arXiv subjects

Sudhir K. Pandey

Publications and source records attributed to Sudhir K. Pandey.

At least 55 records · Page 3Linked to original sources

arXiv2021

An ab-initio study of topological and transport properties of YAuPb

In the last few decades, the study of topological materials has been carried out on an extensive scale. Half-Heusler alloys are well known for their topological behaviours. In this work, we present a detailed study of topological properties of a ternary Half-Heusler alloy, YAuPb, using the tight-binding approach. We have calculated some important topological properties which includes$-$ finding nodes and their chiralities, Berry curvature ($\boldsymbolΩ$) and the surface-states. Based on the study of these properties, we categorise the material as non-trivial topological semimetal. Besides the topological behaviours, we present a comparative study of temperature dependent transport properties corresponding to the chemical potential ($μ$) of the Fermi level and the node points. The results obtained from the calculations of electrical conductivity per unit relaxation time ($\boldsymbolσ/τ$) and the electronic part of thermal conductivity per unit relaxation time ($\boldsymbolκ_0$) indicates the conducting nature of the material to both the heat and electricity. Furthermore, the negative value of $S$ obtained, indicates the n-type behaviour of the compound. The calculated value of electronic specific heat (Pauli magnetic susceptibility) corresponding to Fermi level is $\sim 0.03 \hspace{1mm}(0.18) \times 10^{-2}$ $ Jmol^{-1}K^{-1}$ ($\sim 1.21 \hspace{1mm}(1.14) \times 10^{-10}$ $ m^{3}mol^{-1}$) at 50 (300) K. This work suggests that YAuPb is a promising candidate of non-trivial topological semimetals which can be employed in transmission of heat and electricity, and as n-type material within the temperature range of 50-300 K.

cond-mat.mtrl-sci↗

arXiv2021

First-priniciple based study of transport properties of non-trivial topological fermions of CoSi

Recently, CoSi has been identified to have unconventional electronic topology due to lack of inversion center in its B20 cubic structure. The electronic topology has been reported to be present at three nodal points found in the band structure. Two of these nodal points are situated at the $Γ$ (G1 \& G2) and one at $R$ (R1) point. Based on this, we present a study where various transport coefficients are investigated by using \textit{first-principle} based DFT method for the temperature (T) range 40-300 K. For the chemical potential ($μ$) corresponding to energies of these nodal points and at the Fermi level (E$_F$), 3D constant energy surfaces are constructed. They have shown that the number of states available at energies of these nodal points and the E$_F$ follows an increasing trend as R1 $>$ G2 $>$ E$_F$ $>$ G1 at T = 0 K. Similar increasing behavior seems to follow by other transport coefficients at different $μ$ with T rise such as electrical conductivity($σ$)/relaxation time($τ$) ratio and electronic thermal conductivity ($κ_e/τ$ = $κ^0$). For example, at T = 100 K, $σ/ τ$ $\sim$ 0.13$\times$10$^{20}$ $Ω^{-1} m^{-1} s^{-1}$ at G1 whereas its value reaches $ \sim$ 0.18$\times$10$^{20}$ $Ω^{-1} m^{-1} s^{-1}$ at R1 nodal point. However, Seebeck coefficient ($S$) seems to follow the trend as G2 $>$ E$_F$ $>$ R1 $>$ G1 at any given T. The values of $S$ are obtained to be positive at the $μ$ corresponding to the G2, R1 and E$_F$ (except G1) which is increasing with the rise in T. Also, the dominant charge carriers at G1 point are found to be electrons for T $<$ 225 K whereas for T $>$ 225 K, the charge carriers are obtained to be dominated by holes. Furthermore, the doping concentrations have also been calculated for G1, G2 and R1 points.

cond-mat.str-el↗

arXiv2021

Instrument for simultaneous measurement of Seebeck coefficient and thermal conductivity in the temperature range 300-800 K with python interfacing

Fabrication and characterization of instrument for high-temperature simultaneous measurement of Seebeck coefficient (S) and thermal conductivity ($κ$) has been carried out with python automation. The steady-state based Fourier's law of thermal conduction is employed for $κ$ measurement. The parallel thermal conductance technique is implemented for heat loss measurement. Introducing the thin heater and insulating heater base minimize the heat loss and make the way easier to arrive at high temperature. Measurement of S is carried out using differential method. Same thermocouples are used to measure temperature as well as voltage for S measurement. Care of temperature dependent S of thermocouple has also been taken. Simple design, small size, lightweightmake this instrument more robust. All the components for making sample holder are easily available in the market and can be replaced as per the user demand. This instrument can measure samples with various dimensions and shapes in the temperature range 300 $-$ 800 K. The instrument is validated using different class of samples, such as nickel, gadolinium, Fe$_{2}$VAl and LaCoO$_{3}$. Wide range of S from $\sim$ $-$20 to $\sim$600 $μ$V/K and $κ$ from $\sim$1.1 to $\sim$23.5 W/m-K are studied. The measured values of S and k are in good agreement with the reported data.

cond-mat.mtrl-sci↗

arXiv2021

Studying the lifetime of charge and heat carriers due to intrinsic scattering mechanisms in FeVSb half-Heusler thermoelectric

This work, presents a study of lifetime of carriers due to intrinsic scattering mechanisms $viz.$ electron-electron (EEI), electron-phonon (EPI) and phonon-phonon interactions (PPI) in a promising half-Heusler thermoelectric FeVSb. Using the full-$GW$ method, the effect of EEI and temperature on the valence and conduction band extrema and band gap are studied. The lifetime of carriers with temperature are estimated at these band extrema. At 300 K, estimated value of lifetime at VBM (CBM) is $\sim$1.91 x10$^{-14}s$ ($\sim$2.05 x10$^{-14}s$). The estimated ground state band gap considering EEI is $\sim$378 meV. Next, the effect of EPI on the lifetime of electrons and phonons with temperature are discussed. The comparison of two electron lifetimes suggests that EEI should be considered in transport calculations along with EPI. The average acoustic, optical and overall phonon lifetimes due to EPI are studied with temperature. Further, the effect of PPI is studied by computing average phonon lifetime for acoustic and optical phonon branches. The lifetime of the acoustic phonons are higher compared to optical phonons which indicates acoustic phonons contribute more to lattice thermal conductivity ($κ_{ph}$). The comparison of phonon lifetime due to EPI and PPI suggests that, above 500 K EPI is the dominant phonon scattering mechanism and cannot be ignored in $κ_{ph}$ calculations. Lastly, a prediction of the power factor and figure of merit of n-type and p-type FeVSb is made by considering the temperature dependent carrier lifetime for the electronic transport terms. This study shows the importance of considering EEI in electronic transport calculations and EPI in phonon transport calculations. Our study is expected to provide results to further explore the thermoelectric transport in this material.

cond-mat.str-el↗

arXiv2020

Exploring the suitable theoretical approach for understanding the electronic and magnetic properties of $α$-Iron

We present a comparative electronic structure study using DFT and various beyond-DFT (DFT+$U$, $G_0W_0$, DFT+DMFT) methods for ferromagnetic Iron (Fe) to find better approach for describing the spectral properties of correlated magnetic system. The computed value of $U$ ($W$) is $\sim$5.4 ($\sim$0.8) eV. The calculated spectra of all methods are providing good agreement with experimental spectra (ES) for peaks' positions. But, the proper line shape is only found from DFT+DMFT with correct estimation of incoherent states, which depends on $J$ and form of local Coulomb interactions. The estimation of reduced magnetization as function of reduced temperature using DFT+DMFT shows good agreement with the experimental data. The insight of paramagnetic electronic structure of Fe is also explored. This work suggests that even for simple correlated magnetic metal, we need DFT+DMFT method to reproduce the ES with great accuracy.

cond-mat.str-el↗

arXiv2020

Coexistance of non-Fermi liquid behavior and bi-quadratic exchange coupling in La-substituted CeGe: Non-linear susceptibility and DFT + DMFT study

Studies connected with the investigations of non-Fermi liquid (NFL) systems continue to attract interest in condensed matter physics community. Understanding the anomalous physical properties exhibited by such systems and its related electronic structures is one of the central research topics in this area. In this context, Ce-based and Ce-site diluted (with non-magnetic ions) compounds provide a fertile playground. Here, we present a detailed study of non-linear DC susceptibility and combined density functional theory plus dynamical mean field theory (DFT+DMFT) on Ce0.24La0.76Ge. Theoretical investigation of 4f partial density of states, local susceptibility and self-energy demonstrates the presence of NFL behavior which is associated with fluctuating local moments. Non-linear DC susceptibility studies on this compound reveal that the transition from NFL state to the new phase is due to development of the bi-quadratic exchange coupling and it obeys the non-linear susceptibility scaling. Under the application of magnetic fields, local moments interact spatially through conduction electrons resulting in magnetic fluctuations. Our studies point to the fact that the origin of the observed bi-quadratic exchange coupling is due to the spatial magnetic fluctuations.

cond-mat.str-el↗

arXiv2020

Electronic correlations effect on nontrivial topological fermions in CoSi

The present study has been carried out to understand the effect of electronic correlations on the recently found fermions in CoSi. For which the spectral functions of bulk and (001) surface of CoSi have been investigated by using DFT+DMFT advanced methodology at T = 100 K with and without inclusion of spin-orbit coupling (SOC). All the newly found fermions are observed at $Γ$ and $R$ points similar to other theoretical and experimental reports. Our DFT+DMFT calculations for the bulk states have shown one extra hole pocket at $M$ point. Both coherent and incoherent features have been observed in the spectra of bulk CoSi. This indicates the presence of quasiparticle-quasiparticle (QP-QP) interactions which is eventually affecting the lifetime ($τ$) of exotic fermionic QPs. For instance, the calculated $τ$ for QPs at $ω\sim $ -30 and -186 mev are found to be $\sim 10^{-9}$ s and $\sim 10^{-12}$ s, respectively when SOC is not considered. However, $G_0W_0$ corrections have shown $τ$ for spin-1 fermionic QP at $Γ$ to be infinite while for double Weyl fermionic QP at $R$ point to be $\sim 10^{-12}$ s. Their effective masses ($m^*$) have also been calculated as $\sim$ 1.60 and 1.64 at $Γ$ and $R$ points, respectively. Furthermore, the spectral functions at T = 100 K of (001) surface have also shown both coherent and incoherent features. Consequently, at $ω$ = 0 for surface states $τ$ has been calculated of the order $\sim 10^{-8}$s for both without SOC and with SOC inclusions.

cond-mat.str-el↗

arXiv2020

Understanding the temperature and pressure dependent electronic properties of FeSi: DFT+DMFT study

Electronic structures of FeSi and Fe$_{1.02}$Si$_{0.98}$ under pressure (achieved through volume compression) have been investigated by using DFT+DMFT and KKR-CPA methods, respectively. The widening of band-gap with increasing pressure suggests that the experimentally observed insulator to metal transition temperature should shift towards the higher temperature for FeSi. KKR-CPA calculations have shown the presence of impurity states in the gapped region which predicts the half-metallic nature. The closure of gap (in one spin channel) with pressure increment appears to be responsible for experimentally observed semiconductor to metal transition in Fe excess samples at a temperature below 50 K. Magnetic moments at Fe excess sites are found to be decreasing with increasing pressure from 2.4 $μ_B$ per Fe atom (612 Bohr$^3$) to 1.2 $μ_B$ per Fe atom (507 Bohr$^3$). Moreover, for FeSi the calculated local spin susceptibility has shown decreasing behavior with pressure rise similar to experimental result.

cond-mat.str-el↗

arXiv2020

Dependency of XC functionals and role of 3s(2p) orbitals of Co(Si) as core/valence states on the vibrational and thermodynamic properties of CoSi

First-principles phonon calculations along with density functional theory (DFT) play an important role to study the dynamical and thermal properties of materials. Here, we investigate the effect of exchange correlation (XC) functionals on the vibrational and thermodynamic properties of CoSi. The role of 3s(2p) orbitals of Co(Si) as core/valence states on the phonon properties of this compound is also studied. Phonon calculations are carried out by finite displacement method with supercell approach using equilibrium crystal structures obtained from DFT calculations. The calculated results are compared with the existing experiment. Three XC functionals, viz., LDA, PBEsol and SCAN are used for calculating the phonon dispersion, phonon density of states (DOS)/partial DOS and thermal properties of this compound. SCAN is found to give the highest phonon frequency of $\sim$56 meV which is in good agreement with the experimental value, while LDA (PBEsol) gives $\sim$54 ($\sim$55) meV. The zero-point energy is calculated as $\sim$ 10 kJ/mol for all the functionals. The Debye temperatures ($Θ_{D}$) are computed as $\sim$626 K, $\sim$638 K and $\sim$650 K for LDA, PBEsol and SCAN, respectively. The $Θ_{D}$ obtained from LDA gives the good agreement with the reported value. The phonon dispersion and phonon DOS are found to be dependent whether 3s(2p) orbitals of Co(Si) are considered as core or valence states. But, this orbital dependency is seemed to be insignificant on the thermal properties of this compound.

cond-mat.mtrl-sci↗

arXiv2020

First-principles electronic structure, phonon properties, lattice thermal conductivity and prediction of figure of merit of FeVSb half-Heusler

In this work, we have studied the electronic structure of a promising thermoelectric half-Heusler FeVSb using FP-LAPW method and SCAN meta-GGA including spin-orbit coupling. Using the obtained electronic structure and transport calculations we try to address the experimental Seebeck coefficient $S$ of FeVSb samples. The good agreement between the experimental and calculated $S$ suggests the band gap could be $\sim$0.7 eV. This is supported by the obtained mBJ band gap of $\sim$0.7 eV. Further, we study and report the phonon dispersion, density of states and thermodynamic properties. The effect of long range Coulomb interactions on phonon frequencies are also included by non-analytical term correction. Under quasi-harmonic approximation, the thermal expansion behaviour upto 1200 K is calculated. Using the first-principles anharmonic phonon calculations, the lattice thermal conductivity $κ_{ph}$ of FeVSb is obtained under single-mode relaxation time approximation considering the phonon-phonon interaction. At 300 K, the calculated $κ_{ph}$ is $\sim$18.6 W$m^{-1}K^{-1}$ which is higher compared to experimental value. But, above 500 K the calculated $κ_{ph}$ is in good agreement with experiment. A prediction of figure of merit $ZT$ and efficiency for p-type and n-type FeVSb is made by finding out optimal carrier concentration. At 1200 K, a maximum $ZT$ of $\sim$0.66 and $\sim$0.44 is expected for p-type and n-type FeVSb, respectively. For p-type and n-type materials, maximum efficiency of $\sim$12.2 \% and $\sim$6.0 \% are estimated for hot and cold temperature of 1200 K and 300 K, respectively. A possibility of achieving n-type and p-type FeVSb by various elemental doping/vacancy is also discussed. Our study is expected to help in further exploring the thermoelectric material FeVSb.

cond-mat.str-el↗

arXiv2020

Investigating the effect of temperature dependent many-body interactions on bulk electronic structures and the robust nature of (001) surface states of SnTe

Recently, SnTe has gained attention due to its non-trivial topological nature and eco-friendly thermoelectric applications. We report a detailed temperature dependent electronic structure and thermodynamic properties of this compound using DFT and GW methods. The calculated values of bandgaps by using PBEsol and $G_0W_0$ methods are found to be in good agreement with the experiment, whereas mBJ underestimates the bandgap. The estimated value of fully screened Coulomb interaction ($W$) for Sn (Te) 5$p$ orbitals is $\sim$1.39 ($\sim$1.70) eV. The nature of frequency dependent $W$ reveals that the correlation strength of this compound is relatively weaker and hence the excited electronic state can be properly studied by full-$GW$ many-body technique. The plasmon excitation is found to be important in understanding this frequency dependent $W$. In order to describe the experimental phonon modes, the long range Coulomb forces using nonanalytical term correction is considered. The temperature dependent electron-electron interactions (EEI) reduces the bandgaps with increasing temperature. The value of bandgap at 300 K is obtained to be $\sim$161 meV. The temperature dependent lifetimes of electronic state along W-L-$Γ$ direction are also estimated. This work suggests that EEI is important to explain the high temperature transport behaviour of SnTe. We have also explored the possibility of protecting the (001) surface states via mirror and time-reversal symmetry. These surface states are expected to be robust against the point and line defects.

cond-mat.str-el↗

arXiv2020

Exploring the possibility of enhancing the figure-of-merit ( $>$ 2) of Na$_{0.74}$CoO$_{2}$: A combined experimental and theoretical study

Search of new thermoelectric (TE) materials with high \textit{figure-of-merit} (ZT) is always inspired the researcher in TE field. Here, we present a combined experimental and theoretical study of TE properties of Na$_{0.74}$CoO$_{2}$ compound in high-temperature region. The experimental Seebeck coefficient (S) is found to vary from 64 to 118 $μ$V/K in the temperature range $300-620$ K. The positive values of S are indicating the dominating p-type behaviour of the compound. The observed value of thermal conductivity ($κ$) is $\sim$ 2.2 W/m-K at 300 K. In the temperature region $300-430$ K, the value of $κ$ increases up to $\sim$ 2.6 W/m-K and then decreases slowly till 620 K with the corresponding value of $\sim$ 2.4 W/m-K. We have also carried out the theoretical calculations and the best matching between experimental and calculated values of transport properties are observed in spin-polarized calculation within DFT+\textit{U} by chosen \textit{U} = 4 eV. The maximum calculated value of ZT is found to be $\sim$ 0.67 at 1200 K for p-type conduction. Our computational study suggests that the possibility of n-type behaviour of the compound which can lead to a large value of ZT at higher temperature region. Electron doping of $\sim$ 5.1$\times$10$^{20}$ cm$^{-3}$ is expected to give rise the high ZT value of $\sim$ 2.7 at 1200 K. Using these temperature-dependent ZT values, we have calculated the maximum possible values of efficiency ($η$) of thermoelectric generator (TEG) made by p and n-type Na$_{0.74}$CoO$_{2}$. The present study suggests that one can get the efficiency of a TE cell as high as $\sim$ 11$\%$ when the cold and hot end temperature are fixed at 300 K and 1200 K, respectively. Such high values of ZT and efficiency suggest that Na$_{0.74}$CoO$_{2}$ can be used as a potential candidate for high-temperature TE applications.

cond-mat.mtrl-sci↗

arXiv2019

A detailed electronic structure study of Vanadium metal by using different beyond-DFT methods

We report a detailed electronic structure calculation for Vanadium (V) using DFT, DFT+$U$, $G_0W_0$, $GW_0$ and DFT+DMFT methods. The calculated values of $W$, $U$ and $J$ by cRPA method are $\sim$1.1, $\sim$3.4 and $\sim$0.52 eV, respectively. The comparison between calculated spectra (CS) and experimental spectra (ES) suggests that $W$ ($U$) is more accurate for DFT+$U$ (DFT+DMFT) method. The CS, obtained by these methods, give fairly good agreement with ES for peaks' positions except $GW_0$. The shallowness of the dips lying $\sim$ -1.5 eV and $\sim$1.0 eV in ES are properly explained by DFT+DMFT method only, due to the presence of incoherent $t_{2g}$ states. This work suggests that for the proper explanation of ES, sophisticated many-body theory is needed even for the simple metal.

cond-mat.str-el↗

arXiv2019

Achieving high figure-of-merit in Nb-doped Na$_{0.74}$CoO$_{2}$ compound at high temperature region

We report the thermoelectric (TE) properties of Na$_{0.74}$Co$_{0.95}$Nb$_{0.05}$O$_{2}$ in the temperature range $300-1200$ K, as a potential candidate for p-type thermoelectric material. The experimental values of Seebeck coefficient (S) are $ \sim $ $82-121$ $ μ$V/K measured in the temperature range $300-620$ K. The positive values of S in the entire temperature range indicates p-type behaviour of the compound. At 300 K the experimental value of thermal conductivity ($ κ$) is $ \sim $ 1.88 W/m-K that increases up to $ \sim $ 420 K, then decreases till 620 K with corresponding value $ \sim $ 1.86 W/m-K. To understand the experimentally observed transport properties, we have calculated S and $ρ$ of this compound. Then, based on theoretical understanding, we have estimated \textit{figure-of-merit} (ZT) up to 1200 K by using calculated S and $ρ$ values with extrapolated experimental $κ$. The value of ZT is found to be $\sim$ 0.03 at 300 K, whereas, the highest value is observed as $\sim$ 1.7 at 1200 K. Finally, we have calculated the efficiency ($η$) by keeping the cold end temperature (T$_{c}$) fixed at 500 K and varying hot end temperature (T$_{h}$) from 500 to 1200 K, respectively. The maximum value of $η$ is found to be $\sim$ 8 %, when T$_{c}$ and T$_{h}$ are fixed at 500 and 1200 K, respectively. This result suggests that Na$_{0.74}$Co$_{0.95}$Nb$_{0.05}$O$_{2}$ compound can be used as a p-leg for making high temperature TE generator (TEG).

cond-mat.str-el↗

arXiv2019

Thermoelectric properties, efficiency and thermal expansion of ZrNiSn half-Heusler by first-principles calculations

In this work, we try to understand the experimental thermoelectric (TE) properties of a ZrNiSn sample with DFT and semiclassical transport calculations using SCAN functional. SCAN and mBJ provide the same band gap $E_{g}$ of $\sim$0.54 eV. This $E_{g}$ is found to be inadequate to explain the experimental data. The better explanation of experimental Seebeck coefficient $S$ is done by considering $E_{g}$ of 0.18 eV which suggests the non-stoichiometry and/or disorder in the sample. Further improvement in the $S$ is done by the inclusion of temperature dependence on chemical potential. In order to look for the possible enhanced TE properties obtainable in ZrNiSn with $E_{g}$ of $\sim$0.54 eV, power factor and optimal carrier concentrations are calculated. The optimal electron and hole concentrations required to attain highest power factors are $\sim$7.6x10$^{19}$ cm$^{-3}$ and $\sim$1.5x10$^{21}$ cm$^{-3}$, respectively. The maximum figure of merit $ZT$ calculated at 1200 K for n-type and p-type ZrNiSn are $\sim$0.6 and $\sim$0.7, respectively. The % efficiency obtained for n-type ZrNiSn is $\sim$5.1 % while for p-type ZrNiSn is $\sim$6.1 %. The $ZT$ are expected to be further enhanced to $\sim$1.2 (n-type) and $\sim$1.4 (p-type) at 1200 K by doping with heavy elements for thermal conductivity reduction. The phonon properties are also studied by calculating dispersion, total and partial density of states. The calculated Debye temperature of 382 K is in good agreement with experimental value of 398 K. The thermal expansion behaviour in ZrNiSn is studied under quasi-harmonic approximation. The average linear thermal expansion coefficient $α_{ave}(T)$ of $\sim$7.8x10$^{-6}$ K$^{-1}$ calculated in our work is quite close to the experimental values.

cond-mat.str-el↗

arXiv2019

Two functionals approach in DFT for the prediction of thermoelectric properties of Fe$_{2}$ScX (X = P, As, Sb) full Heusler compounds

In the quest of new thermoelectric (TE) materials with high power factors, full-Heusler compounds having flat band are found to be promising candidates. In this direction, Fe$_{2}$ScX (X=P,As,Sb) compounds are investigated using mBJ for the band gap and SCAN to describe the electronic bands and phonon properties for TE applications. The band gaps obtained from mBJ are 0.81 eV, 0.69 eV and 0.60 eV for Fe$_{2}$ScX compounds, respectively. 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 TE parameters are calculated for both hole and electron dopings under semiclassical theory. We use simple, but reasonable method to estimate phonon relaxation time ($τ_{ph}$). Using the specific heat, estimated $τ_{ph}$ and slopes (phase velocity) of acoustic branches in the linear region, lattice thermal conductivity ($κ_{ph}$) at 300 K is calculated for three compounds. The obtained values of $κ_{ph}$ with constant $τ_{ph}$ 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 TE application. The $ZT$ values for n-type Fe$_{2}$ScX, in 900-1200 K, are 0.34-0.43, 0.40-0.48 and 0.45-0.52, respectively. While, the p-type Fe$_{2}$ScX have $ZT$ 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, Fe$_{2}$ScX compounds can be promising materials in high temperature power generation application on successful synthesis and further $κ_{ph}$ reduction by methods like nanostructuring.

cond-mat.str-el↗

arXiv2018

Effect of density functionals on the vibrational and thermodynamic properties of Fe$_{2}$VAl and Fe$_{2}$TiSn compounds

First-principles phonon calculations along with Kohn-Sham density functional theory (DFT) is an essential tool to study the lattice dynamics, thermodynamical properties and phase-transitions of materials. The two full-Heusler compounds Fe$_{2}$VAl and Fe$_{2}$TiSn are studied for lattice vibration dependent properties using finite displacement method and supercell approach. For the investigation, four density functionals viz., LDA, PBE, PBEsol and meta-GGA SCAN are employed. Using these functionals, phonon dispersion, phonon density of states (DOS), partial density of states (PDOS) thermal propertis and zero-point energy are calculated at equilibrum lattice parameters under harmonic approximation. For the two compounds the Debye temperatures are calculated from the obtained phonon DOS which are $\sim$660 K and $\sim$540 K, respectively. The obtained results from different functionals are compared among each other. The overall phonon energy in the dispersion is found to be $\sim$15 meV higher in Fe$_{2}$VAl than the Fe$_{2}$TiSn compounds. For the two compounds PBE is yielding the lowest phonon frequencies while LDA or SCAN functional is giving the highest. The same pattern is observed in phonon DOS plots of two compounds. The zero-point energy calculated is the highest from SCAN (21.04 and 16.95 J) and the lowest from PBE functionals (20.09 and 16.02 J) obeying the same trend as frequency for both compounds. A general prediction of nature of lattice thermal conductivity is made based on the velocities of acoustic phonons which is in agreement with the qualitative behavior of reported experimental thermal conductivity of two compounds. Phonon spectra obtained from PBE and SCAN have similar general features while those from LDA and PBEsol have resembling features for Fe$_{2}$VAl, while this trend is not observed for the compound Fe$_{2}$TiSn.

cond-mat.str-el↗

arXiv2018

Investigating the electronic structure of MSi (M = Cr, Mn, Fe $\&$ Co) and calculating $\textit{$U_{eff}$}$ $\&$ $\textit{J}$ by using cDFT

We have investigated electronic energy band structures and partial density of states of intermetallic compounds $\textit{viz.}$ CrSi, MnSi, FeSi and CoSi, by using density functional theory (DFT). CrSi \& MnSi, FeSi and CoSi have metallic, indirect band gap semiconducting (band gap $ \sim$ 90 meV) and semi-metallic ground state, respectively. On studying the band structures while going across the series Cr-Co, the occupied bands around the Fermi level are getting narrower while the unoccupied bands are getting wider. Similarly, band edge in partial density of states is shifting away from the Fermi level due to increased hybridizations. The effective mass of holes for FeSi is found to be much larger than that of electrons, giving rise to positive Seebeck coefficient and negative Hall coefficient, which is consistent with experimental results. For different ionic states of 3\textit{d}-metal, the values of $\textit{$U_{eff}$}$ and $\textit{J}$ are evaluated by using constrained DFT method. $\textit{$U_{eff}$}$ ($\textit{J}$) for $2^{+} $ ionic state across the series are $\sim $ 3.3 eV ($\sim $0.65 eV), $\sim $ 3.7 eV ($\sim $0.72 eV), $\sim $ 4.4 eV ($\sim $ 0.82 eV) and $\sim $ 4.5 eV ($\sim $ 0.87 eV). $λ$ and $\textit{J}$ are also calculated by considering Yukawa form of Coulomb interaction. $λ$ values for $2^{+} $ ionic state along the series are $\sim $ 1.97 a.$u^{-1}$, $\sim $ 2.07 a.$u^{-1}$, $\sim $ 2.07 a.$u^{-1}$ and $\sim $ 2.34 a.$u^{-1}$. 4$\textit{s}$ electrons are found to be contributing more in screening the 3$\textit{d}$ electrons as compared to 4$\textit{p}$ electrons of 3$\textit{d}$ metals.

cond-mat.str-el↗