Constant Relaxation Research Articles

Synthesizing the frequency-dependent sound relaxational absorption by three-frequency sound speeds in excitable gases

Capturing the frequency-dependent sound relaxational absorption in excitable gases has the potential of wide applications in gas detection; however, it is still challenging to accurately measure it in real time. With the aid of the relationship between effective compression coefficient and acoustic relaxation process, an approach to synthesize the frequency-dependent sound relaxational absorption is proposed by using the sound speeds at three frequencies under a single pressure that are far easier to be precisely measured in real time. The proposed approach includes three steps. Firstly, the relaxation strength is calculated from the low-frequency and the high-frequency sound speeds measured at two selected frequencies; secondly, the adiabatic constant pressure relaxation time is calculated from the sound speed measured at the third frequency within the range where the sound relaxational absorption is significant; finally, the frequency-dependent sound relaxational absorption and sound speed dispersion are synthesized by the real and imaginary parts of the normalized high-frequency effective adiabatic coefficient. For gases containing CH4, CO2, CL2 and N2, the synthesized results are consistent with the experimental data. Our approach provides a real-time acoustic solution with higher accuracy and implementation convenience for gas detecting.

A DFT study of electronic, magnetic, optical and transport properties of rare earth element (Gd, Sm)-doped GaN material

The main objective of this article is to deal with and establish a new type of transparent conducting material by doping Gallium Nitride (GaN) with rare-earth RE (Gd, Sm) elements. A theoretical study was performed by using the density functional theory (DFT) implemented in WIEN2k code and BoltzTraP package based on semi-classical Boltzmann transport equation (BTE) within constant relaxation time and rigid band approximation (RBA). The application of the modified Becke–Johnson (TB-mBJ) potential has improved the electronic structure, especially the electronic bandgap. Our simulation result matches with other computational results as well. We have compared the total energy of ferromagnetic (FM) and Anti-ferromagnetic (AFM) configurations for the doped system to analyze the magnetic ground state stability. Preferentially the most stable magnetic configuration is found to be ferromagnetic. The low value of formation energy indicates the possibility of the preparation of these materials in the lab. To further compare the results, we have also used the VNL-ATK quantumwise code based on the Linear Combination of Atomic Orbital (LCAO) approach. VNL-ATK incorporate a new functional DFT-1/2 to correct the Kohn–Sham KS eigenvalues around the minima and maxima of the conduction band, and the valence band, respectively, which enhances the bandgap. It shows the high optical absorption and the high electrical conductivity. These characteristics offer that GaN doped with rare earth elements are potential candidates for optoelectronic, solar cell, and spin-based magnetic devices.

Theory of unidirectional magnetoresistance and nonlinear Hall effect

We study the unidirectional magnetoresistance (UMR) and the nonlinear Hall effect (NLHE) in the ferromagnetic Rashba model. For this purpose we derive expressions to describe the response of the electric current quadratic in the applied electric field. We compare two different formalisms, namely the standard Keldysh nonequilibrium formalism and the Moyal–Keldysh formalism, to derive the nonlinear conductivities of UMR and NLHE. We find that both formalisms lead to identical numerical results when applied to the ferromagnetic Rashba model. The UMR and the NLHE nonlinear conductivities tend to be comparable in magnitude according to our calculations. Additionally, their dependencies on the Rashba parameter and on the quasiparticle broadening are similar. The nonlinear zero-frequency response considered here is several orders of magnitude higher than the one at optical frequencies that describes the photocurrent generation in the ferromagnetic Rashba model. Additionally, we compare our Keldysh nonequilibrium expression in the independent-particle approximation to literature expressions of the UMR that have been obtained within the constant relaxation time approximation of the Boltzmann formalism. We find that both formalisms converge to the same analytical formula in the limit of infinite relaxation time. However, remarkably, we find that the Boltzmann result does not correspond to the intraband term of the Keldysh expression. Instead, the Boltzmann result corresponds to the sum of the intraband term and an interband term that can be brought into the form of an effective intraband term due to the f-sum rule.

Effects of topological band structure on thermoelectric transport of bismuthene

Two-dimensional bismuth (Bi) layer, known as bismuthene, exhibits $Z2$ topological bulk states due to large spin-orbit coupling that inverts the bands. Using the tight-binding method, we calculate the band structure of buckled bismuthene to understand its topological and trivial phases. We determine the thermoelectric properties for some considered phases, incorporating the edge states contribution, by using the linearized Boltzmann transport equation with a constant relaxation time approximation. It is shown that the thermoelectric figure of merit, $ZT$, actually drops in undoped topological bismuthene due to the edge effects. Surprisingly, the topological edge states enhance $ZT$ at large doping with the Fermi energy near the bottom of bulk bands when bismuthene is nearly metallic.

Janus Al2STe monolayer: A prospective thermoelectric material

Two dimensional materials based on chalcogenides are showing exciting thermoelectric efficiency. Recent experimental synthesisation of several Janus monolayers inspire investigation of novel materials. In this paper, we study the thermoelectric performance of the novel Janus Al2STe monolayer using Density Functional theory. We found that Janus Al2STe monolayer is an indirect band gap semiconductor with an energy gap of 1.17 eV and is dynamically stable. The transport coefficients such as Seebeck coefficient, electrical conductivity, power factor, and electronic thermal conductivity are evaluated using BoltzTraP2 code based on Boltzmann transport theory within constant relaxation time approximation. The dependence of relaxation time on temperature has been approximated with the deformation potential theory. Further, the lattice component of thermal conductivity has been obtained using ShengBTE code. The calculated value of ZT shows that both n-type and p-type Janus monolayer have outstanding thermoelectric performance. From the evaluated results, we conclude that Janus Al2STe monolayer can be used as a propitious candidate in the fabrication of thermoelectric devices.

Surprisingly good thermoelectric performance of monolayer C3N

The rapid emergence of graphene has attracted numerous efforts to explore other two-dimensional materials. Here, we combine first-principles calculations and Boltzmann theory to investigate the structural, electronic, and thermoelectric transport properties of monolayer C3N, which exhibits a honeycomb structure very similar to graphene. It is found that the system is both dynamically and thermally stable even at high temperature. Unlike graphene, the monolayer has an indirect band gap of 0.38 eV and much lower lattice thermal conductivity. Moreover, the system exhibits obviously larger electrical conductivity and Seebeck coefficients for the hole carriers. Consequently, the ZT value of p-type C3N can reach 1.4 at 1200 K when a constant relaxation time is predicted by the simple deformation potential theory. However, such a larger ZT is reduced to 0.6 if we fully consider the electron–phonon coupling. Even so, the thermoelectric performance of monolayer C3N is still significantly enhanced compared with that of graphene, and is surprisingly good for low-dimensional thermoelectric materials consisting of very light elements.

Elastic, electronic, optical and thermoelectric properties of perovskite: BaTbO3

We investigate the electronic structure, elastic, optical and thermoelectric properties of single-perovskite BaTbO3 by means of the density functional theory approach employing generalized gradient approximation and an on-site coulomb potential. The calculated lattice constants are found to be in good agreement with the available experimental results. The elastic property calculations reveal that BaTbO3 is mechanically stable and ductile in nature. The spin polarized density of states and band structure calculations shows a semiconducting state with band gap ~ 1 eV with anti-ferromagnetic ground state. Based on the obtained values of optical parameters, BaTbO3 is found to be optically active both in the ultra-violet and visible region of the spectrum. Furthermore, thermoelectric properties are evaluated by using BoltzTraP code based on Boltzmann transport theory employing constant relaxation time and rigid band approximation in the temperature range 300 – 1200 K. The noticeable value of power factor is observed at higher temperature region with the maximum value ~ 12 μWcm−1K−2 at 1200 K and the lattice thermal conductivity is significantly low in the entire temperature range. The maximum value of ZT found to be ~ 0.35. The results reveal that this compound can be used as a good optoelectronic application as well as thermoelectric material in the high temperature region.

Comprehensive ab-initio calculations of AlNiX (X = P, As and Sb) half-Heusler compounds: Stabilities and applications as green energy resources

The objective of this work is to explore structural, mechanical and thermal stability including electronic, optical and thermoelectric properties of state-of-the-art AlNiX (X = P, As and Sb) half-Heusler compounds having 18 valence electron count (VEC). Calculations have been carried out in the framework of density functional theory (DFT) implemented in WIEN2k code followed by the solution of the Boltzmann transport equation with constant relaxation time approximation. In the most energetically favorable structure, their lattice constants lie in the range of 5.501 Å – 5.983 Å and band gaps have been found to be 0 eV, which confirm the metallic behavior of these compounds. The different elastic and thermodynamic parameters confirm their mechanical and thermal stability with a mechanically anisotropic and ductile nature. Specific heat and entropy of these materials increase as the atomic number of X atom increases. The calculated optical parameters prove their candidature for promising photovoltaic devices and shields for UV radiation. The thermal conductivity has been computed using Slack's model. This detailed study shows that these compounds are promising for materials scientists due to their very high lattice thermal conductivity and low figure of merit. We deny recommending them as good thermoelectric applications.

Strong electron\u2013phonon coupling influences carrier transport and thermoelectric performances in group-IV/V elemental monolayers

The interactions between electrons and phonons play the key role in determining the carrier transport properties in semiconductors. In this work, comprehensive investigations on full electron–phonon (el–ph) couplings and their influences on carrier mobility and thermoelectric (TE) performances of 2D group IV and V elemental monolayers are performed, and we also analyze the selection rules on el–ph couplings using group theory. For shallow n/p-dopings in Si, Ge, and Sn, ZA/TA/LO phonon modes dominate the intervalley scatterings. Similarly strong intervalley scatterings via ZA/TO phonon modes can be identified for CBM electrons in P, As, and Sb, and for VBM holes, ZA/TA phonon modes dominate intervalley scatterings in P while LA phonons dominate intravalley scatterings in As and Sb. By considering full el–ph couplings, the TE performance for these two series of monolayers are predicted, which seriously downgrades the thermoelectric figures of merits compared with those predicted by the constant relaxation time approximation.

High thermoelectric performance in cubic inorganic halide perovskite material AgCdX3 (X = F and Cl) from first principles

The need for high-performance thermoelectric materials is a hot topic under research to harvest the waste heat generated in the environment. In the present work, we have studied the inorganic halide perovskite AgCdX3 (X = F and Cl) in the cubic phase with the Pm − 3 m space group. We have explored the structural, electronic, elastic, and thermoelectric properties for both materials using first -principles calculations based on density functional theory. The thermoelectric transport coefficients have been calculated from band structure results with the aid of the BoltzTraP2 code based on the semiclassical Boltzmann theory under the constant relaxation time approximation. The deformation potential theory has been implemented to estimate the relaxation time for charge carriers. The Slack equation is solved to evaluate the contribution of lattice thermal conductivity. The results show that the AgCdCl3 has a high value of the figure of merit (ZT) in comparison to that of AgCdF3 at any temperature irrespective of the type of charge carriers. Our findings will provide a guide for the experimentalists to develop high-efficient thermoelectric materials.

Electronic properties of bismuth nanostructures

The passivation of thin Bi(1 1 1) films with hydrogen and oxide capping layers is investigated from first principles. Considering termination-related changes of the crystal structure, we show how the bands and density of states are affected. In the context of the much discussed semimetal-to-semiconductor transition and the band topology of the bulk material, we consider the effects of confinement in the whole Brillouin zone and go beyond standard density functional theory by including many-body interactions via the G$_0$W$_0$ approximation. The conductivity of unterminated films is calculated via the Boltzmann transport equation using the simple constant relaxation time approximation and compared to experimental observations that have suggested a two-channel model.

First-principles calculations of inherent properties of Rb based state-of-the-art half-Heusler compounds: promising materials for renewable energy applications

In the present work, we have studied structural, electronic, optical and thermoelectric properties of Rb based state-of-the-art materials RbYZ (Y = Be, Mg, Ca, Sr and Ba; Z = P, As, Sb and Bi) having 8 valence electron count (VEC) using density functional theory followed by solution of Boltzmann transport equation with constant relaxation time approximation. The exchange and correlation potential are described by the GGA of Wu and Cohen (GGA-WC); the Becke-Johnson approach modified by Tran and Blaha (TB-mBJ) has been used to model the exchange-correlation potential. The bandgap of these materials lies in the range of 0.201 eV—2.591 eV. The various optical parameters are comparable with the state-of-the-art photovoltaic materials. Thermoelectric properties have been computed at 300 K, 600 K and 900 K. At these temperatures lattice thermal conductivity have been computed using Slack’s model. This detailed study shows that these compounds are promising for renewable energy applications.

Electronic and thermo‐physical properties of double antiperovskites X6SOA2 (X = Na, K and A = Cl, Br, I): A non‐toxic and efficient energy storage materials

AbstractWe have explored the structural, electronic and transport parameters of cubic double antiperovskite structure X6SOA2 (X = Na, K and A = Cl, Br, I) using density functional theory followed by the solution of Boltzmann transport equation with constant relaxation time approximation. The exchange and correlation potential are described by the PBE‐GGA; the Becke‐Johnson approach modified by Tran and Blaha (TB‐mBJ) has been used to model the exchange‐correlation potential. Band gap of these materials have been found in between 2.85–4.24 eV. Thermoelectric properties have been computed at 300, 600 and 900 K. It has been found that figure of merit of double antiperovskite materials approaches to unity in both n‐ and p‐type regions. Hence these materials may turn out to be potential thermoelectric candidates. As these properties of the titled compounds have been explored for the very first time, hence, this work may open a new panorama for various detailed experimental and theoretical studies in the quest for non‐toxic, environmentally safe, and efficient energy sources.

Origins of anisotropic transport in the electrically switchable antiferromagnet Fe1/3NbS2

Recent experiments on the antiferromagnetic intercalated transition metal dichalcogenide $\mathrm{Fe_{1/3}NbS_2}$ have demonstrated reversible resistivity switching by application of orthogonal current pulses below its magnetic ordering temperature, making $\mathrm{Fe_{1/3}NbS_2}$ promising for spintronics applications. Here, we perform density functional theory calculations with Hubbard U corrections of the magnetic order, electronic structure, and transport properties of crystalline $\mathrm{Fe_{1/3}NbS_2}$, clarifying the origin of the different resistance states. The two experimentally proposed antiferromagnetic ground states, corresponding to in-plane stripe and zigzag ordering, are computed to be nearly degenerate. In-plane cross sections of the calculated Fermi surfaces are anisotropic for both magnetic orderings, with the degree of anisotropy sensitive to the Hubbard U value. The in-plane resistance, computed within the Kubo linear response formalism using a constant relaxation time approximation, is also anisotropic, supporting a hypothesis that the current-induced resistance changes are due to a repopulating of AFM domains. Our calculations indicate that the transport anisotropy of $\mathrm{Fe_{1/3}NbS_2}$ in the zigzag phase is reduced relative to stripe, consistent with the relative magnitudes of resistivity changes in experiment. Finally, our calculations reveal the likely directionality of the current-domain response, specifically, which domains are energetically stabilized for a given current direction.

Investigations of the electronic, dynamical, and thermoelectric properties of Cd1-xZnxO alloys: First-principles calculations

The structural, dynamical, electronic and thermoelectric properties of rock-salt and wurtzite Cd1-xZnxO alloys were investigated using the density functional theory (DFT). In addition, the semi-classical Boltzmann transport theory was utilized to evaluate the thermoelectric properties with the constant relaxation time approximation. The alloys were found to exhibit a semiconducting behavior with direct and indirect band gaps in their wurtzite and rock-salt structures, respectively, based on the hybrid GGA-mbj functional. The highest Seebeck coefficient values were obtained at 300 K, whereas the highest power factor values were found at 1200 K for all structures. This behavior is promising for thermoelectric applications at high temperatures.

Nonradiative relaxation dynamics in the [Au25-nAgn(SH)18]-1 (n = 1, 12, 25) thiolate-protected nanoclusters.

Evaluation of the electron-nuclear dynamics and relaxation mechanisms of gold and silver nanoclusters and their alloys is important for future photocatalytic, light harvesting, and photoluminescence applications of these systems. In this work, the effect of silver doping on the nonradiative excited state relaxation dynamics of the atomically precise thiolate-protected gold nanocluster [Au25-nAgn(SH)18]-1 (n = 1, 12, 25) is studied theoretically. Time-dependent density functional theory is used to study excited states lying in the energy range 0.0-2.5 eV. The fewest switches surface hopping method with decoherence correction was used to investigate the dynamics of these states. The HOMO-LUMO gap increases significantly upon doping of 12 silver atoms but decreases for the pure silver nanocluster. Doped clusters show a different response for ground state population increase lifetimes and excited state population decay times in comparison to the undoped system. The ground state recovery times of the S1-S6 states in the first excited peak were found to be longer for [Au13Ag12(SH)18]-1 than the corresponding recovery times of other studied nanoclusters, suggesting that this partially doped nanocluster is best for preserving electrons in an excited state. The decay time constants were in the range of 2.0-20 ps for the six lowest energy excited states. Among the higher excited states, S7 has the slowest decay time constant although it occurs more quickly than S1 decay. Overall, these clusters follow common decay time constant trends and relaxation mechanisms due to the similarities in their electronic structures.

Influence of dielectric, Electro-Optic Kerr Effect and spectroscopic characterisation on polar–polar binary liquid mixture

ABSTRACT Complex dielectric spectra of the binary mixture of benzonitrile (BNZ) with acetophenone (ACPH) have been carried out at 11 different concentrations in the frequency range of 10 MHz to 40 GHz at four different temperatures (10°C, 15°C, 20°C and 25°C) using Time Domain Reflectometry technique. Static dielectric constant (ε0), relaxation time (τ) are obtained for above mentioned temperatures using nonlinear least squares fit method and density and refractive index are obtained at room temperature (25°C). These parameters are used to calculate the excess static dielectric constant (ε0 E) and excess inverse relaxation time (1/τ)E, excess molar volume (Vm E), effective Kirkwood correlation factor (geff), Bruggeman factor (fB) and thermodynamic parameters such as enthalpy and entropy of activation of BNZ-ACPH mixture. Static dielectric constant decreases with increasing temperature and increasing concentration of ACPH in BNZ. The deviance in the excess parameters shows the strength of the molecular interactions and the presence of intermolecular interaction between unlike molecules. The measurement of Kerr constant (B) of the binary mixture of BNZ-ACPH have been reported for the wavelengths 632.8 nm and 512 nm for 5 different concentrations at room temperature using Electro-Optic Kerr Effect method. The change in the Kerr constant was observed for the different concentrations associated with the pure liquids. This deviation shows the non-additive behaviour and which is in good agreement with the dielectric parameters of the system under study. Also, the FTIR spectra have been obtained of BNZ-ACPH mixture to have a good agreement with the dielectric study.

Simultaneous memory effects in the stress and in the dielectric susceptibility of a stretched polymer glass.

We report experimental evidence that a polymer stretched at constant strain rate λ[over ̇] presents complex memory effects after λ[over ̇] is set to zero at a specific strain λ_{w} for a duration t_{w}, ranging from 100s to 2.2×10^{5}s. When the strain rate is resumed, both the stress and the dielectric constant relax to the unperturbed state nonmonotonically. The relaxations depend on the observable, on λ_{w} and on t_{w}. Relaxation master curves are obtained by scaling the time and the amplitudes by ln(t_{w}). The dielectric evolution also captures the distribution of the relaxation times, so the results impose strong constraints on the relaxation models of polymers under stress and they can be useful for a better understanding of memory effects in other disorder materials.

Efficient calculation of carrier scattering rates from first principles

The electronic transport behaviour of materials determines their suitability for technological applications. We develop a computationally efficient method for calculating carrier scattering rates of solid-state semiconductors and insulators from first principles inputs. The present method extends existing polar and non-polar electron-phonon coupling, ionized impurity, and piezoelectric scattering mechanisms formulated for isotropic band structures to support highly anisotropic materials. We test the formalism by calculating the electronic transport properties of 23 semiconductors, including the large 48 atom CH3NH3PbI3 hybrid perovskite, and comparing the results against experimental measurements and more detailed scattering simulations. The Spearman rank coefficient of mobility against experiment (rs = 0.93) improves significantly on results obtained using a constant relaxation time approximation (rs = 0.52). We find our approach offers similar accuracy to state-of-the art methods at approximately 1/500th the computational cost, thus enabling its use in high-throughput computational workflows for the accurate screening of carrier mobilities, lifetimes, and thermoelectric power.

Mechanically stable with highly absorptive formamidinium lead halide perovskites [(HC(NH2)2PbX3; X = Br, Cl]: Recent advances and perspectives

AbstractFull potential‐linear augmented plane wave method with two exchange‐correlation potentials Perdew–Burke–Ernzerhof‐generalized gradient approximation and Becke–Johnson have been used to investigate structural, electronic, optical, transport and mechanical anisotropy of formamidinium lead halides (FAPbX3; X = Br, Cl). This computational exploration shows that these materials have a direct band gap, high absorption coefficients and the stability of the compound has been tested using the enthalpy of formation, and elastic stability criteria of the elastic constants. The persistent hybridization ofsstates of Pb andpstates of Br/Cl in valence band contribute significantly in the structural stability. The calculated band gap is 2.26/2.84 eV for FAPbBr3/FAPbCl3and are in concurrence with the experimental and other theoretical studies. As higher absorption promotes higher emissions, optical properties with the peaks of dielectric function spectra with high energy region, and higher absorption peaks show the significant future for these materials to be used in color light‐emitting diode. Parameters of elastic properties like Bulk modulus, Young's modulus, Pugh's ratio and Poisson's ratio show that these have ductile nature and may be deposited as thin films, which is a significant feature in photovoltaic applications. Moreover, electronic transport properties have been calculated within the constant relaxation time approximation. This provided following observations: (a) Seebeck coefficient are noted to decrease with increasing temperature, (b) electrical conductivity are nearly constant within the whole temperature range, (c) thermal conductivity increased with increasing temperature, and (d) power factor and figure of merits are increasing with increasing temperature, and at a given electron and hole concentration (1018–1019 cm−3). The figure of merit signifies that these materials may also be used as thermoelectric devices. These computational observations hereby are of paramount importance for future integrated applications.