arXiv subjects

Sudhir K. Pandey

Publications and source records attributed to Sudhir K. Pandey.

At least 19 recordsLinked to original sources

arXiv2026

Multiple topological electronic and phononic quasiparticle excitations in hexagonal TTe (T=Hf, Zr & Ti) crystals

The exploration of topological fermions and bosons marks a new chapter in condensed matter physics, unveiling rich and unconventional phenomena. Particularly in quantum field theory, exotic quasiparticle excitations such as Dirac and Weyl fermions can serve as direct analogs, and their recent experimental realizations have sparked significant interest in these topological quasiparticles. Although there are various reports on the coexistence of unconventional fermionic quasiparticles such as spin-1/2 (type-I, type-II, & type-III), spin-1 (threefold degeneracy), nodal line (type-I, type-II, & type-III) and others, reports on the simultaneous presence of such quasiparticles in both electronic and phononic spectra within a single material remain extremely limited. Herein, using state-of-theart ab initio calculations, we propose the HfTe class of materials, which hosts coexisting type-I, type-II, and type-III Weyl and nodal line phases, along with pseudospin-1/spin-1 quasiparticles in both electronic and phononic states. We have found that these excitations are robust against variations in exchange-correlation functionals, spin-orbit coupling, and lattice parameters, confirming that multiple topological phases in this class of materials are likely to be observed experimentally.

cond-mat.mtrl-sci↗

arXiv2026

C-BerryTrans: A C++ code for first-principles calculation of Berry-curvature-driven anomalous Hall and Nernst conductivities

We present \textit{C-BerryTrans}, a C++ code for \textit{ab-initio} calculations of Berry-curvature-driven AHC \textit{i.e.}, $σ_{μν}^{AHC}$ and ANC \textit{i.e.}, $α_{μν}^{ANC}$. The code extracts eigenvalues and momentum-matrix from WIEN2k calculations and evaluates the Berry curvature using a Kubo-like formalism. It parallelizes $\boldsymbolΩ$ evaluation over \textbf{\textit{k}}-points and stores band-resolved curvature data in binary format. This design enables rapid post-processing of AHC and ANC over a wide range of temperature ($T$) and chemical potential ($ω$) values in a single run. The code is benchmarked on ferromagnetic materials- Fe, Fe$_3$Ge, Pd, Fe$_3$Al, and Co$_2$FeAl. For Fe, the $σ_{xy}^{AHC}$ is obtained to be $\sim$775 ($\sim$744) $S/cm$ at 0 (300) $K$. For Fe$_3$Ge, the value of $σ_{xy}^{AHC}$ is 311 $S/cm$ at 300 $K$. Nextly, for Co$_2$FeAl, the computed value of $σ_{xy}^{AHC}$ at 2 $K$ is $\sim$56 $S/cm$. Moving further, magnitude of $α_{xy}^{ANC}$ for Pd is found as $\sim$0.97 $AK^{-1}m^{-1}$ at 300 K. For Fe$_3$Al, the maximum magnitude of $α_{xy}^{ANC}$ for $T\leq$500 $K$ is computed as $\sim$2.83 $AK^{-1}m^{-1}$. Lastly, for Co$_2$FeAl, the value of $α_{xy}^{ANC}$ is found to be $\sim$0.10 $AK^{-1}m^{-1}$ at 300 $K$. These results show good agreement with the reported data. With its accuracy and user-friendly workflow, \textit{C-BerryTrans} provides a powerful tool for exploring $\boldsymbolΩ$-driven transport and is well suited for high-throughput materials discovery. The code also enables the evaluation of $\boldsymbolΩ$-derived AHC/ANC contributions along user-defined \textbf{\textit{k}}-point paths. Additionally, the code is equipped with a visualization module that allows analysis of \textbf{\textit{k}}-point contributions to AHC or ANC in any material.

cond-mat.mtrl-sci↗

arXiv2026

Band gap renormalization, carrier mobility, and transport in Mg$_{2}$Si and Ca$_{2}$Si: \textit{Ab initio} scattering and Boltzmann transport equation study

We perform first-principles electron-phonon interaction (EPI) calculations based on many-body perturbation theory to study the temperature-dependent band-gap and charge-carrier transport properties for Mg$_{2}$Si and Ca$_{2}$Si using the Boltzmann transport equation (BTE) under different relaxation-time approximations (RTAs). For a PBE band gap of 0.21 (0.56) eV in Mg$_{2}$Si (Ca$_{2}$Si), a zero-point renormalization correction of 29-33 (37-51) meV is obtained using various approaches, while the gap at 300 K is 0.15-0.154 (0.46-0.5) eV. The electron mobility ($μ_{e}$), with a detailed convergence study at 300 K, is evaluated using linearized (self-energy and momentum RTA, or SERTA and MRTA) and iterative BTE (IBTE) solutions. At 300 K, the $μ_{e}$ values are 351 (100), 573 (197), and 524 (163) cm$^{2}V^{-1}s^{-1}$ from SERTA, MRTA, and IBTE, respectively, for Mg$_{2}$Si (Ca$_{2}$Si). SERTA (MRTA) provides results in better agreement with IBTE at higher (lower) temperatures, while SERTA-derived $μ_{e}$ closely matches experimental $μ_{e}$ values for Mg$_{2}$Si. Thermoelectric (TE) transport coefficients significantly influenced by the choice of RTA, with SERTA and MRTA yielding improved agreement with experimental results compared to constant RTA (CRTA) for Mg$_{2}$Si over an electron concentration range of $10^{17}$ to $10^{20}$ cm$^{-3}$. The lattice thermal conductivity ($κ_{ph}$) at 300 K due to phonon-phonon interactions is estimated to be 22.7 (7.2) W m$^{-1}K^{-1}$ for Mg$_{2}$Si (Ca$_{2}$Si). The highest calculated figure of merit (zT) under CRTA is 0.35 (0.38), which decreases to 0.08 (0.085) when EPI is included using MRTA. This study clearly identifies the critical role of EPI in accurate transport predictions of TE silicides. Finally, we explore strategies to enhance zT by reducing $κ_{ph}$ through nanostructuring and mass-difference scattering.

cond-mat.mtrl-sci↗

arXiv2026

Significant first-principles electron-phonon coupling effects in the LiZnAs and ScAgC half-Heusler thermoelectrics

The half-Heusler (hH) compounds are currently considered promising thermoelectric (TE) materials due to their favorable thermopower and electrical conductivity. Accurate estimates of these properties are therefore highly desirable and require a detailed understanding of the microscopic mechanisms that govern transport. To enable such estimations, we carry out comprehensive first-principles computations of one of the primary factors limiting carrier transport, namely the electron-phonon ($e-ph$) interaction, in LiZnAs and ScAgC. Our study first investigates their electron and phonon dispersions and then examines the temperature-induced renormalization of the electronic states. We then solve the Boltzmann transport equation (BTE) under multiple relaxation-time approximations (RTAs) to evaluate the carrier transport properties. Phonon-limited electron and hole mobilities are comparatively assessed using the linearized self-energy and momentum RTAs (SERTA and MRTA), and the exact or iterative BTE (IBTE) solutions within $e-ph$ coupling. Electrical transport coefficients for TE performance are also comparatively analyzed under the constant RTA (CRTA), SERTA, and MRTA schemes. The lattice thermal conductivity, determined from phonon-phonon interaction, is further reduced through nanostructuring techniques. The bulk LiZnAs (ScAgC) compound achieves the highest figure of merit ($zT$) of 1.05 (0.78) at 900 K with an electron doping concentration of 10$^{18}$ (10$^{19}$) cm$^{-3}$ under the MRTA scheme. This value significantly increases to 1.53 (1.0) for a 20 nm nanostructured sample. The remarkably high $zT$ achieved through inherently present phonon-induced electron scattering effects, combined with grain-boundary engineering, opens a promising path for discovering highly efficient and accurate next-generation hH TEs.

cond-mat.mtrl-sci↗

arXiv2026

Many-body \textit{ab initio} study of quasiparticles, optical excitations, and excitonic properties in LiZnAs and ScAgC for photovoltaic applications

Using first-principles density-functional theory and many-body excited-state calculations, we study the quasiparticle band structure, optical and excitonic properties of two half-Heusler (HH) compounds, namely LiZnAs and ScAgC, for photovoltaic (PV) applications. Our results reveal a direct bandgap semiconducting behavior in LiZnAs (ScAgC) with a value of 1.5 (1.0) eV under an accurate G$_0$W$_0$ calculation. The highest value of the imaginary part of dielectric function is found as 52 (87), 77 (87), 88 (91) using the independent-quasiparticle approximation, local field effects in random-phase approximation, and electron-hole interaction in the Bethe-Salpeter equation, respectively. Both materials demonstrate a high refractive index, high absorption coefficients (1.2-1.6 $\times 10^6 cm^{-1}$), and low reflectivity (< 40%) in active region of the solar spectrum. The triply degenerate bright excitons (exciton A) at the main absorption peak and a considerable number of bright excitonic states in the visible region, are observed; however, the excitons oscillator strength are comparatively weaker in ScAgC than in LiZnAs. We further discuss the exciton character contributing to intense optical interband transitions and reveal that direct band gap is associated to the loosely bound exciton A state with binding energy of 45 (56) meV in LiZnAs (ScAgC). Exciton A is found to be highly localized (delocalized) in momentum (real) space, indicating the presence of Mott-Wannier type excitons at bandgap. Finally, we assess the solar efficiencies using the spectroscopic limited maximum efficiency (SLME) model and find SLME values of 32% for LiZnAs and 31% for ScAgC at a 0.4 $μ$m thin-film thickness. These findings highlight the significant role of excitons in solar energy absorption process and also suggest that both are highly suitable candidates for single-junction thin-film solar cells.

cond-mat.mtrl-sci↗

arXiv2025

Realization of strain induced multiple topological phases in Cu$_2$SnS$_3$: An $ab$-$initio$ study

The search of multiple topological phases (TPs) and their transitions by tuning different parameters through chemical substitutions, electric field, magnetic field, strain and Floquet engineering, etc has garnered a widespread attention in recent time. In spite of great effort, the observations of multiple TPs in a single material and multiple TP transitions in the presence of one parameter remain elusive. Here we demonstrate the presence of multiple TPs and their transitions with uniaxial compressive strain (UCS) in orthorhombic Cu$_2$SnS$_3$ by using $state$-$of$-$the$-$art$ $ab$-$initio$ calculations. In the absence of spin-orbit coupling (SOC), the Cu$_2$SnS$_3$ exhibits a single (type-II) nodal-ring and in the presence of SOC, it hosts Weyl phase with seven Weyl points (three at $Γ$ and four at general positions) along with nodal arcs. On the application of UCS, it remains type-II nodal-ring $<5.5$\%, which further evolves into type-III nodal-ring for $5.5\% \leq$ UCS $<5.6$\%. Interestingly, at 5.6\% of UCS, it shows Weyl phase with four Weyl nodes even in the absence of SOC. All the above-mentioned seven Weyl points persist below $5$\% of UCS. For 5\% $\leq$ UCS $<5.6$\%, four Weyl points (at general positions) disappear and nodal-arcs remain intact in all the studied range of UCS. The TPs observed in the absence of SOC appears to arise due to the presence of strain driven topological flat band, which is typically reported to be seen in kagome and Lieb lattices.

cond-mat.mtrl-sci↗

arXiv2025

Spin-dependent orbital selectivity and partial Kondo-screening in magnetically ordered Hund's metal

Hund's metallicity in 3$d$ transition metal oxides constitutes a rare class of compounds, since they have been long understood considering the dominance of Hubbard $U$. $\mathrm{LiV_2O_4}$\& $\mathrm{Sr_2CoO_4}$ belong to this rare class of metals; among them, $\mathrm{LiV_2O_4}$ has been the subject of extensive investigations for its unconventional heavy-fermion behavior, while studies on $\mathrm{Sr_2CoO_4}$ remain limited despite its anomalous ferromagnetic ground state. In this study, we report an unusual spin-orbital selective localization in $\mathrm{Sr_2CoO_4}$ leading to a sharp Kondo resonance at $\sim$70 K in the spin-$up$ channel of orbitals of $t_{2g}$ symmetry using a combination of Density functional theory and Dynamical mean field theory (DFT+DMFT) calculations. Correspondingly, an appreciable reduction in the magnetization below $T$=100 K further suggests partial Kondo screening of local moments active at low temperatures, explaining its effective spin magnetization state and upturn in its resistivity observed in experimental reports. We note a significant effect of Hund's induced spin-orbital selective incoherence in dictating the temperature evolution of its macroscopic observables e.g. spin-spin correlation function and effective local moment. Our results reveal a potentially distinct/new form of spin-dependent selectivity induced via Hund's coupling in addition to the conventional orbital-selectivity in the Hund's metals, as a plausible key mechanism in stabilizing their long-range magnetic order.

cond-mat.str-el↗

arXiv2025

First-principles many-body study for electronic, optical, and excitonic properties of RbTlCl3 perovskite for solar cells

We present a detailed many-body ab initio study of the valence-skipper RbTlCl$_{3}$ perovskite compound for photovoltaic (PV) applications. The electronic and optical properties, both with and without spin-orbit coupling, have been calculated using density functional theory (DFT) and many-body excited-state calculations. The band gap, which is indirect in nature, is found to be 0.95 eV and 0.89 eV from PBE and PBEsol, respectively. The optical properties have been computed using four different approximations: independent particle approximation (IPA), IPA with scissor correction (IQPA), random phase approximation for local-field effects (LFEs), and the Bethe-Salpeter equation (BSE). The estimated highest value of the imaginary part of the dielectric function using IQPA is 7 at 2 eV, which slightly decreases to 5.7 due to LFEs. Within BSE, the peak value is obtained to be maximum at 1.6 eV with a magnitude of 10.8, which indicates the strong excitonic effect below the optical gap. Large number of bright and dark bound excitons are found, where the binding energies of four main bound bright excitons are found in the range of 299-350 meV. The exciton amplitude in both reciprocal and real space is analyzed. The main bound bright exciton is localized in the reciprocal space, while this exhibits a delocalized nature in real space. The BSE predicts a highest absorption coefficient of 3.6 $\times$ $10^{6}$ cm$^{-1}$ at 1.7 eV, while a minimum reflectivity in the active region of the solar energy spectrum is obtained to be around 2.7\%. Finally, the solar efficiency has been estimated using the spectroscopic limited maximum efficiency approach and obtained highest value is 15.5% at a thickness of 0.5 $μ$m. These findings reveal a significant excitonic effect in the absorption spectra of RbTlCl$_{3}$ and highlight its potential as a promising material for single-junction thin-film solar cells.

cond-mat.mtrl-sci↗

arXiv2025

Strain-tunable type-II to type-III & Gimbal nodal line transition in Imm2-phase of Cu$_2$SnS$_3$: An ab-initio study

Topological nodal line semimetals (NLSMs) represent an intriguing quantum phase, opening new avenues in materials science for practical applications such as anisotropic transport devices, high-mobility conductors, unconventional thermoelectrics, and nonlinear optical devices. Recently, Cu$_2$SnS$_3$ has been theoretically proposed as a type-II NLSM, with its Fermi surface containing only one nodal ring. Here, we demonstrate how uniaxial, equi-biaxial, and equi-triaxial strains affect the nodal line state of the $Imm2$-phase of Cu$_2$SnS$_3$ by using state-of-the-art ab-initio calculations. Under the application of uniaxial compressive strain (UCS) along the a-direction, the plane of the nodal line evolves from the $k_x$-$k_z$ to $k_y$-$k_z$ for 6\%$\leq$UCS$\leq$8\%. In contrast, under uniaxial tensile strain (UTS), the nodal line remains in the ($k_x$-$k_z$) plane across the entire studied range of UTS. Interestingly, on the application of equi-biaxial tensile strain (EBTS) along a-b (a-c) directions, it hosts only one nodal ring below 8\% ($<$6\%), which further evolves into three (seven) nodal-ring for EBTS of 8\% (6\%$\leq$EBTS$\leq$8\%). Upon the application of EBTS along a-c directions, we found two sets of three mutually orthogonal, intersecting nodal loops (topological gimbals). Apart from this, under the application of equi-biaxial compressive strain (EBCS) along the a-b (a-c) directions, it exhibits only one nodal ring up to 8\% (7\%). Beyond this, the nodal line completely vanishes and does not reappear at higher values of EBCS. Under equi-triaxial tensile strain (ETTS), Cu$_2$SnS$_3$ exhibits only one nodal-ring $<$6\%, which subsequently transforms into five nodal-ring for 6\%$\leq$ETTS$\leq$8\%. However, under the application of equi-triaxial compressive strain (ETCS), as in EBCS, only one nodal line exists up to 6\% ETCS.

cond-mat.mtrl-sci↗

arXiv2025

Revisiting the Topological Nature of TaIrTe4, SrSi2, and Cu2XY3: An ab-initio Investigation

Several topological electronic materials have been theoretically predicted, leading to a comprehensive catalog systematically characterized by their band crossings. Researchers have attempted to experimentally verify the topological nature of some materials from the present catalogs, but not all efforts have yielded positive results. Here, we introduce a possible reason for the discrepancies between theoretical and experimental results. In this direction, firstly we have revisited the nature of the well-known topological materials TaIrTe$_4$ and SrSi$_2$ using \textit{state-of-the-art ab-initio} calculations, and found additional Weyl points in both materials that were missing in previously reported studies. Then we have verified the recently predicted topological states of the \textit{Imm2}-phase of Cu$_2$XY$_3$ (X=Si, Ge, Sn \& Y=S, Se, Te). Contrary to previously reported results, we did not find any Weyl points or nodal arcs in Cu$_2$SnTe$_3$. Notably, our theoretical results reveal that Cu$_2$SiTe$_3$, Cu$_2$GeTe$_3$ and Cu$_2$GeSe$_3$ each host four small nodal rings, eight Weyl points, and eight nodal arcs, respectively, which differ from previous studies. Considering Cu$_2$SnS$_3$ as an example, we have also investigated the robustness of the topological phase against local strain. Our study provides insights into the inconsistencies between theoretical predictions and experimental results, and demonstrates how the topological phase is sensitive to changes in lattice parameters, atomic positions, and exchange-correlation functionals.

cond-mat.mtrl-sci↗

arXiv2025

C-BerryANC: A first-principle C++ code to calculate Berry Curvature dependent anomalous Nernst conductivity in any material

The anomalous Nernst conductivity (ANC) is a key transport property in magnetic and topological materials, arising from the Berry curvature ($\boldsymbolΩ$) of electronic bands. It offers deep insight into the underlying topology and thermoelectric behavior. While Wannier interpolation have become popular for calculating ANC due to their computational efficiency, their accuracy critically depends on the quality of the Wannierization, which can be challenging for entangled bands or materials with complex band crossings. These limitations highlight the need for a direct first-principles approach to reliably compute ANC from ab-initio electronic structures. Here, we present a C++ based code named C-BerryANC that calculates $\boldsymbolΩ$-dependent ANC by directly using the eigenvalues and momentum-matrices obtained from DFT calculations. Presently, the code is interfaced with WIEN2k package which uses all-electron approach and full-potential linearized augmented plane wave (FP-LAPW) based method. For efficiently handling dense k-mesh, calculation of $\boldsymbolΩ$ is made parallel over k-points using the OpenMP method. Additionally, the code stores band-resolved components of $\boldsymbolΩ$ in binary files thereby reducing the memory occupancy and providing fast post-process option to compute ANC for any range of chemical potential and temperature values. Also, as compilation of C++ modules produce executable files which are in machine level language, computational speed of C-BerryANC is very fast. The code is benchmarked over some well-known materials exhibiting ANC. These includes- Pd, Fe$_3$Al & Co$_2$FeAl. The obtained values of ANC is found to in good agreement with the previously reported data. This highlights the accuracy, efficieny and reliability of the C-BerryANC code.

cond-mat.mtrl-sci↗

arXiv2025

$\textit{PY-BerryAHC}$: An $\textit{ab-initio}$ python 3 code to calculate Berry Curvature dependent Anomalous Hall Conductivity in any material

The anomalous Hall conductivity (AHC) in materials has long been a topic of debate. Studies reveal that AHC originates from the Berry curvature ($\boldsymbolΩ$) of Bloch states. Accurate computation of AHC is crucial for predicting material properties and guiding experimental studies in topological and spintronic applications. Traditional approaches often rely on wannier interpolation, which can introduce inaccuracies and computational overhead. Also, reliability of the wannierization technique becomes questionable when the bands are highly entangled and dispersive. This demands the calculation of AHC using the $\textit{first-principle}$ approach. Here, we present $\textit{PY-BerryAHC}$, a Python 3 based code that directly computes $\boldsymbolΩ$ and AHC using WIEN2k output. Since, WIEN2k employs an all-electron full-potential linearized augmented plane wave method, $\textit{PY-BerryAHC}$ provides highly accurate AHC results. The code efficiently handles large $\textbf{k}$-grids by parallelizing $\boldsymbolΩ$ computations over $\textbf{k}$-points. Also, it stores band-resolved $\boldsymbolΩ$ in a binary file, thereby greatly reducing the required storage memory and allowing fast post-processing to compute AHC. $\textit{PY-BerryAHC}$ has been validated on well-known materials exhibiting AHC. These include- Fe, Fe$_3$Ge & Co$_2$FeAl. At 300 K, the calculated magnitude of $σ_{xy}$ for Fe & Fe$_3$Ge is found to be 744 $S/cm$ & 311 $S/cm$, respectively. For Co$_2$FeAl, the magnitude of $σ_{xy}$ is obtained to be $\sim$56 $S/cm$ and is found to be constant with the change in temperature from 0-300 K. These results are in good agreement with previously reported theoretical and experimental data. This ensures the accuracy, reliability and efficiency of the code. The code is also provided with a post-processing tool to visualize $\boldsymbolΩ$.

cond-mat.str-el↗

arXiv2025

Studying the Seebeck coefficient and exploring the possibility of enhancing ZT upto 1.8 for NaCo$_2$O$_4$ in high temperature region

Here, we have studied the temperature dependent Seebeck coefficient (S) of the NaCo$_2$O$_4$ (NCO) by using experimental and computational methods. The range of experimentally obtained S is $\sim$55 to 103 $μ$V/K in the temperature range of 300-600 K, which confirms the p-type behaviour of NCO. The electronic structure of this compound is obtained via DFT+U formalism. The band dispersion and partial density of states confirms the magnetic and half metallic nature. Furthermore, in the transport properties, the obtained S using a U = 4 eV gives the best match with experimental data. The temperature and chemical potential dependent S$^{2}$$σ$$/$$τ$ is calculated using the obtained electronic transport properties, in which the maximum value obtained for p(n)-type doping is $\sim$22(61)$\times$10$^1$$^4$ $μ$WK$^{-2}$cm$^{-1}$s$^{-1}$. The possibility of enhancing the ZT is identified, and it is calculated in temperature range 300-1200 K. The maximum calculated value of ZT is 0.64 for p-type and 1.8 for n-type doping at 1200 K. The calculated carrier concentration obtained for p(n) type doping at 1200 K is $\sim$1.17 (1.6)$\times$10$^2$$^2$ cm$^-$$^3$. This study suggests that the careful doping of p and n type can enhance the applicability of this compound in thermoelectric for high temperature application.

cond-mat.mtrl-sci↗

arXiv2025

First-principles study of optoelectronic and thermoelectronic properties of the ScAgC half-Heusler compound

Here, we present a theoretical study in the context of photovoltaic (PV) and thermoelectric (TE) applications of ScAgC. The electronic, optical, and thermoelectric properties have been investigated systematically using density functional theory (DFT) and semi-classical Boltzmann transport theory. DFT calculates a direct band gap of 0.47 eV, whereas the $G_{0}W_{0}$ method estimates a band gap of 1.01 eV. We used parabola fitting to estimate the effective mass values for bands B1 to B4 at $Γ$-point, which are -0.087 (-0.075), -0.17 (-0.27), -0.17 (-0.27), and 0.049 (0.058) along the $Γ$-$X$ ($Γ$-$L$) direction, respectively. Furthermore, the optoelectronic properties are calculated and analyzed over an energy range of 0 to 10 eV. The optical conductivity, refractive index, and dielectric function show strong optical transitions in the visible region. The lowest calculated reflectivity is 0.24 at 4.7 eV, and the highest calculated value of the absorption coefficient is $1.7\times10^{6}$ cm$^{-1}$ at $8.5$ eV. At 300 K, we expect a maximum solar efficiency (SLME) of 33\% at a thickness of $1~μ$m. The lattice part of the thermal conductivity shows a maximum value of 3.8 Wm$^{-1}$K$^{-1}$ at $1200$ K. At 1200 K, for electron doping of $3.9\times10^{21}$ cm$^{-3}$, the maximum value of $S^{2}σ/τ$ is $145 \times 10^{14}$ $μ$WK$^{-2}$cm$^{-1}$s$^{-1}$, while for hole doping of $1.5\times10^{21}$ cm$^{-3}$, it is $123 \times 10^{14}$ $μ$WK$^{-2}$cm$^{-1}$s$^{-1}$. The highest $ZT$ at $1200$ K is expected to be $0.53$, whereas the optimal efficiency is predicted to be $8.5\%$ for cold and hot temperatures of $300$ K and $1200$ K, respectively. The collected results suggest that the ScAgC compound is a potential candidate for renewable energy sources such as solar cell and TE applications

cond-mat.mtrl-sci↗

arXiv2025

Mg$_{2}$Si and Ca$_{2}$Si semiconductors for photovoltaic applications: Calculations based on density-functional theory and the Bethe-Salpeter equation

We conduct a comprehensive assessment of the electronic and optical properties, as well as photovoltaic (PV) performance parameters for Mg$_{2}$Si and Ca$_{2}$Si using density-functional theory and Bethe-Salpeter equation (BSE) based methods. The band-gap for Mg$_{2}$Si (Ca$_{2}$Si) is found to be in the range of 0.25-0.6 (0.57-0.96) eV when PBE, PBEsol and mBJ functionals are used. In the independent-particle approximation (IPA), the real and imaginary parts of dielectric function show maximum values of 50 (16.3) at 2.6 (1.0) eV and 61 (16.2) at $\sim$3.24 (3.4) eV, respectively. Within BSE, these respective values change to 59 (17) at 2.5 (0.86) eV and 65 (16.6) at 2.68 (3.1) eV. The excitonic effect is found to be crucial in understanding the experimental optical spectra of Mg$_{2}$Si. However, this effect is relatively weaker in Ca$_{2}$Si. Present study highlights the importance of different levels of theoretical approximations for obtaining the optical spectroscopy data of silicides with a high level of accuracy. Finally, we have evaluated PV efficiency by using spectroscopic limited maximum efficiency (SLME) calculation. On the top of radiative recombination, we have also incorporated non-radiative carrier recombination at a defect trap state via Shockley-Read-Hall (SRH) mechanism to evaluate the efficiency. Among the studied defects, the interstitial Mg (Si) is identified as the most stable in Mg$_{2}$Si (Ca$_{2}$Si) and this provides SRH lifetime of 2 $μs$ (11.3 $ms$). The estimated maximum SLME using BSE absorption spectrum is 1.3 (31.2)\%, which decreases to 1.2 (28.5)\% due to SRH recombination. The present study suggests that Ca$_{2}$Si (Mg$_{2}$Si) is a potential candidate for single-junction (bottom cell in multi-junction) thin-film PV devices.

cond-mat.mtrl-sci↗

arXiv2024

Enhanced Pauli spin response, failure of Stoner \& spin fluctuation models, and presence of 6 $eV$ plasmonic excitations in Ni metal

We revisit the electronic structure of Ni, using the density functional theory (DFT) and dynamical mean-field theory (DMFT) for the theoretical description of its electronic structure properties along with finite-temperature magnetism. Our study provides a comprehensive account of electronic and magnetic properties with the same set of Coulomb interaction parameters, $U$($J$)=5.78(1.1) $eV$ calculated using first-principles approach. The nature of theoretical magnetization curves obtained from DFT \& DFT+DMFT as well as the experimental curve show deviation from the standard models of magnetism, $viz$ Stoner and spin fluctuation model. The temperature dependent DFT approach is found to well describe the finite-temperature M(T) of Ni below critical temperature ($T$ $\leq$ 631 K). The study finds significant Pauli-spin susceptibility contribution to paramagnetic spin susceptibility. Excluding the Pauli-spin response yields a linear Curie-Weiss dependence of the inverse paramagnetic susceptibility at higher temperatures. Also, the presence of mixed valence electronic configuration (3$d^8$, 3$d^9$ and 3$d^7$) is noted. The competing degrees of both the itinerant and localized moment picture of 3$d$ states are found to dictate the finite-temperature magnetization of the system. Furthermore, the quasiparticle scattering rate is found to exhibit strong deviation from $T^2$ behavior in temperature leading to the breakdown of conventional Fermi-liquid theory. In addition to the 6 $eV$ feature, our calculated electronic excitation spectrum confirms the satellite feature extending $\sim$10 $eV$ binding energy, being consistent with experimental observation. Interestingly, our $G_0W_0$ results find the presence of plasmonic excitation contribution to the intensity of famous 6 $eV$ satellite along with the electronic correlation effects,paving way for its reinterpretation.

cond-mat.str-el↗

arXiv2024

Understanding the transport behaviour of PbSe: A combined experimental and computational study

Lead chalcogenides are the promising thermoelectric (TE) materials having narrow band gap. The present work investigates the TE behaviour of PbSe in the temperature range 300-500 K. The transport properties of the sample have been studied using the Abinit and BoltzTrap code. The experimentally observed value of \textit{S} at 300 and 500 K is found to be $\sim$ 198 and 266 $μ$V K$^{-1}$, respectively. The rate of increase in \emph{S} from 300 to 460 (460 to 500) K is found to be $\sim$ 0.4 (0.09). The temperature dependent electrical conductivity \textit{($σ$)} shows the increasing trend, with values of $\sim $ 0.35 $\times $ 10$^{3}$ and $\sim$ 0.58 $\times$ 10$^{3}$ $Ω$$^{-1}$ m$^{-1}$ at 300 and 500 K, respectively. Further, the value of thermal conductivity \textit{($κ$)} at 300 (500) K is found to be 0.74 (1.07) W m$^{-1}$ K$^{-1}$. The value of \textit{$κ$} is found to be increasing upto 460 K and then starts decreasing. The dispersion plot indicates that PbSe is a direct band gap semiconductor with band gap value of 0.16 (0.27) eV considering spin-orbit coupling (without SOC). The partial density of states (PDOS) plot shows that Pb 6p and Se 4p states have a major contribution in the transport properties. The observed and calculated values of \textit{S} gives a good match for SOC case. The calculated \textit{$σ$} and electronic part of thermal conductivity (\textit{$κ{_e}$}) gives good match with the experimental data. The maximum power factor (PF) value of $\sim$ 4.3 $\times$ 10$^{-5}$ W/mK$^{2}$ is observed at 500 K. This work helps in understanding the TE behaviour of PbSe through a novel and insightful alliance of experimental measurements and DFT approach.

cond-mat.mtrl-sci↗

arXiv2024

An ab-initio study of nodal-arcs, axial strain's effect on nodal-lines & Weyl nodes and Weyl-contributed Seebeck coefficient in TaAs class of Weyl semimetals

This work verifies the existence of dispersive \textit{nodal-arcs} and their evolution into Weyl nodes under the effect of spin-orbit coupling (SOC) in NbAs & NbP. The obtained features mimic the observations as reported for TaAs & TaP in our previous work. In addition, this work reports that the number of nodes in TaAs class of Weyl semimetals (WSMs) can be altered via creating strain along $a$ or $c$ direction of the crystal. For instance, the number of nodes in NbAs under SOC-effect along with 2% (3%) tensile-strain in $a$ direction is found to be 40 (56) in its full Brillouin zone (BZ). Besides the nodes, such strain are found to have considerable impact on the nodal-lines of these WSMs when effect of SOC is ignored. A 3\% tensile (compressive) strain along the $a$ ($c$) direction leads to the partially merging of nodal-lines (without SOC) in the extended BZ of NbAs \& NbP, which is not observed in TaAs & TaP within the range of -3% to 3% strain. Apart from this, the work discusses the role of Weyl physics in affecting the Seebeck coefficient ($S$) of any WSM. In this direction, it is discussed that how a symmetric Weyl cone, even if tilted, will have no contribution to the $S$ of WSMs. Furthermore, the work highlights the conditions under which a Weyl cone can contribute to the $S$ of a given WSM. Lastly, the discussion of Weyl contribution to $S$ is validated over TaAs class of WSMs via investigating the features of their Weyl cones and calculating the contributions of such cones to the $S$ of these semimetals. The value of $S$ contributed from Weyl cone is found to be as large as $\sim$65 $μ$\textit{V}/\textit{K} below 25 K in case of TaAs. The findings of this work present a possibility of engineering the topological properties of TaAs class of WSMs via creating strain in their crystal. It also makes the picture of Weyl physics impact on the $S$ of WSMs a more clear.

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