Dependence Of Energy Gap Research Articles

Perfect control of spin, valley and spin-valley-coupled currents in bilayer graphene on WSe2 magnetic junction: Effect of spin-valley dependent Fermi level and energy gap

We investigate the spin-valley dependent transport properties in a gated bilayer graphene (BG) junction placed on top of WSe2. By means of layer-dependent proximity, the spin–orbit interaction (SOI) is induced in the bottom layer, while the top layer is induced into ferromagnetism by a magnetic insulator. As a result of these differing properties, the Fermi level and energy gap become spin-valley dependent, which could lead to controllable spin- and valley-dependent filtering in the BG system. Our findings predict perfect spin-valley polarization control through gate control, given specific energy and electric field conditions, as well as spin-valley-coupled and spin currents when both the electric-field-induced energy gap and the exchange energy are equal to the SOI strength. We predict that there will be very highly sensitive gate control of −100% to + 100% of spin-valley-coupled polarization for large barrier thickness. Additionally, we predict 100% valley polarization can be controlled by the gate when the exchange energy is equal to the SOI. Furthermore, we found that the performance of the junction to control the polarizations can be enhanced by increasing the thickness of the junction. This work reveals the potential of the BG/WSe2 hybrid structure for spin-valley-current-based electronics, such as spin-, valley-, and spin-valley field-effect transistors.

Topological alloy engineering and locally linearized gap dependence on concentration

Alloy engineering is a well-established approach to tune various materials' properties, but its application to topological alloys remains rudimentary. Of special interest is the band gap, the most defining property of topological materials; however, the concentration dependence of energy gaps in topological alloys remains unknown. Here we systematically investigate the band gap evolution of a topological alloy as a function of alloy concentration, using $\mathrm{KZn}{\mathrm{Sb}}_{1\ensuremath{-}\mathrm{x}}{\mathrm{Bi}}_{\mathrm{x}}$ as a prototype, based on first-principles calculations. In contrast to the well-established smooth bowing curve for a trivial gap in semiconductor alloys, we found that the topological gap evolves generally with a complex fragmented pattern due to topological phase transitions, and most strikingly a linear dependence on concentration locally in each distinct phase. Such gap linearization is fundamentally rooted in the linear dependence on alloy concentration of spin-orbit coupling (SOC) that predominantly determines a topological gap. Furthermore, we demonstrate topological alloy engineering as a general approach to tune the topological order by modulating the band edge composition and degeneracy through the alloying-induced interplay of SOC and atomic orbital on-site energy, while the linear gap dependence on alloy concentration remains independent of the degree of topological order.

The Response of Phonon Frequencies, Sound Velocity, Electronic, Optical, and Mechanical Properties of Indium (Phosphide, Arsenide, and Antimonide) to Hydrostatic Pressure

The pressure dependence of phonon vibrations, the velocity of sound, and some other related physical quantities of and semiconductors have been determined. This investigation is done using the empirical pseudopotential method. We proposed a pressure dependence of the pseudopotential form factors; hence we were able to calculate the pressure dependence of the fundamental energy gaps. Using the values of the fundamental band gaps, we were able to calculate the refractive index, static dielectric constant, high-frequency dielectric constant, longitudinal and transverse phonon frequencies, elastic constants, mechanical moduli, and velocity of sound. Our findings may serve as a reference for experimental work at high pressures.

Facet dependent surface energy gap on magnetic topological insulators

Magnetic topological insulators $({\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}){({\mathrm{Bi}}_{2}{\mathrm{Te}}_{3})}_{n}$ ($n=0,1,2,3$) are promising to realize exotic topological states such as the quantum anomalous Hall effect (QAHE) and axion insulator (AI), where the ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ layer introduces versatility to engineer electronic and magnetic properties. However, whether surface states on the ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ terminated facet are gapless or gapped is debated, and its consequences in thin-film properties are rarely discussed. In this work, we find that the ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ terminated facets are gapless for $n\ensuremath{\ge}1$ compounds by calculations. Although the surface ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ (one layer or more) and underlying ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ layers hybridize and give rise to a gap, such a hybridization gap may overlap with bulk valence bands, leading to a gapless surface after all. Such a metallic surface poses a fundamental challenge to realize QAHE or AI, which requires an insulating gap in thin films with at least one ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ surface. In theory, the insulating phase can still be realized in a film if both surfaces are ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ layers. Otherwise, it requires that the film thickness be less than $10--20\phantom{\rule{4pt}{0ex}}\mathrm{nm}$ to push down bulk valence bands via the size effect. Our work paves the way to understand surface states and design bulk-insulating quantum devices in magnetic topological materials.

DFT investigation on D-π-A structures for n type semiconducting material for the application in organic field effect transistors

Organic Field Effect Transistors have been widely researched since the first reported OFET in 1986 which was made of Polythiophene. The designed two molecular structures are based on D-π-A model and looked for their efficient use in Organic field effect transistors. The designed systems are named as system first (anthracene-pyrrole-propanoic acid) and system second (naphthalene-pyrrole-nitroethane) where anthracene and naphthalene are donors, pyrrole is spacer molecule, propanoic acid and nitroethane are acceptors. Both the structures were optimized under the framework of Density functional theory with B3LYP/6–31 + G(d,p) basis set on a Gaussian16W software package. The calculated charge transport parameters are hole reorganization energy λh, electron reorganization energy λe , HOMO-LUMO energy gap. Further, these have been looked upon their electric field dependence of the HOMO-LUMO energy gap and molecular electrostatic potential (MESP). The λh value for system first is 2.8 × 10-4 eV, and λe value is 4.08 × 10-5 eV. Similarly, the λh value for system second is 1.36 × 10-5 eV and λe value is 1.08 × 10-5 eV. In both the systems the electron reorganization energy λe is smaller as compared to the hole reorganization energy λh. Therefore, this is confirmed that the designed molecules can be utilized as n-type semiconductor for Organic Field Effect Transistors.

Quantum simulations of thermally activated delayed fluorescence in an all-organic emitter.

We investigate the prototypical NAI-DMAC thermally activated delayed fluorescence (TADF) emitter in the gas phase- and high-packing fraction limits at finite temperature, by combining first principles molecular dynamics with a quantum thermostat to account for nuclear quantum effects (NQE). We find a weak dependence of the singlet-triplet energy gap (ΔEST) on temperature in both the solid and the molecule, and a substantial effect of packing. While the ΔEST vanishes in the perfect crystal, it is of the order of ∼0.3 eV in the molecule, with fluctuations ranging from 0.1 to 0.4 eV at 300 K. The transition probability between the HOMOs and LUMOs has a stronger dependence on temperature than the singlet-triplet gap, with a desirable effect for thermally activated fluorescence; such temperature effect is weaker in the condensed phase than in the molecule. Our results on ΔEST and oscillator strengths, together with our estimates of direct and reverse intersystem crossing rates, show that optimization of packing and geometrical conformation is critical to increase the efficiency of TADF compounds. Our findings highlight the importance of considering thermal fluctuations and NQE to obtain robust predictions of the electronic properties of NAI-DMAC.

Engineering the Energy Gap of Cupric Oxide Nanomaterial Using Extreme Learning Machine and Stepwise Regression Algorithms

CuO is a narrow band gap semiconductor with distinct features that render it indispensable in many industrial and technological applications such as environmental friendly catalysts for organic pollutant removal, sensors, photovoltaic, solar cells, batteries, and storage media among others. Engineering of its energy gap becomes imperative and necessary for tailoring its light absorption capacity to a desired level required for a particular application. Elemental doping mechanisms with accompanied lattice distortion symmetry breaking effectively enhance the optical property of this semiconductor and serve as a major route through which material design is achieved. This work develops an extreme learning machine intelligent predictive (ELM-IP) model and stepwise regression (SWR) based model for estimating energy gap of a doped CuO semiconductor. The developed ELM-IP-Sin model which employs sine activation function performs better than the ELM-IP-Sig model (that utilizes sigmoid activation function) and SWR model with a percentage improvement of 14.15% and 50.05%, respectively, using root mean square error (RMSE) metric, while the developed ELM-IP-Sig model outperforms the SWR-based model. The developed models further investigate the dependence of CuO energy gap on iron and cobalt impurity incorporation, and the obtained results agree well with measured values. The outstanding performance of the developed models is highly meritorious in tailoring the light response capacity of CuO semiconductor for photocatalytic and optoelectronics applications at a reduced cost while the experimental stress is circumvented.

Floquet transitions “Insulator – Semimetal – Insulator” in 2D crystals with displaced Dirac points

The Floquet spectrum of charge carriers of ac-driven Dirac 2D material has been calculated by averaging over the incident radiation period. The possibility of ac-tuned transitions between semimetallic and Floquet insulator states has been shown. The dependence of energy gap induced by high frequency field on the radiation power has been derived. This dependence has been shown to be nonmonotoneous. The density of states in conditions of ac-driving has been analyzed.

Deuteration Effects on the Transport Properties of (TMTTF)2X Salts

The electronic properties in the quasi-one-dimensional Fabre salts are strongly affected by electronic correlations along the molecular stacks, but also by the interactions with the anions located in a cage that is formed by the methyl end groups. We systematically compare the charge transport in deuterated and protonated (TMTTF)2X salts with the anions X = Br, PF6, SbF6, and ClO4, ranging from Mott and Efros–Shklovskii variable-range hopping to activated band transport with a temperature dependent energy gap. The strong dependence of charge localization and ordering on the anion size and deuteration confirms the subtle structural involvement of the anions in the charge transport along the TMTTF stack.

Linear response of the indirect J‐coupling alternation to the energy gap of increasing π‐conjugated oligomers

AbstractWe have investigated the dependence of the electronic energy gap of organic conjugated oligomers on the J‐coupling alternation (JCA) parameter, derived from the nuclear magnetic resonance (NMR) spectroscopy. We exploit the systematic and concerted oscillations observed for the indirect spin‐spin coupling constants (SSCCs), between two adjacent 13C nuclei (1JCC), in the π‐conjugated systems. Hence, we define a scalar JCA parameter in terms of the differences of 1JCC, which is further analyzed as a function of the HOMO‐LUMO gaps (the energy difference between the frontier molecular orbitals), as well as other properties of the studied oligomers. Using an adequate exchange‐correlation density functional approach, we show that, for all systems considered here, that is, polyacetylene (PA), poly(para‐phenylene) (PPP), polypyrrole (PPy), and polythiophene (PTh), their calculated electronic energy gaps may exhibit a linear dependence on the JCA. Our results indicate that a single‐valued NMR‐based parameter properly renders the electronic features of highly conjugated oligomers, allowing us to compare the strengths of J‐couplings in different systems. The present description yields a way to capture both the bonding features and the electronic absorption of these systems provided by the SSCCs. The JCA gives evidence for the differences in the electronic structure of distinct classes of conjugated systems, such as PA, and those that can exhibit a quinoid character, such as PPP, PPy, and PTh.

Finite-thickness effect and spin polarization of the even-denominator fractional quantum Hall states

The spin-polarized even-denominator fractional quantum Hall (FQH) states in the second Landau level (LL), i.e. 5/2 and 7/2, may possess novel quasi-particle excitations obeying non-Abelian statistics. However, the spin polarization of the 7/2 FQH state has not been investigated experimentally and the spin polarization of the 5/2 FQH state from tilted field experiments remains controversial. Using a piezo-driven sample rotator with the lowest electron temperature down to 25 mK, we studied the energy gap of the even-denominator FQH states in the second LL by precise control of the tilted angles with a resolution less than 0.1{\deg}. We observed two different energy gap dependences on the in-plane magnetic field for 5/2, 7/2, other FQH states (7/3 and 8/3) in the second LL and reentrant integer quantum Hall (RIQH) states in the third LL. Though the transition fields vary from states, their corresponding in-plane magnetic lengths are comparable to the quantum well thickness of the sample, which may result from the influence of the finite-thickness effect. At low in-plane magnetic fields, before the conjectured finite-thickness effect starts to dominate, the energy gaps of both 5/2 and 7/2 states show a non-decreasing behavior, supporting spin-polarized ground states. Our results also suggest that the 7/3, 8/3 FQH states, and the RIQH states in the third LL are spin-polarized or partially spin-polarized.

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.

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.

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.

The role of edge magnetism on the Kohn-Sham gap and fundamental energy gap of graphene quantum dots with zigzag edges

Energy gap engineering in graphene nanostructures is one of the most important topics towards development of graphene-based electronics. In this work, based on the density functional theory, the role of the edge magnetism on the size dependence of Kohn-Sham gap and fundamental energy gap for C6nnH6n (n=2−16) hexagonal graphene quantum dots (GQDs) with zigzag edges is studied. We found a transition from a nonmagnetic to an antiferromagnetic state at a certain critical diameter (∼ 3 nm), characterized by the opening of a Kohn-Sham gap as a consequence of the exchange interaction between localized edge states. Furthermore, the fundamental gap is obtained from the difference between the calculated vertical ionization and electron affinity energies. Such approximation includes relaxation in the exchange correlation potential when the electron is added to the system, which might be useful for GQDs transport properties interpretation. We found a scaling rule for the fundamental gap dependence on quantum dot size, providing a practical way to predict this property for large GQDs with zigzag edges, which currently in most demanding approaches, such as GW, is unfeasible.

Temperature dependence of the energy band gap of CuSi2P3 semiconductor using PSOPW method

Abstract Theoretical formalism based on the orthogonalized plane wave method supplemented by a potential scaling scheme was used to predict the temperature dependence of energy gap of CuSi2P3 semiconductor. A computer code in Pascal was used to perform the variation of fundamental energy gap with temperature in the range of 150 K to 800 K. The dependence of energy gap on temperature for lattice dilation contribution, lattice vibration contribution and total temperature effect were performed separately. The results revealed that, as temperature increases, the top of the valence band and the bottom of the conduction band increase, while the energy band gap decreases. Generally, at low temperatures, the energy gap varies slowly and exhibits a nonlinear dependence and approaches linearity as temperature increases. The calculated energy gap of CuSi2P3 at T = 300 K is 0.4155 eV. The temperature coefficients in the linear region due to lattice dilation contribution, lattice vibration contribution and total temperature effect were calculated as –1.101 × 10−5 eV/K, –1.637 × 10−4 eV/K and –1.7523 × 10−4 eV/K, respectively. Also, the ratio of temperature coefficient of the energy gap due to LV contribution to its value and LD contribution in the linear region is equal to 14.868. That ratio is compared to those of CuGe2P3 and III-V compounds, where those of the latter show a systematic change with Eg. Moreover, the Eg of all the compounds shows a quadratic dependence on the inverse of mean bond length.

Combining the roles of bose correlations and pairing associations to condensation and its influence on optical conductivity; special superconductors

Bose correlations (BC) and pairing associations (PA) are known as two distinct origins for condensation. For most known condensates, condensation is only attributed to one of them. Recent developments call for research on combining the roles of the two origins. Therefore, we investigate the pairing and condensation in a boson-like system through a quantum microscopic approach based on correlation functions and order parameter equation (OPE). For the first time, we obtain general expressions for complex optical conductivity of such condensates. We focus on an interesting self-consistent solution of the OPE, i.e. a condensate with spherically symmetric order parameter growing linearly up to a radius in the momentum-space, and extract interesting predictions for it. The appearance of a temperature dependent energy gap in the middle of excitation spectrum and the appearance of a pseudogap in the absorption spectrum (without a counterpart in the excitation spectrum) in addition to the possibility of condensation at room temperature are among our predictions. Also, the paper provides several coherent pieces of evidence for the interest: the roles of BC and PA combine for creating a novel superconductor similar to MgB2.

The entrainment matrix of a superfluid nucleon mixture at finite temperatures

It is considered a closed system of non-linear equations for the entrainment matrix of a non-relativistic mixture of superfluid nucleons at arbitrary temperatures below the onset of neutron superfluidity, which takes into account the essential dependence of the superfluid energy gap in the nucleon spectra on the velocities of superfluid flows. It is assumed that the protons condense into the isotropic $^1$S$_0$ state, and the neutrons are paired into the spin-triplet $^3$P$_2$ state. It is derived an analytic solution to the non-linear equations for the entrainment matrix under temperatures just below the critical value for the neutron superfluidity onset. In general case of an arbitrary temperature of the superfluid mixture the non-linear equations are solved numerically and fitted by simple formulas convenient for a practical use with an arbitrary set of the Landau parameters.

DFT studies on solvent dependence of electronic absorption spectra of free-base and protonated porphyrin

Abstract The nature of the solvent and composition of the solution in which optical absorption spectra are acquired can substantially affect the structures of analytes as well as spectra and relative intensities of spectral features. We have computed the effects of the solvent for dilute solutions on the electronic absorption spectra and structures of a prototypical porphyrin—free-base porphyrin (herein referred to as FBP and H2FBP)—and its protonated form (H4FBP) in the ground and the lowest triplet states, both important in device and medical applications. Density functional theory (DFT) and time-dependent-DFT (TD-DFT) have been employed to investigate solvent effects on structure and singlet-singlet (S0 → Sn) and triplet-triplet (T1 → Tn) absorption spectra. We also calculated the solvent dependence of energy gaps between optically allowed singlet and nearest triplet states for the species. Soret band positions are found to be strongly solvent dependent, but less so are Q bands. The finding of dependence of the spectroscopic features of the porphyrin on the dielectric constant of the solvent, from 1.00 (gas-phase) to about 20.493 (acetone)—with the features becoming stable for polar solvents with the dielectric constants e > 20—can be used to monitor micro environmental changes of porphyrin (and its derivatives) embedded in biological systems and nanoparticles; such composites may also be suitable to study and monitor modifications of the chemical environment in different solutions and interactions with biological systems and nanoparticles. This is first systematical study on the solvent-dependence of the porphyrin compounds.