Dependence Of Gap Research Articles

Фотоэлектрические свойства монокристаллов MnGaInS-=SUB=-4-=/SUB=-

The spectral distribution of photoconductivity and the temperature dependence of the photocurrent of monocrystals MnGaInS4 are studied. The intrinsic and extrinsic photoconductivities are revealed in the spectrum of photoconductivity. The wavelength range of 0.640–0.760 μm is shown when manganese deficiency in crystals appears and is due to an acceptor defect. The temperature dependence of the band gap in the monocrystals MnGaInS4 is related to the electron – phonon interaction. In the studied temperatures, increase in the photocurrent is associated with the thermal depletion of the attachment levels. The activation energy of the attachment levels is estimated.

Magnetic field effects in rigidly linked D-A dyads: Extreme on-resonance quantum coherence effect on charge recombination

Charge recombination in the photoinduced charge separated (CS) state of a rigidly linked donor/bridge/acceptor triad with a triarylamine (TAA) donor, a 1,3-diethynyl-2,5-dimethoxy benzene bridge (OMe), and a perylenediimide (PDI) unit as an acceptor, represents a spin chemical paradigm case of a rigid radical ion pair formed with singlet spin and recombining almost exclusively to the locally excited PDI triplet state (3PDI). The magnetic field dependence of the CS state decay and 3PDI formation kinetics are investigated from 0 to 1800 mT by nanosecond laser flash spectroscopy. The time-resolved magnetic field affected reaction yields spectra of the CS state population and 3PDI population exhibit a sharp and deep resonance at 18.9 mT, indicating level crossing of the S and T+ levels separated by an exchange interaction of J = 18.9/2 mT at zero field. The kinetics are biexponential around the resonance field and monoexponential outside that range. The monoexponential behavior can be simulated by a classical kinetic model assuming a single field dependent double Lorentzian function for the energy gap dependence of all spin conversion processes. The full field dependence of the kinetics has been simulated quantum theoretically. It has been shown that incoherent and coherent hyperfine coupling contribute to S/T+ spin conversion at all fields and that the biexponentiality of the kinetics at resonance is due to a partitioning of the overall kinetics into 2/3 of the singlet hyperfine states exhibiting strong isotropic coupling to T+ and 1/3 of the singlet hyperfine states that do not or only weakly couple isotropically to T+.

Cu2MgO3 Electronic and Magnetic Properties – a DFT Study

AbstractWe investigate the properties of Guggenite‐α Cu2MgO3, which has an experimentally measured energy gap of approximately 1 eV. The band gap of Cu2MgO3, which is in the solar spectrum range makes it a potential candidate for photovoltaic applications. We calculate with PBE+U and LDA+U, and the SCAN functionals, the structure, formation energy and electronic structure of Cu2MgO3 in its anti‐ferromagnetic ground state and in the ferromagnetic phase. Finally we discuss the formation energy and band gap dependence on the U‐J Hubbard potential and compare our results to experiment.

Unraveling the topology of ZrTe5 by changing temperature

We study the effects of temperature on the topological nature of ZrTe$_5$, which sits near the phase boundary between strong and weak topological insulating orders. Using first-principles calculations, we show that the band gap and the topological indices of ZrTe$_5$ are extremely sensitive to thermal expansion, and that the electron-phonon interaction accounts for more than a third of the total temperature-dependent effects in ZrTe$_5$. We find that the temperature dependence of the band gap has an opposite sign in the strong and weak topological insulator phases. Based on this insight, we propose a robust and unambiguous method to determine the topological nature of any given sample of ZrTe$_5$: if the band gap decreases with temperature it is a strong topological insulator, and if it increases with temperature it is a weak topological insulator. An analogous strategy is expected to be generally applicable to other materials and to become particularly important in the vicinity of topological phase boundaries where other methods provide ambiguous results.

Pressure and Temperature Dependence of Energy Gap in SiC and Si1-xGex

The effect of pressure on energy gap for IV-IV compound SiC and Si1-xGex alloy have been investigated and evaluated by using Birch-Murnaghan equation of state (EOS) and Bardeen equation of state. Ambiguity in the effect of pressure and temperature on Eg of different SiC polytypes (3C, 4H, 6H) have been investigated and attributed. Variation of Eg in Si1-x Gex evaluated and an interpretation, for it, has been suggested.

Rheology of crystallizing polymers: The role of spherulitic superstructures, gap height, and nucleation densities

A longstanding goal in polymer rheology is to develop a physical picture that relates the growth of mechanical moduli during polymer crystallization to that of a structure. Here, we utilize simultaneous mechanical rheology and optical microscopy, with augmentation by deterministic reconstruction and stochastic simulations, to study isothermal crystallization in isotactic polypropylene. We observe the nucleation and growth of the surface and bulk spherulites, which are initially isolated and then impinge to form clusters and superstructures that eventually span the gap. We find that spherulitic superstructures play a critical role in the rheology, especially in the characteristic sharp upturn in moduli. Both the rheology and the spherulitic superstructures show pronounced gap dependencies, which we explain via finite-size effects in percolation phenomena and via surface-induced nucleation. The modulus-crystallinity relationship can be described through a general effective medium theory. It indicates that for thicker gaps, the viscoelastic liquid to solid transition can be described via percolation, whereas for our thinnest gap, it is best described by the linear mixing rule. We describe our results in terms of dimensionless nucleation rates and spherulite size, which enable the estimation of when gap-dependent superstructure effects can be anticipated.

Ab initio study of optical and bulk properties of cesium lead halide perovskite solid solutions

The first-principles calculations of band gaps and bulk moduli of cesium lead halide perovskite solid solutions, [Formula: see text] and [Formula: see text], are performed at the level of general gradient approximation of the density functional theory. We use supercell approach for computational modeling of disordered systems, which gives a description of the properties of the structure baasing on the average over a set of multiple configurations, namely distributions of different species over a given set of atomic positions. The calculations were performed with the CRYSTAL14 program package. The dependence of the band gap and bulk modulus on the content [Formula: see text] are investigated over the whole range [Formula: see text].

Modeling superconductivity in the background of a spin-vortex checkerboard

We introduce a microscopic model aimed at describing the behavior of fermionic excitations in the background of a magnetic texture called "spin-vortex checkerboard". This texture was proposed previously as a possible alternative to stripes to interpret the experimental phenomenology of spin and charge modulations in 1/8-doped lanthanum cuprates. The model involves two kinds of interacting fermionic excitations residing in spin-rich and spin-poor regions of the modulated structure. It is a generalization of another model developed earlier for the so-called "grid checkerboard". The principal terms of our model describe the decay of fermionic pairs belonging to spin-poor regions into single fermions occupying spin-rich regions and vice versa. These terms induce intricate fermionic correlations throughout the system but fall short of inducing superconductivity unless arbitrarily small hopping terms are added to the model Hamiltonian. We present the mean-field solution of the model, including, in particular, the temperature dependence of the energy gap. The latter is found to be in a good overall agreement with available experimental data for high-$T_c$ cuprate superconductors.

Diagnosing Clause Structure in a Polysynthetic Language: Wh-Agreement and Parasitic Gaps in West Circassian

This article identifies and tests a novel diagnostic for clause structure in West Circassian, a polysynthetic language with ergative alignment. The diagnostic concerns an unusual construction involving multiple wh-agreement in relative clauses. I argue that wh-agreement morphology uniformly tracks agreement with a wh-trace, and sentences with more than one instance of wh-agreement are surface manifestations of a parasitic gap dependency. Once multiple wh-agreement is understood in this theoretically familiar light, it can be used as a powerful tool for diagnosing asymmetries between various constituents in the West Circassian clause. By appealing to well-known constraints on parasitic gap licensing, the article demonstrates that the absolutive DP raises to a position c-commanding other clausemate DPs, and applied objects may undergo optional scrambling to a position above the ergative agent.

Ab initio Study of Optical and Bulk Properties of Cesium Lead Halide Perovskite Solid Solutions

The first principles calculations of band gaps and bulk moduli of cesium lead halide perovskite solid solutions, CsPb($${{{\text{I}}}_{{1 - x}}}{\text{C}}{{{\text{l}}}_{x}}$$)3 and CsPb($${{{\text{I}}}_{{1 - x}}}{\text{B}}{{{\text{r}}}_{x}}$$)3, are performed at the level of general gradient approximation of the density functional theory. We use supercell approach for computational modeling of disordered systems, which gives a description of the properties of the structure basing on the average over a set of multiple configurations, namely distributions of different species over a given set of atomic positions. The calculations were performed with the CRYSTAL14 program package. The dependence of the band gap and bulk modulus on the content $$x$$ are investigated over the whole range $$0 \leqslant x \leqslant 1$$.

Getting the Right Twist: Influence of Donor–Acceptor Dihedral Angle on Exciton Kinetics and Singlet–Triplet Gap in Deep Blue Thermally Activated Delayed Fluorescence Emitter

Here, a novel deep blue emitter SBABz4 for use in organic light-emitting diodes (OLED) is investigated. The molecular design of the emitter enables thermally activated delayed fluorescence (TADF), which we examine by temperature-dependent time-resolved electroluminescence (trEL) and photoluminescence (trPL). We show that the dihedral angle between donor and acceptor strongly affects the oscillator strength of the charge transfer state alongside the singlet-triplet gap. The angular dependence of the singlet-triplet gap is calculated by time-dependent density functional theory (TD-DFT). A gap of 15 meV is calculated for the relaxed ground state configuration of SBABz4 with a dihedral angle between the donor and acceptor moieties of 86{\deg}. Surprisingly, an experimentally obtained energy gap of 72+/-5 meV can only be explained by torsion angles in the range of 70{\deg}-75{\deg}. Molecular dynamics (MD) simulations showed that SBABz4 evaporated at high temperature acquires a distribution of torsion angles, which immediately leads to the experimentally obtained energy gap. Moreover, the emitter orientation anisotropy in a host matrix shows an 80% ratio of horizontally oriented dipoles, which is highly desirable for efficient light outcoupling. Understanding intramolecular donor-acceptor geometry in evaporated films is crucial for OLED applications, because it affects oscillator strength and TADF efficiency.

First principle investigations of structural and optoelectronic features of cubic CdxZn1−xSyTe1-y quaternary semiconductor alloys

In case of technologically important CdxZn1−xSyTe1−y quaternary alloys, structural and optoelectronic properties have been calculated for the first time with DFT based FP-LAPW approach. The PBE-GGA functional for structural properties and mainly the mBJ functional for optoelectronic properties are employed to calculate respective exchange-correlation potentials. Each specimen within CdxZn1−xSyTe1−y quaternary system is a direct band gap (Γ-Γ) semiconductor. The lattice constant reduces, while bulk modulus and band gap enhances nonlinearly with increasing anionic (S) concentration y at each cationic (Cd) concentration x. On the other hand, nonlinear increase in lattice constant, but decrease in bulk modulus and band gap is observed with increase in cationic concentration x at each anionic concentration y. Calculated contour maps for lattice constants and energy band gaps would be useful in fabricating new ternary or quaternary alloys with preferred optoelectronic features. Optical properties of the said specimens within CdxZn1−xSyTe1−y quaternary system show several interesting features. The composition dependence of each calculated zero-frequency limit shows opposite trend, while each calculated critical point shows similar trend of composition dependence of band gap. Moreover, calculations suggest the possibilities of growth of several zinc-blende (B3) CdxZn1−xSyTe1−y quaternary specimens on ZnTe, InP and GaAs substrates.

Electronegativity, symmetry, and bond strength intrinsically control charge transport through five-membered single-molecule junction

Density functional theory and Green’s function incorporated tight binding approach have been implemented to investigate the role of electronegativity, symmetry, and bond strength in charge transport of five-membered aromatic molecules, namely cyclopentadiene, thiophene, furan, pyrrole, and 5,5-dimethylcyclopenta-1,3-diene. A novel dependence of charge transport and band gap on the bond strength of the intact segment of molecule was revealed. It is found that decreasing the strength (increasing the length) of the bonds of the sound branch would enhance the electronic transmission and narrow the band gap. In addition, the electronic transmission of the symmetric systems is higher than the asymmetric counterparts and it increases (decreases) with onsite energy of the hetero-site of the symmetric (asymmetric) system.

Quantum Model of a Trapezoidal Limiting Potential Profile in a Spherical Nanocrystal

The limiting potential model is considered for a spherical quantum dot with three variation parameters–the well depth, external and internal radii. The stationary s-states are computed by way of the exact solution and in the WKB approximation. If the quasiclassical conditions are met, the approximate consideration is applied for the states with l ≠ 0. As a result, the energy values of the lower bound states are presented at the fixed external and varying internal radii in the wells of various depths. The dependence of the energy gap between the levels in the well on the variation parameters is shown.

Coexistence of giant Cooper pairs with a bosonic condensate and anomalous behavior of energy gaps in the BCS-BEC crossover of a two-band superfluid Fermi gas

We investigate Bardeen-Cooper-Schrieffer (BCS)- Bose-Einstein condensation (BEC) crossover in a two-band superfluid Fermi gas with an energy shift between the bands. When the intraband coupling in the cold (first) band is fixed as weak, we find that in the case of vanishing interband interaction and in the strong-coupling limit of the hot (second) band the system undergoes a transition to a single-component configuration with the full suppression of the first energy gap and with the full redistribution of particles between bands. For non-vanishing interband interaction we reveal the non-monotonic dependence of the energy gap in the first band vs intraband coupling in the second band with the presence of a hump. In the case of weak interband coupling the system shows a significant amplification of the intrapair correlation length of the condensate in the first band in the strong-coupling regime of the second band, which clearly indicates the coexistence of giant Cooper pairs and a bosonic condensate even for nonzero temperatures. This can lead to a non-monotonic temperature dependence of the second energy gap with a peak. Here predicted coexistence of the giant Cooper pairs and bosonic molecules can be verified by means of the visualization of vortex cores in the two-component atomic condensates as well as in some iron-based superconductors.

Large Band Edge Tunability in Colloidal Nanoplatelets

We study the impact of organic surface ligands on the electronic structure and electronic band edge energies of quasi-two-dimensional (2D) colloidal cadmium selenide nanoplatelets (NPLs) using density functional theory. We show how control of the ligand and ligand-NPL interface dipoles results in large band edge energy shifts, over a range of 5 eV for common organic ligands with a minor effect on the NPL band gaps. Using a model self-energy to account for the dielectric contrast and an effective mass model of the excitons, we show that the band edge tunability of NPLs together with the strong dependence of the optical band gap on NPL thickness can lead to favorable photochemical and optoelectronic properties.

Structural and optoelectronic properties of cubic MgxZn1−xSyTe1-y semiconductor quaternary alloys-a first principles investigation

Density functional calculations of structural and optoelectronic properties of cubic MgxZn1−xSyTe1−y quaternary alloys are performed with mBJ, EV-GGA and PBE-GGA functional. Each quaternary specimen is direct band gap (Γ-Γ) semiconductor. Lattice constant reduces, while bulk modulus and band gap enhances nonlinearly with increase in S-concentration y at each Mg-concentration x. Again, nonlinear increment in lattice constant and band gap, but reduction in bulk modulus is found with increase in Mg-concentration x at each S-concentrations y. Contour plots for lattice constants and band gaps would be useful in fabricating new quaternary alloys with preferred optoelectronic features. Chalcogen-p→Zn-5s,4p; Mg-4s,4p optical excitations contribute intense peaks in each ε2(ω) spectra. Composition dependence of each calculated zero-frequency limit shows opposite trend, while each calculated critical point shows similar trend of composition dependence of band gap. Calculations also suggested the possibility of growth of several said quaternary alloys on ZnTe, InAs and InP substrates.

Influence of phase composition on optical properties of TiO2: Dependence of refractive index and band gap on formation of TiO2-II phase in thin films

Atomic layer deposition was applied to grow TiO2 thin films containing anatase, rutile and mixture of TiO2-II and rutile phases with the aim to investigate the dependence of optical parameters on the formation of metastable TiO2-II phase. The films were grown at a substrate temperature of 450 °C using TiCl4 and O3 as the precursors. The crystal structure obtained depended on the substrates used. The films deposited on amorphous SiO2 substrates contained only the anatase phase when the film thicknesses did not exceed 88 nm while in thicker films, anatase with inclusions of rutile was formed. The films deposited on α-Al2O3 substrates contained epitaxial rutile without traces of other crystalline phases on α-Al2O3(0 1 1̄ 2) (r-sapphire) and epitaxial TiO2-II with inclusions of rutile phase on α-Al2O3(0 0 0 1) (c-sapphire) substrates. The films that contained TiO2-II had refractive indices of 2.8 at a wavelength of 500 nm and 2.61–2.67 at 633 nm. These values were higher than those of anatase films (2.6 and 2.48–2.49, respectively) and rutile films (2.74 and 2.57–2.62, respectively). Optical band gaps of 3.28–3.36 eV obtained for the films containing TiO2-II were by 0.20 ± 0.08 eV wider than the band gaps of rutile films and by 0.06 ± 0.05 eV narrower than the band gaps of anatase films. A band gap value of 3.00 ± 0.08 eV was measured for a non-epitaxial film that was deposited on amorphous SiO2 and contained anatase with inclusions of rutile.

Structural, optical, dielectric and magnetic properties of Ce-doped strontium hexaferrite synthesized by a hydrothermal process

Samples of Sr1−xCexFe12O19 (x = 0.0, 0.05, 0.1, and 0.15) were synthesized by a hydrothermal process. The X-ray diffraction patterns and the Rietveld refinement method confirmed the formation of the pure nano-crystalline phase of the M-type hexaferrite for all samples, except for the x = 0.15 sample with a small CeO2 phase. Fourier transform-infrared spectroscopy measurements showed the formation of the M-type hexaferrite. Field emission-scanning electron microscopy revealed that the prepared samples were nanoparticles that agglomerated in irregular platelets. The hysteresis loops (M–H) measured at room temperature indicated a decrease in the saturation magnetization with an increase in the Ce content. The band gap dependency on the Ce content was also studied using the UV–Vis spectra, which showed that the band gap energies were in the range of 1.37–1.51 eV. The dielectric constant and dielectric tangent loss measurements showed that the x = 0.1 sample has higher values than others.

A surface‐enhanced Raman‐spectroscopic study: Verification of the interparticle gap dependence of field enhancement by triangulation of spherical gold nanoparticle trimers

AbstractStrong electric field enhancement in the interparticle gap of resonators formed by two spherical Au nanoparticles occurs for polarization of the incoming light parallel to the dimer axis. This enhancement exhibits a strong dependence on gap width and is the basis of surface‐enhanced Raman scattering or tip‐enhanced Raman scattering. The enhancement factor as a function of gap width can be readily calculated solving Maxwellapos;s equations. However, the derived dependence is difficult to verify experimentally, in particular, because of the large uncertainty in determining the interparticle gap value, which is typically one or two orders of magnitude smaller than the diameters of the Au spherical nanoparticles forming the resonator. We demonstrate that a much more reliable verification of the predicted interparticle gap dependence of the electric field enhancement is possible using an approach analogous to triangulation in surveying and mapping.