Value Of Effective Mass Research Articles

Quasinormal modes of magnetized black hole

We investigate charged, massive scalar field around static, spherically symmetric black hole immersed into an external asymptotically uniform magnetic field $B$. It is shown that for given multipole number $\ell$ there are $2\ell+1$ numbers of modes due to the Zeeman effect appearing by an interaction of the external magnetic and charged scalar fields introducing an effective mass of the scalar field $\mu_{\rm eff}=\sqrt{\mu^2-mqB}$ where $m$ is the azimuthal number and $q$ is the charge coupling constant. We calculate threshold value of effective mass in which quasinormal modes are arbitrarily long lived and beyond that value quasinormal modes vanish. In the case of $m qB<0$ quasinormal modes are longer lived with larger oscillation frequencies. Whenever, magnetic and massive scalar fields satisfies condition $\mu_{\rm eff}^2<0$, an instability appears, i.e., if $qB>0$ or $qB<0$ there is an instability for the values of azimuthal number $m>\mu^2/qB$ or $m<\mu^2/qB$, respectively.

Structural, elastic, electronic and optical properties of lead-free halide double perovskite Cs2AgBiX6 (X = Cl, Br, and I)

In search of the lead-free photovoltaic materials that could be used as the potential alternative of the front runner hybrid perovskite CH3NH3PbI3, we have employed first-principles calculations to investigate the structural, elastic, electronic and optical properties of lead-free inorganic halide double perovskite materials Cs2AgBiX6 (X = Cl, Br and I). The optimized lattice parameters are in good agreement with experimental values (for X = Cl and Br) and we predict lattice parameter for not synthesized yet Cs2AgBiI6 to be 12.27 Å. Among the double perovskites under investigation, the Cs2AgBiBr6 composition has the highest elastic constants and elastic modulli values which indicate the large response to uniaxial strain with high incompressibility and hardness of the bromide composition compared to other two compositions. We believe that almost equal sized alternate octahedral in bromide composition reproduce an incompressible and hard structure. All the three halide compositions have indirect nature of the band gap with the high dispersive nature of valence band maximum and conduction band minimum and results in smaller values of effective masses. The energy band gap decreases by replacing smaller Cl ion to larger I ion in Cs2AgBiX6 composition and as a result the tendency of the red shift in all the optical spectra is observed.

Two functionals approach in DFT for the prediction of thermoelectric properties of Fe2ScX (X = P, As, Sb) full-Heusler compounds

In the quest for new thermoelectric materials with high power factors, full-Heusler compounds having flat band are found to be promising candidates. In this direction, Fe2ScX (X = P,As,Sb) compounds are investigated using mBJ for the band gap and SCAN to describe the electronic bands and phonon properties for thermoelectric applications. The band gaps obtained from mBJ are 0.81 eV, 0.69 eV and 0.60 eV for Fe2ScX compounds. The phonon dispersion, phonon density of states (DOS) and partial DOS are calculated. The phonon contributions to specific heat are obtained as a function of temperature under harmonic approximation. The electronic band structutre calculated from mBJ and SCAN functionals are qualitatively compared. The effective mass values are calculated at the band extrema from SCAN functional. The thermoelectric parameters are calculated for both hole and electron dopings under semiclassical theory. We use a simple, but reasonable method to estimate the phonon relaxation time (). Using the specific heat, estimated and slopes (phase velocity) of acoustic branches in the linear region, lattice thermal conductivity () at 300 K is calculated for three compounds. The obtained values of with constant are 18.2, 13.6 and 10.3 Wm−1 K−1, respectively. Finally, the temperature dependent figure of merit ZT values are calculated for optimal carrier concentrations in the doping range considered, to evaluate the materials for thermoelectric application. The ZT values for n-type Fe2ScX, in 900–1200 K, are 0.34–0.43, 0.40–0.48 and 0.45–0.52, respectively. While, the p-type Fe2ScX have ZT values of 0.25–0.34, 0.20–0.28 and 0.18–0.26, respectively in the same temperature range. The ZT values suggest that, Fe2ScX compounds can be promising materials in high temperature power generation application on successful synthesis and further reduction by methods like nanostructuring.

Intervalley polaron in atomically thin transition metal dichalcogenides

We study theoretically intervalley coupling in transition-metal dichalcogenide monolayers due to electron interaction with short-wavelength phonons. We demonstrate that this intervalley polaron coupling results in (i) a renormalization of the conduction band spin splitting and (ii) an increase of the electron effective masses. We also calculate the renormalization of the cyclotron energy and the Landau level splitting in the presence of an external magnetic field. An inter-valley magneto-phonon resonance is uncovered. Similar, but much weaker effects are also expected for the valence band holes. These results might help to resolve the discrepancy between ab initio values of the electron effective masses and the ones deduced from magneto-transport measurements.

From homogeneous matter to finite nuclei: Role of the effective mass

Recent astronomical observations, nuclear-reaction experiments, and microscopic calculations have placed new constraints on the nuclear equation of state (EoS) and revealed that most nuclear structure models fail to satisfy those constraints upon extrapolation to infinite matter. A reverse procedure for imposing EoS constraints on nuclear structure has been elusive. Here we present for the first time a method to generate a microscopic energy density functional (EDF) for nuclei from a given immutable EoS. The method takes advantage of a natural Ansatz for homogeneous nuclear matter, the Kohn-Sham framework, and the Skyrme formalism. We apply it to the realistic nuclear EoS of Akmal-Pandharipande-Ravenhall and describe successfully closed-(sub)shell nuclei. In the process, we provide predictions for the neutron skin thickness of nuclei based directly on the given EoS. Crucially, bulk and static nuclear properties are found practically independent of the assumed effective mass value - a unique result in bridging EDF of finite and homogeneous systems in general.

Effect of S and Se replacement on electronic and thermoelectric features of BaCu2GeQ4 (Q = S, Se) chalcogenide crystals

For the Quaternary Chalcogenides BaCu2GeQ4 (Q = S, Se) optoelectronic, structural and transport properties are explored using accurate all electrons FP–LAPW [full potential linearized augmented plane wave] method. The lattice constants calculated theoretically are established to be in fine agreement with the values that are measured experimentally. It is established that all the titled compounds are direct band gap semiconductors and this energy band gap is situated Γ- Γ symmetry points. At the same time the band gap magnitude decreases during replacement of S by Se. Optical constant dispersion like imaginary and real components of dielectric functions, extinction coefficients, refractive indices, reflectivity's, absorption coefficients, have been also calculated for these compounds. High absorption and the direct band gap characteristics of these compounds in the UV–Visible energy range indicate that these perovskite structures might be used in optoelectronic and optical devices working in the UV–Visible range of the energy spectrum. The calculated birefringence (−0.012 and −0.083) enhances the suitability of BaCu2GeSe4 compared to BaCu2GeS4 usually applied as nonlinear optical materials. The computed transport coefficients exhibit the anisotropic nature of the materials, in agreement with their electronic states. The transport properties show stronger carrier concentrations along the Ge-s, Ba-p and Cu-d orbitals, confirming that these orbitals are principal for the electrical transport features. The values of effective mass of electrons are computed by curvature of the CBM band 124 and 184 for BaCu2GeS4 and BaCu2GeSe4 respectively. Investigating of the thermoelectric power factor shows that BaCu2GeS4 is much better than BaCu2GeSe4 over the entire temperature interval which make it suitable for future technological applications.

Study on fabrication of conductive antimony doped tin oxide thin films (SnOx:Sb) by 3rd generation mist chemical vapor deposition

Transparent conducing antimony doped tin oxide (SnOx:Sb) thin films were fabricated by solution-processed atmospheric-pressure 3rd generation mist chemical vapor deposition (3rd G mist CVD) system with two solution chambers and one mixing chamber to control the atmosphere in reaction area. To study how to reduce the resistivity, the effects of each additive component in dopant solution were investigated; this was followed by identification of components contributing to the fabrication of SnOx:Sb films. Experimentally, the HNO3 clearly affected the resistivity and the formation of SnO2 thin films through a change in the valence state of N derived from HNO3 promoting the transformation of Sn (II) and Sb (III) to Sn (IV) and Sb (V). Moreover, the values of electron and hole effective mass were defined using the specified referred parameters to confirm the band diagram of SnOx:Sb thin films fabricated by 3rd G mist CVD system.

Comparative study of thermoelectric properties of Mg&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;0.3&lt;/sub&gt;Sn&lt;sub&gt;0.7&lt;/sub&gt; doped by Ag or Li

In recent decades, Mg&lt;sub&gt;2&lt;/sub&gt;(Si, Sn) solid solutions have long been considered as one of the most important classes of eco-friendly thermoelectric materials. The thermoelectric performance of Mg&lt;sub&gt;2&lt;/sub&gt;(Si, Sn) solid solutions with outstanding characteristics of low-price, non-toxicity, earth-abundant and low-density has been widely studied. The n-type Mg&lt;sub&gt;2&lt;/sub&gt;(Si, Sn) solid solutions have achieved the dimensionless thermoelectric figure of merit &lt;i&gt;ZT&lt;/i&gt; ~1.4 through Bi/Sb doping and convergence of conduction bands. However, the thermoelectric performances for p-type Mg&lt;sub&gt;2&lt;/sub&gt;(Si, Sn) solid solutions are mainly improved by optimizing the carrier concentration. In this work, the thermoelectric properties for p-type Mg&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;0.3&lt;/sub&gt;Sn&lt;sub&gt;0.7&lt;/sub&gt; are investigated and compared with those for different p-type dopant Ag or Li. The homogeneous Mg&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;0.3&lt;/sub&gt;Sn&lt;sub&gt;0.7&lt;/sub&gt; with Ag or Li doping is synthesized by two-step solid-state reaction method at temperatures of 873 K and 973 K for 24 h, respectively. The transport parameters and the thermoelectric properties are measured at temperatures ranging from room temperature to 773 K for Mg&lt;sub&gt;2(1–&lt;i&gt;x&lt;/i&gt;)&lt;/sub&gt;Ag&lt;sub&gt;2&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Si&lt;sub&gt;0.3&lt;/sub&gt;Sn&lt;sub&gt;0.7&lt;/sub&gt; (&lt;i&gt;x&lt;/i&gt; = 0, 0.01, 0.02, 0.03, 0.04, 0.05) and Mg&lt;sub&gt;2(1–&lt;i&gt;y&lt;/i&gt;)&lt;/sub&gt;Li&lt;sub&gt;2&lt;i&gt;y&lt;/i&gt;&lt;/sub&gt;Si&lt;sub&gt;0.3&lt;/sub&gt;Sn&lt;sub&gt;0.7&lt;/sub&gt; (&lt;i&gt;y&lt;/i&gt; = 0, 0.02, 0.04, 0.06, 0.08) samples. The influences of different dopants on solid solubility, microstructure, carrier concentration, electrical properties and thermal transport are also investigated. The X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images show that the solid solubility for Ag and for Li are &lt;i&gt;x&lt;/i&gt; = 0.03 and &lt;i&gt;y&lt;/i&gt; = 0.06, respectively. Based on the assumption of single parabolic band model, the value of effective mass ~1.2&lt;i&gt;m&lt;/i&gt;&lt;sub&gt;0&lt;/sub&gt; of p-type Mg&lt;sub&gt;2(1–&lt;i&gt;x&lt;/i&gt;)&lt;/sub&gt;Ag&lt;sub&gt;2&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Si&lt;sub&gt;0.3&lt;/sub&gt;Sn&lt;sub&gt;0.7&lt;/sub&gt; and Mg&lt;sub&gt;2(1–&lt;i&gt;y&lt;/i&gt;)&lt;/sub&gt;Li&lt;sub&gt;2&lt;i&gt;y&lt;/i&gt;&lt;/sub&gt;Si&lt;sub&gt;0.3&lt;/sub&gt;Sn&lt;sub&gt;0.7&lt;/sub&gt; are similar to that reported in the literature. The comparative results demonstrate that the maximum carrier concentration for Ag doping and for Li doping are 4.64×10&lt;sup&gt;19&lt;/sup&gt; cm&lt;sup&gt;–3&lt;/sup&gt; for &lt;i&gt;x&lt;/i&gt; = 0.01 and 15.1×10&lt;sup&gt;19&lt;/sup&gt; cm&lt;sup&gt;–3&lt;/sup&gt; for &lt;i&gt;y&lt;/i&gt; = 0.08 at room temperature, respectively; the Li element has higher solid solubility in Mg&lt;sub&gt;2&lt;/sub&gt;(Si, Sn), which leads to higher carrier concentration and power factor &lt;i&gt;PF&lt;/i&gt; ~1.62×10&lt;sup&gt;–3&lt;/sup&gt; &lt;inline-formula&gt;&lt;tex-math id="Z-20190527102739-2"&gt;\begin{document}${\rm W}\cdot{\rm m^{–1}}\cdot{\rm K^{–2}}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20190247_Z-20190527102739-2.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20190247_Z-20190527102739-2.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; in Li doped samples; the higher carrier concentration of Li doped samples effectively suppresses the bipolar effect; the maximum of &lt;i&gt;ZT&lt;/i&gt; ~0.54 for Mg&lt;sub&gt;1.92&lt;/sub&gt;Li&lt;sub&gt;0.08&lt;/sub&gt;Si&lt;sub&gt;0.3&lt;/sub&gt;Sn&lt;sub&gt;0.7&lt;/sub&gt; is 58% higher than that of Mg&lt;sub&gt;1.9&lt;/sub&gt;Ag&lt;sub&gt;0.1&lt;/sub&gt;Si&lt;sub&gt;0.3&lt;/sub&gt;Sn&lt;sub&gt;0.7&lt;/sub&gt; samples. The lattice thermal conductivity of Li or Ag doped sample decreases obviously due to the stronger mass and strain field fluctuations in phonon transport.

CuO–SnO2 reverse cubic heterojunctions as high-performance supercapacitor electrodes

The high value of hole effective mass is the main reason for the high specific capacitance of CuO–SnO2 heterojunctions.

Hydrogenation and Fluorination of 2D Boron Phosphide and Boron Arsenide: A Density Functional Theory Investigation.

First-principles density functional theory calculations are performed to study the stability and electronic properties of hydrogenated and fluorinated two-dimensional sp3 boron phosphide (BP) and boron arsenide (BAs). As expected, the phonon dispersion spectrum and phonon density of states of hydrogenated and fluorinated BX (X = P, As) systems are found to be different, which can be attributed to the different masses of hydrogen and fluorine. Hydrogenated BX systems bear larger and indirect band gaps and are found to be different from fluorinated BX systems. These derivatives can be utilized in hydrogen storage applications and ultrafast electronic devices. Finally, we investigated the stability and electronic properties of stacked bilayers of functionalized BP. Interestingly, we found that these systems display strong interlayer interactions, which impart strong stability. In contrast with the electronic properties determined for the fluorinated/hydrogenated monolayers, we found that the electronic properties of these bilayers can finely be tuned to a narrow gap semiconductor, metallic or nearly semimetallic one by selecting a suitable arrangement of layers. Moreover, the nearly linear dispersion of the conduction band edge and the heavy-, light-hole bands are the interesting characteristics. Furthermore, the exceptional values of effective masses assure the fast electronic transport, making this material very attractive to construct electronic devices.

Unbalanced Superconductivity Induced by the Constant Electron–Phonon Coupling on a Square Lattice

AbstractThe properties of the unbalanced superconducting state on a square lattice with the constant value of the electron–phonon coupling function are analyzed. The analysis is conducted in the framework of the Eliashberg formalism, explicitly considering the k‐dependence of the electron and phonon dispersion relations. It is found that the balanced superconducting state does not induce itself in the system due to the high value of the electron effective mass. However, in the unbalanced case, the thermodynamic properties of the superconducting condensate can be distinctly different from the predictions of the Bardeen–Cooper–Schieffer theory, when the coupling constant value in the diagonal channel of the self‐energy is diminished compared with the non‐diagonal channel. This is due to the reduced dimensionality of the tested system and the strong‐coupling effects included in the Eliashberg formalism.

Database Screening of Ternary Chalcogenides for P-type Transparent Conductors

In this work, we investigated ternary chalcogenide semiconductors to identify promising p-type transparent conducting materials (TCMs). High-throughput calculations were employed to find the compounds that satisfies our screening criteria. Our screening strategy was based on the size of band gaps, the values of hole effective masses, and p-type dopability. Our search led to the identification of seven promising compounds (IrSbS, Ba2GeSe4, Ba2SiSe4, Ba(BSe3)2, VCu3S4, NbCu3Se4, and CuBS2) as potential TCM candidates. In addition, branch point energy and optical absorption spectra calculations support our findings. Our results open a new direction for the design and development of p-type TCMs.

MATHEMATICAL MODEL FOR DETERMINING THE STEP TIME AND DISTANCE COVERED BY THE DROPLETS OF LUQUID FIRE-FIGHTING AGENT DURING THE PROCESS OF FIRE EXTINGUISHING IN THE CLOSED ROOM

The article deals with the mathematical model for determining the step time and distance covered by the droplet of liquid till the moment of contact with the horizontal plane. Numerical values of the step time and distance covered till the moment of contact with the horizontal plane are obtained. Experimental studies aimed on determining the effective diameter of the droplet are performed. According to effective diameter value an effective mass of the droplet is determined. After obtaining the effective mass value the step time and distance covered by the droplet of liquid till the moment of contact with the horizontal plane are calculated using a mathematical model.

Modeling the Reverse Gate-Leakage Current in GaN-Channel HFETs: Realistic Assessment of Fowler–Nordheim and Leakage at Mesa Sidewalls

A model considering different leakage paths for describing the reverse gate leakage in mesa-isolated polar GaN-channel heterostructure field-effect transistors (HFETs) is presented. For AlGaN/GaN HFETs, it is illustrated that for small negative values of gate–source bias, gate leakage happens from the gate-covered mesa sidewalls to the 2-D electron gas (2-DEG). The bias and temperature dependences of the gate current show that the sidewall path to the 2-DEG is associated with the Poole–Frenkel electron emission. As the gate–source bias becomes more negative, electrons choose a different path to the 2-DEG. In this case, results corroborate that the gate leakage is dominated by the Fowler–Nordheim (FN) direct tunneling process through the III-nitride barrier. The novel contribution of the present analysis is that it postulates that in absence of absolute uniformity, FN tunneling takes place through only a small portion of the surface of the barrier, which boasts the highest electric field or the smallest Schottky barrier height. By applying this hypothesis, the origin of the inconsistencies inherent to the previously presented models in selecting the value of the electron effective mass can be explained.

Deposition and characterization of spin-coated n-type ZnO thin film for potential window layer of solar cell

Spin-coating technique has been utilized to deposit ZnO thin films on ITO substrates. The dependence of the structural, optical, and electrical properties of the prepared films on both the sol–pH values and annealing temperatures has been investigated. X-ray diffraction analysis has revealed that the optimal sol–pH value was found to be 8 at annealing temperature 450 °C. At these conditions, the optical transmission and reflection data have also shown a clear enhancement in both the morphology and the grain size of the films. In addition, the analysis of I–V and C–V measurements of the Schottky Au/ZnO diode has shown a high agreement with both the investigated structural and optical properties. Furthermore, a new empirical method based on the calculated electrical parameters in the context of both the thermionic emission and Mott–Schottky models has been proposed to evaluate the approximate value of the electron effective mass in the conduction band of the ZnO (m* = 0.224 mo).

First-principles calculation of the optoelectronic properties of doped methylammonium lead halide perovskites: A DFT-based study

Methylammonium lead halide perovskites have emerged as an exciting material for research by virtue of their attractive optoelectronic properties and easy fabrication techniques leading to their use in optical devices, especially solar cells. An extensive knowledge of these properties along with a study of how they can be potentially tuned to suit requirements and improve the efficiency of devices is imperative in the realization of the full potential of these materials. In this regard, we present first-principles modeling and analysis of lead halide organometallic perovskites to determine their optoelectronic properties using density functional theory. Upon validating our model parameters by comparing the calculated optical constants and bandgaps of pure methylammonium lead halide perovskites with published experimental data, we extended the analysis to perovskites doped with various percentages of halogen atoms. We have determined the values of refractive index and extinction coefficient at different doping concentrations for a wide range of photon energies, establishing a comparison between them. In addition, we have investigated the variation in the bandgap and the effective mass of charge carriers with changes in doping concentration. We have also presented the effect of incorporating spin–orbit coupling on the band structure calculations. Our results show a significant reduction in the calculated value of the bandgap and also substantial changes in the values of effective mass upon incorporation of spin–orbit coupling. The data presented in this paper highlight the favorable optoelectronic properties of methylammonium lead halide perovskites and demonstrate the trends in these properties upon halogen doping, thereby facilitating the realization of perovskite-based high-performance solar cells, lasers and other optoelectronic devices with improved efficiency.

Two-body contributions to the effective mass in nuclear effective interactions

Starting from general expressions of well-chosen symmetric nuclear matter quantities derived for both zero- and finite-range effective theories, we derive the contributions to the effective mass. We first show that, independently of the range, the two-body contribution is enough to describe correctly the saturation mechanism but gives an effective mass value around $m^*/m \simeq 0.4$. Then, we show that the full interaction (by instance, an effective two-body density-dependent term on top of the pure two-body term) is needed to reach the accepted value $m^*/m \simeq 0.7-0.8$.

Phonon-induced renormalization of the electron spectrum of biased bilayer graphene

The effect of the electron-phonon interaction on the electron subsystem of the bilayer graphene has been investigated in the case when there is a potential bias between the graphene layers. The electron-phonon interaction has been shown to lead to increasing of the curvature of the lower dispersion branch of the conduction band of the bigraphene in the vicinity of the Dirac point. The latter corresponds to the decreasing of the absolute value of the electron effective mass. The corresponding correction to the effective mass has been calculated. Dependence of this correction on the bias has been investigated. Influence of such effect on the bigraphene conductivity is discussed.

A comparative study of different exchange-correlation functionals in understanding structural, electronic and thermoelectric properties of Fe2VAl and Fe2TiSn compounds

Fe2VAl and Fe2TiSn are full Heusler compounds with non-magnetic ground state. The two compounds are good thermoelectric materials. PBE and LDA (PW92) are the two most commonly used density functionals to study the Heusler compounds. Along with these two well studied exchange-correlation functionals, recently developed PBEsol, mBJ and SCAN functionals are employed to study the two compounds. Using the five functionals equilibrium lattice parameter and bulk modulus are calculated. Obtained values are compared with experimental reports wherever available. Electronic structure properties are studied by calculating dispersion curves, total and partial density of states. For Fe2VAl, band gap of 0.22 eV is obtained from the mBJ potential which is in reasonable agreement with experimental value while, for Fe2TiSn band gap of 0.68 eV is obtained. Fe2VAl is predicted to be semimetallic with different values of negative gaps from LDA, PBEsol, PBE and SCAN functionals. Whereas, Fe2TiSn is found to be semimetallic (semiconducting) from LDA, PBEsol (PBE,SCAN) functionals employed calculations. From the dispersion curve effective mass values are also computed to see the contribution to the Seebeck coefficient. In Fe2TiSn, a flat band is present along the Γ-X direction with calculated value of effective mass ∼36 more than the mass of electron. The improvements or inadequacies among the functionals in explaining the properties of full Heusler alloys for thermoelectric application are thus observed through this study.

Electrophysical parameters of the space charge region of the low-threshold field cathode

It is shown that the two-dimensional-electronic charge on the cathode surface gives the main contribution to emission currents measured in the nanosecond range. This made it possible to propose a new method for determining the effective mass, the de Broglie wavelength, the charge density, and the concentration of free carriers in the near-surface layer of the space charge region of the cathode. The efficiency and accuracy of the method has been proven for the cathode based on the shock-wave modified graphite by comparison of the estimates of the effective mass values of the free charge carriers with the data obtained by other methods.