composed of dielectric and magnetized plasma layers with arbitrary magnetic declination are theoretically investigated for TM polarized wave based on transfer matrix method. As TM wave propagates in one-dimensional magnetized plasma photonic crystals, the electromagnetic wave can be divided into two modes due to the influence of Lorentz force. The equations for effective dielectric functions of such two modes are theoretically deduced, and the transfer matrix equation and dispersion relations for TM wave are calculated. The influences of relative dielectric constant, plasma collision frequency, incidence angle, plasma filling factor, the angle between external magnetic field and +z axis, external magnetic field and plasma frequency on transmission, and dispersion relation are investigated, respectively, and some corresponding physical explanations are also given. From the numerical results, it has been shown that plasma collision frequency cannot change the locations of photonic band gaps for both modes, and also does not affect the reflection and transmission magnitudes. The characteristics of photonic band gaps for both modes can be obviously tuned by relative dielectric constant, incidence angle, plasma filling factor, the angle between external magnetic field and +z axis, external magnetic field and plasma frequency, respectively. These results would provide theoretical instructions for designing filters, microcavities, and fibers, etc.

Tunable electronic structures of graphene/boron nitride heterobilayers

Using first-principles calculations, we show that the band gap and electron effective mass (EEM) of graphene/boron nitride heterobilayers (C/BN HBLs) can be modulated effectively by tuning the interlayer spacing and stacking arrangement. The HBLs have smaller EEM than that of graphene bilayers (GBLs), and thus higher carrier mobility. For specific stacking patterns, the nearly linear band dispersion relation of graphene monolayer can be preserved in the HBLs accompanied by a small band-gap opening. The tunable band gap and high carrier mobility of these C/BN HBLs are promising for building high-performance nanodevices.

Graphene adhesion on MoS2 monolayer: An ab initio study

The geometric and electronic structures of graphene adsorption on MoS(2) monolayer have been studied by using density functional theory. It is found that graphene is bound to MoS(2) with an interlayer spacing of 3.32 Å and with a binding energy of -23 meV per C atom irrespective of adsorption arrangement, indicating a weak interaction between graphene and MoS(2). A detailed analysis of the electronic structure indicates that the nearly linear band dispersion relation of graphene can be preserved in MoS(2)/graphene hybrid accompanied by a small band-gap (2 meV) opening due to the variation of on-site energy induced by MoS(2). These findings are useful complement to experimental studies of this new synthesize system and suggest a new route to facilitate the design of devices where both finite band-gap and high carrier mobility are needed.

Calculations of Lamb wave band gaps and dispersions for piezoelectric phononic plates using mindlin's theory-based plane wave expansion method

Based on Mindlin's piezoelectric plate theory and the plane wave expansion method, a formulation is proposed to study the frequency band gaps and dispersion relations of the lower-order Lamb waves in two-dimensional piezoelectric phononic plates. The method is applied to analyze the phononic plates composed of solid-solid and airsolid constituents with square and triangular lattices, respectively. Factors that influence the opening and width of the complete Lamb wave gaps are identified and discussed. For solid/solid phononic plates, it is suggested that the filling material be chosen with larger mass density, proper stiffness, and weak anisotropic factor embedded in a soft matrix in order to obtain wider complete band gaps of the lower-order Lamb waves. By comparing to the calculated results without considering the piezoelectricity, the influences of piezoelectric effect on Lamb waves are analyzed as well. On the other hand, for air/solid phononic plates, a background material itself with proper anisotropy and a high filling fraction of air may favor the opening of the complete Lamb wave gaps.

Surface states and spin density wave periodicity in Cr(110) films

Using angle-resolved photoemission, we have mapped the dispersion relations and Fermi contours for surface-localized electron states onto clean and hydrogen-covered Cr(110) surfaces. In particular, we have probed the relationship between hydrogen adsorption and the evolution of the spin density wave (SDW) periodicity in chromium thin films observed previously. We find qualitatively similar surface band dispersion relations to those on W(110) and Mo(110), although with a narrower bandwidth, broader spectral features, and a smaller impact from the spin–orbit interaction. We compare our results to existing first-principles calculations and find a significant disagreement for a surface band that produces a prominent surface Fermi contour. Upon hydrogen adsorption, the Fermi contour for a particular surface band becomes well nested at a wave vector that stabilizes a commensurate SDW. We suggest that a competition between commensurate two-dimensional (2D) and incommensurate 3D Fermi surface nesting plays an important role in the SDW energetics in thin Cr(110) films.

Electron-hole generation and recombination rates for Coulomb scattering in graphene

We calculate electron-hole generation and recombination rates for Coulomb scattering (Auger recombination and impact ionization) in zero band-gap graphene. The conduction and valence band dispersion relation in graphene together with energy and momentum conservation requirements restrict the phase space for Coulomb scattering so that electron-hole recombination times can be much longer than $1\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ for electron-hole densities smaller than ${10}^{12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$.

Electronic and optical properties of ZnO thin film under in-plane biaxial strains: Ab initio calculation

Abstract The full regions of the electronic and optical properties of in-plane biaxial strained thin film ZnO are studied using pseudopotential plane-wave method. The fundamental band gap at the Γ point increase linearly with the increase of tensile strains, but decreased with the compressive ones. The strains affected the local tetrahedral symmetry, and so the splitting of crystal field energy. The band dispersion relation of the valence band maximum changes with the strains, which means the residual strains have effects on the effective hole mass, thus the transportation properties of the p -type ZnO. The changes tendency of optical properties up to full regions under strains have been shown and discussed.

Completed-Band Dispersion Relation in Sodium Cobaltates

We utilize fine-tuned polarization selection coupled with excitation-energy variation of photoelectron signal to image the complete d-band dispersion relation in sodium cobaltates. A hybridization gap anticrossing is observed along the Brillouin zone corner and the full quasiparticle band is found to emerge as a many-body entity lacking a pure orbital polarization. At low dopings, the quasiparticle bandwidth (Fermion scale, many-body E(F) approximately 0.25 eV) is found to be smaller than most known oxide metals. The low-lying density of states is found to be in agreement with bulk-sensitive thermodynamic measurements for nonmagnetic dopings where the 2D Luttinger theorem is also observed to be satisfied.

Modification of Surface States in Ultrathin Films via Hybridization with the Substrate: A Study of Ag on Ge

The Shockley surface state of Ag(111) develops unusual band dispersion relations for Ag films of decreasing thicknesses on Ge(111), as observed by angle-resolved photoemission. Its parabolic dispersion in the thick-film limit shifts toward higher binding energies and splits into multiple bands with dispersions that reflect the valence band structure of Ge including the heavy-hole, light-hole, and split-off bands. The results are explained in terms of a hybridization interaction between the Ag surface state and the Ge substrate states.

Linear Response Theory Around a Localized Impurity in the Pseudogap Regime of an Anisotropic Superconductor

We compare and contrast the polarizability of a d-wave superconductor in the pseudogap regime, within the precursor pairing scenario (dPG), and of a d-density-wave (dDW) state, characterized by a d-wave hidden order parameter, but no pairing. Our study is motivated by STM imaging experiments around an isolated impurity, which may in principle distinguish between precursor pairing and dDW order in the pseudogap regime of the high-\(T_{c}\) superconductors. In both cases, the \({\bf q}\)-dependence of the polarizability is characterized by an azimuthal modulation, consistent with the d-wave symmetry of the underlying state. However, only the dDW result shows the fingerprints of nesting, with nesting wave vector \({{\bf Q}}=(\pi,\pi)\), albeit imperfect, due to a nonzero value of the hopping ratio \(t^\prime /t\) in the band dispersion relation. As a consequence of nesting, the presence of hole pockets is also reflected by the \(({\bf q},\omega)\) dependence of the retarded polarizability.

Analyses of mode coupling in joined parallel phononic crystal waveguides

In this paper, we present an analysis of coupling effects in joined parallel phononic crystal waveguides. The finite difference time domain (FDTD) method with periodic boundary condition is adopted to analyze the band gaps and dispersion relation of phononic waveguides. The defect modes of a single phononic waveguide are analyzed and first discussed to serve as a basis for a joined waveguides system. Then, the dispersion relation and displacement field of supermodes of joined waveguides are calculated and discussed. Both displacement pattern and transmission coefficient of the defect modes are calculated. To transfer the power from one waveguide to another, the coupling lengths are evaluated by numerical experiments and can be understood by the concept of beat length. Finally, we analyze an elastic waveguide coupler and demonstrate that the coupler can potentially be employed as a power switch of the acoustic wave.

Linear response theory around a localized impurity in the pseudogap regime of an anisotropic superconductor: Precursor pairing versus d-density-wave scenario

We derive the polarizability of an electron system in (i) the superconducting phase, with d-wave symmetry, (ii) the pseudogap regime, within the precursor pairing scenario, and (iii) the d-density-wave (dDW) state, characterized by a d-wave hidden order parameter, but no pairing. Such a calculation is motivated by the recent proposals that imaging the effects of an isolated impurity may distinguish between precursor pairing and dDW order in the pseudogap regime of the high-Tc superconductors. In all three cases, the wave-vector dependence of the polarizability is characterized by an azymuthal modulation, consistent with the d-wave symmetry of the underlying state. However, only the dDW result shows the fingerprints of nesting, with nesting wave-vector Q=(pi,pi), albeit imperfect, due to a nonzero value of the hopping ratio t'/t in the band dispersion relation. As a consequence of nesting, the presence of hole pockets is also exhibited by the (q,omega) dependence of the retarded polarizability.

Band dispersion relations of zinc-blende and wurtzite InN

The fundamental band-gap energy of the III-V compound semiconductor InN has been corrected due to recent experiments on high-quality InN samples. The advances in the InN growth technique also open new possibilities to realize high-frequency field effect transistors at elevated temperatures due to the small effective mass in the InN conduction-band minimum and high optical phonon frequencies. For device simulations the correct band dispersion relations around the InN band gap are of great interest and motivate the reexamination of previous theoretical works on InN. We present the band structure on InN in the zinc-blende and wurtzite-type configuration calculated within the empirical pseudopotential method approach by exploiting the transferability of ionic model potential parameters. The modified potential parameters of In support the achieved reliability of the presented InN zinc-blende and wurtzite band structure and related properties, e.g., Luttinger and Luttinger-like $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ parameters.

Nonreciprocal magnetic photonic crystals.

We study band dispersion relations omega(k-->) of a photonic crystal with at least one of the constitutive components being a magnetically ordered material. It is shown that by proper spatial arrangement of magnetic and dielectric components one can construct a magnetic photonic crystal with strong spectral asymmetry (nonreciprocity) omega(k-->) not equal omega(-k-->). The spectral asymmetry, in turn, results in a number of interesting phenomena, in particular, one-way transparency when the magnetic photonic crystal, being perfectly transparent for a Bloch wave of frequency omega, "freezes" the radiation of the same frequency omega propagating in the opposite direction. The frozen radiation corresponds to a Bloch wave with zero group velocity partial differential omega(k)/ partial differential k=0 and, in addition, with partial differential(2)omega(k)/ partial differential k(2)=0.

New organic superconductors beta-(BDA-TTP)2X [BDA-TTP + 2,5-bis(1,3-dithian-2ylidene)-1,3,4,6-tetrathiapentalene; X(-) = SbF6(-), AsF6(-), and PF6(-)

The synthesis, electrochemical properties, and molecular structure of a new pi-electron donor, 2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene (BDA-TTP), is described. In contrast to the hitherto-known tetrachalcogenafulvalene pi-donors providing organic superconductors, this donor contains only the bis-fused 1,3-dithiole-2-ylidene unit as a pi-electron system, yet produces a series of ambient-pressure superconductors beta-(BDA-TTP)2X [X = SbF6 (magnetic T(c) = 6.9 K, resistive T(c) = 7.5 K), AsF6 (magnetic T(c) = 5.9 K, resistive T(c) = 5.8 K), and PF6 (magnetic T(c) = 5.9 K)], which are isostructural. The values of the intermolecular overlap integrals calculated on the donor layers of these superconductors suggest a two-dimensional (2D) electronic structure with loose donor packing. Tight-binding band calculations also indicate that these superconductors have the 2D band dispersion relations and closed Fermi surfaces.